Characterization of oligomers as kinetic intermediates in myosin subfragment 1-induced polymerization of G-actin.

The nature of the kinetic intermediates involved in S1-induced polymerization of G-actin into F-acto-S1-decorated filaments has been investigated using as probes light scattering, the fluorescence of pyrenyl- or NBD-labeled actin, and the anisotropy of fluorescence of N-iodoacetyl-N'-(5-sulfo-1-napthyl)ethylene diamine (AEDANS)-labeled actin. With AEDANS-G-actin, the initial formation of a ternary G2S complex between two G-actin and one S1 molecules (Valentin-Ranc, C., Combeau, C., Carlier, M. F., and Pantaloni, D. (1991) J. Biol. Chem. 266, 17871-17879) has been confirmed. Data obtained with all probes are consistent with the subsequent rapid formation of G-actin-S1 oligomers in which the actin/S1 molar ratio is 2:1. Oligomers form above 0.6 microM G-actin with S1(A1) and above 3.5 microM G-actin with S1(A2), at 20 degrees C. Oligomerization is endothermic with a delta H of 14 kcal/mol. A tentative model is proposed to comprehensively account for the data and the structural features of the F-actin-S1 filament. Within this model, longitudinal actin-actin interactions take place in G2S, and lateral, hydrophobic actin-actin interactions appear upon formation of (G2S)n oligomers.