THE STRUCTURE OF THE GTPASE-ACTIVATING DOMAIN FROM P50RHOGAP

Members of the Rho family of small G proteins transduce signals from p lasma-membrane receptors and control cell adhesion, motility and shape by actin cytoskeleton formation(1-4), They also activate other kinase cascades, Like all other GTPases, Rho proteins act as molecular switc hes, with an active GTP-bound form and an inactive GDP-bound form(5), The active conformation is promoted by guanine-nucleotide exchange fac tors, and the inactive state by GTPase-activating proteins (GAPs) whic h stimulate the intrinsic GTPase activity of small G proteins(6), Rho- specific GAP domains are found in a wide variety of large, multi-funct ional proteins(7), Here we report the crystal structure of an active 2 42-residue C-terminal fragment of human p50rhoGAP(8), The structure is an unusual arrangement of nine alpha-helices, the core of which inclu des a four-helix bundle, Residues conserved across the rhoGAP family a re largely confined to one face of this bundle, which may be an intera ction site for target G protein, In particular, we propose that Arg 85 and Asn 194 are involved in binding G proteins and enhancing GTPase a ctivity.