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

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Actin polymerization drives protrusion of the cell membrane

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Lamellipodia Filopodia

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Lamellipodia are composed of branched networks of short filaments

Model for actin polymerization at membranes in lamellipodia

Filopodia are composed of long, unbranched actin filaments

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 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

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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

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