Lecture 2: Actin Cytoskeleton

Proteins and Their Role in Actin Polymerization

Overview of Actin Microfilaments

  • Actin Microfilaments: Key components of the cytoskeleton, involved in various functions including cell movement and structure.

    • Types of Actin Organizations:

    • Parallel Bundles:

      • Found in filopodia (finger-like projections) of migrating cells.

      • All microfilaments oriented in the same direction (all plus ends at one end and all minus ends at the other).

      • Allow for rapid growth and shrinkage, providing quick power.

    • Stress Fibers:

      • Long actin fibers in the tail region of cells.

      • Organized in anti-parallel fashion.

      • Contain both plus and minus ends at both ends of the fibers.

      • Absorb stress during cell movement.

    • Cell Cortex:

      • Contains a cross-linked network of actin filaments providing stability and rigidity just under the plasma membrane.

Dynamics of the Cytoskeleton

  • The cytoskeleton is dynamic: Constant remodeling occurs in response to various stimuli (environmental cues, cell division, cell migration, differentiation).

  • Actin Polymerization Process:

    • Spontaneous formation of actin filaments from G-actin monomers.

    • Treadmilling: Ability of the cell to adjust the length of pre-existing filaments by adding or removing monomers.

  • Regulatory Mechanisms: Actin binding proteins regulate assembly/disassembly processes in space and time.

Actin Binding Proteins (ABPs)

  • Definition: Proteins that bind to actin, influencing its polymerization and depolymerization.

    • Two categories based on binding affinity:

    • Monomeric Actin Binding Proteins: Affect nucleation and polymerization of monomeric actin.

    • Filamentous Actin Binding Proteins: Interact with assembled actin filaments and impact their stability and organization.

Monomeric Actin Binding Proteins

  • Influence the rate of filament growth and nucleation of new filaments.

  • Examples:

    • Thymosin:

    • A small peptide in metazoans that binds G-actin.

    • Inhibits polymerization by preventing G-actin from joining growing filaments.

    • Prevents nucleotide exchange, leading to an accumulation of ADP-bound G-actin, slowing polymerization.

    • Profilin:

    • Binds ADP-G-actin and promotes nucleotide exchange (ADP for ATP).

    • Enhances polymerization rates and directs filament growth preferentially from the plus end.

    • Formin:

    • High affinity for G-actin and promotes nucleation by binding three G-actin monomers to form a growing nucleus.

    • Arp2/3 Complex:

    • Binds to both monomeric actin and filamentous actin, initiating new filament formation off existing filaments, creating networks.

Filamentous Actin Binding Proteins

  • Functions:

    • Severing Proteins: Break filaments into smaller pieces (e.g., Cofilin: causes rapid disassembly of ADP-bound actin).

    • Capping Proteins:

    • Bind to filament ends, stabilizing or preventing further monomer addition or loss.

    • Capping at plus end allows minus end to grow/shrink, capping at minus end allows plus end to grow/shrink.

  • Gel Solen:

    • A protein that can sever actin and cap filament ends to regulate the disassembly of actin networks.

Cross-Linking Proteins

  • Organize actin into different architectures depending on their structure.

  • Examples:

    • Fimbrin:

    • Monomeric protein with two actin binding domains, responsible for tight parallel bundles in filopodia.

    • Alpha-actinin:

    • Homodimer that creates anti-parallel bundles, affecting structures like stress fibers.

    • Spectrin:

    • Forms a tetramer involved in maintaining the biconcave shape of erythrocytes, crucial for oxygen transport. Defects can lead to anemia.

    • Dystrophin:

    • Connects the cytoskeleton of muscle cells to the extracellular matrix; mutations result in muscular dystrophies.

Myosin Family of Proteins

  • Role: Facilitate movement within the cell, often working in conjunction with actin filaments.

  • Types:

    • Myosin I:

    • Monomeric, links actin to membranes to stabilize structures at the cell periphery.

    • Myosin V:

    • Carries transport vesicles along actin filaments in the cytoplasm.

    • Myosin II:

    • Involved in generating force during muscle contraction and cytokinesis.

  • Structure:

    • Consists of a globular head (binding actin) and a tail (determines function).

    • Myosin II forms thick filaments in muscle, interacting with actin in sarcomeres for contraction.

Mechanism of Muscle Contraction

  1. In resting state, the myosin head is bound to actin.

  2. ATP binds to myosin, causing release of actin.

  3. Hydrolysis of ATP leads to changes in myosin structure and allows rebinding to actin.

  4. This triggers conformational changes that slide the filaments relative to one another, resulting in muscle contraction.

Summary Points

  • Actin and myosin cooperate in various cellular functions and dynamics.

  • Actin binding proteins play crucial roles in regulating actin filament dynamics and organizing filamentous structures within the cell, facilitating movement, structural integrity, and cellular responses to environmental cues.