Active GTP-bound State Research Articles

Structural and functional characterization of fast-cycling RhoF GTPase

Ras superfamily GTPases are molecular switches that cycle between GDP-bound inactive state and GTP-bound active state to control many signaling pathways. Emerging evidence suggests that several Ras superfamily GTPases, including RhoF, do not follow the classical GDP/GTP exchange cycle; they act as constitutively active GTP-bound proteins due to their fast activities of GDP/GTP exchange (termed as ‘fast-cycling’ GTPases). To understand the molecular basis of the fast-cycling GTPases, we generated a GTPase active recombinant RhoF and examined its function and structure. Two point mutations in the switch I/II regions (Q77L and P45S, corresponding to Q61L and P29S of Rac1) significantly reduced the GTPase activity of RhoF, suggesting a conserved mechanism of GTP hydrolysis between RhoF and other RAS superfamily GTPases. However, in contrary to the previous evidence, RhoF represented a slow GDP/GTP exchange activity that dissociates GDP very slowly on a day-to-week time scale, in our experiment using fluorescently labeled GDP. The slow GDP dissociation was accelerated by Mg2+ chelation and canonical fast-cycling mutations, F44L (corresponding to F28L of Rac1) and P45S. NMR and dynamic light scattering data revealed a multimeric structure of RhoF that can switch between different conformations depending on the GTP/GDP-bound state. Overall, our study suggests that (1) RhoF shares a conserved mechanism of GTP hydrolysis with other RAS superfamily GTPases, but (2) RhoF adopts a unique multimeric structure. Our study also argues that (3) the emerging concept of the fast-cycling GTPases for RhoF should be validated using an alternative assay that does not rely on fluorescently labeled GDP (251 words).

ARHGEF10L contributes to liver tumorigenesis through RhoA-ROCK1 signaling and the epithelial-mesenchymal transition

Aberrant activity of Rho small G-proteins and their regulators plays an important role in tumorigenesis. Rho guanine nucleotide exchange factor 10-Like (ARHGEF10L) is a member of the RhoGEF family that promotes the active GTP-bound state of Rho GTPases. This study used the Illumina GoldenGate microassay, Sequenom MassARRAY and TaqMan to analyze possible correlations between tag single nucleotide polymorphisms (tag SNPs) in the ARHGEF10L locus and various tumor risks. The genotyping analyses demonstrated a strong association of rs2244444 and rs12732894 with liver cancer. Western blotting and immunohistochemistry also revealed increased expression of ARHGEF10L in hepatocellular carcinoma tissues. Furthermore, increased cell proliferation, cell migration and RhoA activity; increased expression of Rho-associated coiled-coil kinase-1 (ROCK1), phospho- Ezrin/Radixin/Moesin (ERM), vimentin, N-cadherin and Slug, and decreased E-cadherin expression were detected in hepatocellular carcinoma cell Bel-7402 and HepG2 cells with transfection of ARHGEF10L-expressing plasmids. Opposite results were obtained in the two cell lines with transfection of anti-ARHGEF10L siRNA. Tumor-bearing mice were generated with Bel-7402 cells transfected with lentivirus vectors packaging short hairpin ARHGEF10L RNA. The xenograft tumors with the inhibited ARHGEF10L expression showed decreased tumor growth and expression of vimentin, N-cadherin and Slug. Additionally, decreased phospho-ERM expression was detected in Bel-7402 and HepG2 cells with transfection of anti-ROCK1 siRNA and increased expression of ROCK1 was detected in hepatocellular carcinoma tissues. E-cadherin, vimentin, N-cadherin and Slug are markers of the epithelial-to-mesenchymal transition (EMT). ROCK1, phospho-ERM and EMT have been reported to promote tumor cell proliferation, metastasis and angiogenesis. Our study suggests that increased expression of ARHGEF10L stimulates hepatocellular tumorigenesis by activating the RhoA-ROCK1- phospho ERM pathway and EMT.

Human MICAL1: Activation by the small GTPase Rab8 and small-angle X-ray scattering studies on the oligomerization state of MICAL1 and its complex with Rab8.

Human MICAL1 is a member of a recently discovered family of multidomain proteins that couple a FAD-containing monooxygenase-like domain to typical protein interaction domains. Growing evidence implicates the NADPH oxidase reaction catalyzed by the flavoprotein domain in generation of hydrogen peroxide as a second messenger in an increasing number of cell types and as a specific modulator of actin filaments stability. Several proteins of the Rab families of small GTPases are emerging as regulators of MICAL activity by binding to its C-terminal helical domain presumably shifting the equilibrium from the free - auto-inhibited - conformation to the active one. We here extend the characterization of the MICAL1-Rab8 interaction and show that indeed Rab8, in the active GTP-bound state, stabilizes the active MICAL1 conformation causing a specific four-fold increase of kcat of the NADPH oxidase reaction. Kinetic data and small-angle X-ray scattering (SAXS) measurements support the formation of a 1:1 complex between full-length MICAL1 and Rab8 with an apparent dissociation constant of approximately 8 μM. This finding supports the hypothesis that Rab8 is a physiological regulator of MICAL1 activity and shows how the protein region preceding the C-terminal Rab-binding domain may mask one of the Rab-binding sites detected with the isolated C-terminal fragment. SAXS-based modeling allowed us to propose the first model of the free full-length MICAL1, which is consistent with an auto-inhibited conformation in which the C-terminal region prevents catalysis by interfering with the conformational changes that are predicted to occur during the catalytic cycle.

TBC1D5 controls the GTPase cycle of Rab7b.

Rab GTPases are key regulators of intracellular trafficking, and cycle between a GTP-bound active state and a GDP-bound inactive state. This cycle is regulated by guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Several efforts have been made in connecting the correct GEFs and GAPs to their specific Rab. Here, we aimed to identify GAPs for Rab7b, the small GTPase involved in transport from late endosomes to the trans-Golgi. An siRNA screen targeting proteins containing TBC domains critical for Rab GAPs was performed and coupled to a phenotypic read-out that visualized the distribution of Rab7b. Silencing of TBC1D5 provided the strongest phenotype and this protein was subsequently validated in various in vitro and cell-based assays. TBC1D5 localizes to Rab7b-positive vesicles, interacts with Rab7b and has GAP activity towards Rab7b in vitro, which is further increased by retromer proteins. Similarly to the constitutively active mutant of Rab7b, inactivation of TBC1D5 also reduces the number of CI-MPR- and sortilin-positive vesicles. Together, the results show that TBC1D5 is a GAP for Rab7b in the control of endosomal transport to the trans-Golgi.This article has an associated First Person interview with the first author of the paper.

Rac1 Nanoscale Organization on the Plasma Membrane Is Driven by Lipid Binding Specificity Encoded in the Membrane Anchor.

Rac1 is a small guanine nucleotide binding protein that cycles between an inactive GDP-bound and active GTP-bound state to regulate cell motility and migration. Rac1 signaling is initiated from the plasma membrane (PM). Here, we used high-resolution spatial mapping and manipulation of PM lipid composition to define Rac1 nanoscale organization. We found that Rac1 proteins in the GTP- and GDP-bound states assemble into nonoverlapping nanoclusters; thus, Rac1 proteins undergo nucleotide-dependent segregation. Rac1 also selectively interacts with phosphatidic acid (PA) and phosphoinositol (3,4,5)-trisphosphate (PIP3), resulting in nanoclusters enriched in these lipids. These lipids are structurally important because depleting the PM of PA or PIP3 impairs both Rac1 PM binding and Rac1 nanoclustering. Lipid binding specificity of Rac1 is encoded in the amino acid sequence of the polybasic domain (PBD) of the C-terminal membrane anchor. Point mutations within the PBD, including arginine-to-lysine substitutions, profoundly alter Rac1 lipid binding specificity without changing the electrostatics of the protein and result in impaired macropinocytosis and decreased cell spreading. We propose that Rac1 nanoclusters act as lipid-based signaling platforms emulating the spatiotemporal organization of Ras proteins and show that the Rac1 PBD-prenyl anchor has a biological function that extends beyond simple electrostatic engagement with the PM.

Mitochondria-lysosome contacts regulate mitochondrial fission via RAB7 GTP hydrolysis.

Both mitochondria and lysosomes are essential for maintaining cellular homeostasis, and dysfunction of both organelles has been observed in multiple diseases1–4. Mitochondria are highly dynamic and undergo fission and fusion to maintain a functional mitochondrial network, which drives cellular metabolism 5. Lysosomes similarly undergo constant dynamic regulation by the RAB7 GTPase 1, which cycles from an active GTP-bound state into an inactive GDP-bound state upon GTP hydrolysis. Here we have identified the formation and regulation of mitochondria–lysosome membrane contact sites using electron microscopy, structured illumination microscopy and high spatial and temporal resolution confocal live cell imaging. Mitochondria–lysosome contacts formed dynamically in healthy untreated cells and were distinct from damaged mitochondria that were targeted into lysosomes for degradation 6,7. Contact formation was promoted by active GTP-bound lysosomal RAB7, and contact untethering was mediated by recruitment of the RAB7 GTPase-activating protein TBC1D15 to mitochondria by FIS1 to drive RAB7 GTP hydrolysis and thereby release contacts. Functionally, lysosomal contacts mark sites of mitochondrial fission, allowing regulation of mitochondrial networks by lysosomes, whereas conversely, mitochondrial contacts regulate lysosomal RAB7 hydrolysis via TBC1D15. Mitochondria–lysosome contacts thus allow bidirectional regulation of mitochondrial and lysosomal dynamics, and may explain the dysfunction observed in both organelles in various human diseases.

Guanine Nucleotide Exchange Assay Using Fluorescent MANT-GDP.

GTPases are molecular switches that cycle between the inactive GDP-bound state and the active GTP-bound state. GTPases exchange nucleotides either by its intrinsic nucleotide exchange or by interaction with guanine nucleotide exchange factors (GEFs). Monitoring the nucleotide exchange in vitro, together with reconstitution of direct interactions with regulatory proteins, provides key insights into how a GTPase is activated. In this protocol, we describe core methods to monitor nucleotide exchange using fluorescent N-Methylanthraniloyl (MANT)-guanine nucleotide.

NMR analysis of the backbone dynamics of the small GTPase Rheb and its interaction with the regulatory protein FKBP38.

Ras homolog enriched in brain (Rheb) is a small GTPase that regulates mammalian/mechanistic target of rapamycin complex 1 (mTORC1) and, thereby, cell growth and metabolism. Here we show that cycling between the inactive GDP- and the active GTP-bound state modulates the backbone dynamics of a C-terminal truncated form, RhebΔCT, which is suggested to influence its interactions. We further investigated the interactions between RhebΔCT and the proposed Rheb-binding domain of the regulatory protein FKBP38. The observed weak interactions with the GTP-analogue- (GppNHp-) but not the GDP-bound state, appear to accelerate the GDP to GTP exchange, but only very weakly compared to a genuine GEF. Thus, FKBP38 is most likely not a GEF but a Rheb effector that may function in membrane targeting of Rheb.

RABIF/MSS4 is a Rab-stabilizing holdase chaperone required for GLUT4 exocytosis

Rab GTPases are switched from their GDP-bound inactive conformation to a GTP-bound active state by guanine nucleotide exchange factors (GEFs). The first putative GEFs isolated for Rabs are RABIF (Rab-interacting factor)/MSS4 (mammalian suppressor of Sec4) and its yeast homolog DSS4 (dominant suppressor of Sec4). However, the biological function and molecular mechanism of these molecules remained unclear. In a genome-wide CRISPR genetic screen, we isolated RABIF as a positive regulator of exocytosis. Knockout of RABIF severely impaired insulin-stimulated GLUT4 exocytosis in adipocytes. Unexpectedly, we discovered that RABIF does not function as a GEF, as previously assumed. Instead, RABIF promotes the stability of Rab10, a key Rab in GLUT4 exocytosis. In the absence of RABIF, Rab10 can be efficiently synthesized but is rapidly degraded by the proteasome, leading to exocytosis defects. Strikingly, restoration of Rab10 expression rescues exocytosis defects, bypassing the requirement for RABIF. These findings reveal a crucial role of RABIF in vesicle transport and establish RABIF as a Rab-stabilizing holdase chaperone, a previously unrecognized mode of Rab regulation independent of its GDP-releasing activity. Besides Rab10, RABIF also regulates the stability of two other Rab GTPases, Rab8 and Rab13, suggesting that the requirement of holdase chaperones is likely a general feature of Rab GTPases.

Abstract 4018: Identifying allosteric modulators of KRas using second harmonic generation

Abstract The Ras protein family consists of small GTPases that are critical signaling transducers regulating cellular processes including proliferation, differentiation, and survival. As monomeric GTPases, Ras proteins cycle between an inactive GDP-bound state and an active GTP-bound state. Oncogenic mutations in one of the Ras isoforms, H-, N-, and K-Ras, impair GTP hydrolysis, thus de-regulating Ras signaling. Ras mutations are found in one-third of human cancers, and tumors bearing these mutations are notoriously difficult to treat. Conventional approaches to inhibit Ras with small molecules have been largely unsuccessful leading many to characterize the Ras family as an “undruggable” target. However, there is renewed enthusiasm towards Ras drug discovery in response to a recent surge of reports revealing structural complexities to Ras proteins that provide opportunity for innovative drug discovery efforts. In light of findings suggesting KRas contains transient and dynamic binding pockets, many of which are undetectable with traditional structural techniques, we used a second harmonic generation (SHG) platform to screen a ~2700 chemically diverse fragment collection for allosteric modulators of KRas. This high throughput SHG platform enables real-time measurement of protein conformational change under physiological conditions. We selected ~50 fragments for validation by SPR, and positive binders will be selected for structural and biochemical characterization. Another advantage of the SHG platform is that we can monitor the conformational dynamics of fully processed, farnesylated KRas bound to a phosphatidylserine-enriched membrane that mimics KRas’ physiological interaction with the plasma membrane. No other biophysical method is suitable to monitor the real-time interactions of small molecules or proteins with membrane-bound KRas. This analysis may reveal surface contacts unavailable in solution-based assays. We have successfully implemented SHG as a drug discovery platform against KRas, and we aim to utilize selected KRas conformational modulators towards the identification of novel binding pockets and towards the development of targeted KRas therapies. Citation Format: Elizabeth Donohue Vo, Gabriel Besserer Mercado, Patrick Alexander, Ben Moree, Que Van, Andrew G. Stephen, Joshua Salafsky, Frank McCormick. Identifying allosteric modulators of KRas using second harmonic generation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4018. doi:10.1158/1538-7445.AM2017-4018

Abstract 1366: Structural basis of impaired GTP hydrolysis in oncogenic mutants of KRAS

Abstract RAS mutations are found in one-third of all human cancers. Among the three RAS isoforms - HRAS, KRAS and NRAS, KRAS is the most commonly mutated gene (in 86% of RAS-driven cancers) and the mutations are often detected in pancreatic, colorectal and lung cancers. RAS proteins function as molecular switches by alternating between inactive GDP-bound and active GTP-bound states. The active or inactive state of RAS proteins is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). In the GTP-bound state, RAS proteins interact with a variety of effector proteins, such as Raf, PI3K, and RalGDS, leading to activation of several signaling cascades within the cell. In 98% of the cases, oncogenic RAS mutations are found at amino acid positions G12, G13, and Q61 which impair intrinsic and GAP-mediated GTPase function resulting in accumulation of constitutively GTP-bound RAS in cells. To gain insights into the effect of oncogenic mutations on overall structure and GTP hydrolysis, we solved high-resolution crystal structures of wild type and six oncogenic mutants (G12C, G12D, G12V, G13D, Q61L and Q61R) of KRAS4b in complex with GMPPNP (a non-hydrolysable GTP analog) and magnesium. Comparison of GDP and GMPPNP bound structures of wild type (WT) KRAS4b suggests conformational changes that occur when KRAS4b transitions from inactive to active state. Comparison of mutant and wild-type KRAS structures has shown the appearance of new pockets in some cases that could be exploited for structure-based drug design. Structural superposition of mutants vs. wild type KRAS4b in complex with GMPPNP/Mg2+, and KRAS4b mutants vs. WT-HRAS bound to RASA1-GAP provides a rationale for impaired intrinsic and GAP-mediated GTP hydrolysis in the KRAS mutants. Citation Format: Timothy Tran, Sathiya Dharmaiah, Oleg Chertov, Timothy Waybright, William Gillette, Dominic Esposito, Dwight Nissley, Frank McCormick, Andrew Stephen, Dhirendra Simanshu. Structural basis of impaired GTP hydrolysis in oncogenic mutants of KRAS [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1366. doi:10.1158/1538-7445.AM2017-1366

Abstract 4027: Developing biophysical platforms to study KRAS membrane interaction in artificial membranes

Abstract In our studies, we leverage the techniques that we believe will allow us to better understand and tackle KRAS membrane interaction. RAS proteins are small GTPases that oscillate between an active GTP-bound and inactive GDP-bound state to function as plasma membrane localized molecular switches in growth factor regulated signaling pathways. Mutation in HRAS, KRAS and NRAS are present in nearly 30% of human tumors. In particular, KRAS is mutated in almost 90% of pancreatic adenocarcinomas. Considering the requirement of the plasma membrane for KRAS activity, we are setting up ‘screenable’ biophysical platforms to understand the importance of lipids and find small molecules that could prevent KRAS membrane association in reconstituted systems. The two main techniques that are being developed are Surface Plasmon Resonance and Confocal Imaging in model membranes. Using these two techniques, we have been able to better understand the main lipids that drive KRAS membrane association and set up platforms that can be screened by small/large molecules. Biophysical techniques are frequently used by pharmaceutical companies as primary or secondary screening platforms. Developing such platforms for KRAS membrane interaction could enable the discovery of anti-cancer tool compounds. Citation Format: Frantz L. Jean-Francois, Andrew G. Stephen. Developing biophysical platforms to study KRAS membrane interaction in artificial membranes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4027. doi:10.1158/1538-7445.AM2017-4027

Abstract 5166: Sensitivity of melanoma cells to a novel class of Ras inhibitors

Abstract A high percentage of human cancers arise from mutations in the ras gene that encodes an abnormal Ras protein locked in a constitutively active GTP-bound state that promotes tumor cell proliferation, survival, and metastasis. These gain-in-function mutations in the ras gene or constitutive activation of tyrosine kinase receptors upstream of the Ras protein drive tumor cell growth by activating Raf/MAPK and PI3K/AKT signaling pathways. Ras has been an elusive drug target for which no inhibitors are available to treat Ras-driven cancers. Screening a library of indene derivatives in a differential phenotypic assay identified a novel compound class displaying high potency and selectivity to inhibit the growth of tumor cells harboring mutant Ras relative to tumor cells with wild type (WT) Ras. Lead optimization resulted in a drug development candidate (DC070-547) and several back-up analogs (e.g. ADT-006) with IC50 values in the low nanomolar range and selectivity indices of 100 fold or greater to inhibit the growth of tumor cells with constitutively activated Ras relative to tumor cells with low levels of activated Ras. Sensitivity among a large panel of tumor cell lines to this compound class strongly correlated with levels of activated Ras, but did not appear to be limited to a specific ras gene mutation or Ras protein isoform. Here, we report that DC070-547 and ADT-006 potently and selectively inhibit the growth of the human melanoma cell line, SK-MEL-2, harboring a mutation in the ras gene that encodes the constitutively active N-Ras protein with IC50 values of 7 and 25 nM, respectively. In addition, both compounds potently inhibited the growth of the murine melanoma cell line, B16-F10 with WT ras, but harboring a mutation in a tyrosine kinase receptor upstream of Ras, specifically PDGFRα, which results in high levels of active, GTP-bound Ras as confirmed by Ras-RBD pull-down assays. To determine if treatment can disrupt Ras signaling, B16-F10 cells were incubated with the compounds before being subjected to Western blotting for phosphorylated signaling molecules downstream of Ras. Treatment reduced levels of phosphorylated c-Raf and MEK at concentrations that inhibit tumor cell growth. These findings support further investigation of this novel class of Ras inhibitors for the treatment of Ras-driven melanoma. Citation Format: Kate M. Saville, Kevin Lee, Tyler E. Mattox, Xi Chen, Jacob Valiyaveettil, Kristy Berry, Veronica Ramírez-Alcántara, Bing Zhu, Adam Keeton, Michael Boyd, Gary Piazza, Ashley S. Lindsey. Sensitivity of melanoma cells to a novel class of Ras inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5166. doi:10.1158/1538-7445.AM2017-5166

Interaction of p190A RhoGAP with eIF3A and Other Translation Preinitiation Factors Suggests a Role in Protein Biosynthesis

The negative regulator of Rho family GTPases, p190A RhoGAP, is one of six mammalian proteins harboring so-called FF motifs. To explore the function of these and other p190A segments, we identified interacting proteins by tandem mass spectrometry. Here we report that endogenous human p190A, but not its 50% identical p190B paralog, associates with all 13 eIF3 subunits and several other translational preinitiation factors. The interaction involves the first FF motif of p190A and the winged helix/PCI domain of eIF3A, is enhanced by serum stimulation and reduced by phosphatase treatment. The p190A/eIF3A interaction is unaffected by mutating phosphorylated p190A-Tyr308, but disrupted by a S296A mutation, targeting the only other known phosphorylated residue in the first FF domain. The p190A-eIF3 complex is distinct from eIF3 complexes containing S6K1 or mammalian target of rapamycin (mTOR), and appears to represent an incomplete preinitiation complex lacking several subunits. Based on these findings we propose that p190A may affect protein translation by controlling the assembly of functional preinitiation complexes. Whether such a role helps to explain why, unique among the large family of RhoGAPs, p190A exhibits a significantly increased mutation rate in cancer remains to be determined.

PP17. RHO GTPASE SIGNALLING IN CANCER CELL INVASION AND ANGIOGENESIS IN BRAIN TUMOURS

AbstractINTRODUCTION: Rho GTPases are molecular switches that cycle between GDP-bound inactive and GTP-bound active states. They transduce signals from the extracellular environment and downstream of oncogenes to control the actin cytoskeleton, and cellular functions including cell motility, cell cycle progression, polarity, malignant transformation, invasion and metastasis. Activation of Rho proteins is controlled by guanine nucleotide exchange factors (GEFs), and GTPase activating proteins (GAPs). Rho GEFs and GAPs are frequently overexpressed in tumours and they are tractable targets for cancer therapy. Rho proteins Rac1 and Cdc42 are deregulated in GBM. Our study is focused on understanding the role of Rac GEF DOCK4 and Cdc42 GEF DOCK9 [Abraham S, Scarcia M, ..Mavria G (2015) Nat Commun. 6: 7286] in the tumour cell and stroma cell compartments of glioblastomas. METHOD: i) The role of DOCK4 and DOCK9 in GBM invasion was analysed using organotypic spheroid assays, the U251 model and patient-derived models of GBM ii) DOCK4 expression was analysed in GBM patient samples and related to expression of stem cell marker nestin. iii) Blood vessel types were characterised in GBM patient samples and related to histological parameters and patient outcome following treatments. iii) The role of DOCK4 was investigated in the vasculature of heterozygous Dock4 null mice and syngeneic GBM tumour models implanted orthotopically. RESULTS: i) There was reduced outgrowth of GBM cells in spheroid assays with DOCK4 or DOCK9 knockdown. A complex of DOCK4 with DOCK9 was identified by immunoprecipitation in U251 cells. ii) Paired DOCK4 and nestin scores of immunostaining from 19 GBM patient samples showed DOCK4 scores correlated with nestin scores using two-tailed Spearmans Rho. Oncomine microarray data shows significant increase of DOCK4 levels in cultured GBM cells isolated from tumours (n=25) compared to neural stem cells (p=8.16 x 104). iii) Hyperproliferative blood vessels in GBM show larger diameter compared to normal blood vessels and increased number of glomeruloid structures was observed in recurrences iv) Heterozygous Dock4 genetic deletion results in reduction of blood vessel diameter. CONCLUSIONS: Tumour cell invasion into adjacent normal brain and vascular proliferation are pathologic hallmarks of GBM. Our results show that DOCK4 may be implicated in both vascular growth and glioblastoma invasion. Delineating further the signalling network in which DOCK4 participates may shed considerable light in these processes and identify new targets for treatments.

Inhibition of Ral GTPases Using a Stapled Peptide Approach

Aberrant Ras signaling drives numerous cancers, and drugs to inhibit this are urgently required. This compelling clinical need combined with recent innovations in drug discovery including the advent of biologic therapeutic agents, has propelled Ras back to the forefront of targeting efforts. Activated Ras has proved extremely difficult to target directly, and the focus has moved to the main downstream Ras-signaling pathways. In particular, the Ras-Raf and Ras-PI3K pathways have provided conspicuous enzyme therapeutic targets that were more accessible to conventional drug-discovery strategies. The Ras-RalGEF-Ral pathway is a more difficult challenge for traditional medicinal development, and there have, therefore, been few inhibitors reported that disrupt this axis. We have used our structure of a Ral-effector complex as a basis for the design and characterization of α-helical-stapled peptides that bind selectively to active, GTP-bound Ral proteins and that compete with downstream effector proteins. The peptides have been thoroughly characterized biophysically. Crucially, the lead peptide enters cells and is biologically active, inhibiting isoform-specific RalB-driven cellular processes. This, therefore, provides a starting point for therapeutic inhibition of the Ras-RalGEF-Ral pathway.

Invited review: Small GTPases and their GAPs.

Widespread utilization of small GTPases as major regulatory hubs in many different biological systems derives from a conserved conformational switch mechanism that facilitates cycling between GTP-bound active and GDP-bound inactive states under control of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), which accelerate slow intrinsic rates of activation by nucleotide exchange and deactivation by GTP hydrolysis, respectively. Here we review developments leading to current understanding of intrinsic and GAP catalyzed GTP hydrolytic reactions in small GTPases from structural, molecular and chemical mechanistic perspectives. Despite the apparent simplicity of the GTPase cycle, the structural bases underlying the hallmark hydrolytic reaction and catalytic acceleration by GAPs are considerably more diverse than originally anticipated. Even the most fundamental aspects of the reaction mechanism have been challenging to decipher. Through a combination of experimental and in silico approaches, the outlines of a consensus view have begun to emerge for the best studied paradigms. Nevertheless, recent observations indicate that there is still much to be learned. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 431-448, 2016.

Rap2B GTPase: structure, functions, and regulation.

Rap2B GTPase, a member of Ras-related protein superfamily, was first discovered from a platelet cDNA library in the early 1990s. Since then, it has been reported to play an important role in regulating cellular processes including cytoskeletal organization, cell growth, and proliferation. It can be stimulated and suppressed by a wide range of external and internal inducers, circulating between GTP-bound active state and GDP-bound inactive state. Increasing focus on Ras signaling pathway reveals critical effects of Rap2B on tumorigenesis. In particular, Rap2B behaves in a p53-dependent manner in regulation of apoptosis and migration. Apart from being an oncogenic activator, Rap2B has been found to participate in many other physiological events via diverse downstream effectors. In this review, we present recent studies on the structure, regulation, and multiple biological functions of Rap2B, shedding light on its potential status in treatment of cancer as well as other diseases.

Cdc42 and Rac1 activity is reduced in human pheochromocytoma and correlates with FARP1 and ARHGEF1 expression.

Among small GTPases from the Rho family, Cdc42, RAC, and Rho are well known to mediate a large variety of cellular processes linked with cancer biology through their ability to cycle between an inactive (GDP-bound) and an active (GTP-bound) state. Guanine nucleotide exchange factors (GEFs) stimulate the exchange of GDP for GTP to generate the activated form, whereas the GTPase-activating proteins (GAPs) catalyze GTP hydrolysis, leading to the inactivated form. Modulation of Rho GTPase activity following altered expression of RHO-GEFs and/or RHO-GAPs has already been reported in various human tumors. However, nothing is known about the Rho GTPase activity or the expression of their regulators in human pheochromocytomas, a neuroendocrine tumor (NET) arising from chromaffin cells of the adrenal medulla. In this study, we demonstrate, through an ELISA-based activity assay, that Rac1 and Cdc42 activities decrease in human pheochromocytomas (PCCs) compared with the matched adjacent non-tumor tissue. Furthermore, through quantitative mass spectrometry (MS) approaches, we show that the expression of two RHO-GEF proteins, namely ARHGEF1 and FARP1, is significantly reduced in tumors compared with matched non-tumor tissue, whereas ARHGAP36 expression is increased. Moreover, siRNA-based knockdown of ARHGEF1 and FARP1 in PC12 cells leads to a significant inhibition of Rac1 and Cdc42 activities, respectively. Finally, a principal component analysis (PCA) of our dataset was able to discriminate PCC from non-tumor tissue and indicates a close correlation between Cdc42/Rac1 activity and FARP1/ARHGEF1 expression. Altogether, our findings reveal for the first time the importance of modulation of Rho GTPase activities and expression of their regulators in human PCCs.

Ras Conformational Ensembles, Allostery, and Signaling.

Ras proteins are classical members of small GTPases that function as molecular switches by alternating between inactive GDP-bound and active GTP-bound states. Ras activation is regulated by guanine nucleotide exchange factors that catalyze the exchange of GDP by GTP, and inactivation is terminated by GTPase-activating proteins that accelerate the intrinsic GTP hydrolysis rate by orders of magnitude. In this review, we focus on data that have accumulated over the past few years pertaining to the conformational ensembles and the allosteric regulation of Ras proteins and their interpretation from our conformational landscape standpoint. The Ras ensemble embodies all states, including the ligand-bound conformations, the activated (or inactivated) allosteric modulated states, post-translationally modified states, mutational states, transition states, and nonfunctional states serving as a reservoir for emerging functions. The ensemble is shifted by distinct mutational events, cofactors, post-translational modifications, and different membrane compositions. A better understanding of Ras biology can contribute to therapeutic strategies.