Action of a 19K protein from porcine brain on actin polymerization: a new functional class of actin-binding proteins.  A 19K protein isolated from porcine brain not only inhibits actin polymerization but depolymerizes actin filaments quickly. The protein reacts stoichiometrically with actin in a 1:1 molar ratio. When actin is induced to polymerize with salts in the presence of the brain 19K protein, the lag phase is prolonged, and the extent of polymerization is decreased, but the half-polymerization time is not increased. This can be explained by assuming that the 19K protein severs growing actin filaments and thus causes an increase in the number of filament ends during the polymerization process, thereby accelerating the overall polymerization. Moreover, the low-shear viscosity of actin filaments is reduced much more than the high-shear viscosity by the 19K protein, suggesting that actin filaments become shorter in the presence of the 19K protein than in its absence. Actin filament depolymerization by the 19K protein is much faster than that by brain profilin or than spontaneous depolymerization. This indicates that the 19K protein depolymerizes actin filaments not only by sequestering actin monomers but also by directly attacking the filaments. The number of actin filaments, measured by assaying the nucleating ability, is increased by substoichiometric concentrations of the 19K protein, irrespective of whether the protein is added to actin monomers before polymerization or added to performed actin filaments. These results suggest that the brain 19K protein not only stabilizes actin monomers but also cuts actin filaments, thereby decreasing the extent of actin polymerization and also changing the filament length. The action on actin of the actin-depolymerizing protein from starfish oocytes resembles that of the brain 19K protein, although the molecular weight of the starfish protein is slightly smaller. The brain 19K protein and starfish protein should be classified into a new functional group of actin-binding proteins.