Chapter 9. Cytoskeletal Elements

Cytoskeleton

A dynamic network of protein filaments that provides structural support, intracellular transport, and cell motility.

Functions of the Cytoskeleton

Maintains cell shape, enables intracellular transport, facilitates cell division, supports motility, and anchors organelles.

Three Types of Eukaryotic Cytoskeletal Elements

Microtubules, microfilaments (actin filaments), and intermediate filaments.

Building Blocks for Each

Microtubules: Tubulin dimers (α-tubulin & β-tubulin).

Microfilaments: Actin monomers (G-actin).

Intermediate Filaments: Various fibrous proteins (e.g., keratin, vimentin, lamin).

Bacterial Homologues for Each

Microtubules: FtsZ (cell division).

Microfilaments: MreB (cell shape).

Intermediate Filaments: CreS (curvature).

Ultrastructure of a Microtubule

Hollow tube of 13 protofilaments made of alternating α- and β-tubulin dimers.

Microtubule Assembly Model

Three-phase process:

1. Nucleation - Small tubulin oligomers form.

2. Elongation - Oligomers grow into protofilaments.

3. Steady-state - Growth and disassembly occur dynamically.

Kinetics of Microtubule Assembly

Polymerization occurs faster at the plus (+) end, while the minus (-) end is anchored or depolymerizes.

Initial Microtubule Assembly

Begins at the microtubule-organizing center (MTOC), with γ-tubulin serving as a nucleation site.

Dynamic Instability

The process where microtubules rapidly grow and shrink due to tubulin polymerization and depolymerization.

Treadmilling

A process where tubulin subunits are added at the plus end and removed at the minus end, keeping microtubule length stable.

Importance of GTP

GTP stabilizes tubulin dimers for polymerization; hydrolysis to GDP causes instability and depolymerization.

Functions of Microtubules

Support intracellular transport, mitotic spindle formation, flagella and cilia movement, and organelle positioning.

Function of FtsZ

A bacterial tubulin homologue that forms a contractile ring for cell division (cytokinesis).

Effects of Colchicine

Prevents microtubule polymerization by binding to tubulin, inhibiting mitosis.

Effects of Taxol

Stabilizes microtubules, preventing depolymerization and disrupting cell division.

Microfilaments (Actin Filaments)

Thin, helical filaments composed of actin monomers involved in structural support and motility.

Functions of Microfilaments

Enable cell movement, intracellular transport, cytokinesis, and muscle contraction.

Function of MreB

A bacterial actin homologue that helps maintain cell shape by guiding peptidoglycan synthesis.

Microfilament Assembly

Actin monomers (G-actin) polymerize into filamentous actin (F-actin) with ATP binding for stability.

Microfilaments vs. Microtubules (Similarities)

Both are dynamic, polarized, and involved in intracellular transport and motility.

Microfilaments vs. Microtubules (Differences)

Microtubules are hollow, made of tubulin, and use GTP.

Microfilaments are solid, made of actin, and use ATP.

Requirements for Actin Branching

Arp2/3 complex nucleates new actin branches at a 70-degree angle for movement.

Listeria Monocytogenes Movement

Uses ActA protein to hijack host actin polymerization, forming "comet tails" for intracellular motility.

Bacterial Protein ParM

Homologous to actin; pushes plasmids apart before bacterial cell division.

Plasmid Partitioning

ParM filaments push plasmids to opposite poles before division.

Bacterial Chromosome Partitioning

F