αPIX is a multidomain protein that functions both as a signalingscaffold protein and as an enzyme.[8] 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.[6] 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).[6] 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.[8] 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. [9][8]
^Kutsche K, Yntema H, Brandt A, Jantke I, Nothwang HG, Orth U, Boavida MG, David D, Chelly J, Fryns JP, Moraine C, Ropers HH, Hamel BC, van Bokhoven H, Gal A (2000). "Mutations in ARHGEF6, encoding a guanine nucleotide exchange factor for Rho GTPases, in patients with X-linked mental retardation". Nature Genetics. 26 (2): 247–250. doi:10.1038/80002. PMID11017088. S2CID12325765.
Lower KM, Gecz J (2001). "Characterization of ARHGEF6, a guanine nucleotide exchange factor for Rho GTPases and a candidate gene for X-linked mental retardation: mutation screening in Börjeson-Forssman-Lehmann syndrome and MRX27". Am. J. Med. Genet. 100 (1): 43–8. doi:10.1002/ajmg.1189. PMID11337747.
Flanders JA, Feng Q, Bagrodia S, et al. (2003). "The Cbl proteins are binding partners for the Cool/Pix family of p21-activated kinase-binding proteins". FEBS Lett. 550 (1–3): 119–23. doi:10.1016/S0014-5793(03)00853-6. PMID12935897. S2CID46540220.
Brill LM, Salomon AR, Ficarro SB, et al. (2004). "Robust phosphoproteomic profiling of tyrosine phosphorylation sites from human T cells using immobilized metal affinity chromatography and tandem mass spectrometry". Anal. Chem. 76 (10): 2763–72. doi:10.1021/ac035352d. PMID15144186.
