Formation of actin dimers as studied by small angle neutron scattering

Small angle neutron scattering has been used to study the dimensions of G-actin and the formation of low molecular weight actin oligomers under conditions where rapid polymerization does not take place. In the presence of 200 μM Ca²⁺, actin in solution consists of a single component with a radius of gyration (R(g)) of 19.9 ± 0.4 Å, consistent with the known molecular dimensions of the G-actin molecule. In the presence of 50 μM Mg²⁺, however, formation of an actin species with a larger R(g) occurs over a 4-h period. Multicomponent fits were tried and the data were best fit assuming two components, the monomer and a species with an R(g) of 29 ± 1 Å. This latter value is consistent with the dimensions expected for certain actin dimers. The apparent dissociation constant for dimer formation is approximately 150 μM with forward and reverse rate constants of 6.0 x 10^-7 μM^-1 s^-1 and 8.8 x 10^-5 s^-1, respectively. Kinetic fluorescence experiments show that the dimer formed in the presence of low levels of Mg²⁺ is a nonproductive complex which does not participate in the polymerization process. However, the addition of cytochalasin D to actin in the presence of 50 μM Mg²⁺ rapidly induces the formation of dimers, presumably related to cytochalasin's ability to nucleate actin polymerization.