Molecular basis for Rac1 recognition by guanine nucleotide exchange factors

Rho GTPases are activated by a family of guanine nucleotide exchange factor s (GEFs) known as DbI family proteins. The structural basis for how GEFs re cognize and activate Rho GTPases is presently ill defined. Here, we utilize d the crystal structure of the DH/PH domains of the Rac-specific GEF Tiam1 in complex with Rac1 to determine the structural elements of Rac1 that regu late the specificity of this interaction. We show that residues in the Rac1 beta2-beta3 region are critical for Tiam1 recognition. Additionally, we de termined that a single Rac1-to-Cdc42 mutation (W56F) was sufficient to abol ish Rac1 sensitivity to Tiam1 and allow recognition by the Cdc42-specific D H/PH domains of Intersectin while not impairing Rac1 downstream activities. Oar findings identified unique GEF specificity determinants in Rac1 and pr ovide important insights into the mechanism of DH/PH selection of GTPase ta rgets.