Lysosomal proteinase-sensitive regions in fast and slow skeletal muscle myosins

We investigated the limited proteolysis of fast and slow myosins purified from rabbit psoas major and semimembranosus proprius muscles, respectively, by the main lysosomal proteinases: cathepsins B, H, L, and D. In EDTA containing buffer, cathepsin D cleaved both myosins only at the rod-S1 junction leading to the formation of two S1 fragments of slightly higher Mr than the three forms obtained with chymotrypsin. On addition of MgCl2 instead of EDTA, myosin hydrolysis was markedly reduced. In contrast, irrespective of the presence of either MgCl2 or EDTA, cathepsin B hydrolysed both myosins into HMM and LMM. Cathepsin L digested myosins more extensively than cathepsins B and D and the main fragments generated were, in decreasing order of importance, rod, S1, S2, HMM, and LMM. In the incubation conditions tested, cathepsin H displayed nondetectable action on myosins. As fast and slow myosin digest patterns were compared, the main differences observed concerned the size of the proteolytic products and the rate of hydrolysis, which was about 4-fold higher for the fast than for the slow isoform. This appeared consistent whatever enzyme was considered.