Rap2 as a slowly responding molecular switch in the Rap1 signaling cascade

Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins

The Ras-related GTPases Rap1a and 1b have been implicated in multiple biological events including cell adhesion, free radical production, and cancer. To gain a better understanding of Rap1 function in mammalian physiology, we deleted the Rap1a gene. Although loss of Rap1a expression did not initially affect mouse size or viability, upon backcross into C57BL/6J mice some Rap1a-/- embryos died in utero. T cell, B cell, or myeloid cell development was not disrupted in Rap1a-/- mice. However, macrophages from Rap1a null mice exhibited increased haptotaxis on fibronectin and vitronectin matrices that correlated with decreased adhesion. Chemotaxis of lymphoid and myeloid cells in response to CXCL12 or CCL21 was significantly reduced. In contrast, an increase in FcR-mediated phagocytosis was observed. Because Rap1a was previously copurified with the human neutrophil NADPH oxidase, we addressed whether GTPase loss affected superoxide production. Neutrophils from Rap1a-/- mice had reduced fMLP-stimulated superoxide production as well as a weaker initial response to phorbol ester. These results suggest that, despite 95% amino acid sequence identity, similar intracellular distribution, and broad tissue distribution, Rap1a and 1b are not functionally redundant but rather differentially regulate certain cellular events.

Rap2 regulates androgen sensitivity in human prostate cancer cells

BACKGROUND

Progression of prostate cancer to a fatal androgen-independent disease is associated with activation of MAP kinase, consistent with chronic stimulation of the Ras-signaling pathway. We have previously shown that Ras activation is sufficient to induce androgen-independent growth of prostate cancer cells. One mechanism of MAP kinase regulation is modulation of Ras signaling by other Ras family members, the Rap gene paralogs Rap1a/b and Rap2a/b. Here we ask if Rap proteins play a role in determining androgen sensitivity of human prostate cancer cells either alone or in the context of an activated Ras.

METHODS

To evaluate the role of Rap proteins in androgen responsiveness we use Rap over-expression with or without mutated Ras co-transfection and Rap siRNA knockdown to evaluate androgen-dependent prostate-specific antigen (PSA) promoter reporter expression and cell growth in androgen-dependent LNCaP and independent C4-2 human prostate cancer cells.

RESULTS

Rap1 is equally expressed between LNCaP and C4-2 cells and thus we focused on Rap2 which is minimally expressed in C4-2. Rap2a affects androgen-dependent PSA reporter expression in a dose-dependent manner in LNCaP and C4-2 cells. Low levels of Rap2a enhance PSA reporter expression, whereas higher concentrations inhibit expression. We show that Rap2a antagonizes the enhanced PSA reporter expression conferred by an active RasV12 gene in prostate cancer cells. siRNA knockdown data indicate that Rap2 has a greater effect on androgen-stimulated growth in LNCaP than in C4-2 cells.

CONCLUSIONS

We show that Rap2 is involved in androgen-mediated transcriptional and growth responses of human prostate cancer cells.

Characterization of Interactions of Adapter Protein RAPL/Nore1B with RAP GTPases and Their Role in T Cell Migration

Using a model of integrin-triggered random migration of T cells, we show that stimulation of LFA-1 integrins leads to the activation of Rap1 and Rap2 small GTPases. We further show that Rap1 and Rap2 have distinct roles in adhesion and random migration of these cells and that an adapter protein from the Ras association domain family (Rassf), RAPL, has a role downstream of Rap2 in addition to its link to Rap1. Further characterization of the RAPL protein and its interactions with small GTPases from the Ras family shows that RAPL forms more stable complexes with Rap2 and classical Ras proteins compared with Rap1. The different interaction pattern of RAPL with Rap1 and Rap2 is not affected by the disruption of the C-terminal SARAH domain that we identified as the alpha-helical region responsible for RAPL dimerization in vitro and in cells. Based on mutagenesis and three-dimensional modeling, we propose that interaction surfaces in RAPL-Rap1 and RAPL-Rap2 complexes are different and that a single residue in the switch I region of Rap proteins (residue 39) contributes considerably to the different kinetics of these protein-protein interactions. Furthermore, the distinct role of Rap2 in migration of T cells is lost when this critical residue is converted to the residue present in Rap1. Together, these observations suggest a wider role for Rassf adapter protein RAPL and Rap GTPases in cell motility and show that subtle differences between highly similar Rap proteins could be reflected in distinct interactions with common effectors and their cellular function.

Identification of three novel proteins (SGSM1, 2, 3) which modulate small G protein (RAP and RAB)-mediated signaling pathway

We report a novel protein family consisting of three members, each of which contains RUN and TBC motifs and appears to be associated with small G protein-mediated signal transduction pathway. We named these proteins as small G protein signaling modulators (SGSM1/2/3). Northern blot analysis revealed that human SGSM2/3 are expressed ubiquitously in various tissues, whereas SGSM1 is expressed mainly in brain, heart, and testis. Mouse possessed the same protein family genes, and the in situ hybridization and immunohistochemical staining of tissue sections revealed that mouse Sgsm1/2/3 are expressed in the neurons of central nervous system, indicating the strong association of Sgsm family with neuronal function. Furthermore, endogenous Sgsm1 protein was localized in the trans-Golgi network of mouse Neuro2a cells by immunofluorescence microscopy. Expression of various cDNA constructs followed by immunoprecipitation assay revealed that human SGSM1/2/3 proteins are coprecipitated with RAP and RAB subfamily members of the small G protein superfamily. Based on these results, we postulated that the SGSM family members function as modulators of the small G protein RAP and RAB-mediated neuronal signal transduction and vesicular transportation pathways.

Natural history and early diagnosis of LAD-1/variant syndrome

The syndrome of leukocyte adhesion deficiency (LAD) combined with a severe Glanzmann-type bleeding disorder has been recognized as a separate disease entity. The variability in clinical and cell biological terms has remained largely unclear. We present data on 9 cases from 7 unrelated families, with 3 patients being actively followed for more than 12 years. The disease entity, designated LAD-1/variant syndrome, presents early in life and consists of nonpussing infections from bacterial and fungal origin, as well as a severe bleeding tendency. This is compatible with 2 major blood cell types contributing to the clinical symptoms (ie, granulocytes and platelets). In granulocytes of the patients, we found adhesion and chemotaxis defects, as well as a defect in NADPH oxidase activity triggered by unopsonized zymosan. This last test can be used as a screening test for the syndrome. Many proteins and genes involved in adhesion and signaling, including small GTPases such as Rap1 and Rap2 as well as the major Rap activity-regulating molecules, were normally present. Moreover, Rap1 activation was intact in patients' blood cells. Defining the primary defect awaits genetic linkage analysis, which may be greatly helped by a more precise understanding and awareness of the disease combined with the early identification of affected patients.

Crystal structure of the RUN domain of the RAP2-interacting protein x

Rap2-interacting protein x (RPIPx) is a homolog of RPIP8, a specific effector of Rap2 GTPase. The N-terminal region of RPIP8, which contains the RUN domain, interacts with Rap2. Using cell-free synthesis and NMR, we determined that the region encompassing residues 83-255 of mouse RPIPx, which is 40-residues larger than the predicted RUN domain (residues 113-245), is the minimum fragment that forms a correctly folded protein. This fragment, the RPIPx RUN domain, interacted specifically with Rap2B in vitro in a nucleotide-dependent manner. The crystal structure of the RPIPx RUN domain was determined at 2.0 A of resolution by the multiwavelength anomalous dispersion (MAD) method. The RPIPx RUN domain comprises eight anti-parallel alpha-helices, which form an extensive hydrophobic core, followed by an extended segment. The residues in the core region are highly conserved, suggesting the conservation of the RUN domain-fold among the RUN domain-containing proteins. The residues forming a positively charged surface are conserved between RPIP8 and its homologs, suggesting that this surface is important for Rap2 binding. In the crystal the putative Rap2 binding site of the RPIPx RUN domain interacts with the extended segment in a segment-swapping manner.

Inactivation of Crk SH3 domain-binding guanine nucleotide-releasing factor (C3G) in cervical squamous cell carcinoma

C3G, a Crk SH3 domain-binding guanine nucleotide-releasing factor functions as a guanine nucleotide exchange factor for Rap1. It is activated via the Crk adaptor protein and plays an important role in transducing signals from receptors on the cell surface to the nucleus via the Ras/Raf/MAPK signal transduction pathway. However, since the experimental data result in pleiotropic effects in the cascade manner, its precise function remains unclear. Here we examined the C3G expression in cervical squamous cell carcinomas and found a marked decrease in the expression of C3G in a high incidence of said samples. In addition, we also demonstrated frequent hypermethylation of C3G, which resulted in an inactivation mechanism of the gene. Clinical and pathologic data failed to show any relationship between the human papillomavirus infection and the down-regulation of C3G. These results indicate that inactivation of C3G by de novo methylation plays an important role in the development of cervical squamous cell carcinoma.

Farnesylation or geranylgeranylation? Efficient assays for testing protein prenylation in vitro and in vivo

BACKGROUND

Available in vitro and in vivo methods for verifying protein substrates for posttranslational modifications via farnesylation or geranylgeranylation (for example, autoradiography with 3H-labeled anchor precursors) are time consuming (weeks/months), laborious and suffer from low sensitivity.

RESULTS

We describe a new technique for detecting prenyl anchors in N-terminally glutathione S-transferase (GST)-labeled constructs of target proteins expressed in vitro in rabbit reticulocyte lysate and incubated with 3H-labeled anchor precursors. Alternatively, hemagglutinin (HA)-labeled constructs expressed in vivo (in cell culture) can be used. For registration of the radioactive marker, we propose to use a thin layer chromatography (TLC) analyzer. As a control, the protein yield is tested by Western blotting with anti-GST- (or anti-HA-) antibodies on the same membrane that has been previously used for TLC-scanning. These protocols have been tested with Rap2A, v-Ki-Ras2 and RhoA (variant RhoA63L) including the necessary controls. We show directly that RasD2 is a farnesylation target.

CONCLUSION

Savings in time for experimentation and the higher sensitivity for detecting 3H-labeled lipid anchors recommend the TLC-scanning method with purified GST- (or HA-) tagged target proteins as the method of choice for analyzing their prenylation capabilities in vitro and in vivo and, possibly, also for studying the myristoyl and palmitoyl posttranslational modifications.

Rap1A-deficient T and B cells show impaired integrin-mediated cell adhesion

Studies in tissue culture cells have demonstrated a role for the Ras-like GTPase Rap1 in the regulation of integrin-mediated cell-matrix and cadherin-mediated cell-cell contacts. To analyze the function of Rap1 in vivo, we have disrupted the Rap1A gene by homologous recombination. Mice homozygous for the deletion allele are viable and fertile. However, primary hematopoietic cells isolated from spleen or thymus have a diminished adhesive capacity on ICAM and fibronectin substrates. In addition, polarization of T cells from Rap1-/- cells after CD3 stimulation was impaired compared to that of wild-type cells. Despite this, these defects did not result in hematopoietic or cell homing abnormalities. Although it is possible that the relatively mild phenotype is a consequence of functional complementation by the Rap1B gene, our genetic studies confirm a role for Rap1A in the regulation of integrins.

Supersensitive Ras activation in dendrites and spines revealed by two-photon fluorescence lifetime imaging

To understand the biochemical signals regulated by neural activity, it is necessary to measure protein-protein interactions and enzymatic activity in neuronal microcompartments such as axons, dendrites and their spines. We combined two-photon excitation laser scanning with fluorescence lifetime imaging to measure fluorescence resonance energy transfer at high resolutions in brain slices. We also developed sensitive fluorescent protein-based sensors for the activation of the small GTPase protein Ras with slow (FRas) and fast (FRas-F) kinetics. Using FRas-F, we found in CA1 hippocampal neurons that trains of back-propagating action potentials rapidly and reversibly activated Ras in dendrites and spines. The relationship between firing rate and Ras activation was highly nonlinear (Hill coefficient approximately 5). This steep dependence was caused by a highly cooperative interaction between calcium ions (Ca(2+)) and Ras activators. The Ras pathway therefore functions as a supersensitive threshold detector for neural activity and Ca(2+) concentration.

The initial enzyme for glycosylphosphatidylinositol biosynthesis requires PIG-Y, a seventh component

Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by an unusually complex GPI-N-acetylglucosaminyltransferase (GPI-GnT) consisting of at least six proteins. Here, we report that human GPI-GnT requires another component, termed PIG-Y, a 71 amino acid protein with two transmembrane domains. The Burkitt lymphoma cell line Daudi, severely defective in the surface expression of GPI-anchored proteins, was a null mutant of PIG-Y. A complex of six components was formed without PIG-Y. PIG-Y appeared to be directly associated with PIG-A, implying that PIG-Y is the key molecule that regulates GPI-GnT activity by binding directly to the catalytic subunit PIG-A. PIG-Y is probably homologous to yeast Eri1p, a component of GPI-GnT. We did not obtain evidence for a functional linkage between GPI-GnT and ras GTPases in mammalian cells as has been reported for yeast cells. A single transcript encoded PIG-Y and, to its 5' side, another protein PreY that has homologues in a wide range of organisms and is characterized by a conserved domain termed DUF343. These two proteins are translated from one mRNA by leaky scanning of the PreY initiation site.

PARG1, a protein-tyrosine phosphatase-associated RhoGAP, as a putative Rap2 effector

Rap2 belongs to the Ras family of small GTP-binding proteins, but its specific signaling role is unclear. By yeast two-hybrid screening, we have found that the Caenorhabditis elegans ortholog of Rap2 interacts with a protein containing a Rho-GTPase-activating protein (RhoGAP) domain, ZK669.1a, whose human ortholog PARG1 exhibits RhoGAP activity in vitro. ZK669.1a and PARG1 share a homology region with previously unknown function, designated the ZK669.1a and PARG1 homology (ZPH) region. Here we show that the ZPH region of PARG1 mediates interaction with Rap2. PARG1 interacted with Rap2 in a GTP-dependent manner but not with Ras or Rap1. We also show that PARG1 and its mutant lacking the ZPH region induce typical cytoskeletal changes for Rho inactivation in fibroblasts. Rap2 suppressed this in vivo action of PARG1 but not that of the mutant PARG1. These results suggest that PARG1 is a putative specific effector of Rap2 to regulate Rho.

Expression ofRPIP9 (Rap2 interacting protein 9) is activated in breast carcinoma and correlates with a poor prognosis

MDR1 is upregulated in many tumors. We have previously detected activation of the MDR1 upstream promoter in metastatic breast cancer cells. MDR1 overlaps with an uncharacterized gene transcribed from the opposite strand, coding for Rap2 interacting protein 9 (RPIP9). Rap2 belongs to the Ras superfamily of GTPases, whose role in breast cancer remains unknown. We developed sensitive methods for detecting and quantifying RPIP9 mRNA and used it to identify these transcripts in normal human tissues, 60 biopsies of primary breast carcinoma, in isolated epithelial cells both from the primary tumor and from associated lymph nodes, and from bone marrow biopsies of 74 breast cancer patients. RPIP9 is expressed at high levels in normal testis, brain and adrenal gland, and at very low levels in normal breast. Tumorigenic breast carcinoma cell lines expressed RPIP9, whereas MCF-10A and HBL-100 that do not form tumors in nude mice had undetectable levels of RPIP9 mRNA. RPIP9 was activated in a high proportion of breast carcinomas (61.6%; n = 60) and a significant correlation with metastatic lymph node invasion (N = 0-3 vs. N > 3, where N = number of lymph nodes invaded; p = 0.031) was found. RPIP9 mRNA could be detected in malignant epithelial cells isolated from the primary tumor and from metastasized lymph nodes as well as in the bone marrow of significantly more poor-prognosis (N > 3) than better-prognosis (N = 0-3) patients (p = 0.001). Therefore, activation of RPIP9 occurs during the malignant breast epithelial transformation and increases with progression toward an invasive phenotype.

Activation of the small GTPase Rap2B in agonist-stimulated human platelets

The activation of the small GTPase Rap2B in resting and agonist-stimulated human platelets was investigated. Both thrombin, that stimulates heterotrimeric G-protein-coupled receptors, and the GPVI ligand convulxin, that activates a tyrosine-kinase based signaling pathway, were able to induced the rapid and sustained binding of GTP to Rap2B. Similarly, a number of other agonists tested, previously known to activate the highly related protein Rap1B, were also able to stimulate Rap2B. In contrast, platelet antagonists that increase the intracellular concentration of cAMP did not signal to Rap2B. Thrombin- and convulxin-induced activation of Rap2B was not dependent on thromboxane A2, did not require the interaction of the protein with the cytoskeleton, and was not regulated by integrin alphaIIbbeta3-dependent outside-in signaling. When secreted ADP was neutralized, activation of Rap2B induced by thrombin, but not by convulxin, was significantly reduced. ADP itself was found to induce the rapid and sustained binding of GTP to Rap2B, and this effect was predominantly mediated by stimulation of the Gi-coupled P2Y12 receptor. Activation of Rap2B promoted by both thrombin and convulxin was regulated by intracellular Ca2+, while protein kinase C was found to be involved in convulxin- but not in thrombin-induced activation of Rap2B. Moreover, Rap2B activation induced by thrombin, but not by convulxin, was totally dependent on phosphatidylinositol 3-kinase activity. These results demonstrate that the small GTPase Rap2B is involved in platelet activation, and outline some important differences between the regulation of highly related GTPases Rap2B and Rap1B in human platelets.

SHEP1 Function in Cell Migration Is Impaired by a Single Amino Acid Mutation That Disrupts Association with the Scaffolding Protein Cas but Not with Ras GTPases

SHEP1 is a signaling protein that contains a guanine nucleotide exchange factor-like domain, which binds Ras family GTPases and also forms a stable complex with the scaffolding protein Crk-associated substrate (Cas). SHEP1 and Cas have several common functions, such as increasing c-Jun N-terminal kinase activity, promoting T cell activation, and regulating the actin cytoskeleton. However, it is unclear whether a physical association between SHEP1 and Cas is required for these activities. We reported previously that SHEP1 is tyrosine-phosphorylated downstream of the EphB2 receptor; in this study, we further demonstrate that activated EphB2 inhibits SHEP1 association with Cas. To investigate whether phosphorylation negatively regulates the SHEP1-Cas complex, we have identified by mass spectrometry several SHEP1 tyrosine phosphorylation sites downstream of EphB2; of particular interest among them is tyrosine 635 in the Cas association/exchange factor domain. Mutation of this tyrosine to glutamic acid, but not to phenylalanine, disrupts Cas binding to SHEP1 without inhibiting Ras GTPase binding. The glutamic acid mutation also makes SHEP1 unable to promote Cas-Crk association, membrane ruffling, and cell migration toward epidermal growth factor (EGF), implying that these activities of SHEP1 depend upon a physical interaction with Cas. Association with Cas also seems to be necessary for EGF-induced SHEP1 tyrosine phosphorylation, which is mediated by a Src family kinase. It is noteworthy that EGF stimulation does not cause dissociation of SHEP1 from Cas. These data show that SHEP1 regulates membrane ruffling and cell migration and that binding to Cas is probably critical for these functions. Furthermore, the SHEP1-Cas complex may have different roles downstream of EphB2 and the EGF receptor.

The RGS (regulator of G-protein signalling) and GoLoco domains of RGS14 co-operate to regulate Gi-mediated signalling

RGS (regulator of G-protein signalling) proteins stimulate the intrinsic GTPase activity of the a subunits of heterotrimeric G-proteins, and thereby negatively regulate G-protein-coupled receptor signalling. RGS14 has been shown previously to stimulate the GTPase activities of Ga(o) and Ga(i) subunits through its N-terminal RGS domain, and to down-modulate signalling from receptors coupled to G(i). It also contains a central domain that binds active Rap proteins, as well as a C-terminal GoLoco/G-protein regulatory motif that has been shown to act in vitro as a GDP-dissociation inhibitor for Ga(i). In order to elucidate the respective contributions of the three functional domains of RGS14 to its ability to regulate G(i) signalling, we generated RGS14 mutants invalidated in each of its domains, as well as truncated molecules, and assessed their effects on G(i) signalling via the bg pathway in a stable cell line ectopically expressing the G(i)-coupled M2 muscarinic acetylcholine receptor (HEK-m2). We show that the RGS and GoLoco domains of RGS14 are independently able to inhibit signalling downstream of G(i). Targeting of the isolated GoLoco domain to membranes, by myristoylation/palmitoylation or Rap binding, enhances its inhibitory activity on G(i) signalling. Finally, in the context of the full RGS14 molecule, the RGS and GoLoco domains co-operate to confer maximal activity on RGS14. We therefore propose that RGS14 combines the inhibition of G(i) activation or coupling to receptors via its GoLoco domain with stimulation of the GTPase activity of Ga(i)-GTP via its RGS domain to negatively regulate signalling downstream of G(i).

Distinct cAMP signaling pathways differentially regulate alpha2C-adrenoceptor expression: role in serum induction in human arteriolar smooth muscle cells

The physiological role of alpha(2)-adrenoceptors (alpha(2)-ARs) in cutaneous, arteriolar, vascular smooth muscle cells (VSMs) is to mediate cold-induced constriction. In VSMs cultured from human cutaneous arterioles, there is a selective increase in alpha(2C)-AR expression after serum stimulation. In the present study, we examined the cellular mechanisms contributing to this response. Serum induction of alpha(2C)-ARs was paralleled by increased expression of cyclooxygenase-2 (COX-2), increased release of prostaglandins, and increased intracellular concentration of cAMP. Inhibition of COX-2 by acetyl salicylic acid (1 mM), NS-398 (5 microM), or celecoxib (3 microM) abolished the increase in cAMP and markedly reduced alpha(2C)-AR induction in response to serum stimulation. The cAMP agonists, forskolin (10 microM), isoproterenol (10 microM), and cholera toxin (0.1 microg/ml) each dramatically increased expression of alpha(2C)-ARs in human cutaneous VSMs. The A-kinase inhibitor H-89 (2 microM) inhibited phosphorylation of cAMP response element binding protein, but not the increase in alpha(2C)-AR expression in response to these agonists. cAMP-dependent but A-kinase independent signaling can involve activation of guanine nucleotide exchange factors for the GTP-binding protein, Rap. Indeed, pull-down assays demonstrated Rap1 activation by serum and forskolin in VSM. Transient transfections using alpha(2C)-AR promoter-luciferase reporter construct demonstrated that Rap1 increased reporter activity, whereas the A-kinase catalytic subunit decreased reporter activity. These results indicate that cAMP signaling can have dual effects in cutaneous VSMs:activation of alpha(2C)-AR transcription mediated by Rap1 GTPase and suppression mediated by A-kinase. The former effect predominates in serum-stimulated VSMs leading to a COX-2, cAMP, and Rap 1-dependent increase in alpha(2C)-AR expression. Such increased expression of alpha(2C)-ARs may contribute to enhanced cold-induced vasoconstriction and Raynaud's phenomenon.

The Traf2- and Nck-interacting kinase as a putative effector of Rap2 to regulate actin cytoskeleton

Rap2 belongs to the Ras family of small GTP-binding proteins, but its specific roles in cell signaling remain unknown. In the present study, we have affinity-purified from rat brain a Rap2-interacting protein of approximately 155 kDa, p155. By liquid chromatography tandem mass spectrometry, we have identified p155 as Traf2- and Nck-interacting kinase (TNIK). TNIK possesses an N-terminal kinase domain homologous to STE20, the Saccharomyces cerevisiae mitogen-activated protein kinase kinase kinase kinase, and a C-terminal regulatory domain termed the citron homology (CNH) domain. TNIK induces disruption of F-actin structure, thereby inhibiting cell spreading. In addition, TNIK specifically activates the c-Jun N-terminal kinase (JNK) pathway. Among our observations, TNIK interacted with Rap2 through its CNH domain but did not interact with Rap1 or Ras. TNIK interaction with Rap2 was dependent on the intact effector region and GTP-bound configuration of Rap2. When co-expressed in cultured cells, TNIK colocalized with Rap2, while a mutant TNIK lacking the CNH domain did not. Rap2 potently enhanced the inhibitory function of TNIK against cell spreading, but this was not observed for the mutant TNIK lacking the CNH domain. Rap2 did not significantly enhance TNIK-induced JNK activation, but promoted autophosphorylation and translocation of TNIK to the detergent-insoluble cytoskeletal fraction. These results suggest that TNIK is a specific effector of Rap2 to regulate actin cytoskeleton.

Phosphorylated guanine nucleotide exchange factor C3G, induced by pervanadate and Src family kinases localizes to the Golgi and subcortical actin cytoskeleton

BACKGROUND

The guanine nucleotide exchange factor C3G (RapGEF1) along with its effector proteins participates in signaling pathways that regulate eukaryotic cell proliferation, adhesion, apoptosis and embryonic development. It activates Rap1, Rap2 and R-Ras members of the Ras family of GTPases. C3G is activated upon phosphorylation at tyrosine 504 and therefore, determining the localization of phosphorylated C3G would provide an insight into its site of action in the cellular context.

RESULTS

C3G is phosphorylated in vivo on Y504 upon coexpression with Src or Hck, two members of the Src family tyrosine kinases. Here we have determined the subcellular localization of this protein using antibodies specific to C3G and Tyr 504 phosphorylated C3G (pY504 C3G). While exogenously expressed C3G was present mostly in the cytosol, pY504 C3G formed upon Hck or Src coexpression localized predominantly at the cell membrane and the Golgi complex. Tyrosine 504-phosphorylated C3G showed colocalization with Hck and Src. Treatment of Hck and C3G transfected cells with pervanadate showed an increase in the cytosolic staining of pY504 C3G suggesting that tyrosine phosphatases may be involved in dephosphorylating cytosolic phospho-C3G. Expression of Src family kinases or treatment of cells with pervanadate resulted in an increase in endogenous pY504 C3G, which was localized predominantly at the Golgi and the cell periphery. Endogenous pY504 C3G at the cell periphery colocalized with F-actin suggesting its presence at the subcortical actin cytoskeleton. Disruption of actin cytoskeleton by cytochalasin D abolished phospho-C3G staining at the periphery of the cell without affecting its Golgi localization.

CONCLUSIONS

These findings show that tyrosine kinases involved in phosphorylation of C3G are responsible for regulation of its localization in a cellular context. We have demonstrated the localization of endogenous C3G modified by tyrosine phosphorylation to defined subcellular domains where it may be responsible for restricted activation of signaling pathways.

Rap2B-dependent stimulation of phospholipase C-epsilon by epidermal growth factor receptor mediated by c-Src phosphorylation of RasGRP3

Receptor tyrosine kinase regulation of phospholipase C-epsilon (PLC-epsilon), which is under the control of Ras-like and Rho GTPases, was studied with HEK-293 cells endogenously expressing PLC-coupled epidermal growth factor (EGF) receptors. PLC and Ca(2+) signaling by the EGF receptor, which activated both PLC-gamma1 and PLC-epsilon, was specifically suppressed by inactivation of Ras-related GTPases with clostridial toxins and expression of dominant-negative Rap2B. EGF induced rapid and sustained GTP loading of Rap2B, binding of Rap2B to PLC-epsilon, and Rap2B-dependent translocation of PLC-epsilon to the plasma membrane. GTP loading of Rap2B by EGF was inhibited by chelation of intracellular Ca(2+) and expression of lipase-inactive PLC-gamma1 but not of PLC-epsilon. Expression of RasGRP3, a Ca(2+)/diacylglycerol-regulated guanine nucleotide exchange factor for Ras-like GTPases, but not expression of various other exchange factors enhanced GTP loading of Rap2B and PLC/Ca(2+) signaling by the EGF receptor. EGF induced tyrosine phosphorylation of RasGRP3, but not RasGRP1, apparently caused by c-Src; inhibition of c-Src interfered with EGF-induced Rap2B activation and PLC stimulation. Collectively, these data suggest that the EGF receptor triggers activation of Rap2B via PLC-gamma1 activation and tyrosine phosphorylation of RasGRP3 by c-Src, finally resulting in stimulation of PLC-epsilon.

Amplification, up-regulation and over-expression of C3G (CRK SH3 domain-binding guanine nucleotide-releasing factor) in non-small cell lung cancers

The Ras-CRK-Rap1 cellular signal-transduction system is regulated by guanine nucleotide exchange factors (GEFs). Transcription of C3G on chromosome 9q34 and a key member of the GEF gene family is activated by the CRK-adaptor protein; the C3G product is a CRK SH3 domain-binding guanine nucleotide-releasing factor. We document here the amplification of C3G in five of 18 primary non-small cell lung cancers examined and its increased expression in 18 of 28 tumors in comparison to corresponding non-cancerous lung tissues. Immunohistochemical staining revealed prominent C3G protein in the cytoplasm of cancer cells, associated with faint staining at the nucleolar membrane, but C3G was not detectable in normal bronchial mucoepithelial cells or in broncholoalveolar cells of the bronchial/bronchiolar ducts or alveoli. These data indicate that amplification and increased expression of the C3G gene may play some role in human lung carcinogenesis through derangement of the CRK-Rap1 signaling pathway.

Hormonal regulation of phospholipase Cepsilon through distinct and overlapping pathways involving G12 and Ras family G-proteins

PLCepsilon (phospholipase Cepsilon) is a novel PLC that has a CDC25 guanine nucleotide exchange factor domain and two RA (Ras-association) domains of which the second (RA2) is critical for Ras activation of the enzyme. In the present studies, we examined hormonal stimulation to elucidate receptor-mediated pathways that functionally regulate PLCepsilon. We demonstrate that EGF (epidermal growth factor), a receptor tyrosine kinase agonist, and LPA (lysophosphatidic acid), S1P (sphingosine 1-phosphate) and thrombin, GPCR (G-protein-coupled receptor) agonists, stimulate PLCepsilon overexpressed in COS-7 cells. EGF stimulated PLCepsilon in an RA2-dependent manner through Ras and Rap. In contrast, LPA, S1P and thrombin stimulated PLCepsilon by both RA2-independent and -dependent mechanisms. To determine the G-proteins that mediate the effects of these GPCR agonists, we co-expressed constitutively active G-proteins with PLCepsilon and found that G(alpha12), G(alpha13), Rho, Rac and Ral stimulate PLCepsilon in an RA2-independent manner; whereas TC21, Rap1A, Rap2A and Rap2B stimulate PLCepsilon in an RA2-dependent manner similar to H-Ras. Of these G-proteins, we show that G(alpha12)/G(alpha13) and Rap partly mediate the effects of LPA, S1P and thrombin to stimulate PLCepsilon. In addition, the stimulation by LPA and S1P is also partly sensitive to pertussis toxin. These studies demonstrate diverse hormonal regulation of PLCepsilon by distinct and overlapping pathways.

RalA activation at nascent lamellipodia of epidermal growth factor-stimulated Cos7 cells and migrating Madin-Darby canine kidney cells

RalA, a member of the Ras-family GTPases, regulates various cellular functions such as filopodia formation, endocytosis, and exocytosis. On epidermal growth factor (EGF) stimulation, activated Ras recruits guanine nucleotide exchange factors (GEFs) for RalA, followed by RalA activation. By using fluorescence resonance energy transfer-based probes for RalA activity, we found that the EGF-induced RalA activation in Cos7 cells was restricted at the EGF-induced nascent lamellipodia, whereas under a similar condition both Ras activation and Ras-dependent translocation of Ral GEFs occurred more diffusely at the plasma membrane. This EGF-induced RalA activation was not observed when lamellipodial protrusion was suppressed by a dominant negative mutant of Rac1, a GTPase-activating protein for Cdc42, inhibitors of phosphatidylinositol 3-kinase, or inhibitors of actin polymerization. On the other hand, EGF-induced lamellipodial protrusion was inhibited by microinjection of the RalA-binding domains of RalBP1 and Sec5. Furthermore, we found that RalA activity was high at the lamellipodia of migrating Madin-Darby canine kidney cells and that the migration of Madin-Darby canine kidney cells was perturbed by the microinjection of RalBP1-RalA-binding domain. Thus, RalA activation is required for the induction of lamellipodia, and conversely, lamellipodial protrusion seems to be required for the RalA activation, suggesting the presence of a positive feedback loop between RalA activation and lamellipodial protrusion. Our observation also demonstrates that the spatial regulation of RalA is conducted by a mechanism distinct from the temporal regulation conducted by Ras-dependent plasma membrane recruitment of Ral guanine nucleotide exchange factors.

Activation of the Rap GTPases in B lymphocytes modulates B cell antigen receptor-induced activation of Akt but has no effect on MAPK activation

Signaling by the B cell antigen receptor (BCR) activates the Rap1 and Rap2 GTPases, putative antagonists of Ras-mediated signaling. Because Ras can activate the Raf-1/ERK pathway and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, we asked whether Rap activation limits the ability of the BCR to signal via these pathways. To do this, we blocked the activation of endogenous Rap1 and Rap2 by expressing the Rap-specific GTPase-activating protein RapGAPII. Preventing Rap activation had no effect on BCR-induced activation of ERK. In contrast, BCR-induced phosphorylation of Akt on critical activating sites was increased 2- to 3-fold when Rap activation was blocked. Preventing Rap activation also increased the ability of the BCR to stimulate Akt-dependent phosphorylation of the FKHR transcription factor on negative regulatory sites and decreased the levels of p27Kip1, a pro-apoptotic factor whose transcription is enhanced by FKHR. Moreover, preventing Rap activation reduced BCR-induced cell death in the WEHI-231 B cell line. Thus activation of endogenous Rap by the BCR limits BCR-induced activation of the PI3K/Akt pathway, opposes the subsequent inhibition of the FKHR/p27Kip1 pro-apoptotic module, and enhances BCR-induced cell death. Consistent with the idea that Rap-GTP is a negative regulator of the PI3K/Akt pathway, expressing constitutively active Rap2 (Rap2V12) reduced BCR-induced phosphorylation of Akt and FKHR. Finally, our finding that Rap2V12 can bind PI3K and inhibit its activity in a manner that depends upon BCR engagement provides a potential mechanism by which Rap-GTP limits activation of the PI3K/Akt pathway, a central regulator of B cell growth and survival.

Physical and functional interaction between Hck tyrosine kinase and guanine nucleotide exchange factor C3G results in apoptosis, which is independent of C3G catalytic domain

The hematopoietic cell kinase Hck is a Src family tyrosine kinase expressed in cells of myelomonocytic lineage, B lymphocytes, and embryonic stem cells. To study its role in signaling pathways we used the Hck-SH3 domain in protein interaction cloning and identified C3G, the guanine nucleotide exchange factor for Rap1 and R-Ras, as a protein that associated with Hck. This interaction was direct and was mediated partly through the proline-rich region of C3G. C3G could be co-immunoprecipitated with Hck from Cos-1 cells transfected with Hck and C3G. C3G was phosphorylated on tyrosine 504 in cells when coexpressed with Hck but not with a catalytically inactive mutant of Hck. Phosphorylation of endogenous C3G at Tyr-504 was increased by treatment of human myelomonocytic THP-1 cells with mercuric chloride, which is known to activate Hck tyrosine kinase specifically. Coexpression of Hck with C3G induced a high level of apoptosis in many cell lines by 30-42 h of transfection. Induction of apoptosis was not dependent on Tyr-504 phosphorylation or the catalytic domain of C3G but required the catalytic activity of Hck. Using dominant negative constructs of caspases we found that caspase-1, -8, and -9 are involved in this apoptotic pathway. These results suggest that C3G and Hck interact physically and functionally in vivo to activate kinase-dependent and caspase-mediated apoptosis, which is independent of catalytic domain of C3G.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

Mechanism of the spatio-temporal regulation of Ras and Rap1

Epidermal growth factor (EGF) activates Ras and Rap1 at distinct intracellular regions. Here, we explored the mechanism underlying this phenomenon. We originally noticed that in cells expressing Epac, a cAMP-dependent Rap1 GEF (guanine nucleotide exchange factor), cAMP activated Rap1 at the perinuclear region, as did EGF. However, in cells expressing e-GRF, a recombinant cAMP-responsive Ras GEF, cAMP activated Ras at the peripheral plasma membrane. Based on the uniform cytoplasmic expression of Epac and e-GRF, GEF did not appear to account for the non-uniform increase in the activities of Ras and Rap1. In contrast, when we used probes with reduced sensitivity to GTPase-activating proteins (GAPs), both Ras and Rap1 appeared to be activated uniformly in the EGF-stimulated cells. Furthermore, we calculated the local rate constants of GEFs and GAPs from the video images of Ras activation and found that GAP activity was higher at the central plasma membrane than the periphery. Thus we propose that GAP primarily dictates the spatial regulation of Ras family G proteins, whereas GEF primarily determines the timing of Ras activation.

The high-risk human papillomavirus type 16 E6 counters the GAP function of E6TP1 toward small Rap G proteins

We have recently identified E6TP1 (E6-targeted protein 1) as a novel high-risk human papillomavirus type 16 (HPV16) E6-binding protein. Importantly, mutational analysis of E6 revealed a strong correlation between the transforming activity and its abilities to bind and target E6TP1 for ubiquitin-mediated degradation. As a region within E6TP1 has high homology with GAP domains of known and putative Rap GTPase-activating proteins (GAPs), these results raised the possibility that HPV E6 may alter the Rap small-G-protein signaling pathway. Using two different approaches, we now demonstrate that human E6TP1 exhibits GAP activity for Rap1 and Rap2, confirming recent findings that a closely related rat homologue exhibits Rap-specific GAP activity. Using mutational analysis, we localize the GAP activity to residues 240 to 945 of E6TP1. Significantly, we demonstrate that coexpression of HPV16 E6, by promoting the degradation of E6TP1, enhances the GTP loading of Rap. These results support a role of Rap small-G-protein pathway in E6-mediated oncogenesis.

Novel function of Chat in controlling cell adhesion via Cas-Crk-C3G-pathway-mediated Rap1 activation

Chat (Cas/HEF1-associated signal transducer) is a novel signaling molecule with an N-terminal SH2 domain and C-terminal Cas/HEF1 association domain that is implicated in the regulation of cell adhesion. The Cas/HEF1 association domain also shows sequence similarity with guanine nucleotide exchange factors for Ras family small GTPases. In this study, we found significant activation of Rap1 in Chat-overexpressing cells. Myr-Chat, a membrane-targeted form of Chat, activated Rap1 more efficiently. Interestingly, Chat and Cas synergistically activated Rap1. Certain Cas, Crk or C3G mutants suppressed Rap1 activation by Chat. We also confirmed the ternary complex formation consisting of Chat, Cas and Crk. Thus, it is likely that Chat-induced Rap1 activation was mediated by upregulation of the Cas-Crk-C3G signaling pathway rather than direct guanine nucleotide exchange factor activity of Chat. We further demonstrated that Myr-Chat expression induced cell periphery spreading and cell shape branching and that this activity also depended on the Cas-Crk-C3G pathway and Rap1 activity. Moreover, expression of Myr-Chat enhanced integrin-mediated cell adhesion. Taken together we propose a novel role for the Chat-Cas complex in controlling cell adhesion via the activation of Rap1.

Differential roles of Ras and Rap1 in growth factor-dependent activation of phospholipase C epsilon

Phospholipase C epsilon is a phosphoinositide-specific phospholipase C that selectively associates with Ras and Rap small GTPases as a target. Here we explored the molecular basis of the Rap1- as well as Ras-mediated regulation of phospholipase C epsilon upon platelet-derived growth factor stimulation by using a receptor mutant deficient in its ability to phosphorylate and activate phospholipase C gamma. Following platelet-derived growth factor treatment, this receptor induces persistent activation of ectopically expressed PLC epsilon through activation of Ras and Rap1. The rapid and initial phase of the activation is mediated by Ras, whereas Rap1 is responsible for the prolonged activation. We further demonstrate that the CDC25 homology domain, which exhibits guanine nucleotide exchange factor activity toward Rap1, but not Ras, is critical for the prolonged activation of phospholipase C epsilon. Platelet-derived growth factor prevented the hematopoietic BaF3 cells containing the mutant receptor from undergoing apoptosis, and enabled these cells to proliferate, only when phospholipase C epsilon was expressed. Therefore, the phospholipase C signal is suggested to be critical for survival and growth of BaF3 cells.

Di-Ras, a distinct subgroup of ras family GTPases with unique biochemical properties

The small GTPase Ras family regulates a variety of cell functions including proliferation and differentiation. Here we have identified novel Ras members, human Di-Ras1 and Di-Ras2, belonging to a distinct branch of the GTPase family. Di-Ras1 and Di-Ras2 specifically expressed in heart and brain share 30-40% overall identity with other members of Ras family, however, they have the following characteristic substitutions at highly conserved regions among the Ras family. 1) Thr-63 and Ser-65 in Di-Ras are substituted for Ala-59 and Gln-61 positions in Ha-Ras, respectively, that are known to be critical for GTP hydrolysis. 2) Within the effector domains, Di-Ras has Ile at a position corresponding to Asp-33 in Ha-Ras, which is important for its interaction with the downstream effector Raf. As predicted by these substitutions, Di-Ras has only a quite low level of GTPase activity and exists predominantly as a GTP-bound form upon its expression in living cells. Moreover, Di-Ras fails to interact with the Ras-binding domain of Raf, resulting in no stimulation of mitogen-activated protein kinase. Interestingly, introduction of Di-Ras into HEK293T cells induces large cellular vacuolation. These findings raise the possibility that Di-Ras might regulate cell morphogenesis in a manner distinct from other members of Ras family.

Ras and relatives--job sharing and networking keep an old family together

Many members of the Ras superfamily of GTPases have been implicated in the regulation of hematopoietic cells, with roles in growth, survival, differentiation, cytokine production, chemotaxis, vesicle-trafficking, and phagocytosis. The well-known p21 Ras proteins H-Ras, N-Ras, K-Ras 4A, and K-Ras 4B are also frequently mutated in human cancer and leukemia. Besides the four p21 Ras proteins, the Ras subfamily of the Ras superfamily includes R-Ras, TC21 (R-Ras2), M-Ras (R-Ras3), Rap1A, Rap1B, Rap2A, Rap2B, RalA, and RalB. They exhibit remarkable overall amino acid identities, especially in the regions interacting with the guanine nucleotide exchange factors that catalyze their activation. In addition, there is considerable sharing of various downstream effectors through which they transmit signals and of GTPase activating proteins that downregulate their activity, resulting in overlap in their regulation and effector function. Relatively little is known about the physiological functions of individual Ras family members, although the presence of well-conserved orthologs in Caenorhabditis elegans suggests that their individual roles are both specific and vital. The structural and functional similarities have meant that commonly used research tools fail to discriminate between the different family members, and functions previously attributed to one family member may be shared with other members of the Ras family. Here we discuss similarities and differences in activation, effector usage, and functions of different members of the Ras subfamily. We also review the possibility that the differential localization of Ras proteins in different parts of the cell membrane may govern their responses to activation of cell surface receptors.

Activation of rac and cdc42 video imaged by fluorescent resonance energy transfer-based single-molecule probes in the membrane of living cells

Rho family G proteins, including Rac and Cdc42, regulate a variety of cellular functions such as morphology, motility, and gene expression. We developed fluorescent resonance energy transfer-based probes which monitored the local balance between the activities of guanine nucleotide exchange factors and GTPase-activating proteins for Rac1 and Cdc42 at the membrane. These probes, named Raichu-Rac and Raichu-Cdc42, consisted of a Cdc42- and Rac-binding domain of Pak, Rac1 or Cdc42, a pair of green fluorescent protein mutants, and a CAAX box of Ki-Ras. With these probes, we video imaged the Rac and Cdc42 activities. In motile HT1080 cells, activities of both Rac and Cdc42 gradually increased toward the leading edge and decreased rapidly when cells changed direction. Under a higher magnification, we observed that Rac activity was highest immediately behind the leading edge, whereas Cdc42 activity was most prominent at the tip of the leading edge. Raichu-Rac and Raichu-Cdc42 were also applied to a rapid and simple assay for the analysis of putative guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) in living cells. Among six putative GEFs and GAPs, we identified KIAA0362/DBS as a GEF for Rac and Cdc42, KIAA1256 as a GEF for Cdc42, KIAA0053 as a GAP for Rac and Cdc42, and KIAA1204 as a GAP for Cdc42. In conclusion, use of these single-molecule probes to determine Rac and Cdc42 activity will accelerate the analysis of the spatiotemporal regulation of Rac and Cdc42 in a living cell.

The Rap GTPases regulate B cell migration toward the chemokine stromal cell-derived factor-1 (CXCL12): potential role for Rap2 in promoting B cell migration

Stromal cell-derived factor-1 (SDF-1) is a potent chemoattractant for B cells and B cell progenitors. Although the binding of SDF-1 to its receptor, CXCR4, activates multiple signaling pathways, the mechanism by which SDF-1 regulates cell migration is not completely understood. In this report we show that activation of the Rap GTPases is important for B cells to migrate toward SDF-1. We found that treating B cells with SDF-1 resulted in the rapid activation of both Rap1 and Rap2. Moreover, blocking the activation of Rap1 and Rap2 via the expression of a Rap-specific GTPase-activating protein significantly reduced the ability of B cells to migrate toward SDF-1. Conversely, expressing a constitutively active form of Rap2 increased SDF-1-induced B cell migration. Thus, the Rap GTPases control cellular processes that are important for B cells to migrate toward SDF-1.

Subcellular compartmentalization and trafficking of the insulin-responsive glucose transporter, GLUT4

Insulin increases glucose transport into cells of target tissues, primarily striated muscle and adipose. This is accomplished via the insulin-dependent translocation of the facilitative glucose transporter 4 (GLUT4) from intracellular storage sites to the plasma membrane. Insulin binds to the cell-surface insulin receptor and activates its intrinsic tyrosine kinase activity. The subsequent activation of phosphatidylinositol 3-kinase (PI 3-K) is well known to be necessary for the recruitment of GLUT4 to the cell surface. Both protein kinase B (PKB) and the atypical protein kinase C(lambda/zeta) (PKClambda/zeta) appear to function downstream of PI 3-K, but how these effectors influence GLUT4 translocation remains unknown. In addition, emerging evidence suggests that a second signaling cascade that functions independently of the PI 3-K pathway is also required for the insulin-dependent translocation of GLUT4. This second pathway involves the Rho-family GTP binding protein TC10, which functions within the specialized environment of lipid raft microdomains at the plasma membrane. Future work is necessary to identify the downstream effectors that link TC10, PKB, and PKClambda/zeta to GLUT4 translocation. Progress in this area will come from a better understanding of the compartmentalization of GLUT4 within the cell and of the mechanisms responsible for targeting the transporter to specialized insulin-responsive storage compartments. Furthermore, an understanding of how GLUT4 is retained within and released from these compartments will facilitate the identification of downstream signaling molecules that function proximal to the GLUT4 storage sites.

Role of the CDC25 homology domain of phospholipase Cepsilon in amplification of Rap1-dependent signaling

Phospholipase Cepsilon (PLCepsilon) is a novel class of phosphoinositide-specific PLC characterized by possession of CDC25 homology and Ras/Rap1-associating domains. We and others have shown that human PLCepsilon is translocated from the cytoplasm to the plasma membrane and activated by direct association with Ras at its Ras/Rap1-associating domain. In addition, translocation to the perinuclear region was induced upon association with Rap1.GTP. However, the function of the CDC25 homology domain remains to be clarified. Here we show that the CDC25 homology domain of PLCepsilon functions as a guanine nucleotide exchange factor for Rap1 but not for any other Ras family GTPases examined including Rap2 and Ha-Ras. Consistent with this, coexpression of full-length PLCepsilon or its N-terminal fragment carrying the CDC25 homology domain causes an increase of the intracellular level of Rap1.GTP. Concurrently, stimulation of the downstream kinases B-Raf and extracellular signal-regulated kinase is observed, whereas the intracellular level of Ras.GTP and Raf-1 kinase activity are unaffected. In wild-type Rap1-overexpressing cells, epidermal growth factor induces translocation of PLCepsilon to the perinuclear compartments such as the Golgi apparatus, which is sustained for at least 20 min. In contrast, PLCepsilon lacking the CDC25 domain translocates to the perinuclear compartments only transiently. Further, the formation of Rap1.GTP upon epidermal growth factor stimulation exhibits a prolonged time course in cells expressing full-length PLCepsilon compared with those expressing PLCepsilon lacking the CDC25 homology domain. These results suggest a pivotal role of the CDC25 homology domain in amplifying Rap1-dependent signal transduction, including the activation of PLCepsilon itself, at specific subcellular locations such as the Golgi apparatus.

Requirement for C3G-dependent Rap1 activation for cell adhesion and embryogenesis

C3G is a guanine nucleotide exchange factor (GEF) for Rap1, and is activated via Crk adaptor protein. To understand the physiological role of C3G, we generated C3G knockout mice. C3G(-/-) homozygous mice died before embryonic day 7.5. The lethality was rescued by the expression of the human C3G transgene, which could be excised upon the expression of Cre recombinase. From the embryo of this mouse, we prepared fibroblast cell lines, MEF-hC3G. Expression of Cre abolished the expression of C3G in MEF-hC3G and inhibited cell adhesion-induced activation of Rap1. The Cre-expressing MEF-hC3G showed impaired cell adhesion, delayed cell spreading and accelerated cell migration. The accelerated cell migration was suppressed by the expression of active Rap1, Rap2 and R-Ras. Expression of Epac and CalDAG-GEFI, GEFs for Rap1, also suppressed the accelerated migration of the C3G-deficient cells. This observation indicated that Rap1 activation was sufficient to complement the C3G deficiency. In conclusion, C3G-dependent activation of Rap1 is required for adhesion and spreading of embryonic fibroblasts and for the early embryogenesis of the mouse.

Spatio-temporal images of growth-factor-induced activation of Ras and Rap1

G proteins of the Ras family function as molecular switches in many signalling cascades; however, little is known about where they become activated in living cells. Here we use FRET (fluorescent resonance energy transfer)-based sensors to report on the spatio-temporal images of growth-factor-induced activation of Ras and Rap1. Epidermal growth factor activated Ras at the peripheral plasma membrane and Rap1 at the intracellular perinuclear region of COS-1 cells. In PC12 cells, nerve growth factor-induced activation of Ras was initiated at the plasma membrane and transmitted to the whole cell body. After three hours, high Ras activity was observed at the extending neurites. By using the FRAP (fluorescence recovery after photobleaching) technique, we found that Ras at the neurites turned over rapidly; therefore, the sustained Ras activity at neurites was due to high GTP/GDP exchange rate and/or low GTPase activity, but not to the retention of the active Ras. These observations may resolve long-standing questions as to how Ras and Rap1 induce different cellular responses and how the signals for differentiation and survival are distinguished by neuronal cells.

Rap1 signalling: adhering to new models

Ras-like GTPases are ubiquitously expressed, evolutionarily conserved molecular switches that couple extracellular signals to various cellular responses. Rap1, the closest relative of Ras, has attracted much attention because of the possibility that it regulates Ras-mediated signalling. Rap1 is activated by extracellular signals through several regulatory proteins, and it might function in diverse processes, ranging from modulation of growth and differentiation to secretion, integrin-mediated cell adhesion and morphogenesis.

Membrane recruitment of DOCK180 by binding to PtdIns(3,4,5)P3

DOCK180 was originally identified as one of two major proteins bound to the Crk oncogene product and became an archetype of the CDM family of proteins, including Ced-5 of Caenorhabditis elegans and Mbc of Drosophila melanogaster. Further study has suggested that DOCK180 is involved in the activation of Rac by the CrkII-p130(Cas) complex. With the use of deletion mutants of DOCK180, we found that the C-terminal region containing a cluster of basic amino acids was required for binding to and activation of Rac. This region showed high amino-acid sequence similarity to the consensus sequence of the phosphoinositide-binding site; this led us to examine whether this basic region binds to phosphoinositides. For this purpose we used PtdIns(3,4,5)P(3)-APB beads, as reported previously [Shirai, Tanaka, Terada, Sawada, Shirai, Hashimoto, Nagata, Iwamatsu, Okawa, Li et al. (1998) Biochim. Biophys. Acta 1402, 292-302]. By using various competitors, we demonstrated the specific binding of DOCK180 to PtdIns(3,4,5)P(3). The expression of active phosphoinositide 3-kinase (PI-3K) did not enhance a DOCK180-induced increase in GTP-Rac; however, the expression of PI-3K translocated DOCK180 to the plasma membrane. Thus DOCK180 contained a phosphoinositide-binding domain, as did the other guanine nucleotide exchange factors with a Dbl homology domain, and was translocated to the plasma membrane on the activation of PI-3K.