Function of skeletal muscle myosin heavy and light chain isoforms by an in vitro motility assay.

The functional significance of the large number of myosin isoforms in skeletal muscles is poorly understood. Myosin molecules that have the same heavy chain, but differ in their essential or alkali light chains, have the same actin-activated ATPase activity. Similarly, the many heavy chain isoforms that appear during the course of muscle development do not show any significant differences in enzymatic activity. By means of an in vitro motility assay, we have measured the analogue of unloaded shortening velocity for myosin isoforms in a reconstituted actomyosin system. We find that both light and heavy chain isoforms translocate actin filaments at distinct velocities. These results support the hypothesis that myosin isoforms are the primary determinant for the range of shortening velocities adopted by a muscle in response to changing functional demands.