Crystal structure of the rac activator, Asef, reveals its autoinhibitory mechanism

SH3 domain regulation of RhoGAP activity: Crosstalk between p120RasGAP and DLC1 RhoGAP

RhoGAP proteins are key regulators of Rho family GTPases and influence a variety of cellular processes, including cell migration, adhesion, and cytokinesis. These GTPase activating proteins (GAPs) downregulate Rho signaling by binding and enhancing the intrinsic GTPase activity of Rho proteins. Deleted in liver cancer 1 (DLC1) is a tumor suppressor and ubiquitously expressed RhoGAP protein; its activity is regulated in part by binding p120RasGAP, a GAP protein for the Ras GTPases. In this study, we report the co-crystal structure of the p120RasGAP SH3 domain bound directly to DLC1 RhoGAP, at a site partially overlapping the RhoA binding site and impinging on the catalytic arginine finger. We demonstrate biochemically that mutation of this interface relieves inhibition of RhoGAP activity by the SH3 domain. These results reveal the mechanism for inhibition of DLC1 RhoGAP activity by p120RasGAP and demonstrate the molecular basis for direct SH3 domain modulation of GAP activity.

Microtubules as Major Regulators of Endothelial Function: Implication for Lung Injury

Endothelial dysfunction has been attributed as one of the major complications in COVID-19 patients, a global pandemic that has already caused over 4 million deaths worldwide. The dysfunction of endothelial barrier is characterized by an increase in endothelial permeability and inflammatory responses, and has even broader implications in the pathogenesis of acute respiratory syndromes such as ARDS, sepsis and chronic illnesses represented by pulmonary arterial hypertension and interstitial lung disease. The structural integrity of endothelial barrier is maintained by cytoskeleton elements, cell-substrate focal adhesion and adhesive cell junctions. Agonist-mediated changes in endothelial permeability are directly associated with reorganization of actomyosin cytoskeleton leading to cell contraction and opening of intercellular gaps or enhancement of cortical actin cytoskeleton associated with strengthening of endothelial barrier. The role of actin cytoskeleton remodeling in endothelial barrier regulation has taken the central stage, but the impact of microtubules in this process remains less explored and under-appreciated. This review will summarize the current knowledge on the crosstalk between microtubules dynamics and actin cytoskeleton remodeling, describe the signaling mechanisms mediating this crosstalk, discuss epigenetic regulation of microtubules stability and its nexus with endothelial barrier maintenance, and overview a role of microtubules in targeted delivery of signaling molecules regulating endothelial permeability and inflammation.

Structural insights into the small GTPase specificity of the DOCK guanine nucleotide exchange factors

The dedicator of cytokinesis (DOCK) family of guanine nucleotide exchange factors (GEFs) regulates cytoskeletal dynamics by activating the GTPases Rac and/or Cdc42. Eleven human DOCK proteins play various important roles in developmental processes and the immune system. Of these, DOCK1-5 proteins bind to engulfment and cell motility (ELMO) proteins to perform their physiological functions. Recent structural studies have greatly enhanced our understanding of the complex and diverse mechanisms of DOCK GEF activity and GTPase recognition and its regulation by ELMO. This review is focused on gaining structural insights into the substrate specificity of the DOCK GEFs, and discuss how Rac and Cdc42 are specifically recognized by the catalytic DHR-2 and surrounding domains of DOCK or binding partners.

Advances and Insights of APC-Asef Inhibitors for Metastatic Colorectal Cancer Therapy

In Colorectal cancer (CRC), adenomatous polyposis coli (APC) directly interacts with the Rho guanine nucleotide exchange factor 4 (Asef) and releases its GEF activity. Activated Asef promotes the aberrant migration and invasion of CRC cell through a CDC42-mediated pathway. Knockdown of either APC or Asef significantly decreases the migration of CRC cells. Therefore, disrupting the APC-Asef interaction is a promising strategy for the treatment of invasive CRC. With the growth of structural information, APC-Asef inhibitors have been designed, providing hope for CRC therapy. Here, we will review the APC-Asef interaction in cancer biology, the structural complex of APC-Asef, two generations of peptide inhibitors of APC-Asef, and small molecule inhibitors of APC-Asef, focusing on research articles over the past 30 years. We posit that these advances in the discovery of APC-Asef inhibitors establish the protein-protein interaction (PPI) as targetable and provide a framework for other PPI programs.

Structural Insights into the Regulation Mechanism of Small GTPases by GEFs

Small GTPases are key regulators of cellular events, and their dysfunction causes many types of cancer. They serve as molecular switches by cycling between inactive guanosine diphosphate (GDP)-bound and active guanosine triphosphate (GTP)-bound states. GTPases are deactivated by GTPase-activating proteins (GAPs) and are activated by guanine-nucleotide exchange factors (GEFs). The intrinsic GTP hydrolysis activity of small GTPases is generally low and is accelerated by GAPs. GEFs promote GDP dissociation from small GTPases to allow for GTP binding, which results in a conformational change of two highly flexible segments, called switch I and switch II, that enables binding of the gamma phosphate and allows small GTPases to interact with downstream effectors. For several decades, crystal structures of many GEFs and GAPs have been reported and have shown tremendous structural diversity. In this review, we focus on the latest structural studies of GEFs. Detailed pictures of the variety of GEF mechanisms at atomic resolution can provide insights into new approaches for drug discovery.

The Intermolecular Interaction of Ephexin4 Leads to Autoinhibition by Impeding Binding of RhoG

Ephexin4 is a guanine nucleotide-exchange factor (GEF) for RhoG and is involved in various RhoG-related cellular processes such as phagocytosis of apoptotic cells and migration of cancer cells. Ephexin4 forms an oligomer via an intermolecular interaction, and its GEF activity is increased in the presence of Elmo, an Ephexin4-interacting protein. However, it is uncertain if and how Ephexin4 is autoinhibited. Here, using an Ephexin4 mutant that abrogated the intermolecular interaction, we report that this interaction impeded binding of RhoG to Ephexin4 and thus inhibited RhoG activation. Mutation of the glutamate residue at position 295, which is a highly conserved residue located in the region of Ephexin4 required for the intermolecular interaction, to alanine (Ephexin4^E295A) disrupted the intermolecular interaction and increased binding of RhoG, resulting in augmented RhoG activation. In addition, phagocytosis of apoptotic cells and formation of membrane ruffles were increased more by expression of Ephexin4^E295A than by expression of wild-type Ephexin4. Taken together, our data suggest that Ephexin4 is autoinhibited through its intermolecular interaction, which impedes binding of RhoG.

Rational Design and Structure Validation of a Novel Peptide Inhibitor of the Adenomatous-Polyposis-Coli (APC)-Rho-Guanine-Nucleotide-Exchange-Factor-4 (Asef) Interaction

In colorectal cancer, adenomatous polyposis coli (APC) interacts with Rho guanine-nucleotide-exchange factor 4 (Asef), thereby stimulating aberrant colorectal-cancer-cell migration. Consequently, the APC-Asef interaction represents a promising therapeutic target for mitigating colorectal-cancer migration. In this study, we adopted the rational-design strategy involving the introduction of intramolecular hydrogen bonds and optimization of the lipophilic substituents to improve the binding affinities of peptides, leading to the discovery of MAI-400, the best inhibitor of the APC-Asef interaction known to date ( K_d = 0.012 μM, IC₅₀ = 0.25 μM). Comprehensive evaluation of MAI-400 by biochemical and biophysical assays revealed the formation and effect of an intramolecular hydrogen bond. A cell-based assay showed MAI-400 efficiently blocking the APC-Asef interaction in a dose-dependent manner. Therefore, our study provides a best-in-class inhibitor, MAI-400, based on the rational drug design and structural validation, that can effectively inhibit the APC-Asef interaction.

Microtubules-associated Rac regulation of endothelial barrier: a role of Asef in acute lung injury

The endothelial barrier function regulated by the cytoskeletal reorganizations has been implicated in the pathogenesis of multiple lung diseases including asthma, sepsis, edema, and acute respiratory distress syndrome. The extensive studies have established that activation of small GTPase Rac is a key mechanism in endothelial barrier protection but the role of microtubules-associated Rac in the endothelial functions remains poorly understood. With the emerging evidences that microtubules disassembly also plays a critical role in actin cytoskeleton remodeling leading to endothelial permeability, the knowledge on microtubules-mediated regulation of endothelial barrier is imperative to better understand the etiology of lung injuries as well as to develop novel therapeutics against these disorders. In this regard, our recent studies have revealed some novel aspects of microtubules-mediated regulation of endothelial barrier functions and unraveled a putative role of Rac-specific guanine nucleotide exchange factor Asef in mediating the barrier protective effects of hepatocyte growth factor. In this review, we will discuss the role of this novel Rac activator Asef in endothelial barrier protection and its regulation by microtubules.

Temporal Profiling of Astrocyte Precursors Reveals Parallel Roles for Asef during Development and after Injury

Lineage development is a stepwise process, governed by stage-specific regulatory factors and associated markers. Astrocytes are one of the principle cell types in the CNS and the stages associated with their development remain very poorly defined. To identify these stages, we performed gene-expression profiling on astrocyte precursor populations in the spinal cord, identifying distinct patterns of gene induction during their development that are strongly correlated with human astrocytes. Validation studies identified a new cohort of astrocyte-associated genes during development and demonstrated their expression in reactive astrocytes in human white matter injury (WMI). Functional studies on one of these genes revealed that mice lacking Asef exhibited impaired astrocyte differentiation during development and repair after WMI, coupled with compromised blood-brain barrier integrity in the adult CNS. These studies have identified distinct stages of astrocyte lineage development associated with human WMI and, together with our functional analysis of Asef, highlight the parallels between astrocyte development and their reactive counterparts associated with injury.

SIGNIFICANCE STATEMENT

Astrocytes play a central role in CNS function and associated diseases. Yet the mechanisms that control their development remain poorly defined. Using the developing mouse spinal cord as a model system, we identify molecular changes that occur in developing astrocytes. These molecular signatures are strongly correlated with human astrocyte expression profiles and validation in mouse spinal cord identifies a host of new genes associated with the astrocyte lineage. These genes are present in reactive astrocytes in human white matter injury, and functional studies reveal that one of these genes, Asef, contributes to reactive astrocyte responses after injury. These studies identify distinct stages of astrocyte lineage development and highlight the parallels between astrocyte development and their reactive counterparts associated with injury.

Intermolecular steric inhibition of Ephexin4 is relieved by Elmo1

Ephexin4, a guanine nucleotide-exchange factor for RhoG, promotes engulfment of apoptotic cells and cancer cell migration in a RhoG-dependent manner, which is synergistically augmented by Elmo1, an Ephexin4-interacting protein. However, the underlying molecular mechanism remains elusive. Here, we report a mechanism by which Elmo1 cooperates with Ephexin4 to activate RhoG. We found that Ephexin4 activity was increased by elimination of its SH3 domain which intermolecularly interacts with the N20 region of Ephexin4. This interaction prevented RhoG from binding to Ephexin4 and thus inhibited RhoG activation. Moreover, we also found that Elmo1 associated with the SH3 domain as well as the N20 region and competed with the SH3 domain for binding to the N20 region, interrupting the interaction of the SH3 domain with the N20 region and thereby promoting RhoG binding to Ephexin4. In addition, the activity of Ephexin4 lacking the SH3 domain was comparable to that of Ephexin4 with Elmo1. Taken together, the data suggest that Elmo1 relieves the steric hindrance of Ephexin4 generated by the intermolecular interaction of the SH3 domain and makes Ephexin4 more accessible to RhoG.

Structural and Biochemical Characterization of the Catalytic Core of the Metastatic Factor P-Rex1 and Its Regulation by PtdIns(3,4,5)P3

Phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent Rac exchanger 1 (P-Rex1) is a Rho guanine nucleotide exchange factor synergistically activated by PIP3 and Gβγ that plays an important role in the metastasis of breast, prostate, and skin cancer, making it an attractive therapeutic target. However, the molecular mechanisms behind P-Rex1 regulation are poorly understood. We determined structures of the P-Rex1 pleckstrin homology (PH) domain bound to the headgroup of PIP3 and resolved that PIP3 binding to the PH domain is required for P-Rex1 activity in cells but not for membrane localization, which points to an allosteric activation mechanism by PIP3. We also determined structures of the P-Rex1 tandem Dbl homology/PH domains in complexes with two of its substrate GTPases, Rac1 and Cdc42. Collectively, this study provides important molecular insights into P-Rex1 regulation and tools for targeting the PIP3-binding pocket of P-Rex1 with a new generation of cancer chemotherapeutic agents.

Catching Functional Modes and Structural Communication in Dbl Family Rho Guanine Nucleotide Exchange Factors

Computational approaches such as Principal Component Analysis (PCA) and Elastic Network Model-Normal Mode Analysis (ENM-NMA) are proving to be of great value in investigating relevant biological problems linked to slow motions with no demand in computer power. In this study, these approaches have been coupled to the graph theory-based Protein Structure Network (PSN) analysis to dissect functional dynamics and structural communication in the Dbl family of Rho Guanine Nucleotide Exchange Factors (RhoGEFs). They are multidomain proteins whose common structural feature is a DH-PH tandem domain deputed to the GEF activity that makes them play a central role in cell and cancer biology. While their common GEF action is accomplished by the DH domain, their regulatory mechanisms are highly variegate and depend on the PH and the additional domains as well as on interacting proteins. Major evolutionary-driven deformations as inferred from PCA concern the α6 helix of DH that dictates the orientation of the PH domain. Such deformations seem to depend on the mechanisms adopted by the GEF to prevent Rho binding, i.e. functional specialization linked to autoinhibition. In line with PCA, ENM-NMA indicates α6 and the linked PH domain as the portions of the tandem domain holding almost the totality of intrinsic and functional dynamics, with the α6/β1 junction acting as a hinge point for the collective motions of PH. In contrast, the DH domain holds a static scaffolding and hub behavior, with structural communication playing a central role in the regulatory actions by other domains/proteins. Possible allosteric communication pathways involving essentially DH were indeed found in those RhoGEFs acting as effectors of small or heterotrimeric RasGTPases. The employed methodology is suitable for deciphering structure/dynamics relationships in large sets of homologous or analogous proteins.

The guanine nucleotide exchange factor (GEF) Asef2 promotes dendritic spine formation via Rac activation and spinophilin-dependent targeting

Dendritic spines are actin-rich protrusions that establish excitatory synaptic contacts with surrounding neurons. Reorganization of the actin cytoskeleton is critical for the development and plasticity of dendritic spines, which is the basis for learning and memory. Rho family GTPases are emerging as important modulators of spines and synapses, predominantly through their ability to regulate actin dynamics. Much less is known, however, about the function of guanine nucleotide exchange factors (GEFs), which activate these GTPases, in spine and synapse development. In this study we show that the Rho family GEF Asef2 is found at synaptic sites, where it promotes dendritic spine and synapse formation. Knockdown of endogenous Asef2 with shRNAs impairs spine and synapse formation, whereas exogenous expression of Asef2 causes an increase in spine and synapse density. This effect of Asef2 on spines and synapses is abrogated by expression of GEF activity-deficient Asef2 mutants or by knockdown of Rac, suggesting that Asef2-Rac signaling mediates spine development. Because Asef2 interacts with the F-actin-binding protein spinophilin, which localizes to spines, we investigated the role of spinophilin in Asef2-promoted spine formation. Spinophilin recruits Asef2 to spines, and knockdown of spinophilin hinders spine and synapse formation in Asef2-expressing neurons. Furthermore, inhibition of N-methyl-d-aspartate receptor (NMDA) activity blocks spinophilin-mediated localization of Asef2 to spines. These results collectively point to spinophilin-Asef2-Rac signaling as a novel mechanism for the development of dendritic spines and synapses.

Hepatocyte growth factor-induced Asef-IQGAP1 complex controls cytoskeletal remodeling and endothelial barrier

Hepatocyte growth factor (HGF) attenuates agonist-induced endothelial cell (EC) permeability and increases pulmonary endothelial barrier function via Rac-dependent enhancement of the peripheral actin cytoskeleton. However, the precise mechanisms of HGF effects on the peripheral cytoskeleton are not well understood. This study evaluated a role for Rac/Cdc42-specific guanine nucleotide exchange factor Asef and the multifunctional Rac effector, IQGAP1, in the mechanism of HGF-induced EC barrier enhancement. HGF induced Asef and IQGAP1 co-localization at the cell cortical area and stimulated formation of an Asef-IQGAP1 functional protein complex. siRNA-induced knockdown of Asef or IQGAP1 attenuated HGF-induced EC barrier enhancement. Asef knockdown attenuated HGF-induced Rac activation and Rac association with IQGAP1, and it abolished both IQGAP1 accumulation at the cell cortical layer and IQGAP1 interaction with actin cytoskeletal regulators cortactin and Arp3. Asef activation state was essential for Asef interaction with IQGAP1 and protein complex accumulation at the cell periphery. In addition to the previously reported role of the IQGAP1 RasGAP-related domain in the Rac-dependent IQGAP1 activation and interaction with its targets, we show that the IQGAP1 C-terminal domain is essential for HGF-induced IQGAP1/Asef interaction and Asef-Rac-dependent activation leading to IQGAP1 interaction with Arp3 and cortactin as a positive feedback mechanism of IQGAP1 activation. These results demonstrate a novel feedback mechanism of HGF-induced endothelial barrier enhancement via Asef/IQGAP1 interactions, which regulate the level of HGF-induced Rac activation and promote cortical cytoskeletal remodeling via IQGAP1-Arp3/cortactin interactions.

I'm coming to GEF you: Regulation of RhoGEFs during cell migration

Cell migration is a highly regulated multistep process that requires the coordinated regulation of cell adhesion, protrusion, and contraction. These processes require numerous protein–protein interactions and the activation of specific signaling pathways. The Rho family of GTPases plays a key role in virtually every aspect of the cell migration cycle. The activation of Rho GTPases is mediated by a large and diverse family of proteins; the guanine nucleotide exchange factors (RhoGEFs). GEFs work immediately upstream of Rho proteins to provide a direct link between Rho activation and cell–surface receptors for various cytokines, growth factors, adhesion molecules, and G protein-coupled receptors. The regulated targeting and activation of RhoGEFs is essential to coordinate the migratory process. In this review, we summarize the recent advances in our understanding of the role of RhoGEFs in the regulation of cell migration.

CDC42 inhibition suppresses progression of incipient intestinal tumors

Mutations in the APC or β-catenin genes are well-established initiators of colorectal cancer, yet modifiers that facilitate the survival and progression of nascent tumor cells are not well defined. Using genetic and pharmacologic approaches in mouse colorectal cancer and human colorectal cancer xenograft models, we show that incipient intestinal tumor cells activate CDC42, an APC-interacting small GTPase, as a crucial step in malignant progression. In the mouse, Cdc42 ablation attenuated the tumorigenicity of mutant intestinal cells carrying single APC or β-catenin mutations. Similarly, human colorectal cancer with relatively higher levels of CDC42 activity was particularly sensitive to CDC42 blockade. Mechanistic studies suggested that Cdc42 may be activated at different levels, including at the level of transcriptional activation of the stem cell-enriched Rho family exchange factor Arhgef4. Our results indicate that early-stage mutant intestinal epithelial cells must recruit the pleiotropic functions of Cdc42 for malignant progression, suggesting its relevance as a biomarker and therapeutic target for selective colorectal cancer intervention.

A conformational switch in collybistin determines the differentiation of inhibitory postsynapses

The formation of neuronal synapses and the dynamic regulation of their efficacy depend on the assembly of the postsynaptic neurotransmitter receptor apparatus. Receptor recruitment to inhibitory GABAergic and glycinergic synapses is controlled by the scaffold protein gephyrin and the adaptor protein collybistin. We derived new insights into the structure of collybistin and used these to design biochemical, cell biological, and genetic analyses of collybistin function. Our data define a collybistin-based protein interaction network that controls the gephyrin content of inhibitory postsynapses. Within this network, collybistin can adopt open/active and closed/inactive conformations to act as a switchable adaptor that links gephyrin to plasma membrane phosphoinositides. This function of collybistin is regulated by binding of the adhesion protein neuroligin-2, which stabilizes the open/active conformation of collybistin at the postsynaptic plasma membrane by competing with an intramolecular interaction in collybistin that favors the closed/inactive conformation. By linking trans-synaptic neuroligin-dependent adhesion and phosphoinositide signaling with gephyrin recruitment, the collybistin-based regulatory switch mechanism represents an integrating regulatory node in the formation and function of inhibitory postsynapses.

Phosphorylation of serine 106 in Asef2 regulates cell migration and adhesion turnover

Asef2, a 652-amino acid protein, is a guanine nucleotide exchange factor (GEF) that regulates cell migration and other processes via activation of Rho family GTPases, including Rac. Binding of the tumor suppressor adenomatous polyposis coli (APC) to Asef2 is known to induce its GEF activity; however, little is currently known about other modes of Asef2 regulation. Here, we investigated the role of phosphorylation in regulating Asef2 activity and function. Using high-resolution mass spectrometry (MS) and tandem mass spectrometry (MS/MS), we obtained complete coverage of all phosphorylatable residues and identified six phosphorylation sites. One of these, serine 106 (S106), was particularly intriguing as a potential regulator of Asef2 activity because of its location within the APC-binding domain. Interestingly, mutation of this serine to alanine (S106A), a non-phosphorylatable analogue, greatly diminished the ability of Asef2 to activate Rac, while a phosphomimetic mutation (serine to aspartic acid, S106D) enhanced Rac activation. Furthermore, expression of these mutants in HT1080 cells demonstrated that phosphorylation of S106 is critical for Asef2-promoted migration and for cell-matrix adhesion assembly and disassembly (adhesion turnover), which is a process that facilitates efficient migration. Collectively, our results show that phosphorylation of S106 modulates Asef2 GEF activity and Asef2-mediated cell migration and adhesion turnover.

The C-terminal SH3 domain contributes to the intramolecular inhibition of Vav family proteins

Vav proteins are phosphorylation-dependent guanine nucleotide exchange factors (GEFs) that catalyze the activation of members of the Rho family of guanosine triphosphatases (GTPases). The current regulatory model holds that the nonphosphorylated, catalytically inactive state of these GEFs is maintained by intramolecular interactions among the amino-terminal domains and the central catalytic core, which block the binding of Vav proteins to GTPases. We showed that this autoinhibition is mechanistically more complex, also involving the bivalent association of the carboxyl-terminal Src homology 3 (SH3) region of Vav with its catalytic and pleckstrin homology (PH) domains. Such interactions occurred through proline-rich region-independent mechanisms. Full release from this double-locked state required synergistic weakening effects from multiple phosphorylated tyrosine residues, thus providing an optimized system to generate gradients of Vav GEF activity depending on upstream signaling inputs. This mechanism is shared by mammalian and Drosophila melanogaster Vav proteins, suggesting that it may be a common regulatory feature for this protein family.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

Regulation of small GTPases by GEFs, GAPs, and GDIs

Small GTPases use GDP/GTP alternation to actuate a variety of functional switches that are pivotal for cell dynamics. The GTPase switch is turned on by GEFs, which stimulate dissociation of the tightly bound GDP, and turned off by GAPs, which accelerate the intrinsically sluggish hydrolysis of GTP. For Ras, Rho, and Rab GTPases, this switch incorporates a membrane/cytosol alternation regulated by GDIs and GDI-like proteins. The structures and core mechanisms of representative members of small GTPase regulators from most families have now been elucidated, illuminating their general traits combined with scores of unique features. Recent studies reveal that small GTPase regulators have themselves unexpectedly sophisticated regulatory mechanisms, by which they process cellular signals and build up specific cell responses. These mechanisms include multilayered autoinhibition with stepwise release, feedback loops mediated by the activated GTPase, feed-forward signaling flow between regulators and effectors, and a phosphorylation code for RhoGDIs. The flipside of these highly integrated functions is that they make small GTPase regulators susceptible to biochemical abnormalities that are directly correlated with diseases, notably a striking number of missense mutations in congenital diseases, and susceptible to bacterial mimics of GEFs, GAPs, and GDIs that take command of small GTPases in infections. This review presents an overview of the current knowledge of these many facets of small GTPase regulation.

Structural basis for autoinhibition of the guanine nucleotide exchange factor FARP2

FARP2 is a Dbl-family guanine nucleotide exchange factor (GEF) that contains a 4.1, ezrin, radixin and moesin (FERM) domain, a Dbl-homology (DH) domain and two pleckstrin homology (PH) domains. FARP2 activates Rac1 or Cdc42 in response to upstream signals, thereby regulating processes such as neuronal axon guidance and bone homeostasis. How the GEF activity of FARP2 is regulated remained poorly understood. We have determined the crystal structures of the catalytic DH domain and the DH-PH-PH domains of FARP2. The structures reveal an auto-inhibited conformation in which the GEF substrate-binding site is blocked collectively by the last helix in the DH domain and the two PH domains. This conformation is stabilized by multiple interactions among the domains and two well-structured inter-domain linkers. Our cell-based activity assays confirm the suppression of the FARP2 GEF activity by these auto-inhibitory elements.

Regulation of the DH-PH tandem of guanine nucleotide exchange factor for Rho GTPases by phosphoinositides

Rho GTPases act as molecular switches central in cellular processes such as cytoskeleton dynamics, migration, cell proliferation, growth or survival. Their activation is tightly regulated downstream of cell surface receptors by Guanine nucleotide Exchange Factors (GEFs), that are responsible for the specificity, the accuracy, and the spatial restriction of Rho GTPases response to extracellular cues. Because there is about four time more RhoGEFs that Rho GTPases, and GEFs do not always show a strict specificity for GTPases, it is clear that their regulation depends on specific interactions with the subcellular environment. RhoGEFs bear a peculiar structure, highly conserved though evolution, consisting of a DH-PH tandem, the DH (Dbl homology) domain being responsible for the exchange activity. The function of the PH (Pleckstrin homology) domain known to bind phosphoinositides, however, remains elusive, and reports are in many cases rather confusing. This review summarizes data on the regulation of RhoGEFs activity through interaction of the PH-associated DH domain with phosphoinositides which are considered as critical players in the spatial organization of major signaling pathways.

RHGF-2 is an essential Rho-1 specific RhoGEF that binds to the multi-PDZ domain scaffold protein MPZ-1 in Caenorhabditis elegans

RhoGEF proteins activate the Rho family of small GTPases and thus play a key role in regulating fundamental cellular processes such as cell morphology and polarity, cell cycle progression and gene transcription. We identified a Caenorhabditis elegans RhoGEF protein, RHGF-2, as a binding partner of the C. elegans multi-PDZ domain scaffold protein MPZ-1 (MUPP1 in mammals). RHGF-2 exhibits significant identity to the mammalian RhoGEFs PLEKHG5/Tech/Syx and contains a class I C-terminal PDZ binding motif (SDV) that interacts most strongly to MPZ-1 PDZ domain eight. RHGF-2 RhoGEF activity is specific to the C. elegans RhoA homolog RHO-1 as determined by direct binding, GDP/GTP exchange and serum response element-driven reporter activity. rhgf-2 is an essential gene since rhgf-2 deletion mutants do not elongate during embryogenesis and hatch as short immobile animals that arrest development. Interestingly, the expression of a functional rhgf-2::gfp transgene appears to be exclusively neuronal and rhgf-2 overexpression results in loopy movement with exaggerated body bends. Transient expression of RHGF-2 in N1E-115 neuroblastoma cells prevents neurite outgrowth similar to constitutive RhoA activation in these cells. Together, these observations indicate neuronally expressed RHGF-2 is an essential RHO-1 specific RhoGEF that binds most strongly to MPZ-1 PDZ domain eight and is required for wild-type C. elegans morphology and growth.

Collybistin splice variants differentially interact with gephyrin and Cdc42 to regulate gephyrin clustering at GABAergic synapses

Collybistin (CB) is a guanine-nucleotide-exchange factor (GEF) selectively activating Cdc42. CB mutations cause X-linked mental retardation due to defective clustering of gephyrin, a postsynaptic protein associated with both glycine and GABA(A) receptors. Using a combination of biochemistry and cell biology we provide novel insights into the roles of the CB2 splice variants, CB2(SH3+) and CB2(SH3-), and their substrate, Cdc42, in regulating gephyrin clustering at GABAergic synapses. Transfection of Myc-tagged CB2(SH3+) and CB2(SH3-) into cultured neurons revealed strong, but distinct, effects promoting postsynaptic gephyrin clustering, denoting mechanistic differences in their function. In addition, overexpression of constitutively active or dominant-negative Cdc42 mutants identified a new function of Cdc42 in regulating the shape and size of postsynaptic gephyrin clusters. Using biochemical assays and native brain tissue, we identify a direct interaction between gephyrin and Cdc42, independent of its activation state. Finally, our data show that CB2(SH3-), but not CB2(SH3+), can form a ternary complex with gephyrin and Cdc42, providing a biochemical substrate for the distinct contribution of these CB isoforms in gephyrin clustering at GABAergic postsynaptic sites. Taken together, our results identify CB and Cdc42 as major regulators of GABAergic postsynaptic densities.

Membrane shaping by the Bin/amphiphysin/Rvs (BAR) domain protein superfamily

BAR domain superfamily proteins have emerged as central regulators of dynamic membrane remodeling, thereby playing important roles in a wide variety of cellular processes, such as organelle biogenesis, cell division, cell migration, secretion, and endocytosis. Here, we review the mechanistic and structural basis for the membrane curvature-sensing and deforming properties of BAR domain superfamily proteins. Moreover, we summarize the present state of knowledge with respect to their regulation by autoinhibitory mechanisms or posttranslational modifications, and their interactions with other proteins, in particular with GTPases, and with membrane lipids. We postulate that BAR superfamily proteins act as membrane-deforming scaffolds that spatiotemporally orchestrate membrane remodeling.

Essential role of the NH2-terminal region of Cdc24 guanine nucleotide exchange factor in its initial polarized localization in Saccharomyces cerevisiae

The cortical recruitment and accumulation of the small GTPase Cdc42 are crucial steps in the establishment of polarity, but this process remains obscure. Cdc24 is an upstream regulator of budding yeast Cdc42 that accelerates the exchange of GDP for GTP in Cdc42 via its Dbl homology (DH) domain. Here, we isolated five novel temperature-sensitive (ts) cdc24 mutants, the green fluorescent protein (GFP)-fused proteins of which lose their polarized localization at the nonpermissive temperature. All amino acid substitutions in the mutants were mapped to the NH2-terminal region of Cdc24, including the calponin homology (CH) domain. These Cdc24-ts mutant proteins did not interact with Bem1 at the COOH-terminal PB1 domain, suggesting a lack of exposure of the PB1 domain in the mutant proteins. The cdc24-ts mutants were also defective in polarization in the absence of Bem1. It was previously reported that a fusion protein containing Cdc24 and the p21-activated kinase (PAK)-like kinase Cla4 could bypass the requirement for Bem1 in polarity cue-independent budding (i.e., symmetry breaking). Cdc24-ts-Cla4 fusion proteins also showed ts localization at the polarity site. We propose that the NH2-terminal region unmasks the DH and PB1 domains, leading to the activation of Cdc42 and interaction with Bem1, respectively, to initiate cell polarization.

Insights into the molecular activation mechanism of the RhoA-specific guanine nucleotide exchange factor, PDZRhoGEF

PDZRhoGEF (PRG) belongs to a small family of RhoA-specific nucleotide exchange factors that mediates signaling through select G-protein-coupled receptors via Gα(12/13) and activates RhoA by catalyzing the exchange of GDP to GTP. PRG is a multidomain protein composed of PDZ, regulators of G-protein signaling-like (RGSL), Dbl-homology (DH), and pleckstrin-homology (PH) domains. It is autoinhibited in cytosol and is believed to undergo a conformational rearrangement and translocation to the membrane for full activation, although the molecular details of the regulation mechanism are not clear. It has been shown recently that the main autoregulatory elements of PDZRhoGEF, the autoinhibitory "activation box" and the "GEF switch," which is required for full activation, are located directly upstream of the catalytic DH domain and its RhoA binding surface, emphasizing the functional role of the RGSL-DH linker. Here, using a combination of biophysical and biochemical methods, we show that the mechanism of PRG regulation is yet more complex and may involve an additional autoinhibitory element in the form of a molten globule region within the linker between RGSL and DH domains. We propose a novel, two-tier model of autoinhibition where the activation box and the molten globule region act synergistically to impair the ability of RhoA to bind to the catalytic DH-PH tandem. The molten globule region and the activation box become less ordered in the PRG-RhoA complex and dissociate from the RhoA-binding site, which may constitute a critical step leading to PRG activation.

Structural basis for the recognition of Asef by adenomatous polyposis coli

Adenomatous polyposis coli (APC) regulates cell-cell adhesion and cell migration through activating the APC-stimulated guanine nucleotide-exchange factor (GEF; Asef), which is usually autoinhibited through the binding between its Src homology 3 (SH3) and Dbl homology (DH) domains. The APC-activated Asef stimulates the small GTPase Cdc42, which leads to decreased cell-cell adherence and enhanced cell migration. In colorectal cancers, truncated APC constitutively activates Asef and promotes cancer cell migration and angiogenesis. Here, we report crystal structures of the human APC/Asef complex. We find that the armadillo repeat domain of APC uses a highly conserved surface groove to recognize the APC-binding region (ABR) of Asef, conformation of which changes dramatically upon binding to APC. Key residues on APC and Asef for the complex formation were mutated and their importance was demonstrated by binding and activity assays. Structural superimposition of the APC/Asef complex with autoinhibited Asef suggests that the binding between APC and Asef might create a steric clash between Asef-DH domain and APC, which possibly leads to a conformational change in Asef that stimulates its GEF activity. Our structures thus elucidate the molecular mechanism of Asef recognition by APC, as well as provide a potential target for pharmaceutical intervention against cancers.

Identification of a negative regulatory region for the exchange activity and characterization of T332I mutant of Rho guanine nucleotide exchange factor 10 (ARHGEF10)

The T332I mutation in Rho guanine nucleotide exchange factor 10 (ARHGEF10) was previously found in persons with slowed nerve conduction velocities and thin myelination of peripheral nerves. However, the molecular and cellular basis of the T332I mutant is not understood. Here, we show that ARHGEF10 has a negative regulatory region in the N terminus, in which residue 332 is located, and the T332I mutant is constitutively active. An N-terminal truncated ARHGEF10 mutant, ARHGEF10 ΔN (lacking amino acids 1-332), induced cell contraction that was inhibited by a Rho kinase inhibitor Y27632 and had higher GEF activity for RhoA than the wild type. The T332I mutant also showed the phenotype similar to the N-terminal truncated mutant. These data suggest that the ARHGEF10 T332I mutation-associated phenotype observed in the peripheral nerves is due to activated GEF activity of the ARHGEF10 T332I mutant.

Crystal structure of a rigid four-spectrin-repeat fragment of the human desmoplakin plakin domain

The plakin protein family serves to connect cell-cell and cell-matrix adhesion molecules to the intermediate filament cytoskeleton. Desmoplakin (DP) is an integral part of desmosomes, where it links desmosomal cadherins to the intermediate filaments. The 1056-amino-acid N-terminal region of DP contains a plakin domain common to members of the plakin family. Plakin domains contain multiple copies of spectrin repeats (SRs). We determined the crystal structure of a fragment of DP, residues 175-630, consisting of four SRs and an inserted SH3 domain. The four repeats form an elongated, rigid structure. The SH3 domain is present in a loop between two helices of an SR and interacts extensively with the preceding SR in a manner that appears to limit inter-repeat flexibility. The intimate intramolecular association of the SH3 domain with the preceding SR is also observed in plectin, another plakin protein, but not in α-spectrin, suggesting that the SH3 domain of plakins contributes to the stability and rigidity of this subfamily of SR-containing proteins.

Autoinhibition of GEF activity in Intersectin 1 is mediated by the short SH3-DH domain linker

Intersectin 1L (ITSN1L) acts as a specific guanine nucleotide exchange factor (GEF) for the small guanine nucleotide binding protein Cdc42 via its C-terminal DH domain. Interestingly, constructs of ITSN1L that comprise additional domains, for instance the five SH3 domains amino-terminal of the DH domain, were shown to be inhibited in their exchange factor activity. Here, we investigate the inhibitory mechanism of ITSN1L in detail and identify a novel short amino acid motif which mediates autoinhibition. We found this motif to be located in the linker region between the SH3 domains and the DH domain, and we show that within this motif W1221 acts as key residue in establishing the inhibitory interaction. This assigns ITSN1L to a growing class of GEFs that are regulated by a short amino acid motif inhibiting GEF activity by an intramolecular interaction. Moreover, we quantify the interaction between the ITSN1L SH3 domains and the Cdc42 effector N-WASP using fluorescence anisotropy binding experiments. As the SH3 domains are not involved in autoinhibition, binding of N-WASP does not release inhibition of nucleotide exchange activity in kinetic experiments, in contrast to earlier observations.

Ras superfamily GEFs and GAPs: validated and tractable targets for cancer therapy?

There is now considerable and increasing evidence for a causal role for aberrant activity of the Ras superfamily of small GTPases in human cancers. These GTPases function as GDP-GTP-regulated binary switches that control many fundamental cellular processes. A common mechanism of GTPase deregulation in cancer is the deregulated expression and/or activity of their regulatory proteins, guanine nucleotide exchange factors (GEFs) that promote formation of the active GTP-bound state and GTPase-activating proteins (GAPs) that return the GTPase to its GDP-bound inactive state. In this Review, we assess the association of GEFs and GAPs with cancer and their druggability for cancer therapeutics.

The minimal autoinhibited unit of the guanine nucleotide exchange factor intersectin

Intersectin-1L is a member of the Dbl homology (DH) domain guanine nucleotide exchange factors (GEF) which control Rho-family GTPase signaling. Intersectin-1L is a GEF that is specific for Cdc42. It plays an important role in endocytosis, and is regulated by several partners including the actin regulator N-WASP. Intact intersectin-1L shows low Cdc42 exchange activity, although the isolated catalytic DH domain shows high activity. This finding suggests that the molecule is autoinhibited. To investigate the mechanism of autoinhibition we have constructed a series of domain deletions. We find that the five SH3 domains of intersectin are important for autoinhibition, with the fifth domain (SH3(E)) being sufficient for the bulk of the autoinhibitory effect. This SH3 domain appears to primarily interact with the DH domain. We have determined the crystal structure of the SH3(E)-DH domain construct, which shows a domain swapped arrangement in which the SH3 from one monomer interacts with the DH domain of the other monomer. Analytical ultracentrifugation and gel filtration, however, show that under biochemical concentrations, the construct is fully monomeric. Thus we propose that the actual autoinhibited structure contains the related intramolecular SH3(E)-DH interaction. We propose a model in which this intramolecular interaction may block or distort the GTPase binding region of the DH domain.

Analysis of protein functions through a bacterial cell-free protein expression system

Cell-free protein synthesis is a suitable protein expression method for the high-throughput use because a PCR-amplified linear DNA fragment is utilized as a template for protein synthesis without any cloning procedures. We have developed a two-step PCR method for high-throughput and robust production of linear templates ready for cell-free protein synthesis. A high-throughput protein expression method has been established by combining the batch-mode cell-free protein synthesis with the two-step PCR, which is performed on multiwell plates, and is thus adapted for robotics. In this chapter, our two-step PCR method and the batch-mode cell-free protein synthesis are described.

The adenomatous polyposis coli-associated guanine nucleotide exchange factor Asef is involved in angiogenesis

Mutation of the tumor suppressor adenomatous polyposis coli (APC) is a key early event in the development of most colorectal tumors. APC promotes degradation of beta-catenin and thereby negatively regulates Wnt signaling, whereas mutated APCs present in colorectal tumor cells are defective in this activity. APC also stimulates the activity of the guanine nucleotide exchange factor Asef and regulates cell morphology and migration. Truncated mutant APCs constitutively activate Asef and induce aberrant migration of colorectal tumor cells. Furthermore, we have recently found that Asef and APC function downstream of hepatocyte growth factor and phosphatidylinositol 3-kinase. We show here that Asef is required for basic fibroblast growth factor- and vascular endothelial growth factor-induced endothelial cell migration. We further demonstrate that Asef is required for basic fibroblast growth factor- and vascular endothelial growth factor-induced microvessel formation. Furthermore, we show that the growth as well as vascularity of subcutaneously implanted tumors are markedly impaired in Asef(-/-) mice compared with wild-type mice. Thus, Asef plays a critical role in tumor angiogenesis and may be a promising target for cancer chemotherapy.

Adenomatous polyposis coli and Asef function downstream of hepatocyte growth factor and phosphatidylinositol 3-kinase

Mutations of the tumor suppressor adenomatous polyposis coli (APC) are responsible for sporadic and familial colorectal tumors. APC negatively regulates Wnt signaling by inducing beta-catenin degradation. It has also been shown that APC plays a role in the organization of cytoskeletal networks. APC interacts with Asef and Asef2, Rac1- and Cdc42-specific guanine nucleotide exchange factors (GEFs), and stimulates their GEF activity; thereby regulating cell morphology, adhesion, and migration. Truncated mutant APCs present in colorectal tumor cells activate Asef and Asef2 constitutively and contribute to their aberrant migratory properties. We show here that hepatocyte growth factor (HGF), as well as basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF), induce the accumulation and colocalization of APC and Asef in membrane ruffles and lamellipodia of epithelial cells. Both APC and Asef were found to be required for HGF-induced cell migration. Furthermore, we show that the effects of HGF, bFGF, and EGF on APC and Asef are mediated by the activation of phosphatidylinositol 3-kinase (PI3-kinase) and require the PH domain of Asef. These results suggest that Asef and APC function downstream of HGF and PI3-kinase, and play critical roles in growth factor-mediated regulation of cell morphology and migration.

On the mechanism of autoinhibition of the RhoA-specific nucleotide exchange factor PDZRhoGEF

Background

The Dbl-family of guanine nucleotide exchange factors (GEFs) activate the cytosolic GTPases of the Rho family by enhancing the rate of exchange of GTP for GDP on the cognate GTPase. This catalytic activity resides in the DH (Dbl-homology) domain, but typically GEFs are multidomain proteins containing other modules. It is believed that GEFs are autoinhibited in the cytosol due to supramodular architecture, and become activated in diverse signaling pathways through conformational change and exposure of the DH domain, as the protein is translocated to the membrane. A small family of RhoA-specific GEFs, containing the RGSL (regulators of G-protein signaling-like) domain, act as effectors of select GPCRs via Gα_12/13, although the molecular mechanism by which this pathway operates is not known. These GEFs include p115, LARG and PDZRhoGEF (PRG).

Results

Here we show that the autoinhibition of PRG is caused largely by an interaction of a short negatively charged sequence motif, immediately upstream of the DH-domain and including residues Asp706, Glu708, Glu710 and Asp712, with a patch on the catalytic surface of the DH-domain including Arg867 and Arg868. In the absence of both PDZ and RGSL domains, the DH-PH tandem with additional 21 residues upstream, is 50% autoinhibited. However, within the full-length protein, the PDZ and/or RGSL domains significantly restore autoinhibition.

Conclusion

Our results suggest a mechanism for autoinhibition of RGSL family of GEFs, in which the RGSL domain and a unique sequence motif upstream of the DH domain, act cooperatively to reduce the ability of the DH domain to bind the nucleotide free RhoA. The activation mechanism is likely to involve two independent steps, i.e. displacement of the RGSL domain and conformational change involving the autoinhibitory sequence motif containing several negatively charged residues.

Asef2 and Neurabin2 cooperatively regulate actin cytoskeletal organization and are involved in HGF-induced cell migration

The tumor suppressor adenomatous polyposis coli (APC) is mutated in sporadic and familial colorectal tumors. APC interacts with the Rac1- and Cdc42-specific guanine-nucleotide exchange factors (GEF), Asef and Asef2, which contain an APC-binding region (ABR) in addition to Dbl homology, Pleckstrin homology (PH) and Src homology 3 (SH3) domains. APC stimulates the GEF activity of Asef and Asef2, and thereby regulates cell adhesion and migration. Here we show that Asef2, but not Asef, interacts with Neurabin2/Spinophilin, a scaffold protein that binds to Filamentous actin (F-actin). In response to hepatocyte growth factor (HGF) treatment of HeLa cells, Asef2, Neurabin2 and APC were induced to accumulate and colocalize in lamellipodia and membrane ruffles. Neurabin2 did not affect the GEF activity of Asef2. RNA interference experiments showed that Asef2, Neurabin2 and APC are involved in HGF-induced cell migration. Furthermore, knockdown of Neurabin2 resulted in the suppression of Asef2-induced filopodia formation. These results suggest that Asef2, Neurabin2 and APC cooperatively regulate actin cytoskeletal organization and are required for HGF-induced cell migration.

Phosphorylation and activation of the Rac1 and Cdc42 GEF Asef in A431 cells stimulated by EGF

Rac1 has a crucial role in epidermal growth factor (EGF)-induced membrane ruffling, lamellipodial protrusion, and cell migration. Several guanine nucleotide exchange factors (GEFs) including Sos1, Sos2, Tiam1 and Vav2 have been shown to transduce the growth signal from the EGF receptor to Rac1. To clarify the role of each GEF, we time-lapse imaged the EGF-induced activity change of Rac1 in A431 cells transfected with siRNA targeting each Rac1 GEF. Because knockdown of these GEFs suppressed EGF-induced Rac1 activation only partially, we looked for another Rac1 GEF downstream of the EGF receptor and found that Asef, a Rac1-Cdc42 GEF bound to the tumor suppressor APC, also contributed to EGF-induced Rac1 activation. Intriguingly, EGF stimulation induced phosphorylation of Tyr94 within the APC-binding region of Asef in a manner dependent on Src-family tyrosine kinases. The suppression of EGF-induced Rac1 activation in siRNA-treated cells was restored by wild-type Asef, but not by the Tyr94Phe mutant of Asef. This observation strongly argues for the positive role of Tyr94 phosphorylation in EGF-induced Asef activation following the activation of Rac1.

Role of the C-terminal SH3 domain and N-terminal tyrosine phosphorylation in regulation of Tim and related Dbl-family proteins

Dbl-related oncoproteins are guanine nucleotide exchange factors (GEFs) specific for Rho-family GTPases and typically possess tandem Dbl (DH) and pleckstrin homology (PH) domains that act in concert to catalyze exchange. Although the exchange potential of many Dbl-family proteins is constitutively activated by truncation, the precise mechanisms of regulation for many Dbl-family proteins are unknown. Tim and Vav are distantly related Dbl-family proteins that are similarly regulated; their Dbl homology (DH) domains interact with N-terminal helices to exclude and prevent activation of Rho GTPases. Phosphorylation, substitution, or deletion of the blocking helices relieves this autoinhibition. Here we show that two other Dbl-family proteins, Ngef and Wgef, which like Tim contain a C-terminal SH3 domain, are also activated by tyrosine phosphorylation of a blocking helix. Consequently, basal autoinhibition of DH domains by direct steric exclusion using short N-terminal helices likely represents a conserved mechanism of regulation for the large family of Dbl-related proteins. N-Terminal truncation or phosphorylation of many other Dbl-family GEFs leads to their activation; similar autoinhibition mechanisms could explain some of these events. In addition, we show that the C-terminal SH3 domain binding to a polyproline region N-terminal to the DH domain of the Tim subgroup of Dbl-family proteins provides a unique mechanism of regulated autoinhibition of exchange activity that is functionally linked to the interactions between the autoinhibitory helix and the DH domain.

Protein production and purification

In selecting a method to produce a recombinant protein, a researcher is faced with a bewildering array of choices as to where to start. To facilitate decision-making, we describe a consensus 'what to try first' strategy based on our collective analysis of the expression and purification of over 10,000 different proteins. This review presents methods that could be applied at the outset of any project, a prioritized list of alternate strategies and a list of pitfalls that trip many new investigators.

Release of autoinhibition of ASEF by APC leads to CDC42 activation and tumor suppression

Autoinhibition of the Rho guanine nucleotide exchange factor ASEF is relieved by interaction with the APC tumor suppressor. Here we show that binding of the armadillo repeats of APC to a 'core APC-binding' (CAB) motif within ASEF, or truncation of the SH3 domain of ASEF, relieves autoinhibition, allowing the specific activation of CDC42. Structural determination of autoinhibited ASEF reveals that the SH3 domain forms an extensive interface with the catalytic DH and PH domains to obstruct binding and activation of CDC42, and the CAB motif is positioned adjacent to the SH3 domain to facilitate activation by APC. In colorectal cancer cell lines, full-length, but not truncated, APC activates CDC42 in an ASEF-dependent manner to suppress anchorage-independent growth. We therefore propose a model in which ASEF acts as a tumor suppressor when activated by APC and inactivation of ASEF by mutation or APC truncation promotes tumorigenesis.

Identification and characterization of Asef2, a guanine-nucleotide exchange factor specific for Rac1 and Cdc42

The tumor suppressor adenomatous polyposis coli (APC) is mutated in sporadic and familial colorectal tumors. APC interacts with the Rac1-specific guanine-nucleotide exchange factor (GEF) Asef, which contains an APC-binding region (ABR) in addition to Dbl homology (DH), Pleckstrin (PH) and Src homology 3 (SH3) domains. APC stimulates the GEF activity of Asef, and thereby regulates cell adhesion and migration. Here, we have identified a second Asef, termed Asef2, that shows significant structural and functional similarities to Asef. We found that both the N-terminal ABR and SH3 domains of Asef2 are responsible for its interaction with APC. When expressed in HeLa cells, a mutant Asef2 lacking the ABR and SH3 domains, Asef2-DeltaABR/SH3, induced increases in the levels of the active forms of Rac1 and Cdc42. Full-length Asef2 also showed this activity when co-transfected with truncated mutant APC expressed in colorectal tumor cells. Consistent with this, either Asef2-DeltaABR/SH3 or Asef2 plus truncated mutant APC stimulated lamellipodia formation in MDCK cells and filopodia formation in HeLa cells. Furthermore, RNA interference experiments showed that Asef2 is required for migration of colorectal tumor cells expressing truncated APC. These results suggest that similar to Asef, Asef2 plays an important role in cell migration, and that Asef2 activated by truncated mutant APC is required for aberrant migration of colorectal tumor cells.