Tryptic digestion as a probe of myosin S-1 conformation.

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Abstract
One of the products of the limited tryptic hydrolysis of chymotryptic [rabbit] myosin subfragment 1 is the 27,000-dalton NH2-terminal fragment. This fragment is generated by 2 parallel routes from the 75,000- or 95,000-dalton peptide of the H chain: (1) through a 29,500-dalton precursor or (2) directly without participation of a precursor. Lowering of pH and temperature and increasing of ionic strength inhibited route 1 digestion in comparison to route 2. MgATP and its derivatives in mM concentration substantially suppressed route 2 digestion. Suppression of route 1 digestion depended on the concentration of MgATP. It occurred after a lag phase when the ratio of MgATP to subfragment 1 concentration was > 0.5. The MgATP-induced increase in tryptophan fluorescence of myosin subfragment 1 appeared without a lag phase. The generation of the 27,000-dalton fragment by either route was not affected by F-actin; however, the suppression of route 1 digestion induced by MgADP was abolished when myosin subfragment 1 was in ternary complex with actin and MgADP. The 27,000/50,000-dalton hinge region is a flexible domain of the myosin head. The conformation of this region is sensitive to the presence of nucleotides and actin and to variations in ambient factors.