Cell Junctions and Intercellular Communication

1. Cell Junctions and Intercellular Communication

Structural & Functional Classification of Intercellular Adhesion and Communication

1. Cell Junctions and Intercellular Communication

Multicellular organisms require precise physical coordination and direct communication between adjacent cells to maintain tissue integrity and coordinate physiological processes. Cells in tissues are physically linked to one another and to the surrounding extracellular matrix at highly specialised membrane domains known as cell junctions. These junctions are broadly categorised into three functional groups based on their physiological roles.

Occluding JunctionsCreate selective permeability barriers that seal intercellular spaces.

Anchoring JunctionsMechanically attach cells and their cytoskeletons to neighbouring cells or the extracellular matrix.

Communicating JunctionsMediate the direct passage of chemical or electrical signals between adjacent cytoplasms.

Figure: Cell junction taxonomy. The three functional families sit at the top level; each resolves into a vertebrate/animal form and an invertebrate/plant analog that is structurally distinct but functionally equivalent.

1.1 Occluding Junctions

Occluding junctions seal epithelial cells together in a continuous sheet, preventing even small molecules or ions from leaking between the cells from one side of the sheet to the other (paracellular diffusion). This establishes a highly controlled permeability barrier across epithelial cell sheets.

Tight Junctions (Zonula Occludens)

In vertebrates, tight junctions represent the primary occluding junction. They are located at the most apical region of the lateral plasma membrane in epithelial cells. Under electron microscopy, tight junctions appear as a series of focal fusions where the outer leaflets of adjacent plasma membranes are brought into direct, kissing contact.

Figure: Tight junction seal. The outer leaflets of the two membranes are drawn into kissing contact by claudin, occludin, and JAM strands. Their cytosolic tails dock onto the ZO-1/ZO-2/ZO-3 scaffold, which in turn anchors the whole complex to the actin cytoskeleton.

Molecular architecture

Claudins: a family of small (20–24 kDa) four-pass transmembrane proteins that are the principal structural and functional components of tight junction strands. Their extracellular loops interact homophilically or heterophilically with claudins on adjacent cells to seal the intercellular space.
Selective pores: specific claudins also form selective paracellular pores, allowing the regulated passage of specific ions — e.g. claudin-16 for magnesium ions in the kidney.
Occludin: a four-pass transmembrane protein (~65 kDa) that is not essential for the assembly of tight junction strands but plays a critical role in regulating paracellular permeability and structural stability.
Junctional Adhesion Molecules (JAMs): single-pass transmembrane proteins belonging to the immunoglobulin superfamily that assist in junction assembly and cell polarization.
Intracellular scaffold: the cytosolic tails of claudins and occludins bind to Zonula Occludens proteins (ZO-1, ZO-2, and ZO-3). ZO proteins contain multiple protein-interaction domains (PDZ, SH3, and GUK domains), acting as physical adapters that link the transmembrane junctional complexes directly to the actin cytoskeleton.
Claudin / Occludin tails bind→ ZO-1 / ZO-2 / ZO-3 anchor→ Actin cytoskeleton

Physiological functions

Barrier (Gate) Function: restricts the paracellular movement of water, solutes, ions, and pathogens across epithelial barriers, forcing solutes to undergo highly regulated transcellular transport.
Fence Function: prevents the lateral diffusion of integral membrane proteins and lipids between the apical and basolateral domains of the plasma membrane, preserving strict spatial polarization of transport proteins — e.g. SGLT-1 on the apical face vs. GLUT-2 on the basolateral face of intestinal epithelial cells.

Septate Junctions

Invertebrate analog: in invertebrates, septate junctions serve as the primary occluding junction. Structurally distinct from vertebrate tight junctions, they exhibit a highly regular, ladder-like appearance under electron microscopy, with parallel septate walls spanning a wider intercellular space (15–20 nm). Despite their structural divergence, they perform the identical physiological role of establishing a transepithelial permeability barrier.

1.2 Anchoring Junctions

Anchoring junctions provide mechanical stability to tissues by physically tethering the cytoskeletons of adjacent cells together, or anchoring cells to the underlying extracellular matrix. They are abundant in tissues subjected to severe mechanical stress, such as the heart muscle, skin epidermis, and uterine cervix.

All anchoring junctions share a tripartite molecular architecture: transmembrane adhesion proteins linking the cell surface to extracellular structures, intracellular anchor (plaque) proteins that bind their cytoplasmic tails, and cytoskeletal elements that disperse mechanical forces throughout the tissue.

Figure: Anchoring junction molecular topology. A cytoskeletal filament connects through an intracellular plaque to a transmembrane adhesion protein; matching adhesion proteins on the two membranes bond to one another (cell–cell) or to the matrix (cell–matrix).

Adherens Junctions (Zonula Adherens)

These junctions couple the actin cytoskeleton of one cell to the actin cytoskeleton of an adjacent cell or the extracellular matrix.

Adhesion Belts (Zonula Adherens): in epithelial cells, these form a continuous ribbon-like belt of adhesion just below the apical tight junctions. Transmembrane proteins are classical cadherins (primarily E-cadherin in epithelial sheets).
E-cadherin binds→ β-catenin binds→ α-catenin bundles→ Actin filaments
The cytoplasmic domain of cadherin binds β-catenin (or γ-catenin/plakoglobin), which binds α-catenin; α-catenin then recruits and bundles actin filaments along with other actin-binding proteins such as vinculin, formin, and α-actinin.
Focal Contacts (Adhesion Plaques): discrete, punctate cell-to-matrix junctions that anchor the cell to the extracellular matrix. Transmembrane proteins are integrins — heterodimeric extracellular matrix receptors.
Integrin β-subunit links via→ Talin, Vinculin, α-actinin, Filamin anchors→ Actin filaments

Desmosomes (Macula Adherens)

Desmosomes are button-like intercellular contact points that bond neighbouring cells together, linking their intermediate filament networks to create a continuous, stress-resistant cytoskeletal network across the tissue sheet.

Figure: Desmosome anatomy. Intermediate filaments dock onto a dense desmoplakin/plakoglobin/plakophilin plaque on each side; desmoglein and desmocollin form calcium-dependent homophilic bonds across the intercellular space, welding the two filament networks into one continuous sheet.

Transmembrane proteins are non-classical cadherins — desmoglein and desmocollin — which undergo calcium-dependent homophilic and heterophilic interactions in the intercellular space. Desmoplakin acts as a physical clamp anchoring intermediate filaments to the plaque. In most epithelial tissues the attached filaments are keratins; in cardiac muscle cells they are desmin filaments.

Hemidesmosomes (Half-Desmosomes)

Not simply "half a desmosome": hemidesmosomes morphologically resemble half a desmosome, but they are biochemically and functionally distinct cell-to-matrix junctions that anchor the basal surface of epithelial cells to the underlying basal lamina (extracellular matrix).

Figure: Hemidesmosome anatomy. Keratin filaments dock onto a plectin/BP230 plaque, which anchors α₆β₄ integrin and BP180 (type XVII collagen) in the plasma membrane; their extracellular domains bind laminin in the basal lamina.

Junction type	Transmembrane protein	Intracellular linkage	Cytoskeletal filament	Extracellular target
Zonula adherens	Cadherin	β-catenin, α-catenin, vinculin	Actin filaments	Adjacent cell classical cadherins
Focal contact	Integrin	Talin, vinculin, α-actinin	Actin filaments	Extracellular matrix (e.g. fibronectin)
Desmosome	Cadherin (desmoglein/desmocollin)	Plakoglobin, plakophilin, desmoplakin	Intermediate filaments (keratin/desmin)	Adjacent cell desmoglein/desmocollin
Hemidesmosome	Integrin (α₆β₄), BP180	Plectin, BP230	Intermediate filaments (keratin)	Basal lamina (laminin)

1.3 Communicating Junctions

Communicating junctions bridge the cytoplasm of adjacent cells, permitting the direct, non-selective movement of inorganic ions, metabolites, and small signalling molecules.

Gap Junctions

In animal tissues, gap junctions appear as patches where the membranes of adjacent cells are separated by a highly uniform "gap" of 2–4 nm.

Connexins: the structural subunits of gap junctions in vertebrates. Each connexin is a four-pass transmembrane protein with intracellular N- and C-termini.
Connexon (hemichannel): six connexin molecules oligomerise within the plasma membrane of a single cell to form a hollow, cylindrical channel called a connexon.
Complete junction: when a connexon on one cell aligns end-to-end with a corresponding connexon on an adjacent cell, they dock to form a continuous, hydrophilic channel that spans both plasma membranes.

Figure: Gap junction channel. Docked connexons form a continuous aqueous pore roughly 1.5 nm across, spanning both membranes and letting water-soluble molecules under 1000 Da pass directly between the two cytoplasms.

Biophysical properties

Permeability limit: the open aqueous pore of a connexon allows the free, non-selective passage of water-soluble molecules up to 1000 Da — inorganic ions (H⁺, Na⁺, K⁺, Cl⁻, Ca²⁺), amino acids, nucleotides (ATP, ADP), sugars, and key secondary messengers (cAMP, IP₃). It strictly excludes large macromolecules such as proteins, nucleic acids, and polysaccharides.

Gap junction channels are highly dynamic and undergo rapid conformational changes to open or close in response to physiological signals. They typically remain open under resting conditions but close tightly in response to:

Low intracellular pH (H⁺ accumulation): prevents metabolic catastrophe from spreading from damaged, acidifying cells to healthy neighbours.
High intracellular calcium (Ca²⁺ elevation): damaged cells leak extracellular calcium (~1 mM) into their cytosol; closing adjacent gap junctions prevents this toxic influx from entering surrounding cells.
Voltage differences across the junctional membrane.

Invertebrate analogs — innexins: invertebrates lack sequence homology to connexins. Instead, they use a completely different family of proteins called innexins to construct functionally identical gap junction channels.

Plasmodesmata

In plants, the rigid cellulosic cell wall prevents direct plasma membrane contact. Plant cells solve this problem using plasmodesmata — microscopic cytoplasmic channels that penetrate the thick cell wall, establishing a physical continuum of cytoplasm (the symplast) and plasma membrane throughout the plant body.

Figure: Plasmodesma cross-section. The plasma membrane runs continuously through the pore from one cell to the other; a desmotubule (smooth ER extension) sits at the centre, surrounded by the aqueous annulus through which solutes diffuse. Callose collars at the neck regulate the pore's diameter.

Structural components

Plasma Membrane Sleeve: the plasma membrane of Cell 1 passes continuously through the cell wall channel to become the plasma membrane of Cell 2.
Desmotubule: a narrow, closed cylindrical tube of smooth endoplasmic reticulum suspended directly in the centre of the pore; its lumen is continuous with the ER lumen of both cells.
Aqueous Annulus (Cytoplasmic Sleeve): the ring-like space of open cytoplasm between the outer desmotubule membrane and the inner plasma membrane sleeve. Ions, sugars, amino acids, and small signalling molecules diffuse freely through this annulus.
Regulation of pore size (callose deposition): plants control the diameter of the aqueous annulus by synthesizing or degrading callose (a β-(1→3)-glucan polymer) within the cell wall region immediately surrounding the neck of the plasmodesma.
Restriction: excessive callose deposition constricts the pore, restricting macromolecular transport.

Classification by genesis

Primary plasmodesmata: formed during cytokinesis when the cell plate is laid down; ER strands become trapped within the newly forming cell wall.

Secondary plasmodesmata: synthesised de novo across pre-existing, mature cell walls, often exhibiting complex branched or H-shaped architectures.

Cell Adhesion Molecules & the Extracellular Matrix

2. Cell Adhesion Molecules (CAMs)

Structural Families of Transmembrane Adhesion Receptors

2. Cell Adhesion Molecules (CAMs)

Cell Adhesion Molecules (CAMs) are transmembrane glycoproteins that integrate cells into multicellular units by mediating specific binding in the extracellular space. Adhesion occurs via homophilic binding (the CAM binds to an identical CAM on an adjacent cell) or heterophilic binding (the CAM binds to a different class of molecule). CAMs are classified into four major structural families.

Figure: The four CAM families. Each family is defined by two properties — whether binding requires calcium, and whether it is homophilic or heterophilic — which together predict its physiological role.

2.1 Cadherins (Calcium-Dependent Adhesion)

Cadherins are the primary mediators of calcium-dependent, homophilic cell-to-cell adhesion. The superfamily contains over 350 members.

Classical Cadherins

Include E-cadherin (epithelial), N-cadherin (neural/muscle), and R-cadherin (retinal).

Primary Structure: single-pass transmembrane glycoproteins consisting of 700–750 amino acids.
Extracellular Domain: five repeating extracellular cadherin (EC1–EC5) domains, each containing approximately 100 amino acids.
Calcium-Binding Linkers: conserved calcium-binding sites reside in the linker regions between adjacent EC domains. In the presence of Ca²⁺, three calcium ions bind to each linker, rigidifying the extracellular tail into a stiff, rod-like conformation.
Without calcium: the extracellular tail becomes highly flexible and undergoes rapid proteolytic degradation, abolishing cell–cell adhesion.
Homophilic Interaction: adhesion is mediated by the physical interlocking of the N-terminal tips (specifically the EC1 domains) of cadherin molecules projecting from adjacent plasma membranes.

Figure: Classical cadherin ectodomain. Five EC repeats are rigidified by calcium bound at each inter-domain linker; adhesion occurs when the EC1 tips of cadherins on opposing membranes interlock.

C-Terminus binds→ β-Catenin binds→ α-Catenin bundles→ Actin

Non-Classical Cadherins

These exhibit structurally divergent extracellular domains or distinct intracellular linkages.

Desmosomal Cadherins: desmoglein and desmocollin, which link cell junctions to intermediate filaments.
Protocadherins: abundantly expressed in the central nervous system, where they generate extensive molecular diversity required for synaptic specificity.
T-Cadherin: lacks a transmembrane domain and is anchored to the membrane outer leaflet via a glycosylphosphatidylinositol (GPI) anchor.

2.2 Selectins

Selectins are transmembrane proteins that mediate calcium-dependent, heterophilic cell-to-cell adhesion, specifically regulating the transit of white blood cells (leukocytes) through the bloodstream.

Molecular structure: selectins are characterized by a highly conserved, N-terminal C-type lectin domain that binds specifically to specific carbohydrate groups (oligosaccharides) on glycoproteins or glycolipids in a calcium-dependent manner.

Functional classes

L-Selectin: expressed on the surface of most circulating leukocytes.
P-Selectin: stored intracellularly in secretory granules (Weibel–Palade bodies in endothelial cells, α-granules in platelets) and rapidly mobilized to the plasma membrane outer surface during acute inflammatory responses.
E-Selectin: synthesised de novo and expressed on vascular endothelial cells in response to inflammatory cytokines (such as interleukin-1 and TNF-α).

Figure: Selectin-mediated leukocyte rolling. Low-affinity, fast on/off bonds between endothelial selectins and Sialyl Lewis X on the leukocyte surface slow the cell without stopping it — the initial "rolling" phase of extravasation.

Ligands: selectins bind to sialylated and fucosylated oligosaccharides, most notably Sialyl Lewis X (sLe^x), present on cell-surface glycoproteins of circulating leukocytes. This low-affinity interaction mediates the critical initial rolling phase of leukocyte extravasation through blood vessel walls during inflammation.

2.3 Immunoglobulin (Ig) Superfamily

The Ig superfamily of cell adhesion molecules mediates calcium-independent homophilic and heterophilic cell-to-cell adhesion. Members are defined by the presence of one or more disulfide-bonded immunoglobulin-like loops within their extracellular domains.

NCAM (Neural Cell Adhesion Molecule): expressed widely in the nervous system, where it mediates homophilic binding to guide neurite outgrowth, cell migration, and synaptic plastic remodelling.
ICAM-1 & VCAM-1 (Intercellular / Vascular Cell Adhesion Molecule-1): expressed on vascular endothelial cells. They undergo heterophilic binding to integrins expressed on the surface of activated white blood cells. This strong adhesion stops leukocyte rolling and drives their transendothelial migration into inflamed tissues.

2.4 Integrins

Integrins are heterodimeric transmembrane glycoproteins that act as the primary receptors for cell attachment to the extracellular matrix, while also regulating cell signalling and cell survival.

Heterodimeric assembly: composed of one α-subunit (120–180 kDa) and one β-subunit (90–110 kDa) non-covalently associated at the cell surface. Mammals possess 18 α and 8 β subunits, assembling into 24 distinct functional integrin heterodimers.

Figure: Integrin subunit domain architecture. The α-subunit head (β-propeller) and β-subunit head (I-like domain) pair to form the ligand-binding pocket, stabilised by Ca²⁺/Mg²⁺ at the MIDAS site; each subunit's leg domains carry it through the membrane to its own cytoplasmic tail.

Ligand specificity — the RGD motif

Many integrins (e.g. α₅β₁, α_IIbβ₃) recognize and bind specifically to a conserved tripeptide sequence — Arginine-Glycine-Aspartate (Arg-Gly-Asp, or RGD) — present on extracellular matrix glycoproteins, including fibronectin, vitronectin, and osteopontin.

Cytoplasmic anchoring

β-Subunit C-Terminus binds→ Talin / α-Actinin recruits→ Vinculin anchors→ Actin (Focal Adhesion)

Bidirectional signalling

Outside-In Signalling: extracellular ligand binding induces a conformational change that separates the intracellular tails of the α and β subunits. This exposes cytoplasmic binding sites, recruiting and activating Focal Adhesion Kinase (FAK) and Src kinase, which trigger downstream cascades controlling cell growth, migration, and survival.
Inside-Out Signalling: intracellular signals (e.g. via GPCR pathways) drive the binding of talin to the β-subunit cytoplasmic tail. This disrupts an ionic salt bridge between the intracellular tails, inducing a global conformational extension of the extracellular head domains — transitioning the integrin from a folded, low-affinity state to an extended, high-affinity ligand-binding state.

The leukocyte adhesion cascade — three CAM families in sequence: selectins mediate initial low-affinity rolling (2.2); chemokine-activated integrins then bind ICAM-1/VCAM-1 of the Ig superfamily (2.3) to arrest the leukocyte in firm adhesion; the leukocyte then migrates between endothelial cells into the inflamed tissue.

Selectins (rolling) → ICAM-1 / VCAM-1 + Integrins (firm adhesion) → Transendothelial migration

3. The Extracellular Matrix (ECM) and Plant Cell Wall

Comparative Structural Biology of the Animal ECM and the Plant Cell Wall

3.1 The Extracellular Matrix of Animals

The cells of multicellular animals are embedded within a complex, secreted network of extracellular macromolecules that provides physical support, regulates cell migration, and controls developmental cell signalling. The animal ECM consists of three major molecular classes.

Structural Proteins

Collagen: the single most abundant protein in the animal kingdom. It is characterised by a rigid, right-handed triple-helical structure composed of three polypeptide α-chains. The primary sequence is highly repetitive: Gly-X-Y, where X is typically proline and Y is typically hydroxyproline. Glycine, the smallest amino acid, occupies the crowded central axis of the triple helix, permitting tight packing.

Figure: Collagen triple helix. Three left-handed polypeptide chains wind around one another into a right-handed superhelix; glycine's small side chain (every third residue) lets the three strands pack tightly along the central axis.

Elastin: a highly hydrophobic, non-glycosylated protein rich in proline and glycine. It forms an extensive network of elastic fibres in tissues requiring extreme elasticity (e.g. blood vessels, lungs, skin). Elastin molecules are covalently cross-linked via lysine-derived desmosine linkages, allowing them to stretch under tension and spontaneously recoil to their original length when tension is released.

Proteoglycans

Proteoglycans are a unique subclass of glycoproteins in which a core protein is covalently linked to one or more highly specialized carbohydrate chains called glycosaminoglycans (GAGs) — unbranched, long polysaccharide chains of repeating [acidic sugar – amino sugar]_n disaccharide units. The acidic sugar is typically D-glucuronic acid or L-iduronic acid; the amino sugar is typically N-acetylglucosamine or N-acetylgalactosamine.

Physical chemistry: with the exception of hyaluronic acid, GAG chains undergo extensive enzymatic sulfation in the Golgi apparatus. The combination of carboxyl groups (on the acidic sugars) and sulfate groups imparts an exceptionally high density of negative charges to GAGs. This charge density makes GAGs repel one another and attract a massive cloud of osmotically active Na⁺ cations, drawing water into the ECM and forming a highly hydrated, gel-like ground substance that resists compressive mechanical forces.

Primary GAG classes

Hyaluronan (Hyaluronic Acid): the simplest GAG. Completely unsulfated, contains no core protein, is synthesised directly at the plasma membrane rather than the Golgi, and consists of up to 25,000 repeating disaccharides.
Chondroitin Sulfate & Dermatan Sulfate: heavily sulfated, abundant in cartilage and bone.
Heparan Sulfate & Heparin: structurally diverse, involved in binding growth factors.
Keratan Sulfate: abundant in the cornea and bone matrix.

Adhesion Glycoproteins

Laminins: heterotrimeric glycoproteins composed of three polypeptide chains (α, β, γ) joined by disulfide bonds into a distinctive asymmetric cross-shaped structure. Laminin molecules contain specific domain-binding sites for integrins, sulfated lipids, proteoglycans, and dystroglycans, playing a central role in organizing and establishing the basal lamina (formerly called the basement membrane), which separates epithelial sheets from underlying connective tissues.

Figure: Laminin cross-structure. α, β, and γ chains meet at a coiled-coil core; the long arm ends in globular LG domains that carry the integrin- and dystroglycan-binding sites anchoring the basal lamina to overlying cells.

Fibronectin: an extracellular glycoprotein dimer composed of two nearly identical subunits covalently linked by a pair of C-terminal disulfide bonds. Each subunit is folded into multiple functional domains composed of three repeating modules: fibronectin type I (FN I), type II (FN II), and type III (FN III). The FN III domain contains a flexible loop exposing the conserved Arg-Gly-Asp (RGD) sequence, which interacts directly with cell-surface integrins to drive cell adhesion and migration.

Figure: Fibronectin dimer. Two near-identical subunits, each built from FN I/II/III repeats, run in parallel and are joined near their C-termini by a pair of disulfide bonds; the exposed RGD loop on FN III is the integrin-binding site.

Functional forms: soluble plasma fibronectin is synthesised and secreted by hepatocytes into the blood, where it assists in blood clotting and wound healing; insoluble cellular fibronectin is secreted by fibroblasts and assembled into a fibrillar network within the tissue extracellular matrix.

3.2 The Plant Cell Wall

Unlike animal cells, all plant cells are surrounded by a rigid, protective extracellular wall made of insoluble secreted carbohydrates and structural proteins. The wall is structured into three discrete layers.

Figure: Plant cell wall layers. The middle lamella cements adjacent cells; the flexible primary wall permits turgor-driven growth; the rigid secondary wall (deposited after growth ceases) has three sub-layers — S1, S2, S3 — each with cellulose microfibrils oriented at a different angle to maximise tensile strength.

Middle Lamella: the outermost cementing layer that binds adjacent plant cells together. Rich in pectins (specifically calcium pectate) and hydroxyproline-rich glycoproteins.
Primary Cell Wall: a relatively thin, flexible, and extensible layer deposited by the plant cell while it is growing. It permits cell expansion and turgor-driven cell growth.
Secondary Cell Wall: a thick, highly rigid structural layer deposited between the primary cell wall and the plasma membrane after cell growth has ceased. Composed of three distinct layers (S1, S2, S3), each containing cellulose microfibrils oriented at different angles to maximise tensile strength.

Cellulose (Microfibrillar Polysaccharide)

Cellulose is the primary structural component of the plant cell wall. It is a linear, unbranched polymer of D-glucose residues linked via β-(1→4)-glycosidic bonds.

Cellulose microfibrils: individual cellulose chains line up in parallel (typically 18 to 24 chains) and associate via extensive hydrogen bonding and van der Waals forces to form highly stable crystalline bundles — cellulose microfibrils — with tensile strength comparable to steel.

Biosynthesis (cellulose synthase): cellulose microfibrils are synthesised directly at the plasma membrane by a massive hexameric transmembrane enzyme complex called the cellulose synthase complex (CESA), or "rosette." The rosette uses cytosolic UDP-glucose to polymerise cellulose chains, extruding them directly into the extracellular space.

Figure: Microtubule–microfibril alignment. Cortical microtubules beneath the plasma membrane act as tracks that steer the CESA rosette; the direction it travels sets the orientation of the microfibril it extrudes, and that orientation restricts or promotes cell elongation.

Microtubule–microfibril alignment hypothesis: the direction of plant cell expansion is determined by the orientation of cellulose microfibrils. As CESA rosettes move along cortical microtubule tracks, newly synthesised extracellular microfibrils are laid down parallel to the underlying microtubules, restricting lateral expansion and promoting longitudinal growth.

Matrix Polysaccharides

Hemicelluloses: a heterogeneous group of highly branched polysaccharides (including xylans, xyloglucans, glucomannans, and mannans). Their primary role is to bind non-covalently to the surface of cellulose microfibrils, cross-linking them into a resilient, cohesive structural network.
Pectins: highly branched, hydrated, acidic polysaccharides rich in D-galacturonic acid, forming a highly hydrated, negatively charged gel-like matrix that fills the spaces between cellulose microfibrils. Pectins are rich in the middle lamella, where they bind divalent Ca²⁺ to form sticky calcium pectate gels that cement adjacent cells together.

Lignin

Lignin is a massive, amorphous, and highly hydrophobic phenolic polymer, synthesised from three primary monolignol precursors (p-coumaryl, guaiacyl, and syringyl alcohols) derived from phenylalanine via complex phenylpropanoid pathways. It impregnates the secondary cell wall of vascular plants (especially in wood and xylem vessels), displacing water and forming a highly rigid, chemically inert matrix. This lignification process provides mechanical support for upright growth and acts as a physical barrier against pathogen invasion.

Structural Proteins

Extensins: a major family of hydroxyproline-rich glycoproteins (HRGPs) that accumulate in the cell wall. They undergo oxidative, covalent cross-linking via isodityrosine linkages to form a rigid protein network, reinforcing the cell wall during pathogen attacks or mechanical stress.
Expansins: a family of non-enzymatic, pH-dependent proteins located in the primary cell wall. Under acidic conditions (e.g. during auxin-induced proton pumping), expansins bind to the cell wall and disrupt the non-covalent hydrogen bonds between cellulose microfibrils and hemicelluloses. This slide-and-creep mechanism loosens the wall matrix, allowing turgor pressure to drive cell expansion.

Characteristic	Animals	Plants
Chemical nature	Protein-rich, highly hydrated	Carbohydrate-rich, rigid
Structural fiber	Collagens and elastins	Cellulose microfibrils
Hydrated matrix	Proteoglycans (sulfated GAGs)	Pectins and hemicelluloses
Adhesive molecules	Fibronectins and laminins	Pectins (middle lamella)
Rigidifying agent	Calcium phosphate (bone)	Lignin (wood)

Cell Signalling and Receptor Systems

4. Cell Signalling and Receptor Systems

Modes of Intercellular Communication & the Nuclear Receptor Superfamily

4.1 Classifications of Cell Signalling

Cells communicate by releasing extracellular signalling molecules (ligands) that bind to specific receptors in target cells. Signalling is classified based on the distance over which the signal travels.

Figure: Modes of cell signalling, by distance. Endocrine signals travel furthest (via blood); paracrine signals diffuse locally; autocrine signals loop back onto the sending cell itself; juxtacrine signals require direct membrane-to-membrane contact.

Endocrine Signalling: signalling molecules (hormones) are secreted by specialised endocrine cells and transported through the bloodstream over long distances to act on target cells located throughout the body — e.g. insulin secreted by the pancreas acting on liver and muscle cells.
Paracrine Signalling: signalling molecules are released into the local extracellular fluid and act only on neighbouring cells in close proximity — e.g. neurotransmitters diffusing across a synaptic cleft.
Autocrine Signalling: the signalling cell secretes a ligand that binds to receptors expressed on its own surface, regulating its own physiological activity — e.g. T-lymphocytes secreting interleukin-2 to drive their own clonal proliferation during immune responses.
Juxtacrine (Contact-Dependent) Signalling: the signalling molecule is an integral membrane protein expressed on the surface of the signalling cell. It binds directly to a receptor expressed on the membrane of an adjacent cell, requiring physical cell-to-cell contact — e.g. Notch-Delta signalling, classical cadherin-mediated signalling.

Classification by chemical solubility

Signalling molecules are also classified by their solubility, which dictates where their receptor sits and how fast the response unfolds.

Property	Hydrophilic ligands	Lipophilic ligands
Examples	Epinephrine, insulin, acetylcholine	Steroid hormones, thyroid hormone, retinoic acid, vitamin D
Crosses the lipid bilayer	No — blocked by the hydrophobic core	Yes — diffuses freely
Receptor location	Cell-surface	Intracellular (cytosol or nucleus)
Response	Rapid, short-duration, via secondary-messenger cascades	Slow-onset, long-duration, via changes in gene expression

4.2 Intracellular Receptors (The Nuclear Receptor Superfamily)

Intracellular receptors are soluble proteins that bind lipophilic ligands. They belong to a highly conserved evolutionary family called the nuclear receptor (NR) superfamily.

Figure: Nuclear receptor domain organization. A single polypeptide runs from a variable N-terminal AF-1 region, through the highly conserved DNA-binding domain (C) and ligand-binding domain (E), to a variable C-terminal tail.

Conserved domain architecture

A/B Domain: located at the N-terminus. Contains the Activation Function-1 (AF-1) domain, which initiates transcription in a ligand-independent manner by recruiting the transcription machinery. Highly variable in sequence and length.
C Domain (DNA-Binding Domain — DBD): the most highly conserved domain. Contains two zinc-finger motifs, where each finger coordinates a single Zn²⁺ ion via four precisely positioned cysteine residues. The DBD recognizes and binds with high affinity to specific double-stranded DNA sequences known as Hormone Response Elements (HREs), and also contains structural regions that facilitate receptor dimerization.
D Domain (Hinge Region): a flexible linker region between the DBD and the ligand-binding domain. Contains nuclear localization signals (NLS) that guide the nuclear import of the receptor.
E Domain (Ligand-Binding Domain — LBD): a large hydrophobic pocket that binds the specific lipophilic ligand. Also mediates receptor dimerization and contains the Activation Function-2 (AF-2) domain, which undergoes a dramatic conformational change upon ligand binding to recruit transcription coactivators.
F Domain: a highly variable C-terminal tail of unknown or variable function.

Activation mechanisms: steroid vs. thyroid hormone receptors

The superfamily utilizes two distinct mechanisms of transcription regulation, differing in where the resting receptor sits and what it does while unliganded.

Figure: Two activation mechanisms. Steroid receptors start inactive in the cytosol and only reach DNA once ligand-bound. Thyroid/retinoid receptors sit on DNA constitutively, actively repressing transcription until ligand arrives and flips them from corepressor to coactivator recruitment. Green boxes mark active transcription; the rose box marks active repression.

Classical Steroid Hormone Receptors

Resting state: in the absence of ligand, steroid receptors (e.g. glucocorticoid and progesterone receptors) reside in the cytosol. They are kept in an inactive, unfolded monomeric conformation bound to a complex of chaperone proteins, most notably Heat-Shock Protein 90 (HSP90).

Activation cascade: ligand binding to the LBD induces a conformational change that drives the physical dissociation of the HSP90 chaperone complex. The free receptors homodimerise, translocate into the nucleus via nuclear pores, and bind to specific HREs (typically inverted repeat sequences). They recruit histone acetyltransferases (HATs) and coactivators to open chromatin structure and activate transcription.

Thyroid and Retinoid Hormone Receptors

Resting state: thyroid hormone receptors (TRs) and Retinoid X Receptors (RXRs) do not bind chaperones. Instead, they reside constitutively in the nucleus, bound directly to HREs (typically direct repeats) as TR–RXR heterodimers. In the absence of ligand, the TR–RXR heterodimer binds to corepressor proteins that recruit Histone Deacetylases (HDACs), which remove acetyl groups from histones, packing chromatin tightly and actively repressing transcription.

Activation cascade: upon entering the nucleus, thyroid hormone binds to the TR LBD. This induces a conformational shift that releases the corepressor/HDAC complex. The active receptor subsequently recruits coactivator proteins with Histone Acetyltransferase (HAT) activity, which acetylates local histones, loosening chromatin structure and permitting transcription of target genes.

GPCR Signalling, Second Messengers, Toxins & Phototransduction

5. GPCR and Heterotrimeric G-Protein Signalling

The Heterotrimeric & Monomeric GTPase Activation–Deactivation Cycles

5. GPCR and Heterotrimeric G-Protein Signalling

G-Protein Coupled Receptors (GPCRs) represent the largest and most physiologically diverse family of cell-surface receptors in eukaryotes. They regulate sensory perception (vision, smell, taste), cardiovascular function, and hormone responses.

5.1 GPCR and Heterotrimeric G-Protein Activation Cycle

Receptor structure: GPCRs consist of a single polypeptide chain containing seven transmembrane α-helices (H1–H7), which is why they are also called serpentine receptors.

Heterotrimeric G-proteins: membrane-associated proteins composed of three subunits — α, β, and γ. The α-subunit is a GTPase switch protein with lipid anchors tethering it to the cytosolic leaflet; the β and γ subunits bind tightly together to form a stable, functional Gβγ complex that is also lipid-anchored to the membrane.

Figure: The heterotrimeric G-protein cycle. A ligand-activated GPCR acts as a GEF, catalysing GDP release from Gα and its replacement with GTP; the active subunits dissociate and each signals independently until Gα's intrinsic GTPase activity — sped up by RGS/GAP proteins — resets the switch.

Step 1 — Resting State: in the resting, unstimulated state, the G-protein exists as an inactive heterotrimer (αβγ) with GDP bound to the pocket of the α-subunit (Gα-GDP).
Step 2 — Ligand Binding: extracellular ligand binding induces a conformational shift in the GPCR transmembrane helices, altering the structure of the cytoplasmic loops and transforming the GPCR into an active Guanine Nucleotide Exchange Factor (GEF).
Step 3 — Nucleotide Exchange: the active GPCR GEF binds the inactive heterotrimer, widening the GDP-binding pocket of Gα. GDP dissociates rapidly and is replaced by GTP, present at much higher concentrations in the cytosol.
Step 4 — Subunit Dissociation: GTP binding induces a major conformational change in two flexible regions of Gα known as Switch I and Switch II. This causes active Gα-GTP to dissociate from both the GPCR and the Gβγ heterodimer.
Step 5 — Effector Activation: both free Gα-GTP and the released Gβγ complex are now active and bind to downstream effector enzymes or ion channels to propagate the signal.
Step 6 — Signal Termination (GTP Hydrolysis): Gα has intrinsic GTPase activity that slowly hydrolyses bound GTP to GDP and P_i, acting as a built-in molecular clock. This hydrolysis is dramatically accelerated by Regulators of G-protein Signalling (RGS), which act as GTPase-Activating Proteins (GAPs).
Step 7 — Reassociation: once GTP is hydrolysed to GDP, Gα-GDP loses affinity for its effector and rapidly reassociates with the free Gβγ complex to reform the inactive heterotrimer, readying the system for another cycle.

5.2 The Monomeric GTPase Cycle

Apart from heterotrimeric G-proteins, cells utilise a massive superfamily of small, monomeric GTP-binding proteins (20–40 kDa) that act as molecular switches in various cellular pathways — e.g. Ras in growth signalling, Rho in cytoskeletal rearrangement, Rab in vesicle docking, Ran in nuclear transport, and Sar1/Arf in coatomer assembly.

Figure: The monomeric GTPase switch. The same GDP/GTP toggle logic as the heterotrimeric cycle, but distilled to a single subunit flipped ON by a GEF and OFF by a GAP.

GEFs (Guanine Nucleotide Exchange Factors): turn the G-protein "ON" by promoting the dissociation of GDP, allowing GTP to bind in its place.
GAPs (GTPase-Activating Proteins): turn the G-protein "OFF" by accelerating the G-protein's intrinsic rate of GTP hydrolysis by several orders of magnitude.

6. Secondary Messenger Cascades and Enzymatic Definitions

Effector Enzymes, cAMP, Phosphoinositides & Calcium Signalling

6. Secondary Messenger Cascades and Enzymatic Definitions

Intracellular signalling pathways rely on several key enzymatic activities to process and amplify signals.

Kinases: enzymes that transfer a phosphate group from a high-energy donor molecule (typically ATP) to a specific substrate (phosphorylation).
Substrate–OH + ATP→Substrate–O–PO₃²⁻ + ADP
Phosphatases: enzymes that hydrolytically remove a phosphate group from a substrate (dephosphorylation).
R–O–PO₃²⁻ + H₂O→R–OH + P_i
Phosphodiesterases (PDEs): enzymes that cleave cyclic nucleotide phosphodiester bonds — e.g. converting active cyclic AMP or cyclic GMP into inactive 5′-AMP or 5′-GMP.
Cyclic AMP / GMP + H₂O→5′-AMP / 5′-GMP
Phosphorylases: enzymes that catalyse the addition of an inorganic phosphate (P_i) to an acceptor molecule without utilising ATP.
Acceptor + P_i→Acceptor–O–PO₃²⁻

6.1 The Gα_s Pathway and the cAMP Cascade

The Gα_s pathway is the primary pathway for stimulatory signals that increase intracellular cyclic AMP (cAMP) levels.

Figure: The Gαs → cAMP → PKA → CREB cascade. Each arrow is an activation step; PKA's catalytic subunits are the only components that physically relocate, crossing into the nucleus to phosphorylate CREB.

Adenylyl Cyclase Activation: active Gαs-GTP binds to and activates Adenylyl Cyclase (AC), a large multipass transmembrane glycoprotein located in the plasma membrane.
cAMP Synthesis: active Adenylyl Cyclase catalyses the conversion of ATP into cyclic AMP, releasing inorganic pyrophosphate (PP_i). cAMP acts as a highly diffusible, unstable secondary messenger and is rapidly broken down by cAMP-specific phosphodiesterases (PDEs) into inactive 5′-AMP.
PKA Activation: cAMP exerts its physiological effects by activating Protein Kinase A (PKA), also known as cAMP-dependent protein kinase.
PKA Structure: in its inactive state, PKA is a heterotetramer of two regulatory (R) and two catalytic (C) subunits (R₂C₂). The regulatory subunits block the active sites of the catalytic subunits.
Cooperative Binding: four molecules of cAMP bind cooperatively to the two regulatory subunits (two cAMP molecules per R subunit).
Dissociation: cAMP binding induces a major conformational shift in the R subunits, causing them to dissociate from the catalytic subunits. The free catalytic subunits are now fully active and phosphorylate serine and threonine residues on target proteins.

Figure: PKA activation. Four cAMP molecules binding cooperatively to the R₂C₂ tetramer split it into an inactive regulatory dimer and two fully active, free catalytic subunits.

Transcriptional activation via CREB

Active PKA catalytic subunits translocate through nuclear pores into the nucleus.
In the nucleus, PKA phosphorylates the transcription factor CREB (cAMP Response Element Binding protein) at a single conserved residue: Serine-133.
Phosphorylated CREB dimerises and binds specifically to the CRE (cAMP Response Element), a conserved DNA sequence in the promoter region of cAMP-regulated genes.
Phosphorylated Serine-133 recruits the transcriptional coactivator CBP (CREB-Binding Protein), which has histone acetyltransferase activity that remodels chromatin to initiate transcription.

6.2 The Gα_i Pathway

The G_i pathway opposes the G_s pathway by actively inhibiting adenylyl cyclase activity.

Mechanism: active Gαi-GTP binds to adenylyl cyclase, blocking its catalytic activity and lowering intracellular cAMP levels.

Gβγ effector role: the Gβγ complex released from activated G_i proteins also acts as a primary signalling molecule. In cardiac muscle cells, acetylcholine released from the vagus nerve binds metabotropic M2 muscarinic receptors, activating G_i and releasing Gβγ complexes that bind directly to G-protein-coupled inwardly-rectifying potassium (GIRK) channels. This opens the channels, causing an efflux of K⁺ ions that hyperpolarises the cardiac membrane and slows the heart rate.

Gα class	Representative members	Effector enzyme	Secondary messenger	Physiological output
Gαs	Gαs, Gαolf	Activates adenylyl cyclase	Increases cAMP	Activates PKA; CREB-mediated transcription; olfactory signalling
Gαi	Gαi, Gαo, Gαt	Inhibits adenylyl cyclase; activates phosphodiesterases	Decreases cAMP; decreases cGMP	Lowers PKA; closes ion channels; visual phototransduction
Gαq/11	Gαq, Gα11	Activates Phospholipase C-β	Increases IP₃, DAG, cytosolic Ca²⁺	Activates PKC and CaM-kinases; smooth muscle contraction
Gα12/13	Gα12, Gα13	Activates RhoGEF	Activates monomeric Rho GTPase	Regulates cytoskeletal dynamics and cell migration

6.3 The Gα_q Pathway and the Phosphoinositide Cascade

The Gαq pathway regulates the release of intracellular calcium and the activation of Protein Kinase C.

Figure: The phosphoinositide cascade. PLC-β cleaves PIP₂ into two independent messengers — water-soluble IP₃, which triggers ER calcium release, and membrane-bound DAG, which recruits PKC. The two converge: calcium plus DAG plus membrane lipids together fully activate PKC.

Phospholipase C-β Activation: active Gαq-GTP binds to and activates Phospholipase C-β (PLC-β), a membrane-associated enzyme.
PI(4,5)P₂ Cleavage: active PLC-β catalyses the hydrolysis of Phosphatidylinositol 4,5-bisphosphate, a minor phospholipid of the inner leaflet, generating two secondary messengers: Inositol 1,4,5-trisphosphate (IP₃), a small water-soluble sugar that diffuses into the cytosol, and Diacylglycerol (DAG), a hydrophobic molecule that remains embedded in the inner leaflet.
Calcium Release from the ER: IP₃ diffuses to the ER membrane and binds ligand-gated IP₃ receptors (IP₃R). Binding opens the channels, letting Ca²⁺ stored at high concentration inside the ER lumen (~0.5 mM) flood into the cytosol, raising cytosolic calcium from ~100 nM to over 1 µM.
Protein Kinase C (PKC) Activation: elevated cytosolic calcium causes soluble PKC to bind calcium, increasing its affinity for negatively charged lipids and driving its translocation to the plasma membrane, where it binds membrane-bound DAG and phosphatidylserine. This cooperative interaction — calcium, DAG, and membrane lipids — fully activates PKC, which phosphorylates serine and threonine residues on diverse target proteins.

6.4 The Calcium–Calmodulin Complex

Calcium ions also act as a universal secondary messenger by binding to specific calcium-binding proteins. The most important of these is calmodulin, a highly conserved, 17 kDa acidic monomeric protein present in all eukaryotic cells.

Structure and binding: calmodulin contains four high-affinity calcium-binding sites called EF-hands. Binding of Ca²⁺ is highly cooperative — when cytosolic calcium rises, calmodulin binds up to four calcium ions, undergoing a dramatic conformational change that exposes a hydrophobic patch.

Target activation: the active Ca²⁺–calmodulin complex wraps around and activates specific target proteins, including calcium-calmodulin-dependent protein kinases (CaM-kinases) such as CaMKII. Active CaMKII phosphorylates transcription factors (including CREB) and cytoskeletal proteins, regulating memory formation, cell motility, and secretory pathways.

7. Bacterial Toxins Modulating G-Protein Signalling

How Cholera and Pertussis Toxins Hijack the Gα Switch

7. Bacterial Toxins Modulating G-Protein Signalling

Several pathogenic bacteria secrete protein toxins that target heterotrimeric G-proteins, disrupting host cell physiology to cause severe clinical disease.

Figure: Two toxins, one convergent outcome. Cholera toxin locks Gαs permanently ON; pertussis toxin locks Gαi permanently OFF. Opposite mechanisms, but both remove the brakes on adenylyl cyclase and drive cAMP far outside its normal range.

7.1 Cholera Toxin (Vibrio cholerae)

Structure: an AB₅-class enterotoxin. The five B-subunits form a ring that binds GM1 gangliosides on the surface of intestinal epithelial cells, driving endocytosis and retrograde transport of the active A-subunit to the ER.
Mechanism of Action: the A-subunit is translocated into the host cytosol, where it catalyses the covalent transfer of an ADP-ribose group from intracellular NAD⁺ to a critical arginine residue in the active site of the Gαs subunit.
Biophysical Consequence: this ADP-ribosylation completely abolishes the intrinsic GTPase activity of Gαs, which can no longer hydrolyse its bound GTP — locking it in a permanently active state.
Downstream Cascade: permanently active Gαs drives continuous, unregulated adenylyl cyclase activity. Intracellular cAMP skyrockets, overactivating PKA, which phosphorylates CFTR chloride channels on the apical membrane, keeping them continuously open.
Clinical Presentation: massive, unregulated secretion of Cl⁻ into the intestinal lumen, followed osmotically by Na⁺, HCO₃⁻, and water — a loss of up to 20 litres of fluid per day, causing the severe, life-threatening "rice-water" watery diarrhoea characteristic of cholera.

7.2 Pertussis Toxin (Bordetella pertussis)

Structure: an AB₅-class exotoxin secreted by the causative agent of whooping cough.
Mechanism of Action: the active A-subunit translocates into the cytosol of respiratory epithelial cells, catalysing transfer of an ADP-ribose group from NAD⁺ to a conserved cysteine residue near the C-terminus of Gαi.
Biophysical Consequence: this modification physically blocks Gαi from interacting with its GPCRs. Because the GPCR cannot act as a GEF, Gαi remains permanently locked in its inactive, GDP-bound state.
Downstream Cascade: an inactive Gαi cannot inhibit adenylyl cyclase. This removes the primary inhibitory control on cAMP synthesis, allowing cAMP levels to rise uncontrollably.
Clinical Presentation: the resulting cAMP elevation in airway cells disrupts osmotic balance, driving fluid loss, inflammatory mucosal damage, and hypersecretion of thick mucus — the severe, paroxysmal coughing fits characteristic of whooping cough.

8. Biophysics of Visual Phototransduction in Rods

The GPCR Cascade That Converts Photons into a Neural Signal

8. Biophysics of Visual Phototransduction in Rods

Phototransduction is the biochemical process by which rod photoreceptor cells in the retina capture light energy and convert it into a graded electrical signal. This process is mediated by a highly specialised GPCR pathway.

8.1 The Molecular Components

Rhodopsin: the primary light-activated GPCR, located in the membrane discs of the rod outer segment. It consists of a seven-transmembrane protein called opsin covalently coupled to a light-sensitive chromophore, 11-cis-retinal (a derivative of Vitamin A), bound via a protonated Schiff base to Lysine-296.
Transducin (G_t): a specialized heterotrimeric G-protein expressed exclusively in photoreceptor cells.
cGMP Phosphodiesterase (PDE): a membrane-associated enzyme consisting of active α and β catalytic subunits and two inhibitory γ subunits (αβγ₂).
cGMP-Gated Cation Channels: non-selective cation channels in the outer segment plasma membrane. In the dark, they are kept open by bound cytosolic cGMP.

8.2 The Signalling Cascade

Figure: The phototransduction cascade. A single absorbed photon triggers a large, sharp drop in cGMP, closing cation channels and hyperpolarising the cell — the opposite of most GPCR cascades, where activation excites rather than silences the cell.

Light Absorption and Isomerisation: when a photon strikes rhodopsin, its energy is absorbed by the 11-cis-retinal chromophore, driving the rapid (femtosecond) photo-isomerisation of 11-cis-retinal into all-trans-retinal.
Metarhodopsin II Formation: the rigid structure of all-trans-retinal forces opsin to undergo a conformational change, forming the active state known as Metarhodopsin II.
Transducin Activation: active Metarhodopsin II binds the inactive transducin heterotrimer (Gαt-GDP), acting as a GEF to drive GDP dissociation and GTP binding. Active Gαt-GTP dissociates from the Gβγ complex.
PDE Activation: active Gαt-GTP binds directly to the inhibitory γ subunits of PDE, pulling them away from the catalytic core and fully activating the enzyme.
cGMP Plummets: active PDE rapidly hydrolyses cGMP into inactive 5′-GMP, causing cytosolic cGMP in the outer segment to fall precipitously.
Channel Closure: in the dark, high cytosolic cGMP (~4 mM) keeps the cation channels open, permitting a continuous inward "dark current" that keeps the photoreceptor depolarised at roughly −40 mV and drives baseline glutamate release. In the light, falling cGMP causes it to dissociate from the channels, closing them and stopping the dark current, while potassium leak channels continue pumping K⁺ out.
Membrane Hyperpolarisation: the loss of inward sodium flow hyperpolarises the rod membrane from −40 mV toward −70 mV, approaching the potassium equilibrium potential.
Signal Transmission: hyperpolarisation closes voltage-gated calcium channels at the synaptic terminal, halting glutamate release. Because glutamate is inhibitory on downstream ON-bipolar cells, this decrease depolarises those bipolar cells, generating the electrical signal that travels to the brain via the optic nerve.

Property	Dark state	Light state
Cytosolic cGMP	High (~4 mM)	Plummets
cGMP-gated cation channels	Open (dark current flows)	Closed
Membrane potential	Depolarised (~−40 mV)	Hyperpolarised (toward ~−70 mV)
Glutamate release	Continuous, baseline	Decreased

8.3 Deactivation and Recovery Mechanisms

To reset the visual system for subsequent light detection, the cell utilizes several negative feedback mechanisms.

Rhodopsin Kinase and Arrestin: Rhodopsin Kinase (GRK1) phosphorylates the C-terminal tail of active Metarhodopsin II. Arrestin then binds the phosphorylated tail, physically blocking further transducin activation.
GTP Hydrolysis: transducin Gαt has an exceptionally high intrinsic rate of GTP hydrolysis, accelerated by a photoreceptor-specific RGS9 GAP complex, turning Gαt off and allowing the PDE inhibitory subunits to re-bind and shut down PDE activity.
Guanylyl Cyclase Activation: channel closure also stops calcium entry. As calcium is continuously pumped out by Na⁺/Ca²⁺/K⁺ exchangers, intracellular calcium falls; this low calcium activates Guanylyl Cyclase Activating Proteins (GCAPs), which stimulate Guanylyl Cyclase to synthesise new cGMP, reopening the cation channels and restoring the depolarised dark state.

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