Effect of nucleotide on interaction of the 567-578 segment of myosin heavy chain with actin

To probe the effect of nucleotide on the formation of ionic contacts between actin and the 567-578 residue loop of the heavy chain of rabbit skeletal muscle myosin subfragment 1 (S1), the complexes between F-actin and proteolytic derivatives of S1 were submitted to chemical cross-linking with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. We have shown that in the absence of nucleotide both 45 kDa and 5 kDa tryptic derivatives of the central 50 kDa heavy chain fragment of S1 can be cross-linked to actin, whereas in the presence of MgADP.AlF4, only the 5 kDa fragment is involved in cross-linking reaction. By the identification of the N-terminal sequence of the 5-kDa fragment, we have found that trypsin splits the 50 kDa heavy chain fragment between Lys-572 and Gly-573, the residues located within the 567-578 loop. Using S1 preparations cleaved with elastase, we could show that the residue of 567-578 loop that can be cross-linked to actin in the presence of MgADP.AlF4 is Lys-574. The observed nucleotide-dependent changes of the actin-subfragment 1 interface indicate that the 567-578 residue loop of skeletal muscle myosin participates in the communication between the nucleotide and actin binding sites.