Artificial phosphorylation removes gelsolin's dependence on calcium

Gelsolin is one of the best known actin-binding proteins with several distinct activities regulated by calcium. Using a kinase fraction isolated from mitotic HeLa cells, we found that the plasma form of gelsolin can be phosphorylated at a site located within the NH₂-terminus region which does not exist in the cytoplasmic form. After this phosphorylation, gelsolin no longer requires Ca²⁺ for activity; it severs and subsequently caps actin filaments, and nucleates filament formation in Ca²⁺-free solution. These findings may clarify the mechanism of gelsolin regulation by Ca²⁺, and indicate that changes in electrical interactions between the NH₂- and COOH-terminal ends are important for this regulation. Moreover, since only a single site is phosphorylated, and since the phosphorylated region does not contribute to this protein's own activity, the results suggest that a single chemical charge modification at a site away from the protein's core structure, such as this phosphorylation site, is sufficient to alter the protein's function.