Characterization of H-protein, a component of skeletal muscle myofibrils.

H-protein, a rabbit skeletal muscle myofibrillar component, was isolated and characterized. Its content in the myofibril is about 0.3 to 0.4%. H-protein is located at a specific site in the A-band, which is closer to the M-line than the C-protein zone. Anti-H-protein serum does not react with either C-protein or purified myosin in an Ouchterlony immunodiffusion plate. Immunoblotting experiments show H-protein is an intrinsic component of the myofibril. Its molecular weight, estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is 74,000. One characteristic of its amino acid composition is a high proline content, similar to C-protein. Its sedimentation coefficient is 2.5 S. H-protein binds to myosin; however, C-protein can still bind to myosin even if myosin is saturated by H-protein. Although H-protein itself does not have ATPase activity, it inhibits not only actomyosin ATPase but also acto-heavy meromyosin ATPase.