Cleavage of a smooth muscle myosin heavy chain near its c terminus by α-chymotrypsin: Effect on the properties of myosin

Limited proteolysis of gizzard myosin by α-chymotrypsin converted the heavy chain doublet pattern, seen by gel electrophoresis, to a single band. Light chain degradation was not observed and only minor cleavage occurred at other heavy chain sites. Using a polyclonal antibody raised against a unique sequence from the slower-migrating heavy chain (SM1) it was shown that this conversion was due to the loss of a peptide approximately 4000 daltons from the C terminus of SM1. The peptide was isolated and sequenced, and the cleavage site was identified between phenylalanine 1943 and alanine 1944. Addition of antibody before protease protected SM1 from cleavage. The following changes were observed (a) the Mg²⁺-dependence of actin-activated ATPase of digested phosphorylated myosin was altered and activity was relatively high at low Mg²⁺ levels, i.e. similar to phosphorylated heavy meromyosin; (b) the KCI dependence of Mg²⁺-ATPase of the digested myosin, particularly the phosphorylated form, showed an altered pattern consistent with the stabilization of the 6 S conformation; (c) the tendency for aggregation was increased by proteolysis of phosphorylated myosin. These results show that the C-terminal region of a gizzard myosin heavy chain can modify some of the properties of myosin. It is suggested that the observed modifications reflect an enhanced tendency of the digested myosin to aggregate.