Polymerization of G-actin by hydrodynamic shear stresses

In the absence of Ca²⁺ G-actin can be polymerized by the application of shear stress in low ionic strength buffer. When G-actin in low ionic strength buffer containing EGTA was sheared for predetermined times under different velocity gradients, viscosity attained a maximal value, comparable to that obtained by seeding with F-actin nuclei, at a velocity gradient of 3000 s^-1 after about one hour. Such fiow-polymerized actin was indistinguishable from KCl-polymerized actin. Under similar conditions, EDTA which can bind both Ca²⁺ and Mg²⁺, gave a smaller effect than the Ca²⁺-chelating agent EGTA which binds Mg²⁺ weakly. When an Mg²⁺ salt was added to EDTA- or EGTA-containing buffer to give a free Mg²⁺ concentration of a few micromoles/liter, flow induced polymerization was significantly enhanced. It appears that occupancy of only a small fraction of the high affinity binding sites by Ca²⁺ prevents flow-polymerization while Mg²⁺ may enhance this type of polymerization by replacing Ca²⁺. We speculate that the shear stress induces polymerization by promoting nucleation and that Ca²⁺ bound to the high affinity divalent cation binding site inhibits formation of the nuclei.