Transcript Actin
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 • 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 Example: actin in microvilli Example: actin in the cell cortex 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 QuickTime™ and a TIFF decompressor are needed to see this picture. Lamellipodia QuickTime™ and a TIFF decompressor are needed to see this picture. Filopodia Model for actin polymerization at membranes Actin polymerization powers engulfment during phagocytosis QuickTime™ and a MPEG-4 Video decompressor are needed to see this picture. 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 Listeria move on an actin-based “comettail” QuickTime™ and a Cinepak decompressor are needed to see this picture. 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 Myosins “walk” along actin filaments QuickTime™ and a Video decompressor are needed to see this picture. Myosin I can carry organelles or slide actin filaments along the membrane Myosin II slides actin filaments to produce contractile forces Myosin-based contraction drives cytokinesis QuickTime™ and a Video decompressor are needed to see this picture. QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. Skeletal muscle cells are packed with myofibrils, each of which contains repeating chains of sarcomeres Sarcomeres are contractile units of actin and myosin II In muscle cells, myosin II is a filament of many motors Muscle contraction is driven by myosin II The myosin cycle in muscle 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 Muscle contraction QuickTime™ and a Animation decompressor are needed to see this picture.