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02.15.10 Lecture 12 - The actin cytoskeleton. Actin filaments allow cells to adopt different shapes and perform different functions. Villi. Contractile bundles. Sheet-like & Finger-like protrusions. Contractile ring. Actin filaments are thin and flexible. 7 nm in diameter
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Actin filaments allow cells to adopt different shapes and perform different functions Villi Contractile bundles Sheet-like & Finger-like protrusions Contractile ring
Actin filaments are thin and flexible • 7 nm in diameter • Less rigid than microtubules • Plus end - fast growing • Minus end - slow growing • Monomers polymerize into a helical chain
Actin and microtubules polymerize using similar mechanisms • Monomeric actin binds to ATP • Upon polymerization, actin ATPase activity cleaves ATP to ADP • ATP hydrolysis acts as a molecular “clock” • Older actin filaments with ADP are unstable and disassemble
Actin architecture and function is governed by actin-binding proteins
Actin polymerization can produce “pushing” forces • Polymerization at the front of a cell pushes the leading edge forward • Phagocytosis - formation of pseudopods • Intracellular movement and cell-to-cell spreading of pathogens
Actin polymerization drives protrusion of the cell membrane Filopodia Lamellipodia
Movement of Listeria monocytogenes • Pathogenic bacterium that colonizes the epithelial cells lining the gut • Found in contaminated dairy products • Infection can be lethal to newborns and immunocompromised individuals
Myosins are actin-based motor proteins • Myosins convert ATP hydrolysis into movement along actin filaments • Many different classes of myosins (>30 in humans) • Some myosins move cargoes, other myosins slide actin (as in muscles) • Actin & ATP binding sites in N-terminal head domain
Myosin I can carry organelles or slide actin filaments along the membrane
Myosin II slides actin filaments to produce contractile forces
Skeletal muscle cells are packed with myofibrils, each of which contains repeating chains of sarcomeres
Contraction is activated by calcium release from the sarcoplasmic reticulum
Calcium release channels are opened by a voltage-sensitive transmembrane protein in the T-tubule
Contraction is regulated by a Ca+2-mediated change in the conformation of troponin