Gelsolin has three actin-binding sites

Gelsolin, a Ca2+-modulated actin filament-capping and -severing protein, complexes with two actin monomers. Studies designed to localize binding sites on proteolytic fragments identify three distinct actin-binding peptides. 14NT, a 14-kD fragment that contains the NH2 terminal, will depolymerize F-actin. This peptide forms a 1:1 complex with G-actin which blocks the exchange of etheno-ATP from bound actin. The estimated association and dissociation rates for this complex are 0.3 microM-1 s-1 and 1.35 x 10(-6) s-1 which gives a maximum calculated Kd = 4.5 x 10(-12) M. 26NT, the adjacent peptide on the NH2-terminal half of gelsolin, binds to both G- and F-actin. This fragment has little or no intrinsic severing activity and will bind to F-actin to nearly stoichiometric ratios. The interactions of 14NT and 26NT with actin are largely Ca2+ independent and one of these sites, probably 14NT, is the EGTA-stable site identified in the intact protein. 41CT, the COOH-terminal half of gelsolin, forms a rapidly reversible 1:1 complex with actin, Kd = 25 nM, that slows but does not block etheno- ATP exchange. This interaction is Ca2+ dependent and is the exchangeable site in the intact protein. One of these sites is hidden in the intact protein, but cleavage into half fragments exposes all three and removes the Ca2+ dependence of severing.