# The RHO Family GTPases: Mechanisms of Regulation and Signaling
## 3 Regulation of RHO Family GTPases
### Guanine Nucleotide Exchange Factors (GEFs)
#### 3.2.1 DBL Family GEFs  
RHOGEFs of the diffuse B-cell lymphoma (DBL) family directly activate the proteins of the RHO family [82]. The prototype of this GEF family is the DBL protein, which was isolated as an oncogenic product from diffuse B-cell lymphoma cells in an oncogene screen [83] and was later reported to act on CDC42 [84]. Human DBL family proteins have recently been grouped into functionally distinct categories based on both their catalytic efficiencies and their sequence-structure relationship [47]. Members of the DBL family are characterized by a unique DBL homology (DH) domain [85; 86; 87; 88].  
The DH domain is a highly efficient catalytic machine [80] that is able to accelerate the nucleotide exchange of RHO proteins by as much as 10\({}^{7}\)-fold. The DH domain is often followed by a pleckstrin homology (PH) domain, indicating its essential and conserved function. A model for PH domain-assisted nucleotide exchange has been proposed for some GEFs such as DBL, DBS, and TRIO [80]. Thus, the PH domain serves multiple roles in signaling events by anchoring GEFs to the membrane (via phosphoinosititides) and directing them toward their respective GTPase partners, which are on the membrane [80].  
Through a search for DH domain-containing proteins in the human genome, 74 DBL proteins have been identified (Figure 3) [47]. Interestingly, nine of these DBL proteins lack the C-terminal tandem PH domain, and three of these proteins contain a membrane bending and tubulating BAR (BIN/amphiphysin/RVS) domain, and seven of 20 investigated DBL proteins do not exhibit any GEF activity (Figure 3) [47]. In addition to the DH-PH tandem motif, DBL family proteins are highly diverse and contain additional domains with different functions (Figure 3) including SH2, SH3, CH, RGS, PDZ, and/or IQ domains, which enable their interaction with other proteins; BAR, PH FYVE, C1, and C2 domains, which enable their interaction with membrane lipids; and other functional domains such as Ser/Thr kinase, RASGEF, RHOGAP, and RANGEF [82]. These additional domains have been implicated in autoregulation, subcellular localization, and connection to upstream signaling molecules [40; 49; 50]. Spatiotemporal regulation of DBL proteins has been suggested as a mechanism that specifically initiates the activation of substrate RHO proteins and controls a broad spectrum of normal and pathological cellular functions [89]. Thus, it is evident that members of the DBL protein family are attractive therapeutic targets for a variety of diseases [90; 91].  
Figure 3: Domain organization of DBL family proteins. The DBL family RHOGEFs are mostly multimodular proteins and have a number of functional domains that may mediate cross talk between RHO proteins and other signaling pathways. DH domains are almost always found with a PH domain in the C-terminus. Some DBL proteins contain two DH–PH cassettes, while some DBL proteins lack tandem PH domains. Functional domains, in addition to the catalytic DH domain (green), are probably involved in lipid and membrane binding (blue), protein interactions (bright green), and enzymatic activities (red and orange). A scale of amino acid numbers in increments of 200 is shown at the bottom. Underlined proteins do not exhibit activity under cell-free conditions [47].