R. Ivkov, J. G. Forbes, S. C. Greer
We present results of small angle neutron scattering (SANS) measurements of G-actin solutions in D2O, at various protein concentrations. These data show a peak in the angle dependent scattering intensity indicating sturcture that results from significant interparticle repulsions. Comparison of the data with a model based on standard liquid theory and colloid solution theory suggests that the size of the G-actin molecules, the amount of charge each molecule possesses, and the number of calcium ions bound by each are variable and depend on the protein concentration. G-actin molecules will polymerize in vitro to F-actin under appropriate solution conditions. This polymerization reaction occurs under conditions of equilibrium between G- and F-actin, and is driven by an increasing system-wide entropy. We present results of SANS measurements of actin polymerization, as a function of both temperature and KCl concentration. We show that actin polymerization is analogous to a reversible temperature dependent phase transition. In particular, as the concentration of KCl is decreased, the transition from G-actin to F-actin occurs at a higher temperature, and becomes sharper.