Characterization of the action of porcine brain profilin on actin polymerization.

When porcine brain actin is polymerized in either KCl/MgCl2 or KCl alone, porcine brain profilin prolongs the lag phase and inhibits the rate and extent of polymerization in a concentration-dependent manner. Profilin also decreases the elongation rate in a concentration-dependent manner. Moreover, addition of profilin to steady-state actin filaments causes slow depolymerization. All these actions of profilin are explainable by a monomer sequestering mechanism. The inhibition by profilin of both the extent of polymerization and the elongation rate is stronger in KCl alone than in KCl/MgCl2. Moreover, it was found that brain profilin inhibits the polymerization of brain actin more strongly than that of muscle actin. Brain 88K protein/actin complex (88K/A), which has been shown to cap the barbed end of actin filaments, potentiates the inhibitory action of profilin; i.e. the extent of polymerization is much more reduced by profilin in the presence of 88K/A than in its absence.