Artificial phosphorylation removes gelsolin's dependence on calcium

Gelsolin is one of the best known actin-binding proteins with several disti nct activities regulated by calcium. Using a kinase fraction isolated from mitotic HeLa cells, we found that the plasma form of gelsolin can be phosph orylated at a site located within the NH2-terminus region which does not ex ist in the cytoplasmic form. After this phosphorylation, gelsolin no longer requires Ca2+ for activity; it severs and subsequently caps actin filament s, and nucleates filament formation in Ca2+-free solution. These findings m ay 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 ph osphorylated, and since the phosphorylated region does not contribute to th is protein's own activity, the results suggest that a single chemical charg e modification at a site away from the protein's core structure, such as th is phosphorylation site, is sufficient to alter the protein's function.