Comparison of the tryptic digestion pattern of subfragments 1 from V1 and V3 rat cardiac isomyosins

The limited tryptic digestion patterns of the chymotryptic subfragment 1 (S1) of the two rat ventricular isomyosins V1 and V3, were compared under several conditions. Pure S1V1 was obtained from 3-week-old rats and pure S1V3 from adult rats 6 weeks after hypophysectomy. To localize the sites of trypsin susceptibility and to determine the distribution of the peptides along the S1 molecule, we used, as a probe, antibodies raised against a pig cardiac 29-kDa peptide. We demonstrate that this peptide contains the N-acetyl group located on the N-terminal part of the cardiac myosin molecule. In S1V1 we observed two major sites of proteolysis, independently of the digestion conditions: they are located at 27kDa and 80kDa from the N terminus as in skeletal muscle S1.S1V3 appears much more sensitive to the proteolysis conditions: at least two additional sites of cleavage are present in the 50-kDa peptide when digested at pH 8.0. Decrease in the pH from 8.0 to 7.0 or the presence of Mg-ATP have no effect on the digestion of S1V1 while these ambient factors protect the 50-kDa peptide of S1V3 from breakdown. We conclude that the 50-kDa peptide is a variable portion of the myosin molecule, the conformation of which is sensitive to ambient factors such as the pH or the presence of nucleotides.