THE FACIOGENITAL DYSPLASIA GENE-PRODUCT FGD1 FUNCTIONS AS A CDC42HS-SPECIFIC GUANINE-NUCLEOTIDE EXCHANGE FACTOR

The Rho family of small GTP-binding proteins plays important roles in the regulation of actin cytoskeleton organization and cell growth. Act ivation of these GTPases involves the replacement of bound GDP with GT P, a process catalyzed by the Dbl-like guanine-nucleotide exchange fac tors, all of which seem to share a putative catalytic motif termed the Dbl homology (DH) domain, followed by a pleckstrin homology (PH) doma in, Here we have examined the role of a Dbl-like molecule, the facioge nital dysplasia gene product (FGD1), which when mutated in its Dbl hom ology domain, cosegregates with the developmental disease Aarskog-Scot t syndrome. We report that a polypeptide of FGD1 encompassing the DH a nd PH domains can bind specifically to the Rho family GTPase Cdc42Hs a nd stimulates the GDP-GTP exchange of the isoprenylated form of Cdc42H s. Microinjection of this FGD1 polypeptide into Swiss 3T3 fibroblast c ells induces the formation of peripheral actin microspikes, similar to that previously observed when cells were injected with a constitutive ly active form of Cdc42Hs. This effect of FGD1 on actin organization i s readily inhibited by coinjection of a dominant-negative mutant of Cd c42Hs. Examination of NIH 3T3 cells expressing the FGD1 fragment revea led that similar to cells expressing Dbl, two independent elements dow nstream of Cdc42Hs, the Jun NH2-terminal kinase and the p70 S6 kinase, became activated. Hence, our results indicate that FGD1, through its DH and PH domains, acts as a Cdc42Hs-specific guanine-nucleotide excha nge factor and suggest that the Cdc42Hs GTPase may have a role in mamm alian development.