Cytoskeleton and Actin Filaments

Cytoskeleton Overview

  • Three families of protein filaments:
    1. Actin filaments
    2. Microtubules
    3. Intermediate filaments

Types of Cytoskeleton: Actin Filaments

  • Definition:
    • Actin filament (microfilament) is a 2-stranded helical non-covalent polymer.
    • Composed of globular actin (G-actin).
  • Dimensions:
    • Diameter: 5–9 nm
  • Functions:
    • Maintains cell shape
    • Enables cell movement and force generation

Actin Monomer Structure

  • Actin subunit information:
    • G-actin is a 375 amino acid polypeptide with ATP or ADP.
    • Contains two surfaces that produce monomer polarity (plus and minus ends).
    • ATP-binding cleft is at the minus end.
  • Conservation:
    • Highly conserved among eukaryotes with 90% sequence similarity across species.

Filamentous Actin Formation

  • Protofilament formation:
    • G-actin monomers bind (minus to plus end) to form protofilaments.
    • F-actin consists of two protofilaments that twist along their length.
  • Characteristics of F-actin:
    • Flexible and can be modified by cross-linking and bundling.

Filament Structure and Dynamics

  • Two types of forms:
    • "T form" (ATP bound)
    • "D form" (ADP bound)
  • Cellular condition:
    • Most actin subunits are in T form due to a free ATP concentration that is approximately 10-fold greater than that of ADP.

Behaviors of Cytoskeletal Polymers

  • Dynamic Instability:

    • Microtubules behave with dynamic instability, alternating between periods of growth and rapid disassembly based on GTP or GDP binding.
    • Depolymerization occurs faster from GDP-tubulin ends than from GTP-tubulin ends, which favors growth.

  • Treadmilling:

    • Predominantly observed in actin filaments, involves the addition and loss of subunits at opposite ends of the filament.

Regulation of Actin Cytoskeleton

  • Regulated by:
    • Subunit concentration
    • Accessory proteins that affect filament dynamics:
    • Nucleation
    • Promotion of polymerization or depolymerization
    • Connectivity to other structures
    • Severing filaments
    • Crosslinking, stabilization, capping, and sequestration.

Accessory Proteins

  • Examples of key proteins:
    • Formin:
    • Nucleates assembly and associates with growing plus ends.
    • Arp2/3 complex:
    • Nucleates assembly to form a web; associates with minus ends.
    • Thymosin:
    • Binds subunits to prevent assembly.
    • Capping proteins:
    • Prevent assembly/disassembly at plus ends.
    • Profilin:
    • Binds subunits to speed up elongation.

Myosin Motor Proteins

  • General information:
    • Approximately 40 types in the human genome.
    • Share a myosin head region for F-actin binding.
  • Movement direction:
    • Most myosin proteins move towards the plus end of F-actin.

Myosin Structure

  • Composition:
    • 2 heavy chains (≈ 2000 amino acids each)
    • Contain globular head domain (force-generating machinery).
    • 4 light chains (two distinct types).
  • Structure:
    • Heavy chains coiled-coiled dimerization enables structure integrity and function.

Summary of Myosins

  • Diversity:
    • Different types of myosins are found across eukaryotes.
    • Most function towards the plus end of actin.
    • Example types include:
    • Myosin II: muscle contraction
    • Myosin VI: moves towards the minus end.