Rate constants and equilibrium constants for binding of actin to the 1:1 gelsolin-actin complex

The rate constant and equilibrium constant of association of an actin monomer with 1:1 gelsolin-actin complex isolated from chicken were measured by using fluorescently labeled actin. According to fluorescence stopped-flow experiments, the rate constant of formation of the 1:2 gelsolin-actin complex from 1:1 gelsolin-actin complex and actin was found to be about 2 x 10(7) M-1 s-1 under conditions where gelsolin binds Ca2+. The rate of dissociation of one actin molecule from the 1:2 gelsolin-actin complex was determined by exchange of actin for fluorescently labeled actin. The rate constant of dissociation was about 0.02 s-1. Thus, the equilibrium constant for association of actin with 1:1 gelsolin-actin complex can be calculated to be in the range of 10(9) M-1. The rate of dissociation of actin from 1:2 gelsolin-actin complex was independent of the Ca2+ concentration. Ca2+ affects only the rate of association of actin with 1:1 gelsolin-actin complex.