Type II Myosins Function and Comparison

Type II Myosins

• Myosin II pulls arrays of actin filaments together, resulting in cell contraction
• Resembles kinesin, with globular domains that walk along a protein filament, using ATP hydrolysis to change shape

Kinesins Versus Myosin

• Kinesins transport vesicles over large distances, operating alone or in small numbers
• A single myosin II molecule slides an actin filament about 12–15 nm
• Myosin II molecules move short distances but operate in large arrays, mediating muscle contraction

Microfilament-Based Motility: Muscle Cells in Action

• Muscle contraction is the most familiar example of mechanical work mediated by intracellular filaments
• A muscle consists of parallel muscle fibers, each fiber being a long, thin, highly specialized, multinucleate cell
• Each muscle fiber contains numerous myofibrils, divided along their length into repeating units called sarcomeres

The Sliding-Filament Model Explains Muscle Contraction

• Myosin II moves toward the plus ends, so thick filaments move toward the Z lines during contraction (filaments remain same length)
• The sliding filament model explains muscle contraction, where thin filaments slide past thick filaments, with no change in length of either
• Cross-bridges form between F-actin of thin filaments and myosin heads of thick filaments, requiring lots of ATP to dissociate rapidly

Microfilament Organization and Dynamics

• Cytochalasins prevent the addition of new monomers to existing microfilaments
• Latrunculin A sequesters actin monomers and prevents their addition to microfilaments
• Phalloidin stabilizes microfilaments and prevents their depolymerization
• Actin-binding proteins regulate the organization of actin, controlling where actin assembles and the structure of resulting networks

Cells Can Dynamically Assemble Actin into a Variety of Structures

• The cell cortex has actin crosslinked into a gel of microfilaments
• Cells that adhere tightly to the underlying substratum have organized bundles called stress fibers
• Lamellipodia have a branched network of actin
• Filopodia have highly oriented, polarized cables with actin plus ends toward the tip of the protrusion

Actin-Binding Proteins Regulate Actin Organization

• Cells use actin-binding proteins to control where actin assembles and the structure of the resulting network
• Control occurs at the nucleation, elongation, and severing of microfilaments and at the association of microfilaments into networks

Proteins That Bundle Actin Filaments

• α-Actinin and fascin/fimbrin are proteins that bundle actin filaments
• Actin bundles in filopodia and microvilli are examples of ordered actin structures

Actin Networks and Membrane Interactions

• Actin filaments indirectly connect to the plasma membrane and exert force on it
• This connection requires one or more linking proteins, such as spectrin and ankyrin
• Actin–membrane interactions allow cells to respond to mechanical forces