Mechano-chemical behaviour of F-actin

Abstract

Even at very low ionic strengths, actin assumes a polymerized form of moderate size under sonic vibration. This was deduced from the following facts: (a) rapid formation of long F-actin filaments takes place after sonication for a short period; (b) the actin shows moderately high viscosity at the time of stopping the vibration; and (c) actin placed in a sonic field is not denatured by EDTA. The short actin polymer formed by the vibration splits added ATP enzymically. After stopping the vibration, i.e. when the transformation of the short actin polymers into long F-actin filaments takes place, a large amount of ATP is split. The initial velocity of this ATP splitting is proportional to the actin concentration. This finding was explained by assuming that each short actin polymer contains an appreciable proportion of partially interrupted structures of the F-form and their reformation is coupled with the ATP splitting. From the re-examination of the facts obtained from the sonic experiments, the following conclusions were drawn. (1) The interruption-reformation process is essentially reversible; (2) interruption is accelerated by mechanical forces; (3) reformation is accelerated by the dephosphorylation of added ATP. Taking into account these results, various dynamic properties of an F-actin filament which theoretically follow from the helical polymer model are presented.