Defining Diverse Mechanisms that Regulate the Activity of Dbl-family Guanine Nucleotide Exchange Factors

Rho-family GTPases participate in a diverse range of critical cellular processes such as cytoskeletal rearrangement, gene transcription, and cell-fate determination; consequently, aberrant regulation of Rho-family GTPases has been implicated in diseases such as cancer and many developmental disorders. Dbl-family guanine nucleotide exchange factors (GEFs) activate Rho-family GTPases by promoting exchange of bound GDP for GTP. There have been 69 Dbl-family GEFs identified to date in the human genome, and while they vary significantly in size, domain architecture, and GTPase specificity, they all contain a Dbl-homology (DH) domain followed almost invariably by a tandem pleckstrin-homology (PH) domain. The DH domain is the core catalytic unit necessary for nucleotide exchange while the PH domain has been shown to be involved in lipid binding. The work presented examines two mechanisms that regulate the exchange activity of Dbl-family GEFs by ultimately manipulating the ability of the DH domain to engage its cognate GTPases. We show that the N-terminal DH/PH cassette of the GEF Trio requires direct contact between the DH-associated PH domain and its cognate GTPases Rac1 and RhoG for efficient exchange, adding to a growing number of GEFs that require their PH domain for full exchange activity. In addition, we also investigate mechanisms through which Dbl-family GEFs may act as effectors of active small GTPases. These studies add to our knowledge of the components that regulate the activity of Dbl-family GEFs