Myosin Subfragment 1 Catalyzes the Polymerization of Actin at Low Ionic Strength

Abstract

The interactions of heavy meromyosin (HMM) and of the myosin subfragment 1 (S1) with both G-actin and F-actin have been widely investigated. HMM was reported to accelerate actin polymerization at low ionic strength (1) and to induce in G-actin the EPR spectrum typical of the F-actin conformer, the effect being maximal at an HMM/G-actin ratio of 0.25. S1 in this respect was found to be much less effective, the EPR spectrum changes being maximal for an S1/G-actin ratio of 1 to 1 (2). The interaction of either S1 or HMM with F-actin was also studied by numerous physical techniques. By measuring the quasi elastic light scattering of F-actin solutions it was found that the flexibility of the filament was increased by HMM, the effect being maximal at a 1:6 molar ratio of HMM to actin. S1 did not affect actin flexibility (3). F-actin flexibility was found, on the contrary, to be decreased when the interaction of either HMM or S1 was studied by saturation transfer EPR. In these experiments, the addition of the myosin fragments were found to increase 4 times the correlation time of the spin label bound to actin (4).