The mechanism of limited tryptic proteolysis of heavy meromyosin as revealed by peptide analysis.

To elucidate some ambiguous details in the tryptic fragmentation scheme of HMM as given by Bálint et al. (J. Biol. Chem. 250 (1975) 6168; Arch. Biochem. Biophys. 190 (1978)793), the peptide fragments were isolated by a milligram scale preparative gel electrophoresis procedure. The dansyl-peptide map of the 20 kDal tryptic fragment obtained from tryptic heavy meromyosin (HMM) and that of a similar fragment from papainic subfragment-1 (S-1) were found to be nearly identical. This finding gives unequivocal proof of the location of the 17 kDal peptide stretch lost during digestion in the form of small peptides, at the C terminal part of the heavy chain backbone of HMM. The N terminals of the 150, 74, and 25 kDal fragments of the heavy chain isolated from HMM digested by trypsin under widely differing conditions were shown to be acetylated. The N terminal amino group of the other peptide fragments of HMM remains the same under widely differing conditions of digestion. We conclude that all the fragments are well defined polypeptides and digestion progresses by splitting from the C terminals formed by the primary splits.