Myosin degradation fragments in skeletal muscle

Myosin heavy chain degradation fragments produced in vivo have been identified in chicken pectoralis muscle. The fragments were identified by electrophoresis of unfractionated extracts of chicken pectoralis muscle on sodium dodecyl sulfate/polyacrylamide gels followed by immunoblotting on nitrocellulose sheets. Monoclonal antibodies directed against the S2 and light meromyosin subfragments as well as type II myosin-specific polyclonal antibodies directed against the entire myosin heavy chain were used to characterize the fragments, which range in molecular weight from approximately 80,000 to 180,000. All fragments contain the extreme carboxy-terminal portion of the molecule and are distinct from the classical proteolytic fragments such as heavy and light meromyosin, S1, S2 or rod. These fragments appear to be produced in vivo by proteolytic cleavage of peptides from the amino-terminal (S1) end of the heavy chain while the myosin molecule is still embedded in the thick filament. Fragment concentrations are estimated to be approximately 5 to 10% of that of the intact myosin heavy chain. These fragments are not the result of artifactual damage to myosin, e.g. proteolysis or hydrodynamic shear. The techniques described in this paper provide a probe into the early stages of myosin and thick filament degradation in vivo.