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02.16.11Lecture 12 - The actin cytoskeleton
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 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
During cell migration, actin polymerization pushes the leading edge forward
Actin polymerization drives protrusion of the cell membrane
Lamellipodia Filopodia
Lamellipodia are composed of branched networks of short filaments
11
Model for actin polymerization at membranes in lamellipodia
Filopodia are composed of long, unbranched actin filaments
Actin polymerization powers engulfment during phagocytosis
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”
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
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
Skeletal muscle cells are packed with myofibrils, each of which contains repeating
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
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