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
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
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