Chpt 13 - Cytoskeleton _23

Chapter 13: The Cytoskeleton and Cell Movement

Overview

• Cytoskeleton: Network of protein filaments crucial for cellular structure, movement, and organization.

• Major components include actin filaments, myosin motors, microtubules, microtubule motors, and intermediate filaments.

Structure and Organization of Actin Filaments

• Actin Filaments::

  • Key proteins that provide cell shape and structure.

  • Composed of globular (G) actin monomers that polymerize to form filamentous (F) actin.

• Actin Binding Proteins: Various proteins that regulate the dynamics (growth and shrinkage) of actin filaments.

  • Profilin: Facilitates nucleation of actin filaments by promoting ATP-G actin conversion.

  • Cofilin: Binds to ADP-actin to disassemble actin filaments and inhibit polymerization.

• **Branching and Networks: **Provides structural support and enables movement through cytoskeletal organization.

Myosin Motors

• Function: Motor proteins that interact with actin filaments to mediate contraction and movement.

  • Includes Myosin I (single-headed) for cargo transport, Myosin II (muscle contraction), and Myosin V (double-headed for cargo transport).

• Muscle Contraction: Involves the sliding filament model in striated muscle, where myosin heads bind to actin, pulling filaments together upon ATP hydrolysis.

Microtubules

• Function: Rigid, hollow rods crucial for cell shape, organization, and intracellular transport.

  • Composed of alpha and beta-tubulin heterodimers polymerizing to form protofilaments.

  • Microtubule assembly and disassembly is regulated by dynamic instability, allowing for rapid cellular responses and organization during mitosis.

• Microtubule Associated Proteins (MAPs): Regulate growth/shrinkage and stabilization.

  • Examples include CLASP (stabilizes) and Depolymerases (promote shrinkage).

Intermediate Filaments

• Function: Provide mechanical strength and structural support; not directly associated with cell movement.

  • Comprised of various types including keratins, vimentin, desmin, neurofilament proteins, and nuclear lamins.

• Structure: 8 protofilaments twisted to form a filament; stable and lack polarity, unlike actin and microtubules; assembly can be regulated by phosphorylation.

Role in Cell Junctions

• Desmosomes: Junctions between adjacent cells that utilize intermediate filaments for structural integrity.

• Hemidesmosomes: Connect epithelial cells to connective tissue using intermediate filaments.

Cell Movement

• Pseudopodia, Lamellipodia, Filopodia: Extensions of the cell membrane that facilitate movement and nutrient absorption, formed primarily by actin filament polymerization.

• Mechanism of Cell Motility: Involves the extension of leading edges, attachment to substrates, and retraction of trailing edges through coordinated actin polymerization and myosin motor activity.

Learning Objectives

• Understand the dynamics and functions of actin filaments and motor proteins.

• Illustrate the organization of actin filaments in relation to the plasma membrane.

• Describe structures like microvilli formed by actin interactions that enhance cell functions galore.

• Explain the remodeling of cytoskeletal elements aiding in motility and shape changes of cells.