KINETICS OF MYOSIN SUBFRAGMENT-1-INDUCED CONDENSATION OF G-ACTIN INTOOLIGOMERS, PRECURSORS IN THE ASSEMBLY OF F-ACTIN-S-1 - ROLE OF THE TIGHTLY BOUND METAL-ION AND ATP HYDROLYSIS

In a low ionic strength buffer and in the absence of free ATP, the int eraction of G-actin (G) with myosin subfragment-1 (S-1) leads to the f ormation of arrowhead-decorated F-actin-S-1 filaments, through a serie s of elementary steps. The initial formation of GS and G(2)S complexes is followed by their condensation into short oligomers, The kinetics of formation of G-actin-S-1 oligomers have been monitored in a stopped -flow apparatus using a combination of light scattering and fluorescen ce of NBD-labeled actin, Oligomers appear more stable and are formed a t 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 oligomer s 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 G(2)S into oligomers are very s imilar to lateral actin-actin interactions along the short pitch helix of actin filaments, which are involved in actin nucleation, These int eractions trigger ATP hydrolysis on actin.