Reactive lysyl of myosin subfragment 1: location on the 27K fragment and labeling properties.

The limited tryptic digestion of the heavy chain of chymotryptic myosin subfragment 1 resulted in five peptides with approximate molecular weights of 75K, 50K, 29.5K, 27K, and 20K. Of the five peptides, two, 75K and 29.5K, were transient and disappeared during the digestion. Our data suggest that the 27K fragment is generated by two parallel routes: directly from the 75K fragment and through a 29.5K precursor. A method was developed to isolate the final products, 50K, 27K, and 20K fragments, of the tryptic hydrolysis of the heavy chain of myosin subfragment 1. Using this method, it was found that the reactive lysyl residue, labeled by a trinitrophenyl moiety, resides in the 27K fragment. The reactive lysyl residue was also present in the 29.5K fragment. The trinitrophenylation of the reactive lysyl residue was inhibited by magnesium pyrophosphate in the 27K but not in the 29.5K fragment. This may indicate that the two routes of generating the 27K peptide correspond to the proteolysis of two qualitatively different subfragment-1 heads as suggested by Tonomura [Tonomura Y. (1972) Muscle Proteins, Muscle Contraction and Cation Transport, University of Tokyo Press, Tokyo, and University Park Press, Baltimore].