FUNCTIONAL-SIGNIFICANCE OF THE BINDING OF ONE MYOSIN HEAD TO 2 ACTIN MONOMERS

The functional significance of the interaction of one myosin head (S1) with two actin monomers was investigated by comparing the properties of the cross-linked monomeric and filamentous actin-S1 complexes. S1 w as cross-linked to monomeric actin (G-actin) either in the absence or in the presence of DNase I by 1-ethyl-3-[3-(dimethylamino)propyl] carb odiimide. The binary G-actin-S1 and ternary DNase I-G-actin-S1 complex es were then purified by a combination of ion exchange and gel filtrat ion columns. Both the binary and the ternary complexes were characteri zed by negligible, though different, Mg2+-ATPase activities of 0.018 a nd 0.006 s(-1), respectively. Using fluorescence, light-scattering, an d ultracentrifugation experiments, we found that only the binary G-act in-S1 complex could be polymerized in the presence of 2 mM MgCl2. Elec tron microscopic analysis of the cross-linked filamentous complex show ed fully decorated filaments with the arrowhead pattern characteristic of the non-covalent complex in the rigor state. Such a 100% cross-lin ked F-actin-S1 complex exhibited a Mg2+-ATPase activity of 6.2 +/- 0.8 s(-1), slightly lower than the maximum velocity of the non-cross-link ed complex of 8.6 +/- 0.8 s(-1), but comparable to the 6.9 +/- 0.6 s(- 1) obtained for a partially (35%) cross-linked complex. These results implied that the activation of S1 ATPase by actin requires the interac tion of S1 with a second actin monomer within the thin filament. They also suggested that the full activation of the filamentous complex is not dependent on the degree of saturation of the thin filament by myos in.