Cytoskeleton and Cellular Motility

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The Cytoskeleton & Molecular Motors

The Cytoskeleton, Molecular Motors & Muscle Contraction

1. Comparative Structural Hierarchy of the Cytoskeleton

The eukaryotic cytoskeleton is a highly dynamic, three-dimensional intracellular network of protein filaments spanning the cytoplasm. Far from being a static scaffold, this network is responsible for cell shape, mechanical resistance, intracellular transport, chromosome segregation during cell division, and cellular locomotion. The entire system is built upon three primary filament systems — microtubules, actin filaments (microfilaments), and intermediate filaments. These fibres are polymerised from small protein subunits held together by non-covalent interactions, allowing rapid assembly and disassembly in response to physiological signals.

The three cytoskeletal networks differ in their subunit composition, diameter, mechanical properties, and structural polarity.

PropertyMicrotubulesActin FilamentsIntermediate Filaments
StructureHollow, cylindrical tube composed of 13 protofilamentsTwo intertwined helical chains of filamentous actin (F-actin)Tough, rope-like fibres assembled from coiled-coil dimers
Outer diameter~25 nm~7 nm~10 nm
Subunit compositionα-tubulin & β-tubulin heterodimer (~50 kDa each)G-actin (globular actin) monomer (~43 kDa ATPase)Heterogeneous; tissue-specific proteins (e.g. keratins, lamins)
Structural polarityYes — plus (+) and minus (−) endsYes — plus (+) and minus (−) ends (barbed and pointed ends)No — non-polar tetrameric building blocks prevent polarity
Primary nucleotideGTPATPNone
Primary functionsIntracellular transport, spindle poles, ciliary beating, organelle positioningCell motility, crawling, shape, contractile ring in cytokinesisMechanical strength, structural integrity, nuclear envelope lining

2. Microtubule Structure and Biochemistry

Microtubules are hollow cylinders with an external diameter of approximately 25 nm and a wall thickness of about 5 nm. They are assembled from highly conserved, globular heterodimers of α-tubulin and β-tubulin, each possessing a molecular mass of approximately 50 kDa.

Plus (+) End — fast-growing, β-tubulin exposed β α β α β α β α β α β α β α β α β α SEAM (α–β contact) β α β α β α β α β α β α β α β α β α … 13 total protofilaments Minus (−) End — slow-growing, α-tubulin capped β-tubulin: exchangeable GTP/GDP site (hydrolysed after incorporation) α-tubulin: non-exchangeable GTP site (structural, never hydrolysed)

Figure: Microtubule wall architecture. αβ-tubulin heterodimers polymerise head-to-tail into protofilaments, which associate laterally into a hollow, 13-protofilament cylinder. α-tubulin is oriented toward the minus end and β-tubulin toward the plus end within every protofilament. Lateral contacts are homophilic (α–α, β–β) except along the single helical seam, where the lattice offset forces an α–β lateral contact.

2.1 The Protofilament and Lateral Polarity

A linear chain of αβ-tubulin heterodimers, arranged head-to-tail via longitudinal non-covalent bonds, forms a protofilament. Within a protofilament, α-tubulin is always oriented towards the minus (−) end (slow-growing) of the microtubule, while β-tubulin is exposed at the plus (+) end (fast-growing).

A single microtubule typically comprises 13 laterally associated protofilaments that form a hollow cylinder. The lateral interactions between adjacent protofilaments are predominantly homophilic:

  1. α–α contact: α-tubulin associates laterally with the adjacent α-tubulin subunit in the neighbouring protofilament.
  2. β–β contact: β-tubulin associates laterally with the adjacent β-tubulin subunit in the neighbouring protofilament.
  3. The seam (α–β contact): because the 13 protofilaments do not close as a perfect concentric ring but form a helical lattice, one longitudinal line — the seam — carries a helical shift that forces α-tubulin of one protofilament to associate laterally with β-tubulin of the neighbouring protofilament.
    Significance: the seam is a mechanically discontinuous line in the lattice and is thought to be a preferential site of microtubule breakage and of catastrophe initiation.

2.2 Post-Translational Modifications (PTMs) of Tubulin — The "Tubulin Code"

Tubulin subunits undergo numerous, highly specific post-translational modifications that establish a "tubulin code" to regulate microtubule stability and motor protein recruitment.

  1. Phosphorylation: regulates dimer stability and assembly rates.
  2. Acetylation: occurs on the luminal face of stable, long-lived microtubules — primarily on lysine 40 of α-tubulin — increasing mechanical resilience to bending.
  3. Sumoylation: modulates interaction with regulatory proteins.
  4. Polyglutamylation: covalent addition of lateral glutamate chains of varying lengths to the glutamate side-chains in the C-terminal tails of both α- and β-tubulin, regulating motor binding and axonemal function.
Exam tip: acetylation is a marker of microtubule age and stability, not a cause of it — it accumulates on long-lived microtubules because it requires the modifying enzyme to access the luminal face, which happens more readily once the lattice has been intact long enough to acquire lattice defects or open ends.

3. Structural Variations: Singlet, Doublet, and Triplet Microtubules

While cytosolic microtubules exist primarily as solitary cylinders (singlets), specialised structures utilise modified, fused multi-tubule complexes.

A Singlet 13 protofilaments Cytosolic microtubules A B Doublet A: 13 + B: 10–11 protofilaments Cilia & flagella axonemes A B C Triplet A: 13 + B: 10–11 + C: 10–11 Basal bodies & centrioles

Figure: Singlet, doublet, and triplet microtubule cross-sections. Each successive tubule (B, then C) is incomplete and fuses onto the wall of the preceding complete tubule rather than closing into its own independent 13-protofilament ring.

  1. Singlet microtubules: consist of 13 complete protofilaments. They are highly dynamic and populate the general cytoplasm.
  2. Doublet microtubules: found in the axonemes of motile cilia and flagella. They consist of a complete, 13-protofilament A-tubule fused to an incomplete, 10– or 11-protofilament B-tubule, which shares a wall section with the A-tubule.
  3. Triplet microtubules: found in the basal bodies of cilia and flagella, and within centrioles. They consist of a complete A-tubule (13 protofilaments), an incomplete B-tubule (10–11 protofilaments), and an incomplete C-tubule (10–11 protofilaments) fused sequentially.

4. The Microtubule Organising Centre (MTOC) and Nucleation

De novo synthesis of microtubules requires a template to overcome the high activation energy barrier of spontaneous nucleation. This template is provided by the Microtubule Organising Centre (MTOC).

Centrosome Centriole Pair Mother & daughter centrioles, arranged at 90° to one another Pericentriolar Material Amorphous matrix housing γ-tubulin ring complexes (γ-TuRC) γ-TuRC caps the minus (−) end, leaving the plus (+) end free to elongate

Figure: MTOC organisation in animal cells. The centrosome, positioned near the nucleus, is built from a centriole pair and pericentriolar material (PCM). The PCM's γ-TuRC nucleates and caps the microtubule minus end, leaving the plus end free to grow into the cytoplasm.

γ-Tubulin and nucleation kinetics: γ-tubulin does not polymerise into the main microtubule shaft. Instead, it forms a ring-shaped template (γ-TuRC) mimicking the 13-protofilament geometry. The γ-TuRC binds and caps the minus (−) end of the initiating microtubule, shielding it from depolymerisation and leaving the plus (+) end free to elongate outward into the cytoplasm.

Alternative MTOC architectures:

  1. XMAP215: in certain environments, such as Xenopus oocytes and early embryos, the polymerase XMAP215 cooperates with or acts independently of γ-TuRC as a key microtubule nucleator.
  2. Spindle Pole Bodies (SPBs): found in budding and fission yeasts. These multi-protein plaques are permanently embedded within the nuclear envelope. They lack centrioles but nucleate both the intranuclear mitotic spindle and cytoplasmic microtubules.
  3. Plant cells: completely lack centrioles and discrete centrosomes. Instead, nucleating complexes (including γ-TuRC) are distributed along the nuclear envelope and the cortical cytoskeleton to organise microtubules.

5. Microtubule Polymerisation Kinetics and Dynamic Instability

Microtubule assembly from free αβ-tubulin heterodimers proceeds through a distinct three-phase kinetic pathway.

Polymer Mass Time Nucleation Lag phase Elongation Rapid growth Steady State Polymerisation = Depolymerisation

Figure: Three-phase kinetics of microtubule assembly. Nucleation (lag phase): formation of small oligomeric "nuclei" of tubulin; spontaneous nucleation is highly unfavourable under physiological conditions and is bypassed in vivo by the γ-TuRC. Elongation: tubulin heterodimers add rapidly to the exposed ends of the growing filament, at a rate directly proportional to the concentration of free tubulin heterodimers. Steady state: the rate of subunit addition (polymerisation) is exactly balanced by the rate of subunit loss (depolymerisation).

Critical Concentration (Cc) and Treadmilling

The critical concentration (Cc) is the concentration of free tubulin heterodimers at which polymerisation rate equals depolymerisation rate. Because the plus (+) end is structurally more dynamic, it has a lower critical concentration than the minus (−) end:

Cc(plus end) < Cc(minus end)
Treadmilling: if the concentration of free tubulin heterodimers lies between these two thresholds — Cc(plus) < [T] < Cc(minus) — a steady-state flux occurs. Tubulin subunits are added continuously to the plus end while simultaneously dissociating from the minus end, keeping the overall polymer length constant.

GTP Hydrolysis and the GTP Cap

Each α- and β-subunit binds one molecule of GTP:

  1. α-tubulin (N-site): the bound GTP is trapped at the dimer interface, never hydrolysed or exchanged.
  2. β-tubulin (E-site): the bound GTP is exposed at the surface and can be hydrolysed to GDP.
GROWING — GTP Cap Present SHRINKING — Catastrophe (Cap Lost) GTP cap (plus end) GTP-tubulin: strong lateral & longitudinal contacts — stable wall GDP-tubulin: curved, strained protofilaments peel outward and depolymerise

Figure: The GTP cap and dynamic instability. Free tubulin heterodimers in the cytosol are primarily in the T-form (GTP-bound at the β-subunit). When a dimer attaches to a microtubule, GTP hydrolysis is stimulated. If polymerisation outpaces hydrolysis, a GTP cap persists at the plus end, stabilising the lattice. If free tubulin concentration drops, polymerisation slows below the hydrolysis rate, the cap is lost, and the exposed GDP-tubulin — curved and strained — peels away and depolymerises rapidly.

Dynamic instability — catastrophe and rescue: individual microtubules undergo rapid transitions between a phase of slow growth and a phase of rapid shrinkage.

  1. Catastrophe: the abrupt transition from growth to shrinkage, triggered by the loss of the stabilising GTP cap.
  2. Rescue: the abrupt transition from shrinkage back to growth, initiated when new GTP-tubulin dimers re-cap the depolymerising end.

6. Microtubule-Associated Proteins (MAPs)

The dynamic behaviour, stability, and spatial organisation of microtubules are tightly regulated by a diverse class of non-motor proteins known as Microtubule-Associated Proteins (MAPs).

Microtubule-Associated Proteins Stabilisers MAP4, Tau, MAP2 Bind sides of MT wall, promote polymerisation Destabilisers Katanin, Stathmin, Kinesin-13 (MCAK) Sever / destabilise MTs Capping Proteins Patronin Stabilises the minus end Cross-linkers Tau, MAP2, CLIP170 Bundle parallel MTs; CLIP170 tracks the (+) tip

Figure: Functional classes of microtubule-associated proteins. MAPs regulate microtubules through four broad mechanisms — stabilising the lattice, destabilising or severing it, capping an end to block dynamics, and cross-linking or bundling separate microtubules together.

  1. Stabilisers: proteins such as Tau, MAP2, and MAP4 bind along the sides of microtubules, neutralising negative charges and promoting polymerisation and stability.
    Clinical relevance: Tau phosphorylation reduces its affinity for microtubules, a key pathological hallmark of Alzheimer's disease.
  2. Destabilisers:
    Katanin — an ATP-dependent severing protein that cuts microtubules along their length, exposing new, unstable GDP-bound ends.

    Stathmin — sequesters free tubulin heterodimers, reducing the cytosolic pool available for polymerisation.

    Kinesin-13 (MCAK) — a non-motor kinesin that binds specifically to microtubule ends and actively induces protofilament bending, triggering catastrophe.
  3. Capping proteins: Patronin binds to and stabilises microtubule minus ends, protecting them from depolymerisation.
  4. Cross-linkers / bundlers: proteins like Tau (short spacing) and MAP2 (long spacing) bundle parallel microtubules in axons and dendrites, while CLIP170 tracks growing plus ends as a +TIP (plus-end tracking protein).

7. Microtubule-Based Motor Proteins: Kinesins and Dyneins

Intracellular transport along microtubules is driven by two superfamilies of ATP-dependent motor proteins: kinesins (primarily anterograde / plus-end directed) and dyneins (retrograde / minus-end directed).

Kinesin-1 — Plus (+)-end directed Dynein-Dynactin — Minus (−)-end directed Motor Heads (N-term) Two globular heads — ATP-binding catalytic core Coiled-coil Stalk Long, α-helical dimerisation domain Light Chains Bind the tail domain Tail (C-terminus) Binds light chains; recruits cargo receptors (kinectin) Motor Domain 2 heavy chains, each forming a ring of 6x AAA+ ATPase domains Stalk Domain Coiled-coil arm; MT-binding tip between AAA+ domains 4 & 5 Tail Domain N-terminal dimer; binds intermediate & light chains Dynactin Complex p150ᵝ — binds MT + dynein IC Arp1 filament (~37 nm) — cargo adapter (binds spectrin)

Figure: Domain architecture of kinesin-1 versus the dynein–dynactin complex. Kinesin-1 is a compact heterotetramer whose N-terminal motor heads walk toward the microtubule plus end while the C-terminal tail engages cargo. Cytoplasmic dynein is a much larger homodimer whose AAA+ motor rings walk toward the minus end; because dynein cannot bind membrane-bound cargo directly, it depends on the accessory dynactin complex (p150ᵝ and the Arp1 filament) to link it to cargo.

Kinesins: The Anterograde Motors

The mammalian genome encodes over 40 kinesin proteins grouped into 14 families (Kinesin-1 to Kinesin-14). They are structurally classified by the location of their catalytic motor domain:

  1. N-type kinesins (Kinesin-1 to -12): motor domain at the N-terminus. They are plus end-directed motors.
  2. M-type kinesins (Kinesin-13): motor domain in the middle. They do not walk; instead, they function as microtubule depolymerisers.
  3. C-type kinesins (Kinesin-14): motor domain at the C-terminus. They are minus end-directed motors.

Structure of Conventional Kinesin (Kinesin-1)

Kinesin-1 is a heterotetramer composed of two heavy chains (~120 kDa each) and two light chains (~64 kDa each). It has three functional domains: the motor head domain (N-terminus) containing the ATP-binding catalytic core and MT-binding site; the α-helical stalk domain, a long coiled-coil dimerisation domain; and the tail domain (C-terminus), which binds the light chains and recruits cargo-specific membrane receptors such as kinectin.

Kinesin-5 (bipolar tetramer): comprises four heavy chains forming a bipolar, double-headed tetramer. It cross-links antiparallel microtubules and slides them past one another during mitotic spindle elongation.

Dyneins: The Minus-End Motors

Dyneins are massive, multi-subunit motor complexes that walk exclusively towards the minus (−) end of microtubules. They are divided into two main classes:

  1. Cytoplasmic dyneins:
    Cytoplasmic Dynein 1 — ubiquitous; drives vesicle trafficking, retrograde axonal transport, and mitotic spindle positioning.

    Cytoplasmic Dynein 2 — dedicated to retrograde intraflagellar transport (IFT) in cilia and flagella.
  2. Axonemal dyneins: multi-headed complexes (heterodimers or heterotrimers) anchored within the axoneme of motile cilia and flagella, driving sliding and beating.

Structure of Cytoplasmic Dynein 1

Dynein is a homodimer of two heavy chains (>500 kDa each), associated with a complex of intermediate, light intermediate, and light chains. The motor domain at the C-terminus of each heavy chain forms a large planar ring of six AAA+ ATPase domains. A stalk domain — a coiled-coil extending between AAA+ domains 4 and 5 — terminates in a globular microtubule-binding domain. The tail domain at the N-terminus dimerises and binds the accessory chains that mediate cargo interaction.

The Dynactin Complex

Dynein cannot transport membrane-bound cargo on its own; it requires dynactin, a 23-subunit complex that acts as an adapter. Key components include the Arp1 (Actin-Related Protein 1) filament — a short, actin-like filament (~37 nm) that associates with spectrin on cargo vesicles — and the p150ᵝ subunit, a long, projecting polypeptide dimer that binds directly to microtubules and the dynein intermediate chain, increasing the processivity of the motor.

8. Cilia and Flagella Structure: The Axonemal Architecture

Cilia and flagella are hair-like, membrane-bound projection organelles that extend from the eukaryotic cell surface to drive motility or sense environmental signals.

The Axoneme

The core structural scaffold of both motile cilia and flagella is the axoneme, which displays a characteristic 9 + 2 arrangement.

9 + 2 Axoneme Cross-Section Plasma membrane C1 C2 9 outer doublets + 1 central singlet pairA B Radial spoke Nexin linkerDetail: Arms Reach the NEXT Doublet A B Doublet N B Doublet N+1 Outer dynein arm Nexin linker Inner dynein armA gap separates neighbouring doublets — only dynein arms and nexin links bridge it C1 C2 Radial spoke → central singlet pair

Figure: The 9 + 2 axonemal cross-section. Nine outer doublet microtubules (each a complete 13-protofilament A-tubule fused to an incomplete 10-protofilament B-tubule) form a ring around a central singlet pair (C1, C2) enclosed by a central sheath. Radial spokes project inward from each doublet toward the central pair, while nexin linkers tether neighbouring doublets together. Inner and outer dynein arms (detail inset) project from the A-tubule of one doublet toward the B-tubule of its clockwise neighbour.

  1. 9 outer doublet microtubules: a ring of nine doublets. Each doublet contains a complete A-tubule (13 protofilaments) and an incomplete B-tubule (10 protofilaments).
  2. 1 central singlet pair: two individual, complete singlet microtubules (designated C1 and C2) surrounded by a central sheath.
  3. Tektin: a highly α-helical filament running along the groove of the outer doublets, providing structural support to the junction between the A- and B-tubules.

Linkers and Effectors

  1. Axonemal dynein arms: inner and outer dynein arms project from the A-tubule of one doublet toward the B-tubule of the adjacent doublet. They are oriented with their motor domains facing the minus end (toward the basal body).
  2. Nexin linkers: elastic protein bridges that connect adjacent outer doublets, restraining sliding and converting shear stress into bending.
  3. Radial spokes: rigid protein complexes that project from the A-tubules of the outer doublets toward the central singlet pair, coordinating the beating pattern.

The Basal Body (Kinetosome)

The axoneme is nucleated by and anchored to a basal body located at the cell cortex. The basal body consists of nine triplet microtubules arranged in a barrel shape with a 9 + 0 pattern (lacking the central singlet pair).

PropertyMotile CiliaFlagella
Quantity per cellNumerous (multiciliated)Few (usually 1–4)
LengthShort (5–10 µm)Long (~150 µm)
Motion profileCiliary beating: rapid power stroke followed by recovery strokeWave-like sinusoidal undulations; autonomous, independent beating
Functional outcomeMoves fluid/mucus across tissues or propels single cellsPropels the cell through aqueous media

Non-Motile (Primary/Sensory) Cilia

Most vertebrate cells possess a solitary, non-motile primary cilium.

  1. Ultrastructure: features a 9 + 0 axoneme (lacking the central singlet pair and the inner/outer dynein arms).
  2. Function: serves as a sensory antenna loaded with receptors (e.g. Hedgehog pathway, G-protein coupled receptors) to transmit environmental signals.
  3. Embryonic nodal cilia: an exception — these are motile 9 + 0 cilia present in the embryonic node during development. Although they lack the central singlet pair, they contain active dynein arms that generate a clockwise rotational flow ("nodal flow"), which is critical for establishing left–right organ asymmetry.

9. Ciliary and Flagellar Motility Mechanics: The Sliding Filament Model

Movement of cilia and flagella is driven by the sliding of adjacent outer doublets relative to one another, a mechanism described by the Sliding Filament Model.

No Nexin Linkers — Unrestrained Slide With Nexin Linkers — Axoneme Bends Tip (distal) Doublet N (start) Doublet N+1 Dynein-driven slide toward the tip (unbent, dashed) vs. actual bend (solid) Doublet N Doublet N+1 Nexin linker

Figure: The sliding filament model of axonemal bending. Left: without restraining linkers, dynein-driven walking would let Doublet N+1 slide freely past Doublet N toward the ciliary tip, telescoping the axoneme rather than bending it. Right: because nexin linkers tether neighbouring doublets together, that same sliding force is converted into lateral bending of the whole axoneme.

  1. Dynein walking: the axonemal dynein arms anchored to the A-tubule of Doublet N bind to the B-tubule of the adjacent Doublet N+1. Using ATP hydrolysis, the dynein heads walk towards the minus end (proximal direction) of the adjacent doublet.
  2. Shear force generation: this walking motion attempts to slide Doublet N+1 past Doublet N toward the tip (distal end).
  3. Bending transition: because adjacent doublets are tethered together along their length by elastic nexin linkers, unrestricted sliding is prevented. The shear force generated by dynein walking is instead converted into lateral bending of the axoneme.
  4. Metabolic control: sequential, alternating activation of dyneins on opposite sides of the axoneme (coordinated by radial spokes and the central pair) creates the symmetrical, wave-like beating pattern.

10. Centriole Structure and the Centrosome Cycle

Centrioles are barrel-shaped, polarised organelles built from triplet microtubules arranged in a cylinder with strict ninefold symmetry — the classic 9 + 0 pattern (nine outer triplets, no central pair), which distinguishes a centriole from the 9+2 axoneme of a cilium or flagellum.

10.1 The Proximal Cartwheel

At its proximal (basal) end, a newly assembling centriole displays a transient cartwheel structure that establishes and locks in the ninefold symmetry before the outer microtubule wall is fully built.

A B C A B C A B C A B C A B C A B C A B C A B C A B C SAS-6 HUB Nine radial spokes template nine triplet microtubules (A, B, C tubules)

Figure: The proximal cartwheel. Nine coiled-coil homodimers of the protein SAS-6 self-associate through their head domains to form a highly symmetric, ninefold central hub. Nine radial spokes project outward from this hub, precisely templating the assembly of the nine surrounding triplet microtubules and thereby fixing the ninefold symmetry of the entire organelle.

SAS-6 stoichiometry: SAS-6 monomers first form coiled-coil homodimers; nine of these dimers then self-associate through their head domains to build the ring-like hub, from which the nine radial spokes emerge.

10.2 The Centrosome Cycle

To ensure that a dividing cell enters mitosis with exactly two spindle poles, centrioles and centrosomes undergo semiconservative replication tightly coordinated with the cell cycle, so that duplication occurs once, and only once, per division cycle.

G1 PHASE Tether resolved; centrioles separate slightly S PHASE Procentriole buds at a 90° angle from the mother base G2 PHASE Elongation complete; two full centriole pairs, each with PCM M PHASE Centrosomes split; asters migrate to opposite spindle poles

Figure: The centrosome cycle. Mother and daughter centrioles resolve their tether in G1, a procentriole buds at a right angle in S phase, elongation finishes by G2 giving two complete centriole pairs, and the whole complex splits at the G1/M transition so that each spindle pole nucleates its own set of astral and spindle microtubules.

10.3 Phase-by-Phase Mechanism

  1. G1 Phase (Resolution): The tight cohesion between the mother and daughter centrioles is resolved. The protein tether connecting them is broken, allowing them to separate slightly.
  2. S Phase (Duplication and Elongation): Coinciding with DNA replication, G1/S-CDK activity triggers procentriole budding. A new daughter centriole (procentriole) begins to assemble at a right angle near the base of each pre-existing mother centriole.
  3. G2 Phase (Maturation): Procentriole elongation is completed. The cell now contains two pairs of centrioles, each surrounded by pericentriolar material (PCM).
  4. M Phase (Splitting and Spindle Assembly): At the transition to mitosis, the single centrosomal complex splits. The two centrosomes migrate to opposite poles of the cell, nucleating astral and spindle microtubules to form the mitotic spindle.

11. Actin Filament (Microfilament) Structure

Actin filaments, or microfilaments, are thin, flexible, polarised fibres roughly 7 nm in diameter. They are assembled from monomers of G-actin (Globular Actin), a 43 kDa globular ATPase.

Two protofilaments twisted into a right-handed helix Pointed End (−) Barbed End (+) Slow-growing Fast-growing (up to 10×)

Figure: F-actin polarity. G-actin monomers polymerise head-to-tail, and two parallel strands of F-actin twist around one another in a right-handed helix. The two ends are structurally and kinetically distinct: the barbed (+) end grows rapidly while the pointed (−) end grows slowly, giving the filament an intrinsic polarity that motor proteins and regulatory factors read directly.

11.1 The F-Actin Helix

G-actin monomers polymerise head-to-tail to form F-actin (Filamentous Actin). Two parallel strands of F-actin twist around one another in a right-handed helix to form the final microfilament.

11.2 Filament Polarity and Nucleotide State

Actin filaments possess distinct structural and kinetic polarity:

  1. Plus (+) or Barbed End: The fast-growing end. G-actin monomer addition at the barbed end is up to 10 times faster than at the pointed end.
  2. Minus (−) or Pointed End: The slow-growing end.
Nucleotide binding: each G-actin monomer contains a central binding pocket for a Mg2+ ion complexed with ATP (or ADP). G-actin monomers bound to ATP (T-form) have a high affinity for polymerisation. Shortly after incorporation into the filament, the bound ATP is hydrolysed to ADP. ADP-bound actin (D-form) has a lower binding affinity, destabilising the polymer and promoting depolymerisation at the pointed end.

12. Actin Polymerisation Dynamics and Nucleating Proteins

Like microtubules, actin polymerisation occurs in three sequential phases: nucleation (the kinetically unfavourable formation of a trimeric nucleus), elongation (rapid growth), and steady-state (treadmilling). To bypass the unfavourable nucleation phase and control filament architecture, cells rely on specialised actin nucleator complexes.

Branched Nucleation (Arp2/3 Complex) Daughter filament (+) (−) (+) 70° Arp2/Arp3 mimic an actin dimer, nucleating growth off the mother filament sideUnbranched Nucleation (Formin Dimer) (−) FH2 ring (+) FH1 FH1 FH2 stays on the growing (+) end; FH1 recruits profilin-actin (small circles)

Figure: The two major actin nucleators. The Arp2/3 complex builds dense, branched networks by nucleating a new daughter filament off the side of an existing one at a fixed 70° angle. Formin dimers instead build long, straight, unbranched filaments, with the FH2 ring remaining processively attached to the plus end while FH1 domains feed it profilin-bound monomers.

12.1 The Arp2/3 Complex: Branched Actin

The Arp2/3 complex is a heptameric, 220 kDa complex containing two actin-related proteins: Arp2 and Arp3.

  1. Mechanism: When activated by Nucleation Promoting Factors (NPFs) such as WASP or WAVEs, the Arp2/3 complex binds to the side of a pre-existing actin filament.
  2. Geometry: Arp2 and Arp3 mimic an actin dimer, nucleating a new daughter filament that grows outward at a highly conserved, distinctive 70° angle relative to the mother filament. This creates the dense, branched actin networks typical of the leading edge of crawling cells.

12.2 Formins: Unbranched Actin

Formins are multi-domain homodimers that nucleate straight, unbranched actin filaments.

FH2 domain Dimeric ring→ Nucleates actin; stays processively on the (+) end, protecting it from capping proteins
FH1 domain Proline-rich→ Binds profilin–actin; concentrates G-actin near FH2, accelerating polymerisation up to 10-fold
Regulation: formins are directly regulated by active, membrane-bound Rho GTPases.

13. Physiological Functions and Cell Crawling

Actin filaments are central to cell motility, shape, and mechanical support. They support diverse surface structures, including microvilli (stable, parallel bundles of actin supporting cellular projections in absorptive epithelial cells) and the contractile ring (a transient, myosin-associated ring that drives cytokinesis during cell division).

13.1 The Mechanism of Cell Crawling

Cell crawling across an extracellular substrate is a highly coordinated, three-step physical cycle driven by actin dynamics:

1. Protrusion Substrate Branched actin pushes the lamellipodium forward2. Attachment Substrate Focal adhesions (integrins) anchor the new edge3. Traction & Retraction Substrate Myosin-II contracts; trailing edge de-adheres

Figure: The cell-crawling cycle. A leading-edge protrusion driven by branched actin polymerisation is followed by new focal adhesions anchoring the cell to the substrate, and finally myosin-II-generated traction pulls the cell body forward while the trailing edge releases and retracts.

  1. Protrusion: Branched actin polymerisation (mediated by Arp2/3) at the leading edge pushes the plasma membrane forward, forming flat sheets (lamellipodia), needle-like spikes (filopodia), or large 3D lobes (pseudopodia).
  2. Attachment: The newly protruded membrane adheres to the extracellular substrate through the formation of focal adhesions, containing transmembrane integrins coupled to the internal actin cytoskeleton.
  3. Traction and Retraction: Myosin-II motor proteins pull on the actin network, generating contractile force. This pulls the cell body forward (traction) while simultaneously breaking older adhesions at the rear, causing the trailing edge to retract.

14. Cytoskeletal Pharmacology: Therapeutic and Experimental Drugs

Because of their central roles in cell division and motility, microtubules and actin filaments are major targets for natural toxins and clinical therapeutics.

Filament targetDrug nameSpecific molecular mechanism
Actin filamentsCytochalasin DBinds specifically to the plus (+) ends of actin filaments, blocking further elongation.
Actin filamentsLatrunculinBinds directly to G-actin monomers, preventing their polymerisation into F-actin.
Actin filamentsPhalloidinBinds tightly along the sides of F-actin, blocking depolymerisation and locking filaments in place.
Actin filamentsJasplakinolideInduces actin polymerisation by stimulating nucleation; stabilises F-actin.
Actin filamentsSwinholideSevers actin filaments.
MicrotubulesNocodazoleCauses rapid microtubule depolymerisation to free tubulin subunits.
MicrotubulesColchicine / ColcemidBinds to tubulin dimers, preventing polymerisation; halts cells in mitotic metaphase.
MicrotubulesTaxol (Paclitaxel)Binds and stabilises microtubules, preventing depolymerisation; blocks mitotic spindle dynamics.
MicrotubulesVinblastine / VincristineBinds tubulin dimers, preventing polymerisation; used in cancer chemotherapy.
High-yield distinction: drugs acting on monomers (latrunculin, colchicine/vinblastine) prevent new polymerisation, while drugs acting on the polymer itself either stabilise it (phalloidin, Taxol) or block growth/cause depolymerisation (cytochalasin D, nocodazole) — a distinction frequently tested in exam questions on mechanism.

15. Myosin Motor Biochemistry

Myosins are a superfamily of actin-based motor proteins that use ATP hydrolysis to walk along actin filaments. The classical, muscle-associated motor is Myosin-II.

S1 S1 Two globular heads (N-terminal, motor domain) Light chains (essential + regulatory)Coiled-coil tail (C-terminal) — assembles into bipolar thick filaments HMM (soluble after trypsin) Light Meromyosin (LMM) — insoluble rod; drives tail-tail assembly

Figure: Myosin-II domain architecture. Two heavy chains each contribute a globular S1 head (motor domain), a light-chain-wrapped neck acting as a lever arm, and a long α-helical coiled-coil tail. Trypsin cleavage separates the soluble Heavy Meromyosin (HMM, heads + short tail) from the insoluble Light Meromyosin (LMM, rod).

15.1 Subunit Stoichiometry of Myosin-II

Myosin-II is a hexameric protein (~520 kDa) composed of:

  1. Two Heavy Chains (~220 kDa each): each contains a globular N-terminal head (the motor domain, with actin- and ATP-binding sites), a flexible neck domain, and a long C-terminal tail forming an α-helical coiled-coil.
  2. Two Pairs of Light Chains (~20 kDa each): one pair of essential light chains and one pair of regulatory light chains wrap around the heavy chain neck domains, reinforcing them and acting as lever arms.

15.2 Proteolytic Dissection of Myosin-II

Controlled proteolysis has been critical in defining the functional domains of Myosin-II.

Myosin-II Trypsin→ Light Meromyosin (LMM, insoluble rod; tail-tail assembly) + Heavy Meromyosin (HMM, soluble; heads + necks + light chains)
HMM Papain→ Two Subfragment 1 (S1) heads (ATPase + actin-binding) + one Subfragment 2 (S2) rod-like connector

15.3 Directional Diversity

Almost all myosins (e.g. Myosin-I, Myosin-II, Myosin-V) walk exclusively towards the plus (+) / barbed end of actin filaments.

Exception — Myosin VI: the sole exception. It possesses a unique insertion in its neck region that reverses its orientation, allowing it to walk towards the minus (−) / pointed end of actin filaments.

16. Structural Organisation of the Sarcomere

Skeletal muscle contraction is driven by the ATP-dependent sliding of thick (myosin) and thin (actin) filaments within the sarcomere, the basic contractile unit of striated muscle.

Z-disc Z-disc Thin (actin) Thick (myosin) filament with cross-bridges M-line I-band (thin only) A-band (constant length; spans thick filament) H-zone (thick only)

Figure: Sarcomere architecture. Thin actin filaments anchor at the Z-discs and extend inward; thick myosin filaments are centred on the M-line. The A-band spans the full length of the thick filaments and stays constant in length, while the I-band (thin filaments only) and H-zone (thick filaments only, no overlap) both shorten during contraction as the Z-discs are pulled closer together.

16.1 Sarcomere Zones and Bands

  1. Z-Discs (Z-Lines): the lateral boundaries of each sarcomere. They anchor the plus (+) ends of the thin actin filaments via the cross-linking protein α-actinin.
  2. A-Band (Anisotropic): the dark, central band. It spans the entire length of the thick myosin filaments, including the regions of overlap with thin filaments. Its length remains constant during contraction.
  3. I-Band (Isotropic): the light band, bisected by the Z-disc. It contains only thin actin filaments. During contraction, the I-band shortens.
  4. H-Zone: the pale region in the centre of the A-band, containing only thick myosin filaments with no thin filament overlap. It shortens during contraction.
  5. M-Line: the exact centre of the sarcomere. It contains the protein myomesin, which cross-links and stabilises the thick filaments.

16.2 Sarcomere-Associated Accessory Proteins

Nebulin: a long, non-elastic giant protein that winds around the thin filament, acting as a molecular ruler to dictate its precise length.
Tropomodulin: caps the minus (−) ends of the thin filaments near the M-line, preventing depolymerisation and maintaining actin stability.
Titin: the largest known single polypeptide (~3–4 MDa). It acts as a molecular spring, spanning from the Z-disc to the M-line, anchoring the thick filaments and restoring the sarcomere's resting length during relaxation.
Dystrophin: links the actin cytoskeleton of the myofibril to the extracellular matrix via the dystroglycan complex, protecting the sarcolemma from shearing forces during contraction. Defective dystrophin leads to Duchenne muscular dystrophy.

17. The Biochemical and Cross-Bridge Cycles of Muscle Contraction

Muscle contraction is initiated by an action potential that triggers the release of calcium, removing the steric block on thin filaments.

17.1 Excitation–Contraction Coupling

Resting: Low Ca²⁺ Tropomyosin blocks the myosin-binding site Troponin I holds the inhibitory conformationActive: High Ca²⁺ Ca²⁺ binds Troponin C; tropomyosin is pulled aside Myosin-binding site is now exposed

Figure: The calcium-activated steric block. At rest, tropomyosin sits directly over the myosin-binding groove on actin, held there by troponin I. When calcium floods in and binds troponin C, tropomyosin is displaced away from the groove, exposing the binding site to myosin heads.

  1. An action potential travels along the sarcolemma and propagates deep into the muscle fibre via Transverse (T)-tubules.
  2. Depolarisation of the T-tubules activates voltage-sensitive Dihydropyridine (DHP) Receptors.
  3. Active DHP receptors mechanically open Ryanodine Receptors (RyR) — calcium release channels — in the membrane of the adjacent Sarcoplasmic Reticulum (SR).
  4. Ca2+ rushes out of the SR into the sarcoplasm.
  5. Ca2+ binds to Troponin C, causing a conformational change in the troponin complex (Troponin I, T, C).
  6. This shift pulls tropomyosin out of the myosin-binding groove on the actin filament, exposing the binding sites.

17.2 The Cross-Bridge Cycle

Once binding sites are exposed, Myosin-II heads undergo a highly coordinated, six-stage ATPase cycle that slides the thin filaments toward the M-line:

1. Rigor State No nucleotide; head locked to actin 2. Release State ATP binds; head detaches 3. Cocked State Head rotates 5nm; ADP+Pi remain 4. Weak Attachment Head binds a new actin monomer 5. Power Stroke Pi release; forceful pull on actin 6. Rigor Return ADP released; back to rigor ATP binds ATP hydrolysed Binds actin Pi released Power stroke ADP released

Figure: The six-stage cross-bridge cycle. Starting from rigor, ATP binding detaches the head; hydrolysis cocks it 5 nm forward; the cocked head binds a new actin monomer further along the filament; phosphate release triggers the power stroke that pulls actin toward the M-line; and ADP release returns the head to rigor, ready for the next cycle.

  1. Rigor State: the myosin head lacks bound nucleotides and is locked tightly onto the actin filament.
  2. Release State: ATP binds to the cleft on the back of the myosin head. This causes a conformational change that dramatically reduces the head's affinity for actin, causing it to detach.
  3. Cocked State: the catalytic cleft closes around the bound ATP, triggering ATP hydrolysis to ADP and inorganic phosphate (Pi). The energy released drives a rotation of the lever arm, displacing the head by approximately 5 nm toward the plus (+) end of the actin filament ("cocking"). ADP and Pi remain bound to the head.
  4. Weak Attachment: the cocked myosin head binds to a new actin monomer further along the thin filament.
  5. Power Stroke: Pi is released from the catalytic pocket. This release triggers the power stroke: the lever arm swings forcefully back to its resting conformation, pulling the actin filament towards the centre of the sarcomere (M-line).
  6. ADP Release: ADP is released, leaving the myosin head bound to actin in the low-energy rigor state, ready for a new cycle.

18. Smooth Muscle Contraction: Thick Filament Regulation

Unlike skeletal muscle, vertebrate smooth muscle lacks the troponin complex. It regulates contraction through thick-filament regulation, which is dependent on myosin light-chain phosphorylation.

Ca²⁺ influx Binds Calmodulin Activates MLCK Phosphorylates Regulatory Light Chain Myosin-II Walk (Contraction) Rho kinase inhibits MLCP → sustained contraction MLCP dephosphorylates RLC → relaxation

Figure: Smooth-muscle thick-filament regulation. Calcium influx activates calmodulin, which activates MLCK, which phosphorylates the myosin regulatory light chain to switch on the contractile walk. Relaxation follows MLCP-mediated dephosphorylation, but active Rho kinase can inhibit MLCP directly, sustaining contraction even without a rise in calcium ("calcium sensitisation").

18.1 The MLCK Activation Cascade

  1. An increase in cytosolic Ca2+ (from extracellular influx or SR release) triggers Ca2+ binding to the calcium-sensor protein calmodulin.
  2. The Ca2+–calmodulin complex binds and activates the enzyme Myosin Light Chain Kinase (MLCK).
  3. Active MLCK transfers a phosphate group from ATP to the regulatory light chain of Myosin-II.
  4. Phosphorylation of the regulatory light chain induces a conformational change in the myosin tail, enabling Myosin-II to assemble into active bipolar filaments and interact with actin to initiate contraction.

18.2 Relaxation and the Rho Kinase Pathway

Relaxation: when cytosolic Ca2+ levels fall, Myosin Light Chain Phosphatase (MLCP) dephosphorylates the regulatory light chain, causing myosin filaments to disassemble and muscle to relax.
Calcium-independent contraction (Rho kinase): active Rho kinase (activated by Rho GTPase) can phosphorylate and inactivate MLCP, preventing dephosphorylation of the myosin light chain. This results in sustained muscle contraction ("calcium sensitisation") even at resting calcium concentrations.

19. Intermediate Filaments: Non-Polar Structural Scaffolds

Intermediate filaments (IFs) are tough, rope-like fibres roughly 10 nm in diameter. Unlike microtubules and actin filaments, they do not participate in cell motility and lack structural polarity. Their primary function is to provide mechanical strength and distribute physical stress across tissues.

19.1 Subunit Assembly Pathway

The assembly of intermediate filaments is spontaneous and does not require nucleotide cofactor hydrolysis (ATP or GTP):

Monomer Head—Rod Domain—TailDimer (parallel) Two monomers, same directionTetramer (antiparallel, staggered) Protofilament Tetramers join end-to-end Mature Filament (10 nm) ~8 protofilaments, laterally twisted — non-polar (both ends identical)

Figure: Intermediate filament assembly. A monomer's central rod domain (flanked by head and tail) drives parallel dimer formation; two dimers then associate in an antiparallel, staggered arrangement to form a tetramer that — because it points in opposite directions at each end — is non-polar. Tetramers join end-to-end into protofilaments, and roughly eight protofilaments twist together into the final 10 nm rope.

  1. Monomer: consists of a central, highly conserved α-helical rod domain flanked by non-helical N-terminal head and C-terminal tail domains. The rod domain is characterised by a repeating heptapeptide sequence (heptad repeat) that facilitates coiled-coil formation.
  2. Dimer: two parallel monomers align in register to form a stable, right-handed coiled-coil dimer.
  3. Tetramer: two dimers associate in an antiparallel, staggered orientation. Because the two dimers point in opposite directions, the tetramer lacks structural polarity (both ends are identical).
  4. Protofilament: tetramers associate end-to-end to form protofilaments.
  5. Filament: approximately 8 parallel protofilaments associate laterally and twist together to form the final 10 nm, non-polar intermediate filament rope.

19.2 Six Major Classes of Intermediate Filaments

Intermediate filaments are chemically heterogeneous and expressed in a highly tissue-specific manner:

ClassIntermediate filament proteinPrimary sitePrimary physiological role
Class IAcidic KeratinsEpithelial cellsMechanical integrity of epithelia; forms desmosomes and hemidesmosomes
Class IINeutral or Basic KeratinsEpithelial cellsMechanical integrity of epithelia; forms desmosomes and hemidesmosomes
Class IIIVimentinMesenchymal cells, fibroblastsCellular mechanical support; anchoring of organelles
DesminMuscle cellsIntegrates sarcomeres with plasma membrane in myocytes
Glial Fibrillary Acidic Protein (GFAP)Glial cellsMechanical structure of astrocytes and glial cells
Class IVNeurofilament Proteins (NF-L, NF-M, NF-H)NeuronsControls axonal calibre, diameter, and signal conduction speed
Class VNuclear Lamins (A, B, C)Nucleus (all cells)Forms the nuclear lamina lining the inner nuclear membrane
Class VINestinStem cells of CNSExpressed in neural stem cells during embryonic development
Key distinction from microtubules and actin: intermediate filament assembly requires no nucleotide hydrolysis, and the resulting polymer is non-polar — this rules out any role for IFs as tracks for directional motor-protein movement, unlike microtubules (kinesin/dynein) and actin (myosin).

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