ABSTRACT
We presented, about 40 years ago, a theoretical framework for understanding the polymerization of protein molecules to helical polymers, based on the study on the G-F transformation of actin, a muscle protein [1, 2, 3]. (See Chapter 1.)
In pure water, all actin molecules are in the G-actin (globular actin monomer) state and at the physiological concentration of salts, almost all are in the F-actin (fibrous actin polymer) state. In the intermediate condition of salts, at low concentrations of actin no F-actin exists. At a certain critical concentration, F-actin begins to be formed. With increasing concentration of actin, the amount of F-actin increases, coexisting with G-actin, which is kept at the critical concentration. In this condition, each monomer undergoes a cycle between two states, G-and F-actin. The macroscopic G-F balance is maintained by microscopic cycling. The critical concentration decreases with increasing salt concentration. Thus, the G-F transformation has similar features to gas-liquid condensation or crystallization. Actually, the transformation consists of two processes, nucleation and growth. Usually, nucleation is rate limiting.
