Proteolytic susceptibility of both isolated and bound light chains from various myosins to myopathic hamster protease.

Myopathic hamster protease was incubated with turkey gizzard, scallop adductor, and Loligo mantle retractor myosins in order to establish if the regulatory light chain could be selectively digested. In contrast to cardiac or skeletal muscle myosin in which almost all of the regulatory light chain is degraded, these light chains from smooth and invertebrate muscle myosins were remarkably resistant to proteolysis. In the case of scallop myosin, increasing the protease to myosin ratio resulted in comparable digestions of both the regulatory and essential light chains regardless of the presence of Mg2+. The isolated light chains on the other hand were readily digested into smaller fragments. In addition, it was observed that the myosin heavy chains were extremely sensitive and that it was possible to cleave them quantitatively to produce a new band moving with a mobility on SDS gels corresponding to an Mr of approximately 150,000. This was again at variance with cardiac or skeletal myosin where the breakdown of the heavy chains was shown to be minimal. In spite of the significant extent of heavy chain cleavage, gizzard myosin appears to maintain its tertiary structure as demonstrated by sedimentation velocity and equilibrium ultracentrifugation analysis. Moreover, upon examination by electron microscopy, both intact and cleaved gizzard myosin revealed the characteristic folded structure which had a sedimentation rate of about 10 S when dialyzed into a low salt, Mg X ATP-containing buffer. The effects and implications of such modifications on catalytic activities of gizzard, scallop, and Loligo myosins are discussed in detail.