NUCLEOTIDE-INDUCED CHANGES IN THE INTERACTION OF MYOSIN SUBFRAGMENT-1 WITH ACTIN - DETECTION BY ANTIBODIES AGAINST THE N-TERMINAL SEGMENT OF ACTIN

The binding of myosin subfragment 1 (S-1) to actin in the presence and absence of nucleotides was determined under conditions of partial saturation of actin, up to 80%, by F(ab)(1-7), the antibodies against the first seven N-terminal residues on actin. In the absence of nucleotides, the binding constant of S-1 to actin (2 x 10(7) M-1) was decreased by 1 order of magnitude by F(ab)(1-7). The binding of S-1 to actin caused only limited displacement of F(ab), and between 30 and 50% of actin appeared to bind both proteins. In the presence of MgAMP.PNP, MgADP, and MgPP(i) and at low S-1 concentrations, the same antibodies caused a large decrease in the binding of S-1 to actin. However, the binding of S-1.nucleotide to actin in the presence of F(ab)(1-7) increased cooperatively with the increase in S-1 concentration. Also, in contrast to rigor conditions, there was no indication for the binding of F(ab)(1-7) and S-1.nucleotide to the same actin molecules. These results show a nucleotide-induced transition in the actomysin interface, most likely related to the different roles of the N-terminal segment of actin in the binding of S-1 and S-1.nucleotide. The possible implications of these findings to the regulation of actomyosin interactions are discussed.