Structure and mutagenesis of the Dbl homology domain

Guanine nucleotide exchange factors in the Dbl family activate Rho GTPases by accelerating dissociation of bound GDP, promoting acquisition of the GTP -bound state. Dbl proteins possess a similar to 200 residue catalytic Dbl-h omology (DH) domain, that is arranged in tandem with a C-terminal pleckstri n homology (PH) domain in nearly all cases. Here we report the solution str ucture of the DH domain of human PAK-interacting exchange protein (beta PIX ). The domain is composed of 11 alpha-helices that form a flattened, elonga ted bundle. The structure explains a large body of mutagenesis data, which, along with sequence comparisons, identify the GTPase interaction site as a surface formed by three conserved helices near the center of one face of t he domain. Proximity of the site to the DH C-terminus suggests a means by w hich PH-ligand interactions may be coupled to DH-GTPase interactions to reg ulate signaling through the Dbl proteins in vivo.