The actin filament-severing domain of plasma gelsolin

Gelsolin, a multifunctional actin-modulating protein, has two actin-binding sites which may interact cooperatively. Native gelsolin requires micromolar Ca²⁺ for optimal binding of actin to both sites, and for expression of its actin filament-severing function. Recent work has shown that an NH₂-terminal chymotryptic 17-kD fragment of human plasma gelsolin contains one of the actin-binding sites, and that this fragment binds to and severs actin filaments weakly irrespective of whether Ca²⁺ is present. The other binding site is Ca²⁺ sensitive, and is found in a chymotryptic peptide derived from the COOH-terminal two-thirds of plasma gelsolin; this fragment does not sever F-actin or accelerate the polymerization of actin. This paper documents that larger thermolysin-derived fragments encompassing the NH₂-terminal half of gelsolin sever actin filaments as effectively as native plasma gelsolin, although in a Ca²⁺-insensitive manner. This result indicates that the NH₂-terminal half of gelsolin is the actin-severing domain. The stringent Ca²⁺ requirement for actin severing found in intact gelsolin is not due to a direct effect of Ca²⁺ on the severing domain, but indirectly through an effect on domains in the COOH-terminal half of the molecule to allow exposure of both actin-binding sites.