Crystal structure of Rnd3/RhoE: functional implications

The Role of Fast-Cycling Atypical RHO GTPases in Cancer

Simple Summary

For many years, cancer-associated mutations in RHO GTPases were not identified and observations suggesting roles for RHO GTPases in cancer were sparse. Instead, RHO GTPases were considered primarily to regulate cell morphology and cell migration, processes that rely on the dynamic behavior of the cytoskeleton. This notion is in contrast to the RAS proteins, which are famous oncogenes and found to be mutated at high incidence in human cancers. Recent advancements in the tools for large-scale genome analysis have resulted in a paradigm shift and RHO GTPases are today found altered in many cancer types. This review article deals with the recent views on the roles of RHO GTPases in cancer, with a focus on the so-called fast-cycling RHO GTPases.

Abstract

The RHO GTPases comprise a subfamily within the RAS superfamily of small GTP-hydrolyzing enzymes and have primarily been ascribed roles in regulation of cytoskeletal dynamics in eukaryotic cells. An oncogenic role for the RHO GTPases has been disregarded, as no activating point mutations were found for genes encoding RHO GTPases. Instead, dysregulated expression of RHO GTPases and their regulators have been identified in cancer, often in the context of increased tumor cell migration and invasion. In the new landscape of cancer genomics, activating point mutations in members of the RHO GTPases have been identified, in particular in RAC1, RHOA, and CDC42, which has suggested that RHO GTPases can indeed serve as oncogenes in certain cancer types. This review describes the current knowledge of these cancer-associated mutant RHO GTPases, with a focus on how their altered kinetics can contribute to cancer progression.

Regulation of GTPase function by autophosphorylation

A unifying feature of the RAS superfamily is a conserved GTPase cycle by which these proteins transition between active and inactive states. We demonstrate that autophosphorylation of some GTPases is an intrinsic regulatory mechanism that reduces nucleotide hydrolysis and enhances nucleotide exchange, altering the on/off switch that forms the basis for their signaling functions. Using X-ray crystallography, nuclear magnetic resonance spectroscopy, binding assays, and molecular dynamics on autophosphorylated mutants of H-RAS and K-RAS, we show that phosphoryl transfer from GTP requires dynamic movement of the switch II region and that autophosphorylation promotes nucleotide exchange by opening the active site and extracting the stabilizing Mg2+. Finally, we demonstrate that autophosphorylated K-RAS exhibits altered effector interactions, including a reduced affinity for RAF proteins in mammalian cells. Thus, autophosphorylation leads to altered active site dynamics and effector interaction properties, creating a pool of GTPases that are functionally distinct from their non-phosphorylated counterparts.

Selectivity Determinants of RHO GTPase Binding to IQGAPs

IQ motif-containing GTPase-activating proteins (IQGAPs) modulate a wide range of cellular processes by acting as scaffolds and driving protein components into distinct signaling networks. Their functional states have been proposed to be controlled by members of the RHO family of GTPases, among other regulators. In this study, we show that IQGAP1 and IQGAP2 can associate with CDC42 and RAC1-like proteins but not with RIF, RHOD, or RHO-like proteins, including RHOA. This seems to be based on the distribution of charged surface residues, which varies significantly among RHO GTPases despite their high sequence homology. Although effector proteins bind first to the highly flexible switch regions of RHO GTPases, additional contacts outside are required for effector activation. Sequence alignment and structural, mutational, and competitive biochemical analyses revealed that RHO GTPases possess paralog-specific residues outside the two highly conserved switch regions that essentially determine the selectivity of RHO GTPase binding to IQGAPs. Amino acid substitution of these specific residues in RHOA to the corresponding residues in RAC1 resulted in RHOA association with IQGAP1. Thus, electrostatics most likely plays a decisive role in these interactions.

The RHO Family GTPases: Mechanisms of Regulation and Signaling

Much progress has been made toward deciphering RHO GTPase functions, and many studies have convincingly demonstrated that altered signal transduction through RHO GTPases is a recurring theme in the progression of human malignancies. It seems that 20 canonical RHO GTPases are likely regulated by three GDIs, 85 GEFs, and 66 GAPs, and eventually interact with >70 downstream effectors. A recurring theme is the challenge in understanding the molecular determinants of the specificity of these four classes of interacting proteins that, irrespective of their functions, bind to common sites on the surface of RHO GTPases. Identified and structurally verified hotspots as functional determinants specific to RHO GTPase regulation by GDIs, GEFs, and GAPs as well as signaling through effectors are presented, and challenges and future perspectives are discussed.

Identification and characterization of a new isoform of small GTPase RhoE

The Rho family of GTPases consists of 20 members including RhoE. Here, we discover the existence of a short isoform of RhoE designated as RhoEα, the first Rho GTPase isoform generated from alternative translation. Translation of this new isoform is initiated from an alternative start site downstream of and in-frame with the coding region of the canonical RhoE. RhoEα exhibits a similar subcellular distribution while its protein stability is higher than RhoE. RhoEα contains binding capability to RhoE effectors ROCK1, p190RhoGAP and Syx. The distinct transcriptomes of cells with the expression of RhoE and RhoEα, respectively, are demonstrated. The data propose distinctive and overlapping biological functions of RhoEα compared to RhoE. In conclusion, this study reveals a new Rho GTPase isoform generated from alternative translation. The discovery provides a new scope of understanding the versatile functions of small GTPases and underlines the complexity and diverse roles of small GTPases.

Dai et al. report the identification and characterization of a new isoform of RhoE (RhoEα), a member of the Rho GTPase family, which is generated from the same gene by alternative translation initiation at the downstream ATG codon 46. Compared to RhoE, RhoEα does not differ in the subcellular localization but has increased protein stability and distinct molecular signalling profile.

The pseudoGTPase group of pseudoenzymes

Pseudoenzymes are emerging as significant mediators and regulators of signal transduction. These proteins maintain enzyme folds and topologies, but are disrupted in the conserved motifs required for enzymatic activity. Among the pseudoenzymes, the pseudoGTPase group of atypical GTPases has recently expanded and includes the Rnd and RGK groups, RhoH and the RhoBTB proteins, mitochondrial RhoGTPase and centaurin-γ groups, CENP-M, dynein LIC, Entamoeba histolytica RabX3, leucine-rich repeat kinase 2, and the p190RhoGAP proteins. The wide range of cellular functions associated with pseudoGTPases includes cell migration and adhesion, membrane trafficking and cargo transport, mitosis, mitochondrial activity, transcriptional control, and autophagy, placing the group in an expanding portfolio of signaling pathways. In this review, we examine how the pseudoGTPases differ from canonical GTPases and consider their mechanistic and functional roles in signal transduction. We review the amino acid differences between the pseudoGTPases and discuss how these proteins can be classified based on their ability to bind nucleotide and their enzymatic activity. We discuss the molecular and structural consequences of amino acid divergence from canonical GTPases and use comparison with the well-studied pseudokinases to illustrate the classifications. PseudoGTPases are fast becoming recognized as important mechanistic components in a range of cellular roles, and we provide a concise discussion of the currently identified members of this group. ENZYMES: small GTPases; EC number: EC 3.6.5.2.

The Rho GTPase RND3 regulates adipocyte lipolysis

BACKGROUND

Adipose tissue plays a crucial role in diet- and obesity-related insulin resistance, with implications for several metabolic diseases. Identification of novel target genes and mechanisms that regulate adipocyte function could lead to improved treatment strategies. RND3 (RhoE/Rho8), a Rho-related GTP-binding protein that inhibits Rho kinase (ROCK) signaling, has been linked to diverse diseases such as apoptotic cardiomyopathy, heart failure, cancer and type 2 diabetes, in part by regulating cytoskeleton dynamics and insulin-mediated glucose uptake.

RESULTS

We here investigated the expression of RND3 in adipose tissue in human obesity, and discovered a role for RND3 in regulating adipocyte metabolism. In cross-sectional and prospective studies, we observed 5-fold increased adipocyte levels of RND3 mRNA in obesity, reduced levels after surgery-induced weight loss, and positive correlations of RND3 mRNA with adipocyte size and surrogate measures of insulin resistance (HOMA2-IR and circulating triglyceride/high-density lipoprotein cholesterol (TAG/HDL-C) ratio). By screening for RND3-dependent gene expression following siRNA-mediated RND3 knockdown in differentiating human adipocytes, we found downregulation of inflammatory genes and upregulation of genes related to adipocyte ipolysis and insulin signaling. Treatment of adipocytes with tumor necrosis factor alpha (TNFα), lipopolysaccharide (LPS), hypoxia or cAMP analogs increased RND3 mRNA levels 1.5-2-fold. Functional assays in primary human adipocytes confirmed that RND3 knockdown reduces cAMP- and isoproterenol-induced lipolysis, which were mimicked by treating cells with ROCK inhibitor. This effect could partly be explained by reduced protein expression of adipose triglyceride lipase (ATGL) and phosphorylated hormone-sensitive lipase (HSL).

CONCLUSION

We here uncovered a novel differential expression of adipose RND3 in obesity and insulin resistance, which may at least partly depend on a causal effect of RND3 on adipocyte lipolysis.

Activated Rho GTPases in Cancer-The Beginning of a New Paradigm

Involvement of Rho GTPases in cancer has been a matter of debate since the identification of the first members of this branch of the Ras superfamily of small GTPases. The Rho GTPases were ascribed important roles in the cell, although these were restricted to regulation of cytoskeletal dynamics, cell morphogenesis, and cell locomotion, with initially no clear indications of direct involvement in cancer progression. This paradigm has been challenged by numerous observations that Rho-regulated pathways are often dysregulated in cancers. More recently, identification of point mutants in the Rho GTPases Rac1, RhoA, and Cdc42 in human tumors has finally given rise to a new paradigm, and we can now state with confidence that Rho GTPases serve as oncogenes in several human cancers. This article provides an exposé of current knowledge of the roles of activated Rho GTPases in cancers.

IQGAP1 Interaction with RHO Family Proteins Revisited: KINETIC AND EQUILIBRIUM EVIDENCE FOR MULTIPLE DISTINCT BINDING SITES

IQ motif-containing GTPase activating protein 1 (IQGAP1) plays a central role in the physical assembly of relevant signaling networks that are responsible for various cellular processes, including cell adhesion, polarity, and transmigration. The RHO family proteins CDC42 and RAC1 have been shown to mainly interact with the GAP-related domain (GRD) of IQGAP1. However, the role of its RASGAP C-terminal (RGCT) and C-terminal domains in the interactions with RHO proteins has remained obscure. Here, we demonstrate that IQGAP1 interactions with RHO proteins underlie a multiple-step binding mechanism: (i) a high affinity, GTP-dependent binding of RGCT to the switch regions of CDC42 or RAC1 and (ii) a very low affinity binding of GRD and a C terminus adjacent to the switch regions. These data were confirmed by phosphomimetic mutation of serine 1443 to glutamate within RGCT, which led to a significant reduction of IQGAP1 affinity for CDC42 and RAC1, clearly disclosing the critical role of RGCT for these interactions. Unlike CDC42, an extremely low affinity was determined for the RAC1-GRD interaction, suggesting that the molecular nature of IQGAP1 interaction with CDC42 partially differs from that of RAC1. Our study provides new insights into the interaction characteristics of IQGAP1 with RHO family proteins and highlights the complementary importance of kinetic and equilibrium analyses. We propose that the ability of IQGAP1 to interact with RHO proteins is based on a multiple-step binding process, which is a prerequisite for the dynamic functions of IQGAP1 as a scaffolding protein and a critical mechanism in temporal regulation and integration of IQGAP1-mediated cellular responses.

Pathophysiological Functions of Rnd3/RhoE

Rnd3, also known as RhoE, belongs to the Rnd subclass of the Rho family of small guanosine triphosphate (GTP)-binding proteins. Rnd proteins are unique due to their inability to switch from a GTP-bound to GDP-bound conformation. Even though studies of the biological function of Rnd3 are far from being concluded, information is available regarding its expression pattern, cellular localization, and its activity, which can be altered depending on the conditions. The compiled data from these studies implies that Rnd3 may not be a traditional small GTPase. The basic role of Rnd3 is to report as an endogenous antagonist of RhoA signaling-mediated actin cytoskeleton dynamics, which specifically contributes to cell migration and neuron polarity. In addition, Rnd3 also plays a critical role in arresting cell cycle distribution, inhibiting cell growth, and inducing apoptosis and differentiation. Increasing data have shown that aberrant Rnd3 expression may be the leading cause of some systemic diseases; particularly highlighted in apoptotic cardiomyopathy, developmental arrhythmogenesis and heart failure, hydrocephalus, as well as tumor metastasis and chemotherapy resistance. Therefore, a better understanding of the function of Rnd3 under different physiological and pathological conditions, through the use of suitable models, would provide a novel insight into the origin and treatment of multiple human diseases.

Crystal Structure Analysis of Wild Type and Fast Hydrolyzing Mutant of EhRabX3, a Tandem Ras Superfamily GTPase from Entamoeba histolytica

The enteric protozoan parasite, Entamoeba histolytica, is the causative agent of amoebic dysentery, liver abscess and colitis in human. Vesicular trafficking plays a key role in the survival and virulence of the protozoan and is regulated by various Rab GTPases. EhRabX3 is a catalytically inefficient amoebic Rab protein, which is unique among the eukaryotic Ras superfamily by virtue of its tandem domain organization. Here, we report the crystal structures of GDP-bound fast hydrolyzing mutant (V71A/K73Q) and GTP-bound wild type EhRabX3 at 3.1 and 2.8Å resolutions, respectively. Though both G-domains possess "phosphate binding loop containing nucleoside triphosphate hydrolases fold", only the N-terminal domain binds to guanine nucleotide. The relative orientation of the N-terminal domain and C-terminal domain is stabilized by numerous inter-domain interactions. Compared to other Ras superfamily members, both the GTPase domains displayed large deviation in switch II perhaps due to non-conservative substitutions in this region. As a result, entire switch II is restructured and moved away from the nucleotide binding pocket, providing a rationale for the diminished GTPase activity of EhRabX3. The N-terminal GTPase domain possesses unusually large number of cysteine residues. X-ray crystal structure of the fast hydrolyzing mutant of EhRabX3 revealed that C39 and C163 formed an intra-molecular disulfide bond. Subsequent mutational and biochemical studies suggest that C39 and C163 are critical for maintaining the structural integrity and function of EhRabX3. Structure-guided functional investigation of cysteine mutants could provide the physiological implications of the disulfide bond and could allow us to design potential inhibitors for the better treatment of intestinal amebiasis.

Genetic deletion of Rnd3/RhoE results in mouse heart calcium leakage through upregulation of protein kinase A signaling

RATIONALE

Rnd3, a small Rho GTPase, is involved in the regulation of cell actin cytoskeleton dynamics, cell migration, and proliferation. The biological function of Rnd3 in the heart remains unexplored.

OBJECTIVE

To define the functional role of the Rnd3 gene in the animal heart and investigate the associated molecular mechanism.

METHODS AND RESULTS

By loss-of-function approaches, we discovered that Rnd3 is involved in calcium regulation in cardiomyocytes. Rnd3-null mice died at the embryonic stage with fetal arrhythmias. The deletion of Rnd3 resulted in severe Ca(2+) leakage through destabilized ryanodine receptor type 2 Ca(2+) release channels. We further found that downregulation of Rnd3 attenuated β2-adrenergic receptor lysosomal targeting and ubiquitination, which in turn resulted in the elevation of β2-adrenergic receptor protein levels leading to the hyperactivation of protein kinase A (PKA) signaling. The PKA activation destabilized ryanodine receptor type 2 channels. This irregular spontaneous Ca(2+) release can be curtailed by PKA inhibitor treatment. Increases in the PKA activity along with elevated cAMP levels were detected in Rnd3-null embryos, in neonatal rat cardiomyocytes, and noncardiac cell lines with Rnd3 knockdown, suggesting a general mechanism for Rnd3-mediated PKA signaling activation. β2-Adrenergic receptor blocker treatment reduced arrhythmia and improved cardiac function.

CONCLUSIONS

Rnd3 is a novel factor involved in intracellular Ca(2+) homeostasis regulation in the heart. Deficiency of the protein induces ryanodine receptor type 2 dysfunction by a mechanism that attenuates Rnd3-mediated β2-adrenergic receptor ubiquitination, which leads to the activation of PKA signaling. Increased PKA signaling in turn promotes ryanodine receptor type 2 hyperphosphorylation, which contributes to arrhythmogenesis and heart failure.

Rnd3 haploinsufficient mice are predisposed to hemodynamic stress and develop apoptotic cardiomyopathy with heart failure

Rho family guanosine triphosphatase (GTPase) 3 (Rnd3), a member of the small Rho GTPase family, has been suggested to regulate cell actin cytoskeleton dynamics, cell migration, and apoptosis through the Rho kinase-dependent signaling pathway. The biological function of Rnd3 in the heart is unknown. The downregulation of small GTPase Rnd3 transcripts was found in patients with end-stage heart failure. The pathological significance of Rnd3 loss in the transition to heart failure remains unexplored. To investigate the functional consequence of Rnd3 downregulation and the associated molecular mechanism, we generated Rnd3^+/− haploinsufficient mice to mimic the downregulation of Rnd3 observed in the failing human heart. Rnd3^+/− mice were viable; however, the mice developed heart failure after pressure overload by transverse aortic constriction (TAC). Remarkable apoptosis, increased caspase-3 activity, and elevated Rho kinase activity were detected in the Rnd3^+/− haploinsufficient animal hearts. Pharmacological inhibition of Rho kinase by fasudil treatment partially improved Rnd3^+/− mouse cardiac functions and attenuated myocardial apoptosis. To determine if Rho-associated coiled-coil kinase 1 (ROCK1) was responsible for Rnd3 deficiency-mediated apoptotic cardiomyopathy, we established a double-knockout mouse line, the Rnd3 haploinsufficient mice with ROCK1-null background (Rnd3^{+/−/ROCK1−/−}). Again, genetic deletion of ROCK1 partially but not completely rescued Rnd3 deficiency-mediated heart failure phenotype. These data suggest that downregulation of Rnd3 correlates with cardiac loss of function as in heart failure patients. Animals with Rnd3 haploinsufficiency are predisposed to hemodynamic stress. Hyperactivation of Rho kinase activity is responsible in part for the apoptotic cardiomyopathy development. Further investigation of ROCK1-independent mechanisms in Rnd3-mediated cardiac remodeling should be the focus for future study.

Insights into the GTP/GDP cycle of RabX3, a novel GTPase from Entamoeba histolytica with tandem G-domains

Members of the small GTPase Ras superfamily regulate a host of systems through their ability to catalyze the GTP/GDP cycle. All family members reported thus far possess a single GTPase domain with a P-loop containing a nucleoside triphosphate hydrolase fold. Here for the first time we report a novel member from Entamoeba histolytica, EhRabX3, which harbors two GTPase domains in tandem and exhibits unique biochemical properties. A combination of biochemical and microcalorimetric studies revealed that EhRabX3 binds to a single guanine nucleotide through its N-terminal domain. Unlike most of the members of the Ras superfamily, the dissociation of the nucleotide from EhRabX3 is independent of Mg(2+), perhaps indicating a novel mechanism of nucleotide exchange by this protein. We found that EhRabX3 is extremely sluggish in hydrolyzing GTP, and that could be attributed to its atypical nucleotide binding pocket. It harbors substitutions at two positions that confer oncogenicity to Ras because of impaired GTP hydrolysis. Engineering these residues into the conserved counterparts enhanced their GTPase activity by at least 20-fold. In contrast to most of the members of the Ras superfamily, EhRabX3 lacks the prenylation motif. Using indirect immunofluorescence and biochemical fractionation, we demonstrated that the protein is distributed all over the cytosol in amoebic trophozoites. Collectively, this unique ancient GTPase exhibits a striking evolutionary divergence from the other members of the superfamily.

Deciphering the molecular and functional basis of Dbl family proteins: a novel systematic approach toward classification of selective activation of the Rho family proteins

The diffuse B-cell lymphoma (Dbl) family of the guanine nucleotide exchange factors is a direct activator of the Rho family proteins. The Rho family proteins are involved in almost every cellular process that ranges from fundamental (e.g. the establishment of cell polarity) to highly specialized processes (e.g. the contraction of vascular smooth muscle cells). Abnormal activation of the Rho proteins is known to play a crucial role in cancer, infectious and cognitive disorders, and cardiovascular diseases. However, the existence of 74 Dbl proteins and 25 Rho-related proteins in humans, which are largely uncharacterized, has led to increasing complexity in identifying specific upstream pathways. Thus, we comprehensively investigated sequence-structure-function-property relationships of 21 representatives of the Dbl protein family regarding their specificities and activities toward 12 Rho family proteins. The meta-analysis approach provides an unprecedented opportunity to broadly profile functional properties of Dbl family proteins, including catalytic efficiency, substrate selectivity, and signaling specificity. Our analysis has provided novel insights into the following: (i) understanding of the relative differences of various Rho protein members in nucleotide exchange; (ii) comparing and defining individual and overall guanine nucleotide exchange factor activities of a large representative set of the Dbl proteins toward 12 Rho proteins; (iii) grouping the Dbl family into functionally distinct categories based on both their catalytic efficiencies and their sequence-structural relationships; (iv) identifying conserved amino acids as fingerprints of the Dbl and Rho protein interaction; and (v) defining amino acid sequences conserved within, but not between, Dbl subfamilies. Therefore, the characteristics of such specificity-determining residues identified the regions or clusters conserved within the Dbl subfamilies.

The GTPase-deficient Rnd proteins are stabilized by their effectors

Rnd proteins are Rho family GTP-binding proteins with cellular functions that antagonize RhoA signaling. We recently described a new Rnd3 effector Syx, also named PLEKHG5, that interacts with Rnds via a Raf1-like "Ras-binding domain." Syx is a multidomain RhoGEF that participates in early zebrafish development. Here we demonstrated that Rnd1, Rnd2, and Rnd3 stability is acutely dependent on interaction with their effectors such as Syx or p190 RhoGAP. Although Rnd3 turnover is blocked by treatment of cells with MG132, we provide evidence that such turnover is mediated indirectly by effects on the Rnd3 effectors, rather than on Rnd3 itself, which is not significantly ubiquitinated. The minimal regions of Syx and p190 RhoGAP that bind Rnd3 are not sequence-related but have similar effects. We have identified features that allow for Rnd3 turnover including a conserved Lys-45 close to the switch I region and the C-terminal membrane-binding domain of Rnd3, which cannot be substituted by the equivalent Cdc42 CAAX sequence. By contrast, an effector binding-defective mutant of Rnd3 when overexpressed undergoes turnover at normal rates. Interestingly the activity of the RhoA-regulated kinase ROCK stimulates Rnd3 turnover. This study suggests that Rnd proteins are regulated through feedback mechanisms in cells where the level of effectors and RhoA activity influence the stability of Rnd proteins. This effector feedback behavior is analogous to the ability of ACK1 and PAK1 to prolong the lifetime of the active GTP-bound state of Cdc42 and Rac1.

Mechanism of multi-site phosphorylation from a ROCK-I:RhoE complex structure

The ROCK-I serine/threonine protein kinase mediates the effects of RhoA to promote the formation of actin stress fibres and integrin-based focal adhesions. ROCK-I phosphorylates the unconventional G-protein RhoE on multiple N- and C-terminal sites. These phosphorylation events stabilise RhoE, which functions to antagonise RhoA-induced stress fibre assembly. Here, we provide a molecular explanation for multi-site phosphorylation of RhoE from the crystal structure of RhoE in complex with the ROCK-I kinase domain. RhoE interacts with the C-lobe alphaG helix of ROCK-I by means of a novel binding site remote from its effector region, positioning its N and C termini proximal to the ROCK-I catalytic site. Disruption of the ROCK-I:RhoE interface abolishes RhoE phosphorylation, but has no effect on the ability of RhoE to disassemble stress fibres. In contrast, mutation of the RhoE effector region attenuates RhoE-mediated disruption of the actin cytoskeleton, indicating that RhoE exerts its inhibitory effects on ROCK-I through protein(s) binding to its effector region. We propose that ROCK-I phosphorylation of RhoE forms part of a feedback loop to regulate RhoA signalling.

Function and regulation of Rnd proteins

The Rnd proteins, which form a distinct sub-group of the Rho family of small GTP-binding proteins, have been shown to regulate the organization of the actin cytoskeleton in several tissues. In the brain, they participate in neurite extension, whereas in smooth muscle, they modulate contractility. Recent evidence has shown that Rnd3 (RhoE) is also involved in the regulation of cell-cycle progression and transformation, indicating that these proteins might have other, as yet unexplored roles.

Function and regulation of RhoE

The three Rnd proteins, Rnd1, Rnd2 and RhoE/Rnd3, are a subset of Rho family proteins that are unusual in that they bind but do not hydrolyse GTP, and are therefore not regulated by the classical GTP/GDP conformational switch of small GTPases. Increased expression of each Rnd protein induces loss of stress fibres in cultured fibroblasts and epithelial cells, acting antagonistically to RhoA, which stimulates stress fibre formation. RhoE is farnesylated and localizes partly on membranes, including the Golgi and plasma membrane, and in the cytosol. RhoE inhibits RhoA signalling in part by binding to the RhoA-activated serine/threonine kinase ROCK I (Rho-associated kinase I), thereby preventing it from phosphorylating its targets. RhoE activity is itself regulated by phosphorylation by ROCK I on multiple sites. RhoE phosphorylation enhances its stability, leading to an increase in RhoE levels. In addition, phosphorylation reduces its association with membranes and correlates with its ability to induce loss of stress fibres. RhoE also acts independently of ROCK to inhibit cell cycle progression, in part by preventing translation of cyclin D1, and to inhibit transformation of fibroblasts by oncogenic H-Ras. RhoE is therefore a multifunctional protein whose localization and actions are regulated by phosphorylation.

GDIs: central regulatory molecules in Rho GTPase activation

The GDP dissociation inhibitors (GDIs) are pivotal regulators of Rho GTPase function. GDIs control the access of Rho GTPases to regulatory guanine nucleotide exchange factors and GTPase-activating proteins, to effector targets and to membranes where such effectors reside. We discuss here our current understanding of how Rho GTPase-GDI complexes are regulated by various proteins, lipids and enzymes that exert GDI displacement activity. We propose that phosphorylation mediated by diverse kinases might provide a means of controlling and coordinating Rho GTPase activation.

14-3-3 and calmodulin control subcellular distribution of Kir/Gem and its regulation of cell shape and calcium channel activity

Individual members of the RGK family of Ras-related GTPases, which comprise Rad, Gem/Kir, Rem and Rem2, have been implicated in important functions such as the regulation of voltage-gated calcium channel activity and remodeling of cell shape. The GTPase Kir/Gem inhibits the activity of calcium channels by interacting with the beta-subunit and also regulates cytoskeleton dynamics by inhibiting the Rho-Rho kinase pathway. In addition, Kir/Gem interacts with 14-3-3 and calmodulin, but the significance of this interaction on Kir/Gem function is poorly understood. Here, we present a comprehensive analysis of the binding of 14-3-3 and calmodulin to Kir/Gem. We show that 14-3-3, in conjunction with calmodulin, regulates the subcellular distribution of Kir/Gem between the cytoplasm and the nucleus. In addition, 14-3-3 and calmodulin binding modulate Kir/Gem-mediated cell shape remodeling and downregulation of calcium channel activity. Competition experiments show that binding of 14-3-3, calmodulin and calcium channel beta-subunits to Kir/Gem is mutually exclusive, providing a rationale for the observed regulatory effects of 14-3-3 and calmodulin on Kir/Gem localization and function.

Alternative Splicing of Rac1 Generates Rac1b, a Self-activating GTPase

Rac1b was recently identified in malignant colorectal tumors as an alternative splice variant of Rac1 containing a 19-amino acid insertion next to the switch II region. The structures of Rac1b in the GDP- and the GppNHp-bound forms, determined at a resolution of 1.75 A, reveal that the insertion induces an open switch I conformation and a highly mobile switch II. As a consequence, Rac1b has an accelerated GEF-independent GDP/GTP exchange and an impaired GTP hydrolysis, which is restored partially by GTPase-activating proteins. Interestingly, Rac1b is able to bind the GTPase-binding domain of PAK but not full-length PAK in a GTP-dependent manner, suggesting that the insertion does not completely abolish effector interaction. The presented study provides insights into the structural and biochemical mechanism of a self-activating GTPase.

Structural insights into the interaction of ROCKI with the switch regions of RhoA

The Rho-ROCK pathway modulates the phosphorylation level of a variety of important signaling proteins and is thereby involved in miscellaneous cellular processes including cell migration, neurite outgrowth, and smooth muscle contraction. The observation of the involvement of the Rho-ROCK pathway in tumor invasion and in diseases such as hypertension and bronchial asthma makes it an interesting target for drug development. We herein present the crystal structure of the complex between active RhoA and the Rho-binding domain of ROCKI. The Rho-binding domain structure forms a parallel alpha-helical coiled-coil dimer and, in contrast to the published Rho-protein kinase N structure, binds exclusively to the switch I and II regions of the guanosine 5'-(beta,gamma-imido)triphosphate-bound RhoA. The switch regions of two different RhoA molecules form a predominantly hydrophobic patch, which is complementarily bound by two identical short helices of 13 residues (amino acids 998-1010). The identified ROCK-binding site of RhoA strikingly supports the assumption of a common consensus-binding site for effector recognition.

ARHI is a Ras-related small G-protein with a novel N-terminal extension that inhibits growth of ovarian and breast cancers

Our group recently identified Ras homolog member I (ARHI), a novel maternally imprinted tumor suppressor gene that encodes a 26 kDa GTP-binding protein with high homology to Ras and Rap. Unlike other Ras family members, ARHI exhibits several unusual structural and functional properties. ARHI contains a unique 34 amino-acid extension at the N-terminus, and differs from Ras in residues critical for GTPase activity and in its putative effector domain. Like Ras, ARHI can bind to GTP with high affinity but has low intrinsic GTPase activity. In addition, while Ras is an oncogene, ARHI functions as an inhibitor for cell growth. (32)Phosphorus labeling showed that ARHI is maintained in a constitutively activated GTP-bound state in resting cells, possibly because of impaired GTPase activity. ARHI is associated at the cell membrane through its prenylation at the C-terminal cysteine residue. Mutation of the conserved CAAX box at the C-terminus led to a loss of its membrane association and a decreased ability to inhibit cell growth. Conversion of Ser(51) to Asn decreased GTP binding and reduced ARHI's biological activity. Mutation of Ala(46) to Val increased the ability of ARHI to inhibit cell growth, associated with a further decrease of its intrinsic GTPase activity. Moreover, conversion of residues in ARHI that are conserved in the Ras family for GTPase activity partially restored the GTPase activity in ARHI. Most strikingly, deletion of ARHI's unique N-terminal extension nearly abolished its inhibitory effect on cell growth, suggesting its importance in ARHI's inhibitory function. Thus, ARHI is a unique Ras family member that retains basic small GTPase function, but exhibits many unusual features. In contrast to most other Ras family members, ARHI has a long N-terminal extension, modest GTPase activity, and constitutive GTP binding in resting cells. Furthermore, unlike the Ras oncogene, ARHI inhibits cell growth, and loss of its expression in cells may contribute to the development of breast and ovarian cancers.