The formation of actin oligomers studied by analytical ultracentrifugation.

The small oligomers formed from Mg-G-actin under favorable conditions were studied by sedimentation velocity ultracentrifugation. The critical concentration of actin at pH 7.8 in the presence of 100 microM MgCl2 and 200 microM ATP was 12.5 +/- 2.8 microM. Under these conditions, about 15% of 7.5 microM Mg-actin was converted to oligomers of subunit size four to eight in 5 h at 20 degrees C. In 100 microM MgCl2 and no free ATP, the critical concentration was about 6.5 microM, and about 22% of 7.5 microM Mg-actin was converted to dimers in 80 min. There were no detectable higher oligomers or F-actin present in either case. As determined by the analysis of ATP hydrolysis, most, if not all, of the oligomer subunits contained ATP. When 28.5 microM actin was polymerized to steady state in 100 microM MgCl2 and 200 microM ATP, about 50% of the actin was present as F-actin, consistent with the critical concentration (approximately 12.5 microM), about 50% as oligomers as large as seven subunits, and only about 5% as monomers. When solutions containing oligomers were diluted the oligomers dissociated. Alternatively, when the MgCl2 concentration was raised to 1 mM, the solutions containing oligomers polymerized more rapidly than monomeric Mg-G-actin and to the same final steady state. These data are entirely consistent with the condensation-elongation model for helical polymerization proposed by Oosawa and Kasai (Oosawa, F., and Kasai, M. (1962) J. Mol. Biol. 4, 10-21) according to which, under certain conditions, substantial amounts of short linear and helical oligomers should be formed below the critical concentration and linear oligomers should coexist with monomers and F-actin at steady state.