Cytoskeleton 2

The Cytoskeleton Overview

• The cytoskeleton provides the structural framework for cells.

• It is crucial for maintaining cell shape, enabling movement, and organizing organelles.

• Comprised of three families of protein filaments:

  • Actin filaments (microfilaments)

  • Microtubules

  • Intermediate filaments

Actin Filaments

Structure of Actin Filaments

• Actin filaments (microfilaments):

  • Comprised of 2-stranded helical polymers of globular actin (G-actin).

  • Diameter: 5-9 nm.

  • Form linear bundles and 2-D/3-D networks.

• Functions of actin filaments:

  • Maintain cell shape

  • Enable cell movement and generate force

Actin Monomers

• Each actin subunit (G-actin):

  • 375 amino acids (aa) polypeptide.

  • Carries a tightly associated ATP or ADP molecule.

  • Has two distinct surfaces contributing to filament polarity (plus and minus ends).

  • The ATP-binding cleft is located at the minus end, leading to filament polarity.

Assembly of Actin Filaments

• G-actin monomers dock to form protofilaments (minus to plus end).

• Protofilaments coil to form filamentous actin (F-actin), which is:

  • A flexible and bendable structure.

  • Capable of modification through cross-linking and bundling.

Filament Structures

Differences Between Filament Structures

• Two types of filament structures:

  • "T form": Nucleotide bound (ATP).

  • "D form": Nucleotide bound (ADP).

• In living cells, soluble actin subunits tend to be in T form (ATP concentration exceeds ADP by ~10 times).

Dynamic Instability and Treadmilling

• Dynamic Instability:

  • Associated mainly with microtubules.

  • Characterized by rapid growth and disassembly.

  • Microtubules depolymerize faster from GDP-tubulin than from GTP-tubulin.

• Treadmilling:

  • Predominantly observed in actin filaments.

  • Involves a steady addition of subunits at one end while disassembly occurs at the other end.

Regulation of Actin Cytoskeleton

• The actin cytoskeleton is regulated by:

  • Subunit concentration

  • Accessory proteins that:

    • Nucleate filaments

    • Promote or inhibit polymerization/depolymerization

    • Sever or crosslink filaments

    • Stabilize filaments

    • Cap filament ends

    • Sequester actin subunits

Accessory Proteins in Actin Cytoskeleton

• Key accessory proteins include:

  • Formins and Arp2/3 complex: Nucleate assembly at the plus end.

  • Thymosin: Inhibits assembly of actin subunits.

  • Profilin: Accelerates elongation of actin filaments.

  • Tropomodulin: Stabilizes minus end.

  • Capping proteins and gelsolin: Sever filaments and prevent assembly at the pole ends.

  • Filamin, spectrin, plasma membrane attachments: Provide structural support.

Actin-Nucleating Factors

• Actin-related proteins (ARPs) enhance polymerization by acting as:

  • Nucleators to create branched or straight filament structures.

Actin-Binding Motor Proteins: Myosin

• Myosin proteins:

  • Share a common myosin head region that binds F-actin.

  • Approximately 40 myosin types encoded in the human genome.

  • Most myosins move towards the plus end of actin filaments.

Structure of Myosin

• Composed of:

  • 2 heavy chains (each ~2000 amino acids).

  • Each heavy chain contains a globular head for force generation.

  • 4 light chains of two distinct types on each myosin head.

Summary of Myosin Functions

• Different myosins display various functions throughout eukaryotic cells:

  • Myosin II: Muscle contraction.

  • Myosin VI: Moves towards the minus end of actin filaments.

Conclusion

• Understanding the cytoskeletal components and their dynamics is critical for insights into cellular functions and mechanisms.

• The diversity of actin-binding proteins and myosins highlights the complexity and versatility of cellular movements.