Cytoskeleton and Actin Filaments

Definition: Actin filaments, also known as microfilaments, are helical polymers of the protein actin. They play critical roles in various cellular processes.
Structure:
- Diameter: Approximately 7 nm.
- Polarity: Each filament has a plus (+) end and a minus (−) end, which influences dynamics of growth.
Distribution: Actin filaments are widely dispersed in the cytoplasm but are most concentrated in the cell cortex, the layer just beneath the plasma membrane.
Abundance: Actin is the most abundant protein in eukaryotic cells, constituting about 5-10% of total cellular protein content. In muscle cells, it can make up roughly 20%.
Conservation: Actin is highly conserved evolutionarily, with minimal differences (around 15 amino acids) observed across species including mammals and birds.

Cell Structure and Movement:
- Provides shape and helps with movement (cell crawling).
- Forms structures like microvilli in intestinal cells, promoting nutrient absorption.
Phagocytosis: Actin filaments contribute to the processes that enable cells to engulf particles.
Cell Division: They are essential during cytokinesis, where they form a contractile ring that helps separate divided cells.

Polymerization/Depolymerization:
- Actin filaments are dynamic; they are constantly being synthesized (polymerized) and broken down (depolymerized).
- ATP hydrolysis plays a crucial role in this process. Monomeric actin binds to ATP before incorporating into the filament. After incorporation, ATP is hydrolyzed to ADP, leading to decreased stability of the filament and promoting depolymerization.
Treadmilling:
- A dynamic equilibrium where actin filaments grow at the plus end while simultaneously losing subunits from the minus end, depending on the concentration of actin monomers.

Role: ABPs modulate the behavior and dynamics of actin filaments, influencing cell functions such as motility and shape.
Types of ABPs:
- Capping Proteins: Bind to the ends of the filament to regulate growth.
- Severing Proteins: Cut actin filaments into shorter pieces.
- Bundling and Cross-Linking Proteins: Help form structures like bundles and networks to facilitate specific cellular tasks.
Practical Examples:
- Phalloidin (from mushrooms) that stabilizes actin filaments and Cytochalasin (from fungi) that inhibits polymerization.

Cell Crawling: A process driven by the polymerization of actin filaments at the leading edge, forming structures like lamellipodia and filopodia to propel the cell forward.
Polarization of Cells: Cells establish polarity through actin dynamics, which is crucial in processes such as migration and asymmetric cell division.

Myosin Interaction: Actin filaments interact with myosin motors to produce muscle contraction through the sliding filament mechanism.
Sarcomere Structure: The repeating unit in muscle where actin and myosin filaments overlap. Interaction results in contraction influenced by calcium signals.
Calcium Functionality: Release of calcium ions triggers conformational changes in tropomyosin, exposing myosin binding sites and enabling contraction.

Cytoskeleton Framework: Comprised of microtubules, intermediate filaments, and actin filaments that support cell structure and organization.
Polar Nature of Actin: Understanding the polarity of actin filaments is critical for grasping how they grow and function within cells.
Physical Forces in Cytoskeleton: Actin polymerization generates physical forces that drive movement and transport processes within cells.
Signaling Pathways: Proteins like Rho, Rac, and Cdc42 regulate actin dynamics through signaling pathways that influence cellular morphology and movement.