Myosin, the motor protein in skeletal muscle, is composed of two subunits, myosin heavy chain and myosin light chain. All vertebrates express a family of myosin heavy chain and myosin light chain isoforms that together are primary determinants of force, velocity, and power in muscle fibers. Therefore, appropriate expression of myosin isoforms in skeletal muscle is critical to proper motor function. Myosin isoform expression is highly plastic and undergoes significant changes in response to muscular injury, muscle disuse, and disease. Therefore, myosin isoform function and plasticity are highly relevant to clinical orthopaedic research, musculoskeletal surgery, and sports medicine. Muscle from frogs offers a special opportunity to study the structural basis of contractile protein function because single intact fibers can be isolated that maintain excellent mechanical stability, allowing for high-resolution studies of contractile performance in intact cells. The current authors summarize recent studies defining the myosin isoforms in muscle from frogs and the relationship between myosin isoforms and mechanical performance of intact single muscle cells. Preliminary studies also are described that show the potential for simple plasmid-based in vivo gene transfer approaches as a model system to elucidate the structural basis of muscle protein function in intact cells.

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