ACTIVATION OF THE EXCHANGE FACTOR RAS-GRF BY CALCIUM REQUIRES AN INTACT DBL HOMOLOGY DOMAIN

Ras-GRF is a guanine nucleotide exchange factor that activates Ras pro teins, Its activity on Pas in cells is enhanced upon calcium influx, A ctivation follows calcium-induced binding of calmodulin to an IQ motif near the N-terminus of Ras-GRF, Ras-GRF also contains a Dbl homology (DH) domain C-terminal to the IQ motif, In many proteins, DH domains a ct as exchange factors for Rho-GTPase family members, However, we fail ed to detect exchange activity of this domain on well characterized Rh o family members, Instead, we found that mutations analogous to those that block exchange activity of Dbl prevented Ras-GRF activation by ca lcium/ calmodulin in vivo, All DH domains are followed immediately by a pleckstrin homology (PH) domain, We found that a mutation at a conse rved site within the PH domain following the DH domain also prevented Ras-GRF activation by calcium in vivo. These results suggest that in a ddition to playing a role as activators of Rho proteins, DH domains ca n also contribute to the coupling of cellular signals to Ras activatio n. (C) 1997 Federation of European Biochemical Societies.