Myosin subfragment-1-induced polymerization of G-actin. Formation of partially decorated filaments at high actin-S1 ratios.

Myosin subfragment-1-induced polymerization of G-actin into arrowhead-decorated F-actin-myosin subfragment-1 (S1) filaments has been studied at low ionic strength and in the absence of ATP, using a combination of light scattering, fluorescence of 4-nitrobenz-2-oxa-1,3-diazol-7-yl- or pyrenyl-labeled actin, sedimentation, and electron microscopy techniques. When G-actin is in excess over myosin subfragment-1, the initial formation of fully decorated F-actin-S1 filaments, in which the actin:S1 molar ratio is 1:1, is followed by further incorporation of G-actin subunits in the polymer concomitant with the redistribution of the myosin heads along the polymer, leading to partially decorated filaments containing less than one S1/actin, in equilibrium with G-actin. This process leads to an overshoot in the light-scattering polymerization curves at high actin:S1 ratios. The concentration of G-actin at equilibrium with partially decorated filaments is a nonlinear function of the molar fraction of S1 in the polymer, indicating that actin-actin-S1 interactions are energetically more favorable than actin-actin or actin-S1-actin-S1 interactions.