28ATPase Cycle “The Movie” PG-13 All myosins have a similar mechanochemical cyclesATPase Cycle “The Movie” PG-13So how do myosins differ?
29Myosin V transports vesicles in cells Myosin V is a “processive” motor Myosin V HMMYFPYFP
30Myosin V is a high (> 50%) duty ratio motor 36 nm
31Myosin II is a low duty ratio (< 10%) motor Myosin V-long neck (6IQ motifs)-organelle motor-functional unit: two headsMyosin II-short neck (2IQ motifs)-drives muscle contraction-functional unit: ~20 heads
32Many myosin II molecules required to propel actin filament In vitro motility assayReal time fluorescence microscopy
33In muscle, myosin II molecules are assembled into a thick filament. Filaments form by association of hydrophobic regions in the tail.
34Muscle sarcomere is the fundamental contractile unit in muscle. The sarcomere contracts when myosin thick filaments and actin thin filaments slide past each other.
35Myofibril consists of a series of sarcomeres Light and dark banding pattern gives rise to “striated” muscle.A band I bandFigure Molecular Biology of the Cell
36Sarcomere shortening (i. e Sarcomere shortening (i.e., muscle contraction) shortens the I band but not the A band.
37“Crowns” of crossbridges project from thick filament at 14 “Crowns” of crossbridges project from thick filament at 14.3 nm intervals and successive crowns are rotated.The result is a thick filament with six rows of crossbridges along its length.
38Thick and thin filaments in an insect flight muscle Thick filamentMyosinThin filamentActin
39Accessory proteins in muscle CapZ and tropomodulin cap ends of actin to keep filament length constant.Z disc contains -actinin and other proteins that stably join sarcomeres.Titin maintains thick filament position in the sarcomere.Nebulin sets the length of the thin filaments.Figure Molecular Biology of the Cell