Experimental studies of the equilibrium polymerization of muscle actin

Actin is an important structural protein found in a wide variety of organisms. In cells, it typically exists as either a globular protein, G-actin, or as its polymer, the filamentous protein, F-actin. G-actin molecules will polymerize in vitro to F-actin if the solution temperature, pH, and added salts are appropriate. 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 small angle neutron scattering measurements of actin polymerization, as a function of temperature, from three solutions of 3.00 mg/ml actin, each possessing a different KCl concentration. We show that actin polymerization is analogous to a reversible temperature dependent phase transition. The Ornstein-Zernike expression was used to obtain the zero-angle scattering intensity, ;In addition, small angle neutron scattering measurements of G-actin solutions were conducted. These data show a peak in the angle dependent scattering intensity, ;Finally, we present preliminary ultraviolet absorption results of the extent of polymerization as a function of temperature. These results, and the small angle neutron scattering data give polymerization temperatures that are considerably higher than those predicted from thermodynamic data currently available.