Function of Heavy Meromyosin in the Acceleration of Actin Polymerization.

The function of myosin A in the acceleration of actin polymerization was first observed by Laki and Clark (1) who noted the catalytic effect of myosin A. Martonosi and Gouvea (2) used a light scattering method to demonstrate that an enzymically active fragment of myosin A, heavy meromyosin, prepared by tryptic digestion, also accelerated the actin polymerization. Yagi and Sakakibara (3) have shown that, in the presence of myosin A (or heavy meromyosin) with divalent cations, the velocity of orthophosphate liberation accompanying the actin polymerization was proportional to the concentration of G-actin and myosin -4 (or heavy meromyosin) under their experimental conditions. However, it is still not known whether the actin polymerization was reinforced by the adenosine triphosphatase activity of heavy meromyosin or by heavy meromyosin and Gactin having formed a particular complex. Bbr&ny, BBrBny, and Oppenheimer (4) have shown that sonic vibration of myosin A partly diminishes the ATPase activity of this protein, but not its ability to bind F-actin. I f actin binding abilii;y is essential in acceleration of actin polymerization, this result suggests a possible method of differentiating between acceleration ability and ATPase activity in heavy meromyosin. The present paper concerns investigation of the accelerative function of heavy meromyosin in actin polymerization with experimental conditions where heavy meromyosin was required for actin polymerization at a low ionic strength. It was concluded from the experiments that salt was required for G-actin-G-actin interaction, and that the accelerative effect of heavy meromyosin resulted from its specific binding with G-actin. A preliminary report has been previously presented (5).