ARHGEF6 explained

Rho guanine nucleotide exchange factor 6 is a protein that, in humans, is encoded by the ARHGEF6 gene.^{[1] [2] [3]}

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.^[2]

Domains and functions

αPIX is a multidomain protein that functions both as a signaling scaffold protein and as an enzyme.^[4] 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.^[2] 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).^[2] 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.^[4] 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. ^{[5] [4]}

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, ^[6] as can mutations affecting its binding partner PAK3 whose gene is also located on the X chromosome.^[7] In animal models, loss of ARHGEF6 gene function is associated with neuronal synapse defects,^[8] immune T-cell migration and maturation defects,^[9] and hearing loss.^[10]

Interactions

αPIX has been reported to interact with over 40 proteins.^{[4] [11]}

Major interacting proteins include:

• Itself, or the highly-related ARHGEF7/βPIX via a trimeric coiled-coil interaction.
• GIT1 or GIT2 dimers via GIT-binding domain.
• p21-activated kinases (PAKs) 1, 2 and 3 via SH3 domain.
• c-Cbl via SH3 domain.
• Rho family GTP-binding protein family members Rac1 and Cdc42, activated via DHPH RhoGEF domain.

See also

• Rho family of GTPases
• Scaffold protein
• Guanine nucleotide exchange factor
• X-linked intellectual disability

Further reading

• Yntema HG, Hamel BC, Smits AP . Localisation of a gene for non-specific X linked mental retardation (MRX46) to Xq25-q26 . J. Med. Genet. . 35 . 10 . 801–5 . 1999 . 9783701 . 10.1136/jmg.35.10.801 . 1051453 . etal.
• Yoshii S, Tanaka M, Otsuki Y . alphaPIX nucleotide exchange factor is activated by interaction with phosphatidylinositol 3-kinase . Oncogene . 18 . 41 . 5680–90 . 1999 . 10523848 . 10.1038/sj.onc.1202936 . etal. free .
• Brown A, Wang X, Sawai E, Cheng-Mayer C . Activation of the PAK-related kinase by human immunodeficiency virus type 1 Nef in primary human peripheral blood lymphocytes and macrophages leads to phosphorylation of a PIX-p95 complex . J. Virol. . 73 . 12 . 9899–907 . 1999 . 10559302 . 10.1128/JVI.73.12.9899-9907.1999. 113039 .
• Premont RT, Claing A, Vitale N . The GIT family of ADP-ribosylation factor GTPase-activating proteins. Functional diversity of GIT2 through alternative splicing . J. Biol. Chem. . 275 . 29 . 22373–80 . 2000 . 10896954 . 10.1074/jbc.275.29.22373 . etal. free .
• Lower KM, Gecz J . 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 . 2001 . 11337747 . 10.1002/ajmg.1189 .
• Yoshii S, Tanaka M, Otsuki Y . Involvement of alpha-PAK-interacting exchange factor in the PAK1-c-Jun NH(2)-terminal kinase 1 activation and apoptosis induced by benzo[a]pyrene . Mol. Cell. Biol. . 21 . 20 . 6796–807 . 2001 . 11564864 . 10.1128/MCB.21.20.6796-6807.2001 . 99857 . etal.
• Vikis HG, Li W, Guan KL . The plexin-B1/Rac interaction inhibits PAK activation and enhances Sema4D ligand binding . Genes Dev. . 16 . 7 . 836–45 . 2002 . 11937491 . 10.1101/gad.966402 . 186329 .
• Flanders JA, Feng Q, Bagrodia S . The Cbl proteins are binding partners for the Cool/Pix family of p21-activated kinase-binding proteins . FEBS Lett. . 550 . 1–3 . 119–23 . 2003 . 12935897 . 10.1016/S0014-5793(03)00853-6 . 46540220 . etal.
• Brill LM, Salomon AR, Ficarro SB . 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 . 2004 . 15144186 . 10.1021/ac035352d . etal.
• Beausoleil SA, Jedrychowski M, Schwartz D . Large-scale characterization of HeLa cell nuclear phosphoproteins . Proc. Natl. Acad. Sci. U.S.A. . 101 . 33 . 12130–5 . 2004 . 15302935 . 10.1073/pnas.0404720101 . 514446 . 2004PNAS..10112130B . etal. free .
• Jin J, Smith FD, Stark C . Proteomic, functional, and domain-based analysis of in vivo 14-3-3 binding proteins involved in cytoskeletal regulation and cellular organization . Curr. Biol. . 14 . 16 . 1436–50 . 2004 . 15324660 . 10.1016/j.cub.2004.07.051 . 2371325 . etal. free .

Notes and References

1. Nomura N, Nagase T, Miyajima N, Sazuka T, Tanaka A, Sato S, Seki N, Kawarabayasi Y, Ishikawa K, Tabata S . Prediction of the coding sequences of unidentified human genes. II. The coding sequences of 40 new genes (KIAA0041-KIAA0080) deduced by analysis of cDNA clones from human cell line KG-1 (supplement) . DNA Res . 1 . 5 . 251–62 . Dec 1995 . 7584048 . 10.1093/dnares/1.5.251 . free .
2. Manser E, Loo TH, Koh CG, Zhao ZS, Chen XQ, Tan L, Tan I, Leung T, Lim L . PAK kinases are directly coupled to the PIX family of nucleotide exchange factors . Mol Cell . 1 . 2 . 183–92 . Jul 1998 . 9659915 . 10.1016/S1097-2765(00)80019-2 . free .
3. Web site: Entrez Gene: ARHGEF6 Rac/Cdc42 guanine nucleotide exchange factor (GEF) 6.
4. Zhou W, Li X, Premont RT . Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT-PIX complexes . Journal of Cell Science . 129 . 10 . 1963–1974 . May 2016 . 27182061 . 6518221 . 10.1242/jcs.179465 .
5. Rosenberger G, Jantke I, Gal A, Kutsche K . Interaction of alphaPIX (ARHGEF6) with beta-parvin (PARVB) suggests an involvement of alphaPIX in integrin-mediated signaling . Human Molecular Genetics . 12 . 2 . 155–167 . 2003 . 12499396 . 10.1093/hmg/ddg019 . free .
6. 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 . 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 . 2000 . 11017088 . 10.1038/80002 . 12325765 .
7. Allen KM, Gleeson JG, Bagrodia S, Partington MW, MacMillan JC, Cerione RA, Mulley JC, Walsh CA . PAK3 mutation in nonsyndromic X-linked mental retardation . Nature Genetics . 20. 1 . 25–30 . September 1998 . 9731525 . 10.1038/1675 . 16041444 .
8. Ramakers GJ, Wolfer D, Rosenberger G, Kuchenbecker K, Kreienkamp HJ, Prange-Kiel J, Rune G, Richter K, Langnaese K, Masneuf S, Bösl MR, Fischer KD, Krugers HJ, Lipp HP, van Galen E, Kutsche K . Dysregulation of Rho GTPases in the αPix/Arhgef6 mouse model of X-linked intellectual disability is paralleled by impaired structural and synaptic plasticity and cognitive deficits . Human Molecular Genetics . 21. 2 . 268–286 . January 2012 . 21989057 . 10.1093/hmg/ddr457 . 20.500.11850/41564 . free .
9. Korthals M, Schilling K, Reichardt P, Mamula D, Schlüter T, Steiner M, Langnäse K, Thomas U, Gundelfinger E, Premont RT, Tedford K, Fischer KD . αPIX RhoGEF supports positive selection by restraining migration and promoting arrest of thymocytes . Journal of Immunology . 192. 7 . 3228–3238 . April 2014 . 24591366 . 10.4049/jimmunol.1302585 . free .
10. Zhu C, Cheng C, Wang Y, Muhammad W, Liu S, Zhu W, Shao B, Zhang Z, Yan X, He Q, Xu Z, Yu C, Qian X, Lu L, Zhang S, Zhang Y, Xiong W, Gao X, Xu Z, Chai R . Loss of ARHGEF6 Causes Hair Cell Stereocilia Deficits and Hearing Loss in Mice . Frontiers in Molecular Neuroscience . 11. 362 . October 2018 . 30333726 . 6176010 . 10.3389/fnmol.2018.00362 . free .
11. Web site: ARHGEF6 Result Summary .