Kinetics of Myosin Subfragment-1-Induced Condensation of G-Actin into Oligomers, Precursors in the Assembly of F-Actin−S₁. Role of the Tightly Bound Metal Ion and ATP Hydrolysis

In a low ionic strength buffer and in the absence of free ATP, the interaction of G-actin (G) with myosin subfragment-1 (S1) leads to the formation of arrowhead-decorated F-actin-S1 filaments, through a series of elementary steps. The initial formation of GS and G2S complexes is followed by their condensation into short oligomers. The kinetics of formation of G-actin-S1 oligomers have been monitored in a stopped-flow apparatus using a combination of light scattering and fluorescence of NBD-labeled actin. Oligomers appear more stable and are formed at a faster rate from MgATP-G-actin than from CaATP-G-actin. The actin-bound ATP is hydrolyzed when oligomers are formed from MgATP-G-actin, not when they are formed from CaATP-G-actin. The formation of oligomers is energetically favored in the presence of cytochalasin D. All data are consistent with the view that the actin-actin interactions which take place upon condensation of GS and G2S into oligomers are very similar to lateral actin-actin interactions along the short pitch helix of actin filaments, which are involved in actin nucleation. These interactions trigger ATP hydrolysis on actin.