Effect of Tryptic Digestion of Myosin Subfragment-1 on Its Binding to F-Actin1

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Abstract
We investigated the effect of tryptic digestion of S-1 (95K) into three segments (27K, 50K, and 20K) on the binding of F-actin with S-1, using an ultracentrifugal separation method and a light-scattering method. The tryptic digestion of S-1 decreased the affinity of S-1 for F-actin both in the absence of nucleotide and the presence of AMPPNP or ATP, suggesting that the peptide cutting impairs the structures participating in the binding of S-1 or S-1-nucleotide complex with F-actin. Although nucleotides markedly weakened the affinity of S-l for F-actin, the ratios of affinity of digested S-1 for F-actin to that of intact S-1 were all about 1/10 both in the absence and presence of nucleotides. This may be understood if we accept the assumption that two kinds of structure participate in the binding of S-1 with F-actin; one is independent of nucleotides and the other is dependent on them, with only the former being affected by the tryptic digestion of S-1.