Ca2+ control of actin filament length. Effects of macrophage gelsolin on actin polymerization.

Gelsolin complexes with calcium (gelsolin-Ca2+) binds to the ends of actin filaments to which monomers add preferentially during elongation. It forms a stable complex with actin in a low ionic strength solution which does not normally favor the polymerization of actin. Gelsolin-Ca2+ increases the rate of nucleation of actin which precedes polymerization, but decreases the rate of elongation of the filaments. The final average length of filaments formed in the presence of gelsolin-Ca2+ is shorter and the equilibrium monomer concentration increases relative to actin polymerized in the absence of gelsolin-Ca2+. Gelsolin-Ca2+ also increases the number of actin filaments because the magnitude of the increase in monomer concentration is disproportionately small compared with the reduction in polymer length. In these respects, the population of actin filaments formed during polymerization in the presence of gelsolin-Ca2+ is similar to that resulting from the action of gelsolin on previously assembled actin filaments (Yin, H. L., Zaner, K. S., and Stossel, T. P. (1980) J. Biol. Chem. 255, 9494-9500). The calcium-dependent shortening of ects, the population of actin filaments formed during polymerization in the presence of gelsolin-Ca2+ is similar to that resulting from the action of gelsolin on previously assembled actin filaments (Yin, H. L., Zaner, K. S., and Stossel, T. P. (1980) J. Biol. Chem. 255, 9494-9500). The calcium-dependent shortening of ects, the population of actin filaments formed during polymerization in the presence of gelsolin-Ca2+ is similar to that resulting from the action of gelsolin on previously assembled actin filaments (Yin, H. L., Zaner, K. S., and Stossel, T. P. (1980) J. Biol. Chem. 255, 9494-9500). The calcium-dependent shortening of actin filaments is the primary mechanism for the dissolution of an actin gel by gelsolin. Therefore, the ability of gelsolin to produce short filaments irrespective of the initial state of assembly of the actin offers flexibility for controlling the network structure of the cytoplasm in which either the monomeric or polymeric form of actin molecules might predominate at different times.