Role of the 50-kilodalton tryptic peptide of myosin subfragment 1 as a communicating apparatus between the adenosinetriphosphatase and actin binding sites.

Limited glutaraldehyde modification of tryptic myosin subfragment 1, which mainly consists of 26-, 50-, and 20-kilodalton (kDa) peptides, resulted in the selective cross-linking of the 20- and 50-kDa peptides. The cross-linking pattern was altered by nucleotides, depending on the base structure. Neither the reactive thiols on the 20-kDa peptide nor the reactive lysyl residue on the 26-kDa peptide was modified with the reagent, regardless of the presence or absence of nucleotide. Glutaraldehyde treatment of the protein resulted in marked increases in its Mg2+-ATPase activity and affinity for actin. High ATPase activity and actin affinity were not produced if the treatment was conducted in the presence of ATP. These ATPase and actin binding properties of the protein derivatives are explained by assuming that glutaraldehyde "freezes" the existing interactions between the 20- and 50-kDa peptides in the activated and nonactivated conformational states, respectively. Taking into account the previous reports that the ATPase site resides between the 26- and 50-kDa peptides, and the 50-kDa peptide binds either ATP or actin, the present results suggest that the 50-kDa peptide acts as a communicating apparatus between the ATPase and actin binding sites of myosin. A simple model for the intersite communication is also proposed.