Protease-sensitive regions in myosin subfragment 1.

Proteolytic digestions of myosin subfragment 1 (S-1) with elastase, subtilisin, papain, thermolysin, and Staphylococcus aureus protease reveal that the two trypsin-sensitive regions in S-1 have broad protease susceptibility. The cleavage of S-1 by these enzymes yields products that correspond within 1-2 kilodaltons (kDa) to the 25-, 50-, and 20-kDa fragments produced by trypsin. Papain and thermolysin cut preferentially at the 26-kDa/70-kDa junction, whereas elastase, subtilisin, and S. aureus protease cleave both the 26-kDa/70-kDa and 75-kDa/22-kDa junctions in S-1. Binding of actin to S-1 decreases the rate of all proteolytic reactions in the 95-kDa heavy chain. The protection of the 26-kDa/70-kDa junction by actin is greatest against papain and thermolysin attack. The reaction times of elastase, subtilisin, and S. aureus protease with S-1 increase 2-fold in the presence of actin. However, in contrast to similar reactions with trypsin, they proceed at both junctions and lead to formation of the 50- and 22-kDa fragments. The cleavage of the 22-kDa/50-kDa junction by elastase increases the Km value for the actin-activated ATPase. The presence of the two protease-sensitive regions in S-1 is consistent with a three-domain structure of the myosin head and may have important implications to the mode of intersite communication in this protein.