Dock4 is regulated by RhoG and promotes Rac-dependent cell migration

Cell migration is essential for normal development and many pathological processes including tumor metastasis. Rho family GTPases play important roles in this event. In particular, Rac is required for lamellipodia formation at the leading edge during migration. Dock4 is a member of the Dock180 family proteins, and Dock4 mutations are present in a subset of human cancer cell lines. However, the function and the regulatory mechanism of Dock4 remain unclear. Here we show that Dock4 is regulated by the small GTPase RhoG and its effector ELMO and promotes cell migration by activating Rac1. Dock4 formed a complex with ELMO, and expression of active RhoG induced translocation of the Dock4-ELMO complex from the cytoplasm to the plasma membrane and enhanced the Dock4- and ELMO-dependent Rac1 activation and cell migration. On the other hand, RNA interference-mediated knockdown of Dock4 in NIH3T3 cells reduced cell migration. Taken together, these results suggest that Dock4 plays an important role in the regulation of cell migration through activation of Rac1, and that RhoG is a key upstream regulator for Dock4.     

Dock8 interacts with Nck1 in mediating Schwann cell precursor migration

During embryonic development of the peripheral nervous system (PNS), Schwann cell precursors migrate along neuronal axons to their final destinations, where they will myelinate the axons after birth. While the intercellular signals controlling Schwann cell precursor migration are well studied, the intracellular signals controlling Schwann cell precursor migration remain elusive. Here, using a rat primary cell culture system, we show that Dock8, an atypical Dock180-related guanine-nucleotide exchange factor (GEF) for small GTPases of the Rho family, specifically interacts with Nck1, an adaptor protein composed only of Src homology (SH) domains, to promote Schwann cell precursor migration induced by platelet-derived growth factor (PDGF). Knockdown of Dock8 or Nck1 with its respective siRNA markedly decreases PDGF-induced cell migration, as well as Rho GTPase activation, in precursors. Dock8, through its unique N-terminal proline-rich motif, interacts with the SH3 domain of Nck1, but not with other adaptor proteins composed only of SH domains, e.g. Grb2 and CrkII, and not with the adaptor protein Elmo1. Reintroduction of the proline-rich motif mutant of Dock8 in Dock8 siRNA-transfected Schwann cell precursors fails to restore their migratory abilities, whereas that of wild-type Dock8 does restore these abilities. These results suggest that Nck1 interaction with Dock8 mediates PDGF-induced Schwann cell precursor migration, demonstrating not only that Nck1 and Dock8 are previously unanticipated intracellular signaling molecules involved in the regulation of Schwann cell precursor migration but also that Dock8 is among the genetically-conservative common interaction subset of Dock family proteins consisting only of SH domain adaptor proteins.     

Identification of tyrosine residues on ELMO1 that are phosphorylated by the Src-family kinase Hck

The SH3 and SH2 domains of hematopoietic cell kinase (Hck) play important roles in substrate targeting. To identify new components of Hck signaling pathways, we identified proteins that bind to the SH3 domain of Hck (Scott et al. (2002) J. Biol. Chem. 277, 28238). One such protein was ELMO1, the mammalian orthologue of the Caenorhabditis elegans gene, ced-12. ELMO1 is an approximately 80-kD protein containing a PH domain and a C-terminal Pro-rich sequence. In C. elegans, ced-12 is required for the engulfment of dying cells and for cell migration. In mammalian fibroblasts, ELMO1 binds to Dock180, and functions upstream of Rac during phagocytosis and cell migration. We previously showed that ELMO1 binds directly to the Hck SH3 domain and is phosphorylated by Hck. In this study, we used mass spectrometry to identify the following sites of ELMO1 phosphorylation: Tyr 18, Tyr 216, Tyr 511, Tyr 395, and Tyr 720. Mutant forms of ELMO1 lacking these sites were defective in their ability to promote phagocytosis and migration in fibroblasts. Single tyrosine mutations showed that Tyr 511 is particularly important in mediating these biological effects. These mutants displayed comparable binding to Dock180 and Crk as wild-type ELMO1, but gave a lowered activation of Rac. The data suggest that Src family kinase mediated tyrosine phosphorylation of ELMO1 might represent an important regulatory mechanism that controls signaling through the ELMO1/Crk/Dock180 pathway.     

ARF6 Promotes the Formation of Rac1 and WAVE-Dependent Ventral F-Actin Rosettes in Breast Cancer Cells in Response to Epidermal Growth Factor

Coordination between actin cytoskeleton assembly and localized polarization of intracellular trafficking routes is crucial for cancer cell migration. ARF6 has been implicated in the endocytic recycling of surface receptors and membrane components and in actin cytoskeleton remodeling. Here we show that overexpression of an ARF6 fast-cycling mutant in MDA-MB-231 breast cancer-derived cells to mimick ARF6 hyperactivation observed in invasive breast tumors induced a striking rearrangement of the actin cytoskeleton at the ventral cell surface. This phenotype consisted in the formation of dynamic actin-based podosome rosette-like structures expanding outward as wave positive for F-actin and actin cytoskeleton regulatory components including cortactin, Arp2/3 and SCAR/WAVE complexes and upstream Rac1 regulator. Ventral rosette-like structures were similarly induced in MDA-MB-231 cells in response to epidermal growth factor (EGF) stimulation and to Rac1 hyperactivation. In addition, interference with ARF6 expression attenuated activation and plasma membrane targeting of Rac1 in response to EGF treatment. Our data suggest a role for ARF6 in linking EGF-receptor signaling to Rac1 recruitment and activation at the plasma membrane to promote breast cancer cell directed migration.     

Ephexin4 and EphA2 mediate cell migration through a RhoG-dependent mechanism

EphA2, a member of the Eph receptor family, is frequently overexpressed in a variety of human cancers, including breast cancers, and promotes cancer cell motility and invasion independently of its ligand ephrin stimulation. In this study, we identify Ephexin4 as a guanine nucleotide exchange factor (GEF) for RhoG that interacts with EphA2 in breast cancer cells, and knockdown and rescue experiments show that Ephexin4 acts downstream of EphA2 to promote ligand-independent breast cancer cell migration and invasion toward epidermal growth factor through activation of RhoG. The activation of RhoG recruits its effector ELMO2 and a Rac GEF Dock4 to form a complex with EphA2 at the tips of cortactin-rich protrusions in migrating breast cancer cells. In addition, the Dock4-mediated Rac activation is required for breast cancer cell migration. Our findings reveal a novel link between EphA2 and Rac activation that contributes to the cell motility and invasiveness of breast cancer cells.     

The N-terminal DH-PH domain of Trio induces cell spreading and migration by regulating lamellipodia dynamics in a Rac1-dependent fashion

The guanine-nucleotide exchange factor Trio encodes two DH-PH domains that catalyze nucleotide exchange on Rac1, RhoG and RhoA. The N-terminal DH-PH domain is known to activate Rac1 and RhoG, whereas the C-terminal DH-PH domain can activate RhoA. The current study shows that the N-terminal DH-PH domain, upon expression in HeLa cells, activates Rac1 and RhoG independently from each other. In addition, we show that the flanking SH3 domain binds to the proline-rich region of the C-terminus of Rac1, but not of RhoG. However, this SH3 domain is not required for Rac1 or RhoG GDP-GTP exchange. Rescue experiments in Trio-shRNA-expressing cells showed that the N-terminal DH-PH domain of Trio, but not the C-terminal DH-PH domain, restored fibronectin-mediated cell spreading and migration defects that are observed in Trio-silenced cells. Kymograph analysis revealed that the N-terminal DH-PH domain, independent of its SH3 domain, controls the dynamics of lamellipodia. Using siRNA against Rac1 or RhoG, we found that Trio-D1-induced lamellipodia formation required Rac1 but not RhoG expression. Together, we conclude that the GEF Trio is responsible for lamellipodia formation through its N-terminal DH-PH domain in a Rac1-dependent manner during fibronectin-mediated spreading and migration.     

Design,synthesis and biological evaluation of new carbazole derivatives as anti-cancer and anti-migratory agents

Based on the efficacy of EHop-016 as an inhibitor of migration and Rac1 activation, a new series of carbazole derivatives has been synthesized. Cytotoxic and anti-migratory effects of these compounds were evaluated in MCF-7 and MDA-MB-231 breast cancer cell lines. Preliminary investigations of their anticancer activity demonstrated that several compounds have moderate antiproliferative effects on cancer cell lines with GI₅₀ values in the range of 13–50?µM. Furthermore, compounds 3b and 11b inhibit migration activity of metastatic cell line MDA-MB-231 by 32% and 34%, respectively. Compound 11b was shown to inhibit activation of the Rho GTPase Rac1 by 55% at 250?nM in both MDA-MB-231 and MDA-MB-435 cell lines. Compared with the IC₅₀ of Rac1 inhibition by lead compound EHop-016 of 1.1?µM, compound 11b demonstrates 4X improved in vitro efficacy.     

Dock180 and ELMO1 proteins cooperate to promote evolutionarily conserved Rac-dependent cell migration

Cell migration is essential throughout embryonic and adult life. In numerous cell systems, the small GTPase Rac is required for lamellipodia formation at the leading edge and movement ability. However, the molecular mechanisms leading to Rac activation during migration are still unclear. Recently, a mammalian superfamily of proteins related to the prototype member Dock180 has been identified with homologues in Drosophila and Caenorhabditis elegans. Here, we addressed the role of Dock180 and ELMO1 proteins, which function as a complex to mediate Rac activation, in mammalian cell migration. Using mutants of Dock180 and ELMO1 in a Transwell assay as well as transgenic rescue of a C. elegans mutant lacking CED-5 (Dock180 homologue), we identified specific regions of Dock180 and ELMO1 required for migration in vitro and in a whole animal model. In both systems, the Dock180.ELMO1 complex formation and the ability to activate Rac were required. We also found that ELMO1 regulated multiple Dock180 superfamily members to promote migration. Interestingly, deletion mutants of ELMO1 missing their first 531 or first 330 amino acids that can still bind and cooperate with Dock180 in Rac activation failed to promote migration, which correlated with the inability to localize to lamellipodia. This finding suggests that Rac activation by the ELMO.Dock180 complex at discrete intracellular locations mediated by the N-terminal 330 amino acids of ELMO1 rather than generalized Rac activation plays a role in cell migration.     

The Atypical Guanine Nucleotide Exchange Factor Dock4 Regulates Neurite Differentiation through Modulation of Rac1 GTPase and Actin Dynamics

Precise regulation of neurite growth and differentiation determines accurate formation of synaptic connections, whose disruptions are frequently associated with neurological disorders. Dedicator of cytokinesis 4 (Dock4), an atypical guanine nucleotide exchange factor for Rac1, is found to be associated with neuropsychiatric diseases, including autism and schizophrenia. Nonetheless, the neuronal function of Dock4 is only beginning to be understood. Using mouse neuroblastoma (Neuro-2a) cells as a model, this study identifies that Dock4 is critical for neurite differentiation and extension. This regulation is through activation of Rac1 and modulation of the dynamics of actin-enriched protrusions on the neurites. In cultured hippocampal neurons, Dock4 regulates the establishment of the axon-dendrite polarity and the arborization of dendrites, two critical processes during neural differentiation. Importantly, a microdeletion Dock4 mutant linked to autism and dyslexia that lacks the GEF domain leads to defective neurite outgrowth and neuronal polarization. Further analysis reveals that the SH3 domain-mediated interaction of Dock4 is required for its activity toward neurite differentiation, whereas its proline-rich C terminus is not essential for this regulation. Together, our findings reveal an important role of Dock4 for neurite differentiation during early neuronal development.     

Enhanced intrinsic migration of aggressive breast cancer cells by inhibition of Rac1 GTPase

Rac GTPases are known to play a crucial role in regulating cytoskeletal changes necessary for cell migration. Migration has been shown to be positively regulated by Rac in most cell types. However, there is also a large body of conflicting evidence in some other cell types with respect to the role of Rac in migration, suggesting that Rac GTPases regulate cell migration in a cell type-dependent manner. In the present study, we have characterized the effects of Rac1 GTPase inhibition on the migratory abilities of a number of breast cancer cell lines with differential degrees of tumorigenic and metastatic potentials. We show that Rac1 inhibition in non-metastatic (MCF-7, T-47D) or moderately metastatic (Hs578T) cell lines results in inhibition of migration, whereas in highly metastatic cell lines (MDA-MB-435, MDA-MB-231, and C3L5) Rac1 inhibition results in stimulation of migration. This stimulation of migration following Rac1 inhibition is also accompanied by the enhanced RhoA activity, suggesting a possible existence of a dominating role of RhoA over Rac1 in regulating intrinsic migration of the highly metastatic breast cancer cells.     

Activation of H-Ras and Rac1 correlates with epidermal growth factor-induced invasion in Hs578T and MDA-MB-231 breast carcinoma cells

There is considerable experimental evidence that hyperactive Ras proteins promote breast cancer growth and development including invasiveness, despite the low frequency of mutated forms of Ras in breast cancer. We have previously shown that H-Ras, but not N-Ras, induces an invasive phenotype mediated by small GTPase Rac1 in MCF10A human breast epithelial cells. Epidermal growth factor (EGF) plays an important role in aberrant growth and metastasis formation of many tumor types including breast cancer. The present study aims to investigate the correlation between EGF-induced invasiveness and Ras activation in four widely used breast cancer cell lines. Upon EGF stimulation, invasive abilities and H-Ras activation were significantly increased in Hs578T and MDA-MB-231 cell lines, but not in MDA-MB-453 and T47D cell lines. Using small interfering RNA (siRNA) to target H-Ras, we showed a crucial role of H-Ras in the invasive phenotype induced by EGF in Hs578T and MDA-MB-231 cells. Moreover, siRNA-knockdown of Rac1 significantly inhibited the EGF-induced invasiveness in these cells. Taken together, this study characterized human breast cancer cell lines with regard to the relationship between H-Ras activation and the invasive phenotype induced by EGF. Our data demonstrate that the activation of H-Ras and the downstream molecule Rac1 correlates with EGF-induced breast cancer cell invasion, providing important information on the regulation of malignant progression in mammary carcinoma cells.     

Rab35 is required for Wnt5a/Dvl2-induced Rac1 activation and cell migration in MCF-7 breast cancer cells

The small GTPases regulate many major biological processes in both tumorigenesis and tumor progression such as cell survival, actin cytoskeleton organization, cell polarity and movement. Wnt5a, a non-canonical Wnt family member, is implicated in the activation of small GTPases in breast cancer. We previously demonstrated that Wnt5a signaling stimulates the migration of breast cancer cells MDA-MB-231 via activating RhoA. However, we found here that RhoA activation was not enhanced by Wnt5a in breast cancer cells MCF-7. The conflicting results prompted us to further probe novel small GTPases in response to Wnt5a and investigate the mechanisms whereby cell migration is regulated. We showed here that Wnt5a dose dependently activated Dvl2, Rab35 and Rac1 and subsequently promoted the migration of MCF-7 cells, which was, however, abolished by knocking down Wnt5a expression via small interfering RNA (siRNA) transfection. Dvl2 siRNA significantly decreased background and Wnt5a-induced Rab35/Rac1 activation and, consequently, cell migration. Rab35 short hairpin RNA (shRNA) remarkably inhibited background and Wnt5a-induced Rac1 activation and cell migration. Additionally, blockade of Rac1 activation with Rac1 siRNA suppressed background and Wnt5a-induced cell migration. Co-immunoprecipitation and immunofluorescence assays showed that Dvl2 bound to Rab35 in mammalian cells. Taken together, we demonstrated that Wnt5a promotes breast cancer cell migration via the Dvl2/Rab35/Rac1 signaling pathway. These findings implicate Wnt5a signaling in regulating small GTPases, which could be targeted for manipulating breast cancer cell migration.     

A Steric-inhibition model for regulation of nucleotide exchange via the Dock180 family of GEFs

CDM (CED-5, Dock180, Myoblast city) family members have been recently identified as novel, evolutionarily conserved guanine nucleotide exchange factors (GEFs) for Rho-family GTPases . They regulate multiple processes, including embryonic development, cell migration, apoptotic-cell engulfment, tumor invasion, and HIV-1 infection, in diverse model systems . However, the mechanism(s) of regulation of CDM proteins has not been well understood. Here, our studies on the prototype member Dock180 reveal a steric-inhibition model for regulating the Dock180 family of GEFs. At basal state, the N-terminal SH3 domain of Dock180 binds to the distant catalytic Docker domain and negatively regulates the function of Dock180. Further studies revealed that the SH3:Docker interaction sterically blocks Rac access to the Docker domain. Interestingly, ELMO binding to the SH3 domain of Dock180 disrupted the SH3:Docker interaction, facilitated Rac access to the Docker domain, and contributed to the GEF activity of the Dock180/ELMO complex. Additional genetic rescue studies in C. elegans suggested that the regulation of the Docker-domain-mediated GEF activity by the SH3 domain and its adjoining region is evolutionarily conserved. This steric-inhibition model may be a general mechanism for regulating multiple SH3-domain-containing Dock180 family members and may have implications for a variety of biological processes.     

SRC kinase activator CDCP1 promotes hepatocyte growth factor–induced cell migration/invasion of a subset of breast cancer cells

Cancer invasion and metastasis are the major causes of cancer patient mortality. Various growth factors, including hepatocyte growth factor (HGF), are known to promote cancer invasion and metastasis, but the regulatory mechanisms involved are not fully understood. Here, we show that HGF-promoted migration and invasion of breast cancer cells are regulated by CUB domain–containing protein 1 (CDCP1), a transmembrane activator of SRC kinase. In metastatic human breast cancer cell line MDA-MB-231, which highly expresses the HGF receptor MET and CDCP1, we show that CDCP1 knockdown attenuated HGF-induced MET activation, followed by suppression of lamellipodia formation and cell migration/invasion. In contrast, in the low invasive/nonmetastatic breast cancer cell line T47D, which had no detectable MET and CDCP1 expression, ectopic MET expression stimulated the HGF-dependent activation of invasive activity, and concomitant CDCP1 expression activated SRC and further promoted invasive activity. In these cells, CDCP1 expression dramatically activated HGF-induced membrane remodeling, which was accompanied by activation of the small GTPase Rac1. Analysis of guanine nucleotide exchange factors revealed that ARHGEF7 was specifically required for CDCP1-dependent induction of HGF-induced invasive ability. Furthermore, immunofluorescence staining demonstrated that CDCP1 coaccumulated with ARHGEF7. Finally, we confirmed that the CDCP1-SRC axis was also crucial for HGF and ARHGEF7-RAC1 signaling in MDA-MB-231 cells. Altogether, these results demonstrate that the CDCP1-SRC-ARHGEF7-RAC1 pathway plays an important role in the HGF-induced invasion of a subset of breast cancer cells.     

Characterization of EHop-016, novel small molecule inhibitor of Rac GTPase

The Rho GTPase Rac regulates actin cytoskeleton reorganization to form cell surface extensions (lamellipodia) required for cell migration/invasion during cancer metastasis. Rac hyperactivation and overexpression are associated with aggressive cancers; thus, interference of the interaction of Rac with its direct upstream activators, guanine nucleotide exchange factors (GEFs), is a viable strategy for inhibiting Rac activity. We synthesized EHop-016, a novel inhibitor of Rac activity, based on the structure of the established Rac/Rac GEF inhibitor NSC23766. Herein, we demonstrate that EHop-016 inhibits Rac activity in the MDA-MB-435 metastatic cancer cells that overexpress Rac and exhibits high endogenous Rac activity. The IC(50) of 1.1 μM for Rac inhibition by EHop-016 is ～100-fold lower than for NSC23766. EHop-016 is specific for Rac1 and Rac3 at concentrations of ≤5 μM. At higher concentrations, EHop-016 inhibits the close homolog Cdc42. In MDA-MB-435 cells that demonstrate high active levels of the Rac GEF Vav2, EHop-016 inhibits the association of Vav2 with a nucleotide-free Rac1(G15A), which has a high affinity for activated GEFs. EHop-016 also inhibits the Rac activity of MDA-MB-231 metastatic breast cancer cells and reduces Rac-directed lamellipodia formation in both cell lines. EHop-016 decreases Rac downstream effects of PAK1 (p21-activated kinase 1) activity and directed migration of metastatic cancer cells. Moreover, at effective concentrations (<5 μM), EHop-016 does not affect the viability of transformed mammary epithelial cells (MCF-10A) and reduces viability of MDA-MB-435 cells by only 20%. Therefore, EHop-016 holds promise as a targeted therapeutic agent for the treatment of metastatic cancers with high Rac activity.     

MRTF-A and STAT3 synergistically promote breast cancer cell migration

Breast cancer is the leading cause of cancer death in women worldwide which is closely related to metastasis. But the exact molecular mechanism on metastasis is still not fully understood; we now report that both MRTF-A and STAT3 play important role in breast cancer migration of MDA-MB-231 cells. Moreover, MRTF-A and STAT3 synergistically increased MDA-MB-231 cell migration by promoting the expression of migration markers Myl-9 and Cyr-61. Importantly, we identified a detailed molecular mechanism of MDA-MB-231 cell migration controlled via physical interaction between MRTF-A and STAT3, which synergistically promote the transactivity of the migration marker Myl-9 and Cyr-61 by CArG box binding. Interestingly, the two signaling pathways RhoA-MRTF-A and JAK-STAT3 across talk to regulate MDA-MB-231 cell migration. Our data thus provide important and novel insights into the roles of MRTF-A and STAT3 in regulating MDA-MB-231 cell migration.     

ErbB2 directly activates the exchange factor Dock7 to promote Schwann cell migration

The cellular events that precede myelination in the peripheral nervous system require rapid and dynamic morphological changes in the Schwann cell. These events are thought to be mainly controlled by axonal signals. But how signals on the axons are coordinately organized and transduced to promote proliferation, migration, radial sorting, and myelination is unknown. We describe that the axonal signal neuregulin-1 (NRG1) controls Schwann cell migration via activation of the atypical Dock180-related guanine nucleotide exchange factor (GEF) Dock7 and subsequent activation of the Rho guanine triphosphatases (GTPases) Rac1 and Cdc42 and the downstream c-Jun N-terminal kinase. We show that the NRG1 receptor ErbB2 directly binds and activates Dock7 by phosphorylating Tyr-1118. Dock7 knockdown, or expression of Dock7 harboring the Tyr-1118-to-Phe mutation in Schwann cells, attenuates the effects of NRG1. Thus, Dock7 functions as an intracellular substrate for ErbB2 to promote Schwann cell migration. This provides an unanticipated mechanism through which ligand-dependent tyrosine phosphorylation can trigger the activation of Rho GTPase-GEFs of the Dock180 family.     

ARF1 controls Rac1 signaling to regulate migration of MDA-MB-231 invasive breast cancer cells

ADP-ribosylation factors (ARFs) are monomeric G proteins that regulate many cellular processes such as reorganization of the actin cytoskeleton. We have previously shown that ARF1 is overexpressed in highly invasive breast cancer cells and contribute to their enhanced migration. In this study, we propose to define the molecular mechanism by which ARF1 regulates this complex cellular response by investigating the role of this ARF GTPase on the activation process of Rac1, a Rho GTPase, associated with lamellipodia formation during cell migration. Here, we first show that inhibition of ARF1 or Rac1 expression markedly impacts the ability of MDA-MB-231 cells to migrate upon EGF stimulation. However, the effect of ARF1 depletion can be reversed by overexpression of the Rac1 active mutant, Rac1 Q⁶¹L. Depletion of ARF1 also impairs the ability of EGF stimulation to promote GTP-loading of Rac1. To further investigate the possible cross-talk between ARF1 and Rac1, we next examined whether they could form a complex. We observed that the two GTPases could directly interact independently of the nature of the nucleotide bound to them. EGF treatment however resulted in the association of Rac1 with its effector IRSp53, which was completely abrogated in ARF1 depleted cells. We present evidences that this ARF isoform is responsible for the plasma membrane targeting of both Rac1 and IRSp53, a step essential for lamellipodia formation. In conclusion, this study provides a new mechanism by which ARF1 regulates cell migration and identifies this GTPase as a promising pharmacological target to reduce metastasis formation in breast cancer patients.     

Characterization of a novel interaction between ELMO1 and ERM proteins

ERMs are closely related proteins involved in cell migration, cell adhesion, maintenance of cell shape, and formation of microvilli through their ability to cross-link the plasma membrane with the actin cytoskeleton. ELMO proteins are also known to regulate actin cytoskeleton reorganization through activation of the small GTPbinding protein Rac via the ELMO-Dock180 complex. Here we showed that ERM proteins associate directly with ELMO1 as purified recombinant proteins in vitro and at endogenous levels in intact cells. We mapped ERM binding on ELMO1 to the N-terminal 280 amino acids, which overlaps with the region required for binding to the GTPase RhoG, but is distinct from the C-terminal Dock180 binding region. Consistent with this, ELMO1 could simultaneously bind both radixin and Dock180, although radixin did not alter Rac activation via the Dock180-ELMO complex. Most interestingly, radixin binding did not affect ELMO binding to active RhoG and a trimeric complex of active RhoG-ELMO-radixin could be detected. Moreover, the three proteins colocalized at the plasma membrane. Finally, in contrast to most other ERM-binding proteins, ELMO1 binding occurred independently of the state of radixin C-terminal phosphorylation, suggesting an ELMO1 interaction with both the active and inactive forms of ERM proteins and implying a possible role of ELMO in localizing or retaining ERM proteins in certain cellular sites. Together these data suggest that ELMO1-mediated cytoskeletal changes may be coordinated with ERM protein crosslinking activity during dynamic cellular functions.     

Unconventional Rac-GEF activity is mediated through the Dock180-ELMO complex

Mammalian Dock180 and ELMO proteins, and their homologues in Caenorhabditis elegans and Drosophila melanogaster, function as critical upstream regulators of Rac during development and cell migration. The mechanism by which Dock180 or ELMO mediates Rac activation is not understood. Here, we identify a domain within Dock180 (denoted Docker) that specifically recognizes nucleotide-free Rac and can mediate GTP loading of Rac in vitro. The Docker domain is conserved among known Dock180 family members in metazoans and in a yeast protein. In cells, binding of Dock180 to Rac alone is insufficient for GTP loading, and a Dock180 ELMO1 interaction is required. We can also detect a trimeric ELMO1 Dock180 Rac1 complex and ELMO augments the interaction between Dock180 and Rac. We propose that the Dock180 ELMO complex functions as an unconventional two-part exchange factor for Rac.