Module 7 - Cytoskeleton and Motility

What is the cytoskeleton?

A highly dynamic network of filamentous proteins (microtubules, intermediate filaments, microfilaments) extending throughout the cytoplasm of eukaryotic cells. Functions: support, shape, division, transport, organelle positioning.

Microtubules: structure & polarity

Hollow rigid tubes, 25 nm diameter. Made of α+β tubulin dimers → protofilaments (×13) → microtubule. Polar: minus end (at centrosome) and plus end (growing end in cytoplasm).

What is dynamic instability?

Microtubules rapidly alternate between polymerization (growth) and depolymerization (shrinkage). Controlled by GTP hydrolysis: GTP-tubulin cap = growth; loss of cap (GDP-tubulin) = rapid shrinkage (catastrophe).

Kinesin vs. Dynein

Kinesin: moves toward the PLUS end (away from centrosome; toward periphery/axon terminal). Dynein: moves toward the MINUS end (toward centrosome; toward cell body). Both use ATP hydrolysis.

How do microtubules position the Golgi and ER?

Dynein pulls Golgi vesicles toward the minus end (centrosome) → vesicles fuse to form Golgi near nucleus. Kinesin pulls ER membranes toward the plus end → ER extends outward toward plasma membrane.

Intermediate filaments: structure & assembly

Rope-like, ~10 nm, flexible, very tough. Assembly: fibrous monomers → coiled-coil dimer → antiparallel staggered tetramer (loses polarity) → 8 tetramers laterally → rope segment → join end-to-end.

Why are intermediate filaments non-polar?

When two dimers associate antiparallelly to form a tetramer, amino terminals face carboxyl terminals on both ends. Both ends become structurally identical — no plus or minus end.

Four classes of intermediate filaments

1. Keratins (epithelial cells, skin, hair, nails) 2. Vimentin-related (connective tissue, muscle, glial cells) 3. Neurofilaments (nerve axons) 4. Nuclear lamins (nuclear lamina — meshwork, not rope)

Progeria + intermediate filaments

Mutation in Lamin A (nuclear intermediate filament). Disrupts nuclear lamina → nuclear instability → impaired cell division & repair → premature aging, cardiovascular disease (atherosclerosis), death in teens.

Microfilaments: structure & polarity

Thin flexible threads, ~7 nm. Made of G-actin monomers → helical filament (×2 strands) → microfilament. Polar: plus end (ATP-actin added, fast growth) and minus end (ADP-actin lost, slow end). Mainly in cell cortex.

What is actin treadmilling?

At intermediate actin concentrations, ATP-actin is added at the plus end while ADP-actin dissociates from the minus end at the same rate. Filament length stays constant but subunits move through it from plus to minus.

Functions of microfilaments (4 key ones)

1. Muscle contraction (actin + myosin II sliding) 2. Cytokinesis (contractile ring of actin + myosin II) 3. Microvilli formation (bundled actin supports intestinal projections) 4. Cell crawling / migration (actin polymerization pushes membrane forward)

How does a cell crawl?

Signal → actin polymerizes at leading edge (plus ends push membrane forward, forming lamellipodia/filopodia) → integrins anchor protrusions to substrate → myosin contracts actin at rear → cell body dragged forward. Rho GTPases coordinate this.

Cilia & flagella: structure & movement

'9+2' arrangement: 9 outer doublet microtubules around 2 central singlets. Dynein arms between doublets power movement. Alternating inhibition of dynein on opposite sides causes bending. Grow from basal bodies.

Atherosclerosis & the cytoskeleton (big picture)

Oxidized LDL → endothelial cells secrete chemokine → monocytes receive signal → reorganize actin cytoskeleton → change shape → crawl between endothelial cells → become macrophages → engulf LDL → foam cells → plaque → hardened arteries → heart attack / stroke.

Contractile ring in cytokinesis

At cell equator: ring of actin filaments + myosin II assembles. Myosin II walks toward actin plus ends → ring tightens → plasma membrane pinches inward → two daughter cells form.

γ-tubulin ring complex

Found in the centrosome (microtubule-organizing center). 13 γ-tubulin molecules form a ring = nucleation site for one microtubule. The minus end of the new microtubule is anchored here; growth occurs only at the plus end.