Regulation and functions of the RhoA regulatory guanine nucleotide exchange factor GEF-H1

Abstract

Since the discovery by Madaule and Axel in 1985 of the first Ras homologue (Rho) protein in Aplysia and its human orthologue RhoB, membership in the Rho GTPase family has grown to 20 proteins, with representatives in all eukaryotic species. These GTPases are molecular switches that cycle between active (GTP bound) and inactivate (GDP bound) states. The exchange of GDP for GTP on Rho GTPases is facilitated by guanine exchange factors (GEFs). Approximately 80 Rho GEFs have been identified to date, and only a few GEFs associate with microtubules. The guanine nucleotide exchange factor H1, GEF-H1, is a unique GEF that associates with microtubules and is regulated by the polymerization state of microtubule networks. This review summarizes the regulation and functions of GEF-H1 and discusses the roles of GEF-H1 in human diseases.

Keywords: GEF-H1; Rho GTPase; RhoA; ARHGEF2; cancer; guanine nucleotide exchange factor.

Figure 1.

Effects of GEF-H1 activation. The activity of Rho GTPases is determined by bound guanine nucleotides. When GEF-H1 facilitates the exchange of GDP for GTP on RhoA, RhoA is activated and signals this changed activation state via its downstream effectors. Of the RhoA effectors, Rho-associated coiled-coil containing protein kinases 1 and 2 (ROCK1 and ROCK2) are amongst the most important as they regulate the organization of the actin cytoskeleton and influence cellular processes (e.g. cell migration) that are mediated by actin cytoskeleton dynamics. Abbreviations: MYPT1, myosin phosphatase target subunit 1; MLC, myosin light chain; LIMK, LIM kinase

Figure 2.

Regulation of GEF-H1. GEF-H1 has a C1 domain that contains a zinc-finger motif near the N terminus, and is followed by Pleckstrin homology (PH), Dbl-homology domain (DH), and coiled-coil domains. GEF-H1 can bind to microtubules directly via its N- and C-termini, as well as via its PH domain. Alternatively, the dynein light- chain Tctex-1 can mediate the association of GEF-H1 with microtubules by directly binding to dynein intermediate chain (DIC)-dynein heavy chain (DHC) complexes. GEF-H1 can be activated when microtubules depolymerize or when GEF-H1 dissociates from Tctex-1 as a result of GPCR activation. Upon ligand binding, Gα subunits directly bind to GEF-H1 and dissociate GEF-H1 from Tctex-1, while Gβγ subunits interact with Tctex-1 to disrupt its association with DIC. Adapted from [23,26]

Figure 3.

Phosphorylation sites on GEF-H1. A number of sites on GEF-H1 are phosphorylated by various kinases. Some of the more well-characterized phosphorylation sites are listed along with the kinases responsible for phosphorylating the sites. Abbreviations: AurA, Aurora kinase A; Cdk1, cyclin-dependent kinase 1; Erk1/2, extracellular-signal-regulated kinase 1/2; MARK 3, MAP/microtubule affinity-regulated kinase 3; Pak1, p21-activated kinase 1; PKA, protein kinase A. Image is not drawn to scale

Figure 4.

GEF-H1 amplification in cancers. A. Analysis of ARHGEF2 copy number variations in the TCGA PanCancer Atlas set of 32 studies [139] using cBioPortal [140] revealed 6 cancer types with > 5% frequency of gene amplification. B. For uterine corpus endometrial carcinoma, ARHGEF2 amplification in 40 out of 529 patients (7.6%) was associated with significantly reduced overall survival (p < 0.0001, Mantel-Cox test)