Physical chemistry of actin: past, present and future.

History of actin research is reviewed with special emphasis on dynamics of the G-F transformation and flexibility or intrafilamentous mobility of F-actin. Good correlation was found between the flexibility of F-actin and its activity in cell motility. In molecular machines such as the flagellar motor and the sliding machine of F-actin and myosin, the coupling between influx and efflux seems to be loose. F-actin would assume multiple active states during sliding on myosin with hydrolysis of ATP. Recently, the three-dimensional structure of actin molecule in crystals has been determined. Actin research is expected to give an answer to the question on the physiological significance of internal mobility of protein molecules and their assemblies and the structural origin of such mobility.