RhoG (Ras homology Growth-related) (or ARGH) is a small (~21 kDa) monomeric GTP-binding protein (G protein), and is an important component of many intracellular signalling pathways. It is a member of the Rac subfamily of the Rho family of small G proteins [5] and is encoded by the gene RHOG. [6]
RhoG was first identified as a coding sequence upregulated in hamster lung fibroblasts upon stimulation with serum. [7] Expression of RhoG in mammals is widespread and studies of its function have been carried out in fibroblasts, [8] leukocytes, [9] [10] neuronal cells, [11] endothelial cells [12] and HeLa cells. [13] RhoG belongs to the Rac subgroup and emerged as a consequence of retroposition in early vertebrates. [14] RhoG shares a subset of common binding partners with Rac, Cdc42 and RhoU/V members but a major specificity is its inability to bind to CRIB domain proteins such as PAKs. [8] [15]
Like most small G proteins RhoG is involved in a diverse set of cellular signalling mechanisms. In mammalian cells these include cell motility (through regulation of the actin cytoskeleton), [13] gene transcription, [10] [16] endocytosis, [17] neurite outgrowth, [11] protection from anoikis [18] and regulation of the neutrophil NADPH oxidase. [9]
As with all small G proteins RhoG is able to signal to downstream effectors when bound to GTP (Guanosine triphosphate) and unable to signal when bound to GDP (Guanosine diphosphate). Three classes of protein interact with RhoG to regulate GTP/GDP loading. The first are known as Guanine nucleotide exchange factors (GEFs) and these facilitate the exchange of GDP for GTP so as to promote subsequent RhoG-mediated signalling. The second class are known as GTPase activating proteins (GAPs) and these promote hydrolysis of GTP to GDP (via the intrinsic GTPase activity of the G protein) thus terminating RhoG-mediated signalling. A third group, known as Guanine nucleotide dissociation inhibitors (GDIs), inhibit dissociation of GDP and thus lock the G protein in its inactive state. GDIs can also sequester G proteins in the cytosol which also prevents their activation. The dynamic regulation of G protein signalling is necessarily complex and the 130 or more GEFs, GAPs and GDIs described thus far for the Rho family are considered to be the primary determinants of their spatiotemporal activity.
There are a number of GEFs reported to interact with RhoG, although in some cases the physiological significance of these interactions has yet to be proven. Well characterised examples include the dual specificity GEF TRIO which is able to promote nucleotide exchange on RhoG and Rac [19] (via its GEFD1 domain) and also on RhoA [20] via a separate GEF domain (GEFD2). Activation of RhoG by TRIO has been shown to promote NGF-induced neurite outgrowth in PC12 cells [21] and phagocytosis of apoptotic cells in C. elegans . [22] Another GEF, known as SGEF (Src homology 3 domain-containing Guanine nucleotide Exchange Factor), is thought to be RhoG-specific and has been reported to stimulate macropinocytosis (internalisation of extracellular fluid) in fibroblasts [23] and apical cup assembly in endothelial cells (an important stage in leukocyte trans-endothelial migration). [12] Other GEFs reported to interact with RhoG include Dbs, ECT2, VAV2 and VAV3. [15] [24] [25]
There have been very few interactions reported between RhoG and negative regulators of G protein function. Examples include IQGAP2 [15] and RhoGDI3. [26]
Activated G proteins are able to couple to multiple downstream effectors and can therefore control a number of distinct signalling pathways (a characteristic known as pleiotropy). The extent to which RhoG regulates these pathways is poorly understood thus far, however, one specific pathway downstream of RhoG has received much attention and is therefore well characterised. This pathway involves RhoG-dependent activation of Rac via the DOCK (dedicator of cytokinesis)-family of GEFs. [27] This family is divided into four subfamilies (A-D) and it is subfamilies A and B that are involved in the pathway described here. Dock180, the archetypal member of this family, is seen as an atypical GEF in that efficient GEF activity requires the presence of the DOCK-binding protein ELMO (engulfment and cellmotility) [28] which binds RhoG at its N-terminus. The proposed model for RhoG-dependent Rac activation involves recruitment of the ELMO/Dock180 complex to activated RhoG at the plasma membrane and this relocalisation, together with an ELMO-dependent conformational change in Dock180, is sufficient to promote GTP-loading of Rac. [29] [30] RhoG-mediated Rac signalling has been shown to promote neurite outgrowth [11] and cell migration [13] in mammalian cells as well as phagocytosis of apoptotic cells in C. elegans. [22]
Other proteins known to bind RhoG in its GTP-bound state include the microtubule-associated protein kinectin, [31] Phospholipase D1 and the MAP Kinase activator MLK3. [15]
GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a protein domain common to many GTPases.
Small GTPases, also known as small G-proteins, are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a guanosine triphosphate (GTP) to form guanosine diphosphate (GDP). The best-known members are the Ras GTPases and hence they are sometimes called Ras subfamily GTPases.
Guanine nucleotide exchange factors (GEFs) are proteins or protein domains that activate monomeric GTPases by stimulating the release of guanosine diphosphate (GDP) to allow binding of guanosine triphosphate (GTP). A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity. Some GEFs can activate multiple GTPases while others are specific to a single GTPase.
The Rho family of GTPases is a family of small signaling G proteins, and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic kingdoms, including yeasts and some plants. Three members of the family have been studied in detail: Cdc42, Rac1, and RhoA. All G proteins are "molecular switches", and Rho proteins play a role in organelle development, cytoskeletal dynamics, cell movement, and other common cellular functions.
Cell division control protein 42 homolog is a protein that in humans is encoded by the CDC42 gene. Cdc42 is involved in regulation of the cell cycle. It was originally identified in S. cerevisiae (yeast) as a mediator of cell division, and is now known to influence a variety of signaling events and cellular processes in a variety of organisms from yeast to mammals.
Rac1, also known as Ras-related C3 botulinum toxin substrate 1, is a protein found in human cells. It is encoded by the RAC1 gene. This gene can produce a variety of alternatively spliced versions of the Rac1 protein, which appear to carry out different functions.
Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the RHOA gene. While the effects of RhoA activity are not all well known, it is primarily associated with cytoskeleton regulation, mostly actin stress fibers formation and actomyosin contractility. It acts upon several effectors. Among them, ROCK1 and DIAPH1 are the best described. RhoA, and the other Rho GTPases, are part of a larger family of related proteins known as the Ras superfamily, a family of proteins involved in the regulation and timing of cell division. RhoA is one of the oldest Rho GTPases, with homologues present in the genomes since 1.5 billion years. As a consequence, RhoA is somehow involved in many cellular processes which emerged throughout evolution. RhoA specifically is regarded as a prominent regulatory factor in other functions such as the regulation of cytoskeletal dynamics, transcription, cell cycle progression and cell transformation.
Rho guanine nucleotide exchange factor TIAM1 is a protein that in humans is encoded by the TIAM1 gene.
RhoGEF domain describes two distinct structural domains with guanine nucleotide exchange factor (GEF) activity to regulate small GTPases in the Rho family. Rho small GTPases are inactive when bound to GDP but active when bound to GTP; RhoGEF domains in proteins are able to promote GDP release and GTP binding to activate specific Rho family members, including RhoA, Rac1 and Cdc42.
Dedicator of cytokinesis protein 1 (Dock1), also (DOCK180), is a large protein encoded in the human by the DOCK1 gene, involved in intracellular signalling networks. It is the mammalian ortholog of the C. elegans protein CED-5 and belongs to the DOCK family of guanine nucleotide exchange factors (GEFs).
Dedicator of cytokinesis protein 2 (Dock2) is a protein encoded in the human by the DOCK2 gene. Dock2 is a large protein involved in intracellular signalling networks. It is a member of the DOCK-A subfamily of the DOCK family of guanine nucleotide exchange factors (GEFs) which function as activators of small G-proteins. Dock2 specifically activates isoforms of the small G protein Rac.
Dedicator of cytokinesis protein (Dock7) is a large protein encoded in the human by the DOCK7 gene, involved in intracellular signalling networks. It is a member of the DOCK-C subfamily of the DOCK family of guanine nucleotide exchange factors (GEFs) which function as activators of small G-proteins. Dock7 activates isoforms of the small G protein Rac.
Rac is a subfamily of the Rho family of GTPases, small signaling G proteins. Just as other G proteins, Rac acts as a molecular switch, remaining inactive while bound to GDP and activated once GEFs remove GDP, permitting GTP to bind. When bound to GTP, Rac is activated. In its activated state, Rac participates in the regulation of cell movement, through its involvement in structural changes to the actin Cytoskeleton. By changing the cytoskeletal dynamics within the cell, Rac-GTPases are able to facilitate the recruitment of neutrophils to the infected tissues, and to regulate degranulation of azurophil and integrin-dependent phagocytosis.
Dedicator of cytokinesis protein 4 (Dock4), is a large protein encoded in the human by the DOCK4 gene, involved in intracellular signalling networks. It is a member of the DOCK-B subfamily of the DOCK family of guanine nucleotide exchange factors (GEFs) which function as activators of small G-proteins. Dock4 activates the small G proteins Rac and Rap1.
Dedicator of cytokinesis protein 3 (Dock3), also known as MOCA and PBP, is a large protein encoded in the human by the DOCK3 gene, involved in intracellular signalling networks. It is a member of the DOCK-B subfamily of the DOCK family of guanine nucleotide exchange factors (GEFs) which function as activators of small G-proteins. Dock3 specifically activates the small G protein Rac.
ELMO is a family of related proteins involved in intracellular signalling networks. These proteins have no intrinsic catalytic activity and instead function as adaptors which can regulate the activity of other proteins through their ability to mediate protein-protein interactions.
CED-12 is a cytoplasmic, PH-domain containing adaptor protein found in Caenorhabditis elegans and Drosophila melanogaster. CED-12 is a homolog to the ELMO protein found in mammals. This protein is involved in Rac-GTPase activation, apoptotic cell phagocytosis, cell migration, and cytoskeletal rearrangements.
Dedicator of cytokinesis protein 6 (Dock6), also known as Zir1 is a large protein encoded in the human by the DOCK6 gene, involved in intracellular signalling networks. It is a member of the DOCK-C subfamily of the DOCK family of guanine nucleotide exchange factors which function as activators of small G-proteins.
Alan Hall FRS was a British cell biologist and a biology professor at the Sloan-Kettering Institute, where he was chair of the Cell Biology program. Hall was elected a Fellow of the Royal Society in 1999.
Pleckstrin homology domain containing, family G member 2 (PLEKHG2) is a protein that in humans is encoded by the PLEKHG2 gene. It is sometimes written as ARHGEF42, FLJ00018.
