SMOOTH CARDIAC AND SKELETAL-MUSCLE MYOSIN FORCE AND MOTION GENERATIONASSESSED BY CROSS-BRIDGE MECHANICAL INTERACTIONS IN-VITRO

Differences in the mechanical properties of mammalian smooth, skeletal , and cardiac muscle have led to the proposal that the myosin isozymes expressed by these tissues may differ in their molecular mechanics. T o test this hypothesis, mixtures of fast skeletal, V1 cardiac, V3 card iac and smooth muscle (phosphorylated and unphosphorylated) myosin wer e studied in an in vitro motility assay in which fluorescently-labelle d actin filaments are observed moving over a myosin coated surface. Pu re populations of each myosin produced actin filament velocities propo rtional to their actin-activated ATPase rates. Mixtures of two myosin species produced actin filament velocities between those of the faster and slower myosin alone. However, the shapes of the myosin mixture cu rves depended upon the types of myosins present. Analysis of myosin mi xtures data suggest that: (1) the two myosins in the mixture interact mechanically and (2) the same force-velocity relationship describes a myosin's ability to operate over both positive and negative forces. Th ese data also allow us to rank order the myosins by their average forc e per cross-bridge and ability to resist motion (phosphorylated smooth > skeletal =V3 cardiac > V1 cardiac). The results of our study may re flect the mechanical consequence of multiple myosin isozyme expression in a single muscle cell.