Family Of Guanine Nucleotide Exchange Factors Research Articles

Guanine nucleotide exchange factors and colon neoplasia.

Despite many diagnostic and therapeutic advances, colorectal cancer (CRC) remains the second leading cause of cancer death for men and women in the United States. Alarmingly, for reasons currently unknown, the demographics of this disease have shifted towards a younger population. Hence, understanding the molecular mechanisms underlying CRC initiation and progression and leveraging these findings for therapeutic purposes remains a priority. Here, we review critically the evidence that canonical and noncanonical actions of guanine nucleotide exchange factors (GEFs) play important roles in CRC evolution. Rho GEF GTPases, which switch between inactive GDP-bound and active GTP-bound states, are commonly overexpressed and activated in a variety of cancers, including CRC, and may be tractable therapeutic targets. In addition to comprehensively reviewing this field, we focus on Rho/Rac GEFs that are involved in regulating key functions of normal and neoplastic cells like cell polarity, vesicle trafficking, cell cycle regulation, and transcriptional dynamics. Prime examples of such Rho/Rac GEFs include βPak-interacting exchange factor (βPix), a Rho family GEF for Cdc42/Rac1, Tiam1, GEF-H1, RGNEF, and other GEFs implicated in CRC development and progression. Throughout this analysis, we explore how these findings fill key gaps in knowledge regarding the molecular basis of colon carcinogenesis and how they may be leveraged to treat advanced CRC. Lastly, we address potential future directions for research into the role of GEFs as CRC biomarkers and therapeutic targets. In this regard, leveraging the noncanonical actions of GEFs appears to provide a relatively unexplored opportunity requiring further investigation.

New Mechanisms Underlying Oncogenesis in Dbl Family Rho Guanine Nucleotide Exchange Factors.

Transmembrane signaling is a critical process by which changes in the extracellular environment are relayed to intracellular systems that induce changes in homeostasis. One family of intracellular systems are the guanine nucleotide exchange factors (GEFs), which catalyze the exchange of GTP for GDP bound to inactive guanine nucleotide binding proteins (G proteins). The resulting active G proteins then interact with downstream targets that control cell proliferation, growth, shape, migration, adhesion, and transcription. Dysregulation of any of these processes is a hallmark of cancer. The Dbl family of GEFs activates Rho family G proteins, which, in turn, alter the actin cytoskeleton and promote gene transcription. Although they have a common catalytic mechanism exercised by their highly conserved Dbl homology (DH) domains, Dbl GEFs are regulated in diverse ways, often involving the release of autoinhibition imposed by accessory domains. Among these domains, the pleckstrin homology (PH) domain is the most commonly observed and found immediately C-terminal to the DH domain. The domain has been associated with both positive and negative regulation. Recently, some atomic structures of Dbl GEFs have been determined that reemphasize the complex and central role that the PH domain can play in orchestrating regulation of the DH domain. Here, we discuss these newer structures, put them into context by cataloging the various ways that PH domains are known to contribute to signaling across the Dbl family, and discuss how the PH domain might be exploited to achieve selective inhibition of Dbl family RhoGEFs by small-molecule therapeutics. SIGNIFICANCE STATEMENT: Dysregulation via overexpression or mutation of Dbl family Rho guanine nucleotide exchange factors (GEFs) contributes to cancer and neurodegeneration. Targeting the Dbl homology catalytic domain by small-molecule therapeutics has been challenging due to its high conservation and the lack of a discrete binding pocket. By evaluating some new autoinhibitory mechanisms in the Dbl family, we demonstrate the great diversity of roles played by the regulatory domains, in particular the PH domain, and how this holds tremendous potential for the development of selective therapeutics that modulate GEF activity.

DENND6A links Arl8b to a Rab34/RILP/dynein complex, regulating lysosomal positioning and autophagy

Lysosomes help maintain cellular proteostasis, and defects in lysosomal positioning and function can cause disease, including neurodegenerative disorders. The spatiotemporal distribution of lysosomes is regulated by small GTPases including Rabs, which are activated by guanine nucleotide exchange factors (GEFs). DENN domain proteins are the largest family of Rab GEFs. Using a cell-based assay, we screened DENND6A, a member of the DENN domain protein family against all known Rabs and identified it as a potential GEF for 20 Rabs, including Rab34. Here, we demonstrate that DENND6A activates Rab34, which recruits a RILP/dynein complex to lysosomes, promoting lysosome retrograde transport. Further, we identify DENND6A as an effector of Arl8b, a major regulatory GTPase on lysosomes. We demonstrate that Arl8b recruits DENND6A to peripheral lysosomes to activate Rab34 and initiate retrograde transport, regulating nutrient-dependent lysosomal juxtanuclear repositioning. Loss of DENND6A impairs autophagic flux. Our findings support a model whereby Arl8b/DENND6A/Rab34-dependent lysosomal retrograde trafficking controls autophagy.

A Syx-RhoA-Dia1 signaling axis regulates cell cycle progression, DNA damage, and therapy resistance in glioblastoma.

Glioblastomas (GBM) are aggressive tumors that lack effective treatments. Here, we show that the Rho family guanine nucleotide exchange factor Syx promotes GBM cell growth both in vitro and in orthotopic xenografts derived from patients with GBM. Growth defects upon Syx depletion are attributed to prolonged mitosis, increased DNA damage, G2/M cell cycle arrest, and cell apoptosis, mediated by altered mRNA and protein expression of various cell cycle regulators. These effects are phenocopied by depletion of the Rho downstream effector Dia1 and are due, at least in part, to increased phosphorylation, cytoplasmic retention, and reduced activity of the YAP/TAZ transcriptional coactivators. Furthermore, targeting Syx signaling cooperates with radiation treatment and temozolomide (TMZ) to decrease viability in GBM cells, irrespective of their inherent response to TMZ. The data indicate that a Syx-RhoA-Dia1-YAP/TAZ signaling axis regulates cell cycle progression, DNA damage, and therapy resistance in GBM and argue for its targeting for cancer treatment.

CD16+ fibroblasts foster a trastuzumab-refractory microenvironment that is reversed by VAV2 inhibition.

The leukocyte Fcγ receptor (FcγR)-mediated response is important for the efficacy of therapeutic antibodies; however, little is known about the role of FcγRs in other cell types. Here we identify a subset of fibroblasts in human breast cancer that express CD16 (FcγRIII). An abundance of these cells in HER2+ breast cancer patients is associated with poor prognosis and response to trastuzumab. Functionally, upon trastuzumab stimulation, CD16+ fibroblasts reduce drug delivery by enhancing extracellular matrix stiffness. Interaction between trastuzumab and CD16 activates the intracellular SYK-VAV2-RhoA-ROCK-MLC2-MRTF-A pathway, leading to elevated contractile force and matrix production. Targeting of a Rho family guanine nucleotide exchange factor, VAV2, which is indispensable for the function of CD16 in fibroblasts rather than leukocytes, reverses desmoplasia provoked by CD16+ fibroblasts. Collectively, our study reveals a role for the fibroblast FcγR in drug resistance, and suggests that VAV2 is an attractive target to augment the effects of antibody treatments.

Deciphering the conformational dynamics of gephyrin-mediated collybistin activation

Efficient neuronal signaling depends on the proper assembly of the postsynaptic neurotransmitter machinery. The majority of inhibitory synapses feature γ-aminobutyric acid type A (GABAA) receptors. The function of these GABAergic synapses is controlled by the scaffolding protein gephyrin and collybistin, a Dbl family guanine nucleotide exchange factor and neuronal adaptor protein. Specifically, collybistin interacts with small GTPases, cell adhesion proteins, and phosphoinositides to recruit gephyrin and GABAA receptors to postsynaptic membrane specializations. Collybistin usually contains an N-terminal SH3 domain and exists in closed/inactive or open/active states. Here, we elucidate the molecular basis of the gephyrin-collybistin interaction with newly designed collybistin Förster resonance energy transfer (FRET) sensors. Using fluorescence lifetime-based FRET measurements, we deduce the affinity of the gephyrin-collybistin complex, thereby confirming that the C-terminal dimer-forming E domain binds collybistin, an interaction that does not require E domain dimerization. Simulations based on fluorescence lifetime and sensor distance distributions reveal at least a two-state equilibrium of the SH3 domain already in the free/unbound collybistin, thereby illustrating the accessible volume of the SH3 domain. Finally, our data provide strong evidence for a tightly regulated collybistin-gephyrin interplay, where, unexpectedly, switching of collybistin from closed/inactive to open/active states is efficiently triggered by gephyrin.

A commercial ARHGEF17/TEM4 antibody cross-reacts with Nuclear Mitotic Apparatus protein 1 (NuMA).

The Rho family Guanine nucleotide exchange factor (GEF) ARHGEF17 (also known as TEM4) is a large protein with only 3 annotated regions: an N-terminal actin-binding domain, a Rho-specific dbl homology (DH)- pleckstrin homology (PH) type GEF domain and a seven bladed β propeller fold at the C-terminus with unknown function. TEM4 has been implicated in numerous activities that rely on regulation of the cytoskeleton including cell migration, cell-cell junction formation and the spindle assembly checkpoint during mitosis. Here we have assessed the specificity of a TEM4 polyclonal antibody that has been commonly used as a Western blotting and immunocytochemistry probe for TEM4 in mammalian cells. We find that this antibody, in addition to its intended target, cross-reacts with the Nuclear Mitotic Apparatus Protein 1 (NuMA) in Western blotting and immunoprecipitation, and detects NuMA preferentially in immunocytochemistry. This cross-reactivity, with an abundant chromatin- and mitotic spindle-associated factor, is likely to affect the interpretation of experiments that make use of this antibody probe, in particular by immunocytochemistry and immunoprecipitation.

Structural/functional studies of Trio provide insights into its configuration and show that conserved linker elements enhance its activity for Rac1

Trio is a large and highly conserved metazoan signaling scaffold that contains two Dbl family guanine nucleotide exchange factor (GEF) modules, TrioN and TrioC, selective for Rac and RhoA GTPases, respectively. The GEF activities of TrioN and TrioC are implicated in several cancers, especially uveal melanoma. However, little is known about how these modules operate in the context of larger fragments of Trio. Here we show via negative stain electron microscopy that the N-terminal region of Trio is extended and could thus serve as a rigid spacer between the N-terminal putative lipid-binding domain and TrioN, whereas the C-terminal half of Trio seems globular. We found that regions C-terminal to TrioN enhance its Rac1 GEF activity and thus could play a regulatory role. We went on to characterize a minimal, well-behaved Trio fragment with enhanced activity, Trio1284–1959, in complex with Rac1 using cryo-electron microscopy and hydrogen-deuterium exchange mass spectrometry and found that the region conferring enhanced activity is disordered. Deletion of two different strongly conserved motifs in this region eliminated this enhancement, suggesting that they form transient intramolecular interactions that promote GEF activity. Because Dbl family RhoGEF modules have been challenging to directly target with small molecules, characterization of accessory Trio domains such as these may provide alternate routes for the development of therapeutics that inhibit Trio activity in human cancer.

Tandem C2 domains mediate dynamic organelle targeting of a DOCK family guanine nucleotide exchange factor.

Multicellular organisms use dedicator of cytokinesis (DOCK) family guanine nucleotide exchange factors (GEFs) to activate Rac/Rho-of-plants small GTPases and coordinate cell shape change. In developing tissues, DOCK signals integrate cell-cell interactions with cytoskeleton remodeling, and the GEFs cluster reversibly at specific organelle surfaces to orchestrate cytoskeletal reorganization. The domain organizations among DOCK orthologs are diverse, and the mechanisms of localization control are poorly understood. Here, we use combinations of transgene complementation and live-cell imaging assays to uncover an evolutionarily conserved and essential localization determinant in the DOCK-GEF named SPIKE1. The SPIKE1-DHR3 domain is sufficient for organelle association in vivo, and displays a complicated lipid-binding selectivity for both phospholipid head groups and fatty acid chain saturation. SPIKE1-DHR3 is predicted to adopt a C2-domain structure and functions as part of a tandem C2 array that enables reversible clustering at the cell apex. This work provides mechanistic insight into how DOCK GEFs sense compositional and biophysical membrane properties at the interface of two organelle systems.

A cell-based GEF assay reveals new substrates for DENN domains and a role for DENND2B in primary ciliogenesis.

Primary cilia are sensory antennae crucial for cell and organism development, and defects in their biogenesis cause ciliopathies. Ciliogenesis involves membrane trafficking mediated by small guanosine triphosphatases (GTPases) including Rabs, molecular switches activated by guanine nucleotide exchange factors (GEFs). The largest family of Rab GEFs is the DENN domain–bearing proteins. Here, we screen all 60 Rabs against two major DENN domain families using a cellular GEF assay, uncovering 19 novel DENN/Rab pairs. The screen reveals Rab10 as a substrate for DENND2B, a protein previously implicated in cancer and severe mental retardation. Through activation of Rab10, DENND2B represses the formation of primary cilia. Through a second pathway, DENND2B functions as a GEF for RhoA to control the length of primary cilia. This work thus identifies an unexpected diversity in DENN domain–mediated activation of Rabs, a previously unidentified non-Rab substrate for a DENN domain, and a new regulatory protein in primary ciliogenesis.

Variable Distribution of DOCK-D Proteins between Cytosol and Nucleoplasm in Cell Lines, Effect of Interleukin-4 on DOCK10 in B-Cell Lymphoid Neoplasms, and Validation of a New DOCK10 Antiserum for Immunofluorescence Studies.

Dedicator-of-cytokinesis (DOCK), a family of guanine-nucleotide exchange factors (GEFs), comprises four subfamilies, named from A to D. DOCK-D comprises DOCK9, DOCK10, and DOCK11. The GEF activity involves translocation from the cytoplasm to the plasma membrane (PM), as assessed by the transfection of tagged proteins. However, the cellular localization of endogenous DOCK proteins is poorly understood. In this paper, to gain a better understanding of the role of the DOCK-D proteins, we studied their distribution between cytosol and nucleoplasm in 11 cell lines. DOCK-D proteins were distributed with variable cytosolic or nuclear predominance, although the latter was common for DOCK9 and DOCK11. These results suggest that the DOCK-D proteins may perform new nuclear functions, which remain to be discovered. Furthermore, we found that DOCK10 levels are increased by interleukin-4 (IL-4) in B-cell lymphoid neoplasms other than chronic lymphocytic leukemia (CLL) such as mantle cell lymphoma and diffuse large B-cell lymphoma. We also found evidence for an induction of the cytosolic levels of DOCK10 by IL-4 in CLL. Finally, we obtained a valid DOCK10 antiserum for immunofluorescence (IF) microscopy that, as an antibody against the hemagglutinin (HA) tag, marked PM ruffles and filopodia in HeLa cells with inducible expression of HA-DOCK10.

Structural Design and Analysis of the RHOA-ARHGEF1 Binding Mode: Challenges and Applications for Protein-Protein Interface Prediction.

The interaction between two proteins may involve local movements, such as small side-chains re-positioning or more global allosteric movements, such as domain rearrangement. We studied how one can build a precise and detailed protein-protein interface using existing protein-protein docking methods, and how it can be possible to enhance the initial structures using molecular dynamics simulations and data-driven human inspection. We present how this strategy was applied to the modeling of RHOA-ARHGEF1 interaction using similar complexes of RHOA bound to other members of the Rho guanine nucleotide exchange factor family for comparative assessment. In parallel, a more crude approach based on structural superimposition and molecular replacement was also assessed. Both models were then successfully refined using molecular dynamics simulations leading to protein structures where the major data from scientific literature could be recovered. We expect that the detailed strategy used in this work will prove useful for other protein-protein interface design. The RHOA-ARHGEF1 interface modeled here will be extremely useful for the design of inhibitors targeting this protein-protein interaction (PPI).

More evidence on TRIO missense mutations in the spectrin repeat domain causing severe developmental delay and recognizable facial dysmorphism with macrocephaly.

TRIO is a Dbl family guanine nucleotide exchange factor (GEF) and an important regulator of neuronal development. Most truncating and missense variants affecting the Dbl homology domain of TRIO are associated with a neurodevelopmental disorder with microcephaly (MIM617061). Recently, de novo missense variants affecting the spectrin repeat region of TRIO were associated with a novel phenotype comprising severe developmental delay and macrocephaly (MIM618825). Here, we provide more evidence on this new TRIO-associated phenotype by reporting two severely affected probands with de novo missense variants in TRIO affecting the spectrin repeat region upstream of the typically affected GEF1 domain of the protein.

NET1 promotes HCC growth and metastasis in vitro and in vivo via activating the Akt signaling pathway.

Neuroepithelial cell transforming gene 1 (NET1), a member of the guanine nucleotide exchange factor family, is involved in various cancers, including gastric cancer, breast cancer and glioma. However, the role of NET1 in hepatocellular carcinoma (HCC) remains largely uncovered. In this study, we found that NET1 expression was upregulated in HCC, and that upregulated NET1 expression was closely associated with poor prognosis and some clinical characteristics in HCC patients. Whilst forced expression of NET1 in HCC cells was observed to significantly promote cell growth and metastasis in vitro and in vivo; downregulation of NET1 was shown to exhibit an opposite inhibitory effect. RNA-seq analysis and gene set enrichment analysis demonstrated that knockdown of NET1 significantly suppressed the level of Akt phosphorylation level and the expression of Akt downstream genes in HCC cells. Moreover, MK2206, a potent Akt inhibitor was shown to block the NET1-induced effects in HCC. Taken together, this study demonstrated that, through the Akt signaling pathway, NET1 plays an oncogenic role in HCC progression and metastasis. Hence, NET1 may potentially be used as a potential therapeutic target and prognostic marker of HCC.

ARHGEF10L Promotes Cervical Tumorigenesis via RhoA-Mediated Signaling.

Background Rho guanine nucleotide exchange factor 10-like protein (ARHGEF10L) is a member of the guanine nucleotide exchange factor family, which regulates Rho GTPase activities, thus contributing to tumorigenesis. Our previous study demonstrated a strong association between the ARHGEF10L gene and the risk of cervical carcinoma. This study investigated the pathogenic role and mechanism of ARHGEF10L in cervical tumors. Methods The HeLa cell line, which was derived from cervical carcinoma, was transfected with ARHGEF10L-overexpressing plasmids or anti-ARHGEF10L siRNA. Cell counting kit-8 assays, wound-healing assays, and cell apoptosis assays were performed to investigate the effects of ARHGEF10L on cell activities. A Rho pull-down assay and RNA-sequencing analysis were performed to investigate the pathogenic pathway of ARHGEF10L involvement in cervical tumors. Results ARHGEF10L overexpression promoted cell proliferation and migration, reduced cell apoptosis, and induced epithelial-to-mesenchymal transition (EMT) via downregulation of E-cadherin and upregulation of N-cadherin and Slug in transfected HeLa cells. The overexpression of ARHGEF10L also upregulated GTP-RhoA, ROCK1, and phospho-ezrin/radixin/moesin (ERM) expression in HeLa cells. RNA-sequencing analysis detected altered transcription of 31 genes in HeLa cells with ARHGEF10L overexpression. Kyoto Encyclopedia of Genes and Genomes (KEGG) and gene ontology (GO) pathway analyses identified significant differences in cyclin-dependent protein serine/threonine kinase activity, cell responses to vitamin A, and Toll-like receptor signaling pathways. Both real-time PCR and Western blotting verified the increased expression of heat shock 70 kDa protein 6 (HSPA6) in ARHGEF10L-overexpressing HeLa cells. Since we reported that ARHGEF10L played a role through RhoA-ROCK1-ERM signaling, an important pathway in tumorigenesis, and stimulated EMT and HSPA6 expression in liver tumors and gastric tumor cells, we suggest that ARHGEF10L is a novel oncogene in many tumors.

Double inhibition and activation mechanisms of Ephexin family RhoGEFs

Ephexin family guanine nucleotide exchange factors (GEFs) transfer signals from Eph tyrosine kinase receptors to Rho GTPases, which play critical roles in diverse cellular processes, as well as cancers and brain disorders. Here, we elucidate the molecular basis underlying inhibition and activation of Ephexin family RhoGEFs. The crystal structures of partially and fully autoinhibited Ephexin4 reveal that the complete autoinhibition requires both N- and C-terminal inhibitory modes, which can operate independently to impede Ras homolog family member G (RhoG) access. This double inhibition mechanism is commonly employed by other Ephexins and SGEF, another RhoGEF for RhoG. Structural, enzymatic, and cell biological analyses show that phosphorylation of a conserved tyrosine residue in its N-terminal inhibitory domain and association of PDZ proteins with its C-terminal PDZ-binding motif may respectively relieve the two autoinhibitory modes in Ephexin4. Our study provides a mechanistic framework for understanding the fine-tuning regulation of Ephexin4 GEF activity and offers possible clues for its pathological dysfunction.

SPIKE1 Activates the GTPase ROP6 to Guide the Polarized Growth of Infection Threads in Lotus japonicus

In legumes, rhizobia attach to root hair tips and secrete nodulation factor to activate rhizobial infection and nodule organogenesis. Endosymbiotic rhizobia enter nodule primordia via a specialized transcellular compartment known as the infection thread (IT). The IT elongates by polar tip growth, following the path of the migrating nucleus along and within the root hair cell. Rho-family ROP GTPases are known to regulate the polarized growth of cells, but their role in regulating polarized IT growth is poorly understood. Here, we show that LjSPK1, a DOCK family guanine nucleotide exchange factor (GEF), interacts with three type I ROP GTPases. Genetic analyses showed that these three ROP GTPases are involved in root hair development, but only LjROP6 is required for IT formation after rhizobia inoculation. Misdirected ITs formed in the root hairs of Ljspk1 and Ljrop6 mutants. We show that LjSPK1 functions as a GEF that activates LjROP6. LjROP6 enhanced the plasma membrane localization LjSPK1 in Nicotiana benthamiana leaf cells and Lotus japonicus root hairs, and LjSPK1 and LjROP6 interact at the plasma membrane. Taken together, these results shed light on how the LjROP6-LjSPK1 module mediates the polarized growth of ITs in L. japonicus.

Identification of P-Rex1 in the Regulation of Liver Cancer Cell Proliferation and Migration via HGF/c-Met/Akt Pathway.

BackgroundRho-GTPases and their activators, guanine nucleotide exchange factors (GEFs), are increasingly being recognized as essential mediators of oncogenic signaling. Although it is known that P-Rex1, a member of the Dbl family of GEFs for the Rac small GTPase, contributes to the migration of cancer cells, its exact role in liver cancer and the underlying mechanisms remain unclear.Materials and MethodsPublic datasets from the Gene Expression Omnibus database (GEO) and clinical liver cancer samples were analyzed to explore the expression of P-Rex1. P-Rex1 knockdown and overexpression cell lines were established using a recombinant lentiviral transfection system. BrdU and colony formation assays were performed to determine cell viability. Migratory capacity was analyzed using a transwell migration assay and an in vitro wound-healing assay. Nude mice bearing subcutaneous xenograft tumors were established to determine the effects of P-Rex1 on tumorigenesis in vivo. The role of P-Rex1 in hepatocarcinogenesis was determined through Western blot and co-immunoprecipitation.ResultsInduced expression of endogenous P-Rex1 was identified in liver cancer tumors when compared with adjacent nonmalignant tissues from clinical data. In response to HGF treatment, P-Rex1-knockdown cells displayed reduced proliferation and migration in vitro as well as reduced xenograft tumor growth in vivo. Overexpression of P-Rex1 promoted liver cancer cell proliferation and migration. P-Rex1 primarily acts as a downstream effector of GPCR signaling. This study demonstrated that downregulation of P-Rex1 led to a significant decrease in the phosphorylation of Akt and Erk1/2 by reducing the phosphorylation of the tyrosine kinase receptor c-Met. Furthermore, a physical association between P-Rex1 and c-Met was observed after HGF treatment, suggesting that P-Rex1 may be involved in the HGF/c-Met signaling pathway.ConclusionThese results support the role of P-Rex1 as a novel player in liver cancer, which suggest that targeting P-Rex1 may provide a potential strategy for liver cancer treatment.

A mitochondrial recruitment assay to characterize a family of DENN‐domain proteins, Rab GDP/GTP exchange factors.

Intracellular membrane trafficking controls the levels, localization and functional activity of a myriad of proteins and is thus critical for cellular function. Rab GTPases are molecular switches that regulate membrane trafficking by toggling between GTP‐bound (active; membrane‐bound) and GDP‐bound (inactive; cytosolic) states. These GTPases are controlled largely by guanine nucleotide exchange factors (GEFs) that mediate the exchange of GDP for GTP. Mammalian cells possess 61 Rabs but many of their GEFs are unknown. One of the largest family of Rab GEFs contain an evolutionarily conserved protein module called the DENN domain. There are 18 DENN domain proteins but many of their corresponding Rab GTPase partner(s) are either not known or are controversial. This is due to the fact that all studies concerning DENN domains have used biochemical and in‐vitro assays, which do not reflect physiological conditions. Here we describe a cell‐based GEF assay as a method to identify GTPase substrates of DENN domain proteins. By overexpressing DENN domains fused to a mitochondrial targeting sequence we can drive the recruitment of physiological GTPase substrates to the mitochondrial membrane, which can then be detected by immunofluorescence analysis. We validated our assay with a known DENN/Rab pair, DENND1A and Rab35. Using a panel of GFP‐tagged Rabs we have resolved controversies over previously reported DENN/Rab pairs and have identified new Rab substrates for DENN domains. Identifying the full complement of DENN/Rab pairs will provide new insights into membrane trafficking and cellular function.Support or Funding Information1) Canada First Research Excellence Fund, awarded to McGill University for Healthy Brains for Healthy Lives. 2) ALS Canada

Vav2 lacks Ca2+ entry-promoting scaffolding functions unique to Vav1 and inhibits T cell activation via Cdc42.

Vav family guanine nucleotide exchange factors (GEFs) are essential regulators of immune function. Despite their structural similarity, Vav1 promotes and Vav2 opposes T cell receptor (TCR)-induced Ca2+ entry. By using a Vav1-deficient Jurkat T cell line, we find that Vav1 facilitates Ca2+ entry via non-catalytic scaffolding functions that are encoded by the catalytic core of Vav1 and flanking linker regions. We implicate, in this scaffolding function, a previously undescribed polybasic motif that is strictly conserved in Vav1 and absent from Vav2 in tetrapods. Conversely, the catalytic activity of Vav2 contributes to the suppression of TCR-mediated Ca2+ entry. By performing an in vivo 'GEF trapping' assay in intact cells, we demonstrate that Cdc42 interacts with the catalytic surface of Vav2 but not Vav1, and that Vav1 discriminates Cdc42 from Rac1 via F56 (W56 in Rac1). Finally, the Cdc42-specific inhibitor ZCL278 and the shRNA-mediated suppression of Cdc42 each prevent the inhibition of TCR-induced Ca2+ entry by Vav2. These findings define stark differences in the functions of Vav1 and Vav2, and provide an explanation for the differential usage of these Vav isoforms by immune subpopulations.