Serine protease which degrades the myosin heavy chain but not actin or alpha-actinin. Selectively cleaves Arg-|-Xaa bonds. Inhibited by the serine protease inhibitors Pefabloc SC, STI, aprotinin, benzamidine, leupeptin and antipain, and by the cysteine proteinase inhibitors DTNB and pCMB. Not inhibited by the cysteine protease inhibitor E-64, the aspartic proteinase inhibitor pepstatin A and the metalloproteinase inhibitor EDTA. Optimum pH is 8.0. Active from pH 5.0 to 11.0. Optimum temperature is 55 degrees Celsius. Active from 30 to 65 degrees Celsius. Detected in muscle (at protein level). Belongs to the peptidase S1 family.