02.16.11 lecture 12 - the actin...
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02.16.11Lecture 12 - The actin cytoskeleton
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Actin filaments allow cells to adopt different shapes and perform different functions
Villi Contractilebundles
Sheet-like &Finger-like protrusions
Contractilering
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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
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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
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Actin architecture and function is governed by actin-binding proteins
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Example: actin in microvilli
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Example: actin in the cell cortex
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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
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During cell migration, actin polymerization pushes the leading edge forward
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Actin polymerization drives protrusion of the cell membrane
Lamellipodia Filopodia
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Lamellipodia are composed of branched networks of short filaments
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Model for actin polymerization at membranes in lamellipodia
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Filopodia are composed of long, unbranched actin filaments
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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
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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
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Myosins “walk” along actin filaments
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Myosin I can carry organelles or slide actin filaments along the membrane
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Myosin II slides actin filaments to produce contractile forces
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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
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Sarcomeres are contractile units of actin and myosin II
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In muscle cells, myosin II is a filament of many motors
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Muscle contraction is driven by myosin II
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The myosin cycle in muscle
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Contraction is activated by calcium release from the sarcoplasmic reticulum
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Calcium release channels are opened by a voltage-sensitive transmembrane protein in the T-tubule
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Contraction is regulated by a Ca+2-mediated change in the conformation of troponin
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Muscle contraction