Interaction of monomeric and polymeric actin with myosin subfragment 1

Abstract

The abilities of F-actin and G-actin to activate the ATPase of subfragment 1 of myosin have been compared at 15 °C in a medium of low ionic strength in which polymerization of G-actin does not occur. F-actin gives a very much larger activation than G-actin, but the activation by G-actin is still appreciable. At 25 °C the difference between G-actin and F-actin is accentuated. At higher ionic strengths the rate of hydrolysis of ATP by subfragment 1 and G-actin increases with time, due to polymerization of G-actin. Photo-oxidized G-actin, which is incapable of polymerization, has a similar effect to unmodified G-actin. The activation by G-actin is destroyed by treatment with urea or guanidinium chloride, and other proteins do not activate. Experiments demonstrate that the activation by G-actin is not due to an indirect effect on the ionic composition of the medium. It is concluded that the activation by G-actin is due to protein-protein interaction, and that the structure of G-actin is unlikely to differ very greatly from an F-actin subunit.