Cellular Compartmentalisation, Ribosomes & Protein Trafficking

Organelle Biochemistry · Ribosome Structure · The Endoplasmic Reticulum · Co-Translational Translocation

Chapter 1

1. Cellular Compartmentalisation and Ribosome Biochemistry

The eukaryotic cell is defined by intracellular compartmentalisation. Dividing the protoplasm into membrane-bound organelles establishes distinct chemical microenvironments, each physically and chemically optimised for specific metabolic reactions.

1.1 The Thermodynamic and Kinetic Imperative of Compartmentalisation

Compartmentalisation provides key kinetic and thermodynamic advantages that a single undivided cytoplasm could not offer:

Concentration of Reactants: enclosing enzymes and substrates within a microscopic volume increases collision frequency, accelerating reaction rates without requiring massive global concentrations.
Maintenance of pH and Ionic Gradients: active transport systems maintain specific electrochemical conditions — for example, lysosomal acidity at pH 4.5–5.0 versus cytosolic neutrality at pH 7.2 — required for compartment-specific enzyme activity.
Separation of Incompatible Processes: catabolic processes such as lysosomal macromolecular hydrolysis are physically isolated from anabolic pathways such as ribosomal protein synthesis, preventing futile metabolic cycles.
Electrophysiological Storage: bounding membranes act as capacitors, accumulating ion gradients that store free energy to drive transport or transmit biological signals.

Figure: Hierarchy of the eukaryotic protoplast. The protoplasm divides into cytoplasm and nucleus; the cytoplasm in turn resolves into membrane-bound organelles and the surrounding aqueous cytosol. Cytoplasmic inclusions such as glycogen, starch, and lipid droplets are non-living ergastic deposits, distinct from the living protoplasm.

1.2 Eukaryotic Ribosome Structure and Macromolecular Composition

Ribosomes are large ribonucleoprotein complexes that translate mRNA nucleotide sequences into polypeptides. They consist of two unequal subunits designated by their sedimentation coefficient in Svedberg units (S):

S = v / (ω²r)

Svedberg unit: a non-additive measure of sedimentation velocity under centrifugal acceleration, where v is sedimentation velocity, ω is angular velocity, and r is radial distance from the rotation axis. One Svedberg unit equals 10⁻¹³ seconds.

The cytosolic ribosome of eukaryotes is an 80S complex (~4.2 MDa), dissociating into a small 40S and large 60S subunit. Prokaryotic cytosolic ribosomes, along with eukaryotic chloroplast and mitochondrial ribosomes, are smaller 70S complexes (mitochondrial ribosomes show specialised variations).

Complex	Subunits	rRNA components	Proteins
80S (eukaryotic cytosol)	40S + 60S	18S (2,300 nt) · 28S (4,718 nt) · 5.8S (160 nt) · 5S (120 nt)	33 (S1–S33) + 46 (L1–L46)
70S (bacterial / plastid)	30S + 50S	16S (1,542 nt) · 23S (2,904 nt) · 5S (120 nt)	21 (S1–S21) + 31 (L1–L31)
55S (human mitochondrial)	Small + large	12S + 16S (no 5S equivalent)	High protein-to-RNA ratio

1.3 Genomic Organisation and Processing of Eukaryotic rDNA

The eukaryotic genome contains several hundred tandemly repeated rDNA gene clusters. In humans these are located on the short arms of five acrocentric chromosomes — 13, 14, 15, 21, and 22 — which aggregate to form the Nucleolar Organiser Regions (NORs) that organise the nucleolus.

Figure: The 45S/47S rDNA transcription unit. RNA Polymerase I transcribes the 18S, 5.8S, and 28S genes together with their flanking/internal spacers as one long 47S precursor, which is then cleaved and trimmed within the nucleolus to yield the mature rRNAs.

The 45S/47S Transcription Unit: the genes for 18S, 5.8S, and 28S rRNAs are organised into a single unit transcribed by RNA Pol I in the nucleolus.
Processing Pathways: the 47S pre-rRNA undergoes coordinated endonucleolytic cleavage and exonucleolytic trimming, removing the External and Internal Transcribed Spacers (ETS1/ETS2, ITS1/ITS2) to yield mature 18S, 5.8S, and 28S rRNAs.
Chemical Modifications: small nucleolar ribonucleoproteins (snoRNPs) direct 2′-O-methylation (C/D box snoRNAs) and pseudouridylation (H/ACA box snoRNAs), protecting the rRNA and aiding folding.
The 5S rRNA Exception: encoded by a separate gene cluster on chromosome 1, transcribed by RNA Polymerase III in the nucleoplasm, then imported into the nucleolus for 60S assembly.

Chapter 2

2. Protein Targeting, Translocation, and Topology

2.1 Spatial Segregation of Translation

Eukaryotic cells contain two populations of 80S ribosomes in the cytosol: membrane-bound ribosomes attached to the cytosolic face of the ER, and free ribosomes unattached in the cytosol. The two populations are structurally identical, differing only in the sorting signal of the nascent chain they are translating.

Figure: Fate of a nascent chain. Translation always begins in the cytosol. The presence or absence of a sorting signal on the emerging polypeptide determines whether the ribosome stays free (soluble/cytosolic and organelle-targeted post-translational routes) or docks at the ER for co-translational import.

2.2 Structural Nature of Sorting Signals

Signal Sequences (Leader Peptides): linear stretches of 15–60 amino acids, typically N-terminal (ER, mitochondrial import) but sometimes C-terminal (peroxisomal) or internal. N-terminal signals are often cleaved by peptidases once translocation is complete.
Signal Patches: 3D arrangements of surface atoms that form only once the polypeptide folds into its native tertiary structure — constituent residues may be widely separated in sequence but converge in space. Non-cleavable; used for processes like lysosomal hydrolase targeting.

2.3 Three Modes of Intracellular Transport

2.4 Co-translational vs. Post-translational Translocation

Co-translational Translocation: the default pathway for ER targeting in mammalian cells. The ribosome associates with the ER membrane so translation and translocation occur simultaneously — the growing chain is pushed directly through the translocator by the force of translation, preventing premature cytosolic folding.

Post-translational Translocation: occurs when proteins are fully synthesised in the cytosol before import into an organelle (mitochondria, chloroplasts, peroxisomes, or yeast ER). Cytosolic chaperones such as Hsp70 keep the protein unfolded before it is threaded across the membrane using ATP hydrolysis or electrochemical gradients.

Chapter 3

3. Structural and Functional Anatomy of the Endoplasmic Reticulum

3.1 Structural Organisation

The ER is the largest single membrane-bound compartment in eukaryotic cells, often exceeding half of the cell's total membrane area. It forms an extensive, continuous network of cisternae enclosing a single internal space — the ER lumen — continuous with the perinuclear space via the outer nuclear membrane.

Microsomes: when cells are disrupted, ER fragments seal into small spherical vesicles. Rough microsomes (from RER) remain ribosome-studded and have high buoyant density, separable from smooth microsomes (from SER and plasma membrane fragments) by equilibrium density-gradient centrifugation.

3.2 Biosynthetic Pathways of the Smooth ER

3.2.1 Phosphatidic Acid Synthesis (The Bilayer Foundation)

Glycerophospholipid synthesis begins on the cytosolic leaflet of the ER membrane:

Acylation at C1:
Glycerol-3-Phosphate + Fatty Acyl-CoA (R1)Glycerol-3-P Acyltransferase→Lysophosphatidic Acid
Acylation at C2:
Lysophosphatidic Acid + Fatty Acyl-CoA (R2)1-Acylglycerol-3-P Acyltransferase→Phosphatidic Acid
Phosphatidic acid carries a hydrophobic diacylglycerol tail anchored in the cytosolic leaflet, with a highly polar phosphate head group.

3.2.2 Head-Group Attachment Strategies

Strategy 1 — Activation of the Lipid Backbone (e.g. phosphatidylinositol):

Phosphatidic Acid + CTP→CDP-Diacylglycerol + PP_i

CDP-Diacylglycerol + Inositol→Phosphatidylinositol + CMP

Strategy 2 — Activation of the Head Group (e.g. phosphatidylcholine):

Choline + ATP→Phosphocholine + ADP

Phosphocholine + CTP→CDP-Choline + PP_i

CDP-Choline + Diacylglycerol→Phosphatidylcholine + CMP

3.2.3 Maintenance of Membrane Asymmetry via Scramblases

Because synthesis occurs on the cytosolic leaflet, new lipids would otherwise accumulate asymmetrically. Scramblases are non-specific, ATP-independent translocators that flip phospholipids bidirectionally down their concentration gradients, keeping both leaflets equally populated.

3.3 The Cytochrome P450 Monooxygenase System

The SER of hepatocytes detoxifies hydrophobic drugs and xenobiotics via the Cytochrome P450 (CYP) family of mixed-function oxygenases — haemeproteins that absorb light strongly at 450 nm when CO-bound.

Figure: The Cytochrome P450 catalytic cycle. Substrate (RH) first binds the oxidised Fe³⁺ enzyme; a first electron from the NADPH–reductase system reduces it to Fe²⁺, which binds O₂. A second electron drives O–O bond cleavage, forming a reactive Fe–O₂ intermediate that hydroxylates the substrate, releasing ROH and H₂O while regenerating the oxidised Fe³⁺ enzyme to begin the cycle again.

RH + O₂ + NADPH + H⁺Cytochrome P450→ROH + H₂O + NADP⁺

Electrons flow from NADPH via NADPH–Cytochrome P450 reductase (FAD/FMN) to the CYP haem iron. Reduced Fe²⁺ binds O₂, transferring one oxygen atom to the substrate (forming the polar –OH) and reducing the other to water — converting hydrophobic toxins into hydrophilic, renally excretable products.

3.4 Calcium Sequestration and Regulation

The SER is the primary intracellular calcium store. Dense SERCA pumps (Ca²⁺-ATPases) actively pump calcium from the cytosol into the ER lumen against a steep gradient.

Within the lumen, calcium is bound by high-capacity, low-affinity buffering proteins such as calsequestrin and reticulocalbin. Controlled release via IP₃ receptors or ryanodine receptors triggers responses including muscle contraction and secretory cascades.

Chapter 4

4. Molecular Mechanics of Co-Translational Translocation

4.1 The Signal Hypothesis and the Translocation Pathway

The Signal Hypothesis, proposed by Günter Blobel and David Sabatini in 1971, states that proteins targeted to the ER contain an N-terminal signal sequence acting as a physical postcode, directing the ribosome–nascent chain complex to the ER membrane.

Figure: SRP-mediated co-translational translocation. The signal sequence emerges, SRP binds and arrests translation, the complex docks at the SRP receptor via GTP hydrolysis, and the growing chain is threaded through the Sec61 translocon into the ER lumen, where signal peptidase later cleaves the signal and BiP pulls the chain through.

4.2 Structure of the Signal Recognition Particle

The SRP is a cytosolic ribonucleoprotein complex classified in mammals as an 11S RNP: a single ~300-nucleotide 7SL scRNA (homologous to genomic Alu elements) plus six polypeptide subunits — P9, P14, P19, P54, P68, P72.

4.3 SRP Receptor and Translocon Assembly

SRP Receptor (SR) α-subunit: a peripheral 70 kDa GTPase projecting into the cytosol to interact with the SRP S-domain.
SRP Receptor (SR) β-subunit: a 30 kDa transmembrane GTPase anchoring the α-subunit to the ER membrane.
Sec61α: a large, 10-membrane-spanning subunit forming the main aqueous translocation pore.
Sec61β / Sec61γ: single-pass accessory subunits stabilising the complex.

The channel is gated by a helical plug on the luminal side (keeps the pore closed when vacant) and a lateral gate — a vertical slit allowing hydrophobic transmembrane domains to slide into the lipid bilayer. TRAM (translocating chain-associating membrane protein) assists membrane insertion.

4.4 Steps of the Translocation Cycle

Targeting: as the signal sequence emerges from the ribosome, it is bound by the SRP P54 subunit.
Arrest: the Alu domain of SRP binds the ribosome's A-site, pausing translation to prevent premature cytosolic folding.
Docking: the ribosome–nascent chain–SRP complex associates with the SRP receptor (SRαβ), stabilised by GTP binding to both SRP and its receptor.
Transfer: the ribosome is transferred to the Sec61 translocon; GTP hydrolysis causes SRP to dissociate from the signal sequence and receptor, returning to the cytosol.
Translocation: translation resumes, pushing the chain through Sec61; the signal sequence remains anchored via the lateral gate.
Cleavage: signal peptidase cleaves the signal peptide from the growing chain on the luminal side.
Lumenal Pulling: BiP (an Hsp70 ATPase), working with co-chaperone Sec63, binds the emerging polypeptide and pulls it through in an ATP-dependent manner.

4.5 Retrotranslocation (Dislocation) of Misfolded Proteins — ERAD

Misfolded ER proteins must be removed and degraded to prevent cellular stress, via ER-Associated Degradation (ERAD):

Recognition: chaperones identify misfolded proteins by exposed hydrophobic regions or altered glycan profiles.
Export (Retrotranslocation): the misfolded protein is exported back into the cytosol through a Sec61-associated channel.
Deglycosylation: the cytosolic enzyme N-glycanase cleaves N-linked oligosaccharide chains from the polypeptide.
Ubiquitylation: ubiquitin ligases on the ER membrane attach ubiquitin chains to the protein.
Degradation: the polyubiquitylated protein is recognised and degraded by the 26S proteasome.

Membrane Protein Biology: Topology, Glycosylation & Trafficking

Membrane Protein Biology

Topology · N-Linked Glycosylation · Vesicular Trafficking · Golgi Processing · Lysosomal Sorting

5. Topological Classes of Transmembrane and Membrane-Linked Proteins

Insertion Mechanisms · Orientation · Lipid Anchoring

5. Topological Classes of Transmembrane and Membrane-Linked Proteins

5.1 Thermodynamics of Membrane Insertion

Integrating membrane proteins into the lipid bilayer is governed by thermodynamic constraints. Transmembrane domains are made of 20 to 25 hydrophobic amino acids arranged in an α-helix, which maximizes hydrogen bonding between peptide bonds and exposes hydrophobic side chains to the lipid core. Hydrophobic regions of a nascent protein are guided into the bilayer through the lateral gate of the translocon, while hydrophilic regions remain in the aqueous cytosol or ER lumen.

5.2 Classification of Transmembrane Proteins

Transmembrane proteins are divided into four main topological classes based on their insertion mechanism, the presence of cleavable signals, and the orientation of their N- and C-termini.

Figure: The four topological classes of transmembrane proteins. Type I and Type III proteins both place the N-terminus in the lumen, but differ in whether that N-terminus is cleaved (Type I) or part of a retained signal-anchor (Type III). Type II proteins are the mirror image, with an N-terminus retained on the cytosolic side. Type IV proteins cross the membrane repeatedly via alternating start-transfer and stop-transfer signals.

Type I — Cleavable N-terminal signal: Initiated by a cleavable N-terminal signal sequence (start-transfer signal). As translation proceeds, a hydrophobic stop-transfer anchor sequence enters the translocon, arrests translocation, and slips through the lateral gate into the bilayer — leaving the N-terminus in the ER lumen and the C-terminus in the cytosol.
Examples: LDL receptor, insulin receptor, growth hormone receptor.
Type II — Internal signal-anchor, cytosolic N: Lacks a cleavable N-terminal signal. An internal, non-cleavable signal-anchor sequence acts as both targeting signal and transmembrane anchor, orienting with its N-terminus facing the cytosol while the C-terminal chain is translocated into the lumen.
Examples: Transferrin receptor, Golgi galactosyltransferase.
Type III — Internal signal-anchor, luminal N: Also uses a single internal, non-cleavable signal-anchor sequence, but inserts in the opposite orientation to Type II — N-terminus into the lumen, C-terminus remaining in the cytosol.
Examples: Cytochrome P450 monooxygenases.
Type IV — Multipass proteins: Driven by an alternating series of internal start-transfer and stop-transfer signals. Each start-transfer signal initiates translocation of the following loop, which continues until a stop-transfer signal releases the segment laterally into the bilayer. Final orientation depends on the first start-transfer signal's initial orientation.
Examples: GPCRs (7 passes), CFTR (12 passes), GLUT1 (12 passes).

5.3 The Positive-Inside Rule

The orientation of single-pass membrane proteins (Type II vs. Type III) is largely determined by the Positive-Inside Rule. Positively charged amino acids (Lysine and Arginine) in the segments flanking the transmembrane domain are predominantly kept on the cytosolic side of the membrane. This orientation is guided by electrostatic interactions with the negatively charged phospholipids on the cytosolic face of the bilayer and charges within the translocon itself.

5.4 Glycosylphosphatidylinositol (GPI) Anchored Proteins

Some proteins are covalently attached to the membrane via a lipid anchor rather than a transmembrane helix. In the ER, these proteins are initially synthesized as transmembrane proteins with a C-terminal hydrophobic domain.

Figure: GPI-anchor attachment. The membrane-bound enzyme transamidase recognizes a specific C-terminal signal sequence, cleaves the protein near this sequence, and transfers the new carboxyl terminus to the amino group of a pre-formed GPI anchor in the luminal leaflet of the ER bilayer. The resulting protein is spatially restricted to the non-cytosolic luminal leaflet, destined for the outer face of the plasma membrane.

6. The N-Linked Glycosylation Pathway and Quality Control

Dolichol Precursor Assembly · En Bloc Transfer · Calnexin/Calreticulin Cycle

6. The N-Linked Glycosylation Pathway and Quality Control

6.1 Biosynthesis of the Dolichol Precursor

N-linked glycosylation involves the covalent attachment of a pre-formed oligosaccharide precursor to the nitrogen atom of an Asparagine (Asn) residue. The precursor, Glc₃Man₉GlcNAc₂, is synthesized on dolichol phosphate, a long-chain membrane lipid of 75–95 carbon atoms.

Figure: Dolichol-linked oligosaccharide biosynthesis. Synthesis begins on the cytosolic leaflet with transfer of GlcNAc-1-phosphate to Dol-P (blocked by tunicamycin), followed by addition of a second GlcNAc and five mannose residues. The seven-residue intermediate flips to the luminal leaflet, where four further mannose and three glucose residues complete the precursor, using Dol-P-Man and Dol-P-Glc carriers synthesized on the cytosolic face and flipped inward.

6.2 En Bloc Transfer and Consensus Sequences

The fully assembled core oligosaccharide is transferred en bloc from the dolichol pyrophosphate carrier to the side-chain amide nitrogen of an Asparagine residue in the nascent polypeptide, catalyzed by the membrane-bound enzyme oligosaccharyl transferase as the chain emerges into the ER lumen.

Dol-P-P-Glc₃Man₉GlcNAc₂ + Asn-X-Ser/Thr Oligosaccharyl Transferase→ Glycoprotein + Dol-P-P

Consensus sequence: the enzyme targets Asparagine residues within Asn-X-Ser or Asn-X-Thr, where X may be any amino acid except Proline.

6.3 The ER Quality Control Cycle (Calnexin/Calreticulin Chaperone Pathway)

Immediately after transfer, glycosidases begin trimming the oligosaccharide, starting with removal of the three terminal glucose residues — a signal that directs the glycoprotein through ER quality control.

Figure: Calnexin/calreticulin quality control cycle. Glucosidases I and II trim two glucoses, leaving a mono-glucosylated glycoprotein bound by calnexin (membrane-bound) or calreticulin (soluble), which prevent aggregation and assist folding. Glucosidase II then removes the final glucose, releasing the protein to be checked by glucosyltransferase: misfolded protein is reglucosylated and returned to the chaperones, while correctly folded protein proceeds toward COPII export.

Lectin association: Glucosidases I and II remove two of the three terminal glucose residues; the mono-glucosylated protein (Glc₁Man₉GlcNAc₂) is bound by calnexin and calreticulin to prevent aggregation and assist folding.
De-glucosylation: Once folding is complete, glucosidase II cleaves the final glucose, releasing the glycoprotein from the chaperones.
Misfolding sensing: The released protein is monitored by glucosyltransferase, a soluble ER sensor of misfolding.
Outcome — misfolded: glucosyltransferase adds back a glucose from UDP-glucose, restoring calnexin/calreticulin affinity and returning the protein to the cycle.
Outcome — correctly folded: glucosyltransferase ignores it; ER mannosidases remove a specific mannose to signal maturity, and the protein is packaged into COPII vesicles for Golgi transport.

7. Vesicular Trafficking: ER to Golgi and Back

COPII · COPI · Clathrin · Retrograde Retrieval Signals

7. Vesicular Trafficking: ER to Golgi and Back

7.1 Coatomer Systems

Vesicular transport within the endomembrane system is mediated by protein-coated vesicles. The assembly of these coat proteins drives membrane deformation and vesicle budding, while selectively packaging cargo.

7.2 COPII Coat Assembly (Anterograde Pathway)

The assembly of COPII-coated vesicles for anterograde transport from the ER to the cis-Golgi is regulated by the small GTPase Sar1.

Figure: COPII coat assembly. Sec12 exchanges GDP for GTP on Sar1, exposing an amphipathic helix that inserts into the ER membrane. Membrane-bound Sar1-GTP recruits the Sec23/Sec24 inner coat, with Sec24 capturing cargo via di-acidic sorting motifs, followed by the Sec13/Sec31 outer coat, whose polymerization curves the membrane and buds the vesicle. Sec23-stimulated GTP hydrolysis on Sar1 then disassembles the coat before Golgi fusion.

Activation: Soluble Sar1-GDP is recruited to the ER membrane, where the GEF Sec12 catalyzes GDP→GTP exchange.
Membrane insertion: GTP binding exposes an N-terminal amphipathic α-helix that inserts into the cytosolic leaflet of the ER bilayer.
Inner coat recruitment: Sar1-GTP recruits the Sec23/Sec24 heterodimer; Sec23 binds Sar1-GTP directly, while Sec24 captures cytosolic cargo sorting signals such as di-acidic Asp-X-Glu motifs.
Outer coat recruitment: Sec23/Sec24 recruits the heterotetrameric Sec13/Sec31 complex, which polymerizes into a cage-like lattice that drives membrane curvature and budding.
Fission and uncoating: Sec23 promotes GTP hydrolysis on Sar1-GTP; the resulting Sar1-GDP retracts its helix and dissociates, disassembling the coat before fusion with the cis-Golgi network.

7.3 COPI Coat Assembly and Retrograde Transport

The COPI coat mediates retrograde transport from the Golgi back to the ER, as well as bidirectional transport between Golgi cisternae, regulated by the small GTPase Arf1, activated by a Golgi-associated GEF.

Coatomer complex: once active, Arf1-GTP inserts a fatty acid tail into the membrane and recruits the pre-assembled heptameric coatomer complex (α, β, β′, γ, δ, ε, ζ subunits) to drive vesicle budding.

7.4 Retrograde Retrieval Signals

Proteins that belong in the ER but escape to the Golgi are retrieved by specific retrograde pathways.

Pathway	Signal	Mechanism
KDEL Pathway	C-terminal Lys-Asp-Glu-Leu (soluble proteins, e.g. BiP, PDI)	Escaped protein binds the KDEL Receptor with high affinity at Golgi pH (~6.5); complex is packaged into COPI vesicles and returns to the ER, where the neutral pH (~7.2) triggers release
KKXX Pathway	C-terminal Lys-Lys-X-X (transmembrane proteins)	Motif is recognized and bound directly by the COPI coatomer complex, packaging the protein into retrograde vesicles without an intermediate receptor

8. Compartmentalised Processing in the Golgi Complex

Cisternal Organization · Glycan Maturation · Cisternal Transport Models

8. Compartmentalised Processing in the Golgi Complex

8.1 Structural Architecture of the Dictyosome

The Golgi complex (dictyosome) consists of flattened, membrane-bound cisternae organized into distinct functional domains. Proteins enter at the cis-Golgi Network (CGN), move sequentially through the cis-, medial-, and trans-cisternae, and exit at the trans-Golgi Network (TGN).

8.2 Segmented Glycosylation and Processing Pathways

N-linked glycan processing proceeds through three main glycan outcomes as glycoproteins move through the stack:

Glycan type	Where processed	Structure
High-mannose	cis-cisterna	Retains the original mannose core, no further sugar addition
Complex	medial & trans cisternae	Mannosidases trim the core to five residues; GlcNAc, galactose, and terminal sialic acid (NANA, negatively charged) are added
Hybrid	medial & trans cisternae	One branch retains high-mannose residues while the other is modified with complex sugars

O-linked glycosylation: occurs primarily in the Golgi, attaching sugars to the hydroxyl group of Serine or Threonine (occasionally hydroxyproline/hydroxylysine). It begins with GalNAc transfer by polypeptide GalNAc transferase, followed by sequential sugar addition — without a dolichol carrier.

Sulfation: in the TGN, sulfotransferases transfer sulfate groups from PAPS (3′-phosphoadenosine-5′-phosphosulfate) to tyrosine residues or carbohydrate chains, increasing negative charge.

8.3 Models of Cisternal Transport

Cisternal Maturation Model: Golgi cisternae are dynamic, transient structures that physically mature and migrate from the cis to the trans face. A new cis-cisterna forms at the CGN by fusion of ER-derived vesicles, then matures through medial and trans stages as it moves through the stack, while resident enzymes are recycled backward by retrograde COPI vesicles. Vesicular Transport Model: cisternae are static, stable compartments with a fixed set of resident enzymes; cargo is packaged into anterograde COPI vesicles that bud from one cisterna and fuse with the next.

9. Protein Sorting from the trans-Golgi Network to Lysosomes

Mannose 6-Phosphate Tagging · Receptor Dissociation · Retromer Recycling

9. Protein Sorting from the trans-Golgi Network to Lysosomes

9.1 The Mannose 6-Phosphate Sorting Signal

Lysosomal enzymes (acid hydrolases) are synthesized in the RER and targeted to lysosomes by a specific sorting signal: Mannose 6-Phosphate (M6P).

Figure: The mannose 6-phosphate pathway. In the CGN, GlcNAc phosphotransferase recognizes a signal patch on the acid hydrolase and adds a phosphodiester intermediate; phosphodiesterase then unmasks the active M6P tag. In the TGN, M6P-tagged hydrolases bind M6P Receptors and are packaged into clathrin/AP1 vesicles bound for late endosomes, where low pH dissociates the hydrolase from its receptor before a luminal phosphatase removes the phosphate.

Tagging in the CGN: UDP-GlcNAc phosphotransferase recognizes a signal patch on the newly synthesized hydrolase and transfers GlcNAc-1-phosphate to the C-6 hydroxyl of a terminal mannose, forming a phosphodiester intermediate.
Unmasking: Phosphodiesterase cleaves the terminal GlcNAc residue, leaving an active M6P tag.
Receptor binding in the TGN: M6P-tagged hydrolases bind transmembrane M6P Receptors at pH ~6.5.
Vesicle transport: The receptor-cargo complex is packaged into clathrin-coated vesicles (with the AP1 adaptor complex) bound for late endosomes.
Dissociation: The acidic late-endosome lumen (pH ~5.5) causes the hydrolase to dissociate; a luminal phosphatase then removes the phosphate, preventing rebinding.
Lysosomal maturation: The late endosome containing active acid hydrolases matures into a lysosome.

9.2 Retromer-Mediated Receptor Recycling

Once the hydrolase has dissociated, the empty M6P receptor must return to the TGN to capture more cargo — retrieval mediated by the heteropentameric Retromer Complex.

Figure: Retromer complex architecture. The Cargo-Selective Trimer (VPS26–VPS35–VPS29) scaffolds onto the cytosolic tail of the empty M6P receptor via VPS35, while VPS26 and VPS29 stabilize the interaction. The Sorting Nexin heterodimer (VPS5–VPS17) uses Phox domains to bind PI(3)P in endosomal membranes and BAR domains to drive membrane tubulation, together packaging receptors into retrograde tubules back to the TGN.

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Cell Structure and Protein Sorting

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