SMOOTH-MUSCLE AND SKELETAL-MUSCLE MYOSINS PRODUCE SIMILAR UNITARY FORCES AND DISPLACEMENTS IN THE LASER TRAP

Purified smooth muscle myosin in the in vitro motility assay propels a ctin filaments at 1/10 the velocity, yet produces 3-4 times more force than skeletal muscle myosin. At the level of a single myosin molecule , these differences in force and actin filament velocity may be reflec ted in the size and duration of single motion and force-generating eve nts, or in the kinetics of the cross-bridge cycle. Specifically, an in crease in either unitary force or duty cycle may explain the enhanced force-generating capacity of smooth muscle myosin. Similarly, an incre ase in attached time or decrease in unitary displacement may explain t he reduced actin filament velocity of smooth muscle myosin. To discrim inate between these possibilities, we used a laser trap to measure uni tary forces and displacements from single smooth and skeletal muscle m yosin molecules. We analyzed our data using mean-variance analysis, wh ich does not rely on scoring individual events by eye, and emphasizes periods in the data with constant properties. Both myosins demonstrate d multiple but similar event populations with discrete peaks at approx imately +11 and -11 nm in displacement, and 1.5 and 3.5 pN in force. M ean attached times for smooth muscle myosin were longer than for skele tal-muscle myosin. These results explain much of the difference in act in filament velocity between these myosins, and suggest that an increa sed duty cycle is responsible for the enhanced force-generating capaci ty of smooth over skeletal-muscle myosin.