How Vav proteins discriminate the GTPases Rac1 and RhoA from Cdc42

Vav proteins are GDP/GTP exchange factors for Rho/Rac GTPases that are acti vated by tyrosine phosphorylation. These proteins activate Rac1, RhoG, and RhoA but not the highly related Cdc42 protein. At present, there is no avai lable information to explain this substrate selectivity at the structural l evel. Here we show that the selection of Vav proteins substrates is achieve d at two different levels. On one hand, Vav proteins utilize some residues of the beta2/beta3 region of Rho/Rac GTPases (D49 and E54) to assure the sp ecific binding to its substrate. In addition, these exchange factors need a second structural signal located in the beta5 region of Rho/Rac proteins ( residue K118) to promote proper GDP/GTP exchange. These results identify th e amino acid residues that allow the discrimination of the Vav family subst rates from Cdc42 and, in addition, demonstrate that the activation of speci fic Rho/Rac GTPases by these GEFs requires two concatenated events, binding and subsequent enzyme reaction, whose specificities are determined by two separate regions of Rho proteins.