02.16.11 Lecture 12 - The actin cytoskeleton. Actin filaments allow cells to adopt different shapes...
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Transcript of 02.16.11 Lecture 12 - The actin cytoskeleton. Actin filaments allow cells to adopt different shapes...
Actin filaments allow cells to adopt different shapes and perform different functions
Villi Contractilebundles
Sheet-like &Finger-like protrusions
Contractilering
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 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
During cell migration, actin polymerization pushes the leading edge forward
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Actin polymerization drives protrusion of the cell membrane
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Lamellipodia Filopodia
Actin polymerization powers engulfment during phagocytosis
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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 “comet-tail”
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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
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Myosin-based contraction drives cytokinesis
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Skeletal muscle cells are packed with myofibrils, each of which contains repeating
chains of sarcomeres