Gelsolin is a six-domain protein with a wide array of actin regulating activities. Despite the growing body of structural data on this protein, little is known about how it binds F-actin during severing and capping, In this paper we have combined data from X-ray crystallography, NMR, and electron microscopy to develop a model of an actin filament capped by a severing protein. The protein which we have modeled is G1/alpha A1-2, a genetically engineered molecule containing domains from both gelsolin and alpha-actinin. In the capped filament, domains G1 and alpha A1-2 of the hybrid severing protein bind two adjacent monomers along the long-pitch F-actin helix. The distance spanning these domains suggests the need for a flexible linker between them. By analogy, this implies that the gelsolin deletion mutant G1-3 contacts the same two monomers in the capped filament and suggests that the linker between G1 and G2 plays a crucial role in severing and capping. (C) 1995 Academic Press, Inc.