Cytoskeleton

Label a prokaryotic and eukaryotic cell

How do cytoskeletal polymers assemble?

• Non-covalent protein-protein interactions, driven by self-assembly and head-to-tail subunit addition

• Assembly is regulated by nucleotide binding and hydrolysis

• Intermediate filament assemble without nucleotides, relying on coiled-coil hydrophobic interactions and lateral contacts for strength.

What are the cytoskeletal polymers?

• Actin filaments (microfilaments)

• Microtubules

• Intermediate filaments

Actin

Subunit: globular actin

Polymer structure: Head to tail → 2 stranded helix

Polarity: Yes +/- , growth occurs at + ends

Nucleotide: ATP

Microtubules

Subunit: αβ-tubulin heterodimer

Assembly: Head to tail protofilaments → 13-protofilament tube

Polarity: Yes +/-

Nucleotide: GTP (β-tubulin hydrolyses)

Intermediate filaments

Subunit: IF monomer (coiled coil)

Assembly: Dimer → Tetramer → unit-length filament → mature IF

Polarity: No

Nucleotide: None

Branched Actin network

• ARP2/3 binds the side of an existing filament

• Creates a 70° branch

• Produces a dense meshwork at the leading edge

• Plus ends face forward → protrusion

Sarcomere

Thin actin filaments on the outside, thich myosin filaments in the middle

Formin-nucleated linear filaments

• Formins bind the plus end

• Promote rapid monomer addition

• Produce long, unbranched filaments

• Bundled by fascin in filopodia

Contractile bundles

• Actin filaments assemble spontaneously

• Myosin II forms bipolar filaments

• Crosslinkers (α‑actinin, etc.) organise anti‑parallel arrays

• Myosin sliding → contraction

Muscle contraction

• No nucleotide (rigor state): Myosin head tightly bound to actin.

• ATP binding: Reduces affinity → myosin releases actin.

• ATP hydrolysis: Myosin head “cocks” into a high‑energy conformation.

• Pi release: Triggers the power stroke—myosin head pivots, pulling actin filament.

• ADP release: Returns to rigor state, ready for another cycle.

Cell migration

• Myosin I:

  • Anchored to the plasma membrane.

  • Walks along actin filaments using its motor domain.

  • Generates local tension and helps pull the membrane forward.

• Myosin II:

  • Forms small bipolar “minifilaments”.

  • Uses its motor domains to contract actin meshwork at the rear of the cell.

  • Drives retraction of the trailing edge and forward movement of the cell body.

Microtubule nucleation

• The centrosome is the primary site

• It contains centrioles surrounded by pericentriolar material rish in γ-tubulin ring complexes (γ‑TuRC) which act as templates for MT assembly

• γ‑TuRC binds αβ‑tubulin dimers.

• The first ring of tubulin forms around the γ‑TuRC.

• The minus end of the MT remains anchored at the centrosome.

• The plus end extends outward into the cytoplasm.

Dynamic Instability

• Each tubulin dimer binds GTP, but only β‑tubulin’s GTP is hydrolysed after incorporation.

• Growing MTs have a GTP‑tubulin cap at the plus end.

• When the cap is lost (due to hydrolysis outpacing addition), the lattice becomes unstable → catastrophe (rapid depolymerisation).

• Regaining a GTP cap → rescue (growth resumes).

Dynamic instability allows MTs to “search and capture” cellular targets:

• MTs grow and shrink randomly until they encounter specific structures (e.g., kinetochores, cell cortex).

• Stabilisation occurs upon binding → defines cell polarity, spindle formation, or organelle positioning.

Microtubules in axons

• relativley stable

• long parallel bundles

• Unifrom polarity allows directional transport: Kinesin (+ ended) → anterograde transport, Dynein (- ended) → retrograde transport

Microtubules in cilia/flagella

• 9+2 arrangment: Nine outer doublet MTs, Two central singlet MTs

• All MTs have + ends at the top and - ends at basal body

• Extremely stable

• Dynein arms on A‑tubules “walk” along adjacent B‑tubules.

• Nexin links convert sliding into bending → ciliary beating.

Microtubules on mitotic spindle

• Kinetochore MTs

  • Attach to chromosomes

  • Plus ends at kinetochores

  • Minus ends at centrosomes

• Interpolar MTs

  • Overlap in the spindle midzone

  • Stabilised by MAPs

• Astral MTs

  • Radiate outward to the cortex

• - ends at centrosomes, + ends extend outwards

• Dynamic instability is essential

• MT dynamics + motor proteins generate:

  • Chromosome alignment

  • Chromosome segregation

  • Spindle elongation