ARHGEF6
Rho guanine nucleotide exchange factor 6 is a protein that, in humans, is encoded by the "ARHGEF6" gene. ARHGEF6 is commonly known as the p21-activated protein kinase exchange factor alpha (alpha-PIX or αPIX), because it was identified by binding to p21-activated kinase (PAK) and also contains a guanine nucleotide exchange factor domain. Domains and functions. αPIX is a multidomain protein that functions both as a signaling scaffold protein and as an enzyme. αPIX shares this domain structure and signaling function with the highly similar ARHGEF7/βPIX protein. αPIX contains a central DH/PH RhoGEF domain that  functions as a guanine nucleotide exchange factor (GEF) for small GTPases of the Rho family, and specifically Rac and Cdc42. Like other GEFs, αPIX can promote both release of GDP from an inactive small GTP-binding protein and binding of GTP to promote its activation. Signaling scaffolds bind to specific partners to promote efficient signal transduction by arranging sequential elements of a pathway near each other to facilitate interaction/information transfer, and also by holding these partner protein complexes in specific locations within the cell to promote local or regional signaling. In the case of αPIX, its SH3 domain binds  to partner proteins with appropriate polyproline motifs, and particularly to group I p21-activated kinases (PAKs) (PAK1, PAK2 and PAK3). PAK is bound to the αPIX SH3 domain in the inactive state, and activated Rac1 or Cdc42 binding to this PAK stimulates its protein kinase activity leading to downstream target protein phosphorylation; since αPIX can activate the “p21’’ small GTPases Rac1 or Cdc42 through its GEF activity, this αPIX/PAK/Rac complex exemplifies a scaffolding function. Structurally, αPIX assembles as a trimer through its carboxyl-terminal coiled-coil domain, and further interacts with dimers of GIT1 or GIT2 through a  nearby GIT-binding domain to form oligomeric GIT-PIX complexes. Through this GIT-PIX complex, the scaffolding function of αPIX is amplified by also being able to hold GIT partners in proximity to αPIX partners. αPIX contains an amino-terminal Calponin Homology (CH) domain whose functions remain relatively poorly defined, but interacts with parvin/affixin family proteins. Because the ARHGEF6 gene is located on the X chromosome so that males have only one copy, mutations in this gene in humans can cause X-chromosome-linked non-specific intellectual disability, as can mutations affecting its binding partner PAK3 whose gene is  also located on the X chromosome. In animal models, loss of ARHGEF6 gene function is associated with neuronal synapse defects, immune T-cell migration and maturation defects, and hearing loss. Interactions. αPIX has been reported to interact with over 40 proteins. Major interacting proteins include: