Cytoskeletal Filaments and Functions

Cytoskeletal Filaments

• Cytoskeletal filaments include actin, microtubules, and intermediate filaments.
• Actin filaments are long, thin protein fibers, crucial for cell structure and movement.
• Microtubules are hollow tubes composed of protein subunits, forming a dynamic scaffold.
• Intermediate filaments provide mechanical strength and support to cells.

Functions of Cytoskeleton

• Cytoskeleton acts as a scaffold supporting cell shape.
• It positions organelles within the cell.
• Transports cellular materials like organelles, vesicles, and mRNA.
• It's an apparatus generating force for cell movement.
• Essential for cell division machinery.

Microtubules

• Microtubules are composed of tubulin heterodimers
• Characterized by dynamic instability, rapidly growing and shrinking.
• Microtubule growth is initiated by microtubule-associated proteins (MAPs).
• MAPs regulate microtubule organization.

Actin Filaments

• Actin filaments are composed of actin monomers.
• Actin filaments polymerize to form microfilaments.
• Actin filaments growth at their plus ends is faster.
• Actin filaments are stabilized by actin-binding proteins.
• Actin polymerization and depolymerization are regulated by actin-binding proteins.

Myosin Motors

• Myosin II is vital in muscle contraction.
• Myosin is attached to the actin filament and moves along it.
• ATP binding and hydrolysis drive myosin's movement.
• Myosin's head's conformational changes are essential for sliding filaments during muscle contraction.

Actin Bundles & Networks

• Actin bundles are stabilized and organized into bundles or networks by actin-binding proteins, like cross-linking proteins
• Actin filaments, projecting from the cell surface, are involved in cell surface projections like microvilli, filopodia, and cell locomotion.

Cell Migration

• Cell movement across a surface involves three steps: leading edge extension, attachment to the substratum, and retraction of the trailing edge.

Focal Adhesions and Adherens Junctions

• Focal adhesions attach stress fibers to the plasma membrane.
• Integrins bind to an extracellular matrix, facilitating cell attachment and spreading.
• Adherens junctions connect cells to each other, providing cell-cell contact.
• Actin bundles are anchored to adherens junctions via cadherins.

Microvilli

• These are actin-based cell protrusions.
• They're common in epithelial cells lining the gut.
• They significantly increase the surface area for nutrient absorption.

Structure of Spectrin

• Spectrin is a tetramer, composed of two α and two β polypeptide chains.
• Each β-chain has an actin-binding domain (ABD) at its amino terminus.
• The α and β chains contain multiple a-helical spacer domains, providing structural stability to the network.

Stress Fibers and Focal Adhesions

• Many cells have specialized plasma membrane regions for forming contacts with the extracellular matrix.
• These regions, called focal adhesions, are crucial attachments for stress fiber bundles of actin filaments.
• Stress fibers attach cells to their surroundings.

Filament Severing by Cofilin

• Cofilin binds to and severs actin filaments, crucial for cell motility.
• The newly formed filament ends are available for polymerization or depolymerization.
• This process is essential for cell motility and division.