Molecular cloning of cDNAs encoding a guanine-nucleotide-releasing factor for Ras p21

CDC25(Mm)/Ras-GRF1 regulates both Ras and Rac signaling pathways

The Ras-GRF1 exchange factor molecule contains in addition to the catalytic domain two pleckstrin homology (PH1 and PH2), one IQ and one Dbl homology (DH) domains. In this study we investigated the role of such additional domains. We found that a Ras-GRF1 mutant lacking PH1 and IQ domains is sufficient to activate c-fos promoter in response to lysophosphatidic acid (LPA). The same mutant did not increase external stimuli-regulated kinase (ERK) activity, suggesting an additional mechanism for the induction of gene transcription. Isolated DH-PH2 module activates c-Jun NH(2)-terminal kinase and the c-fos promoter in response to LPA, providing the basis for an ERK-independent mechanism. These results provide evidence that Ras-GRF1 acts as a bifunctional molecule on both ERK-dependent and independent pathways.

Signal transduction elements of TC21, an oncogenic member of the R-Ras subfamily of GTP-binding proteins

TC21 is a Ras-like GTPase with high oncogenic potential that is found mutated in some human tumors and overexpressed in breast cancer cell lines. We have conducted cellular and biochemical studies in order to understand the role of this protein in signal transduction and to unveil the signaling elements that participate in the TC21 pathway. Using gene transfer experiments, we demonstrate here that the TC21 oncogene can induce both cellular transformation in mouse fibroblasts and neuronal-like differentiation in rat PC12 cells. Interestingly, the proto-oncogenic version of TC21 shows also a lower, but significant, activity in both biological processes. We also demonstrate that the similarity of the cellular responses induced by TC21 and Ras derive from the utilization of overlapping pathways. Thus, the exchange of guanosine nucleotides in wild type TC21 is catalyzed by Ras exchange factors. Moreover, TC21 binds physically to c-Raf-1 in a GTP-dependent manner. Finally, overexpression of TC21G23V in NIH3T3 cells results in the activation of c-Raf-1 and the MAPK and the JNK branches of serine/threonine cascades. From these results, we conclude that TC21 promotes Ras-like responses in diverse cell types due to the use of overlapping, if not identical, signaling elements of the Ras oncogenic pathway.

Mouse semaphorin H induces PC12 cell neurite outgrowth activating Ras-mitogen-activated protein kinase signaling pathway via Ca(2+) influx

We recently showed that mouse semaphorin H (MSH), a secreted semaphorin molecule, acts as a chemorepulsive factor on sensory neurites. In this study, we found for the first time that MSH induces neurite outgrowth in PC12 cells in a dose-dependent manner. Comparison of Ras-mitogen-activated protein kinase (MAPK) signaling pathways between MSH and nerve growth factor (NGF) revealed that these pathways are crucial for MSH action as well as NGF. K-252a, an inhibitor of tyrosine autophosphorylation of tyrosine kinase receptors (Trks), did not inhibit the action of MSH, suggesting that MSH action occurs via a different receptor than NGF. L- and N-types of voltage-dependent Ca(2+) channel blockers, diltiazem and omega-conotoxin, inhibited MSH-induced neurite outgrowth and MAPK phosphorylation in a Ca(2+)-dependent manner. A transient elevation in intracellular Ca(2+) level was observed upon MSH stimulation. These findings suggest that extracellular Ca(2+) influx, followed by activation of the Ras-MAPK signaling pathway, is required for MSH induced PC12 cell neurite outgrowth.

The N-terminal half of Cdc25 is essential for processing glucose signaling in Saccharomyces cerevisiae

Saccharomyces cerevisiae Cdc25 is the prototype Ras GDP/GTP exchange protein. Its C-terminal catalytic domain was found to be highly conserved in the homologues p140(ras-GRF) and Sos. The regulatory domains in each Ras exchanger mediate the signals arriving from upstream elements such as tyrosine kinases for Sos, or Ca2+ and G proteins for p140.(Ras-GRF) In this study, we show that the N-terminal half (NTH) of S. cerevisiae Cdc25, as well as the C-terminal 37 amino acids, is essential for processing the elevation of cAMP in response to glucose. The mammalian p140(ras-GRF) catalytic domain (CGRF) restores glucose signaling in S. cerevisiae only if tethered between the N-terminal half (NTH) of S. cerevisiae Cdc25 and the C-terminal 37 amino acids. The glucose-induced transient elevation in cAMP is nullified or severely hampered by the deletion of domains within the NTH of Cdc25. These deletions, however, do not modify the intrinsic GDP/GTP exchange activity of mutant proteins as compared to native Cdc25. We also show that 7 Ser to Ala mutations at the cAMP-dependent protein kinase putative phosphorylation sites within the NTH of Cdc25 eliminate the descending portion of the glucose response curve, responsible for signal termination. These findings support a dual role of the NTH of Cdc25 in both enabling the glucose signal and being responsible for its attenuation.

GRFbeta, a novel regulator of calcium signaling, is expressed in pancreatic beta cells and brain

By screening for genes expressed differentially in pancreatic beta cells, we have isolated a cDNA encoding GRFbeta, a novel 178-amino acid protein whose N terminus is identical to that of GRF1, a calcium-dependent guanine nucleotide exchange factor, and whose C terminus is unrelated to known proteins. We show that both GRF1 and GRFbeta are expressed selectively in beta cell lines, pancreatic islet cells and brain. Treatment of beta cell lines (betaTC1 and HIT) with calcium ionophore led to a significant elevation in activity of the Ras signal transduction pathway, as determined by phosphorylation of extracellular signal-related kinase (ERK). Transfection of beta cells with a plasmid encoding a dominant negative variant of GRF1 led to 70% reduction in ERK phosphorylation, consistent with a role for GRF1 in calcium-dependent Ras signaling in these cells. To examine the possible function of GRFbeta, cultured cells were transfected with a GRFbeta expression vector. This led to a significant reduction in both GRF1-dependent ERK phosphorylation and AP1-dependent reporter gene activity. The results suggest that GRF1 plays a role in mediating calcium-dependent signal transduction in beta cells and that GRFbeta represents a novel dominant negative modulator of Ras signaling.

Activation of the Ras-GRF/CDC25Mm exchange factor by lysophosphatidic acid

The Ras-GRF exchange factor can activate Ras-dependent responses following the activation of heterotrimeric G-protein and calcium signalling. In stable lines of NIH-3T3 fibroblasts that express Ras-GRF, the agonist lysophosphatidic acid (LPA) increases the phosphorylation state and activity of Ras-GRF. The stimulation of Ras-GRF can be demonstrated in vitro, in an assay using recombinant Ras substrate, and in situ, by a selective increase in the ability of LPA to stimulate mitogen-activated protein (MAP) kinase. The increase in Ras-GRF phosphorylation state, which occurs on serine residues, and the increase in exchange factor activity are blocked by pretreatment with pertussis toxin. Activation of Ras-GRF by LPA can also be inhibited by chelation of intracellular calcium and treatment of the Ras-GRF with protein phosphatase 1 (PP1), supporting a model in which Ras-GRF serves to integrate signals from multiple transduction pathways.

BCR binds to the xeroderma pigmentosum group B protein

The BCR gene is involved in the formation of the BCR-ABL oncogene responsible for the pathogenesis of Philadelphia chromosome-positive human leukemias. We have previously shown that P210 BCR-ABL binds to the xeroderma pigmentosum group B protein (XPB) through the portion of BCR that is homologous to the catalytic domain of GDP-GTP exchangers such as yeast CDC24 and Dbl. In the baculovirus overexpression system which facilitates binding of coexpressed proteins, we now show that XPB binds to the intact BCR protein efficiently but not to CDC24 or Dbl, suggesting specificity of this interaction. The binding of endogenous BCR and XPB proteins was also detected in Hela cells, and this was inhibited by a blocking peptide. Full-length (1-782) XPB and its truncated form (203-782), which does not contain the nuclear localization signal, were tagged with glutathione S-transferase (GST) and were expressed in Rat1 fibroblasts. GST-XPB(203-782) was localized predominantly in the cytoplasm and bound to BCR but not to p62, one of the other components in TFIIH. GST-XPB(1-782) was largely in the nucleus and bound to p62 and BCR. Although the biological significance of the binding remains to be uncovered, BCR binds to the XPB/p62 complex.

Ras-specific exchange factor GRF: oligomerization through its Dbl homology domain and calcium-dependent activation of Raf

The full-length versions of the Ras-specific exchange factors Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2), which are expressed in brain and a restricted number of other organs, possess an ionomycin-dependent activation of Erk mitogen-activated protein kinase activity in 293T cells (C. L. Farnsworth et al., Nature 376:524-527, 1995; N. P. Fam et al., Mol. Cell. Biol. 17:1396-1406, 1996). Each GRF protein contains a Dbl homology (DH) domain. A yeast two-hybrid screen was used to identify polypeptides that associate with the DH domain of GRF1. In this screen, a positive cDNA clone from a human brain cDNA library was isolated which consisted of the GRF2 DH domain and its adjacent ilimaquinone domain. Deletion analysis verified that the two-hybrid interaction required only the DH domains, and mutation of Leu-263 to Gln (L263Q) in the N terminus of the GRF1 DH domain abolished the two-hybrid interaction, while a cluster of more C-terminally located mutations in the DH domain did not eliminate the interaction. Oligomers between GRF1 and GRF2 were detected in a rat brain extract, and forced expression of GRF1 and GRF2 in cultured mammalian cells formed homo- and hetero-oligomers. Introduction of the L263Q mutation in GRF1 led to a protein that was deficient in oligomer formation, while GRF1 containing the DH cluster mutations formed homo-oligomers with an efficiency similar to that of wild type. Compared to wild-type GRF1, the focus-forming activity on NIH 3T3 cells of the GRF1 DH cluster mutant was reduced, while the L263Q mutant was inactive. Both mutants were impaired in their ability to mediate ionomycin-dependent Erk activity in 293T cells. In the absence of ionomycin, 293T cells expressing wild-type GRF1 contained much higher levels of Ras-GTP than control cells; the increase in Erk activity induced by ionomycin in the GRF1-expressing cells also induced a concomitant increase in Raf kinase activity, but without a further increase in the level Ras-GTP. We conclude that GRF1 and GRF2 can form homo- and hetero-oligomers via their DH domains, that mutational inactivation of oligomer formation by GRF1 is associated with impaired biological and signaling activities, and that in 293T cells GRF1 mediates at least two pathways for Raf activation: one a constitutive signal that is mainly Ras-dependent, and one an ionomycin-induced signal that cooperates with the constitutive signal without further augmenting the level of GTP-Ras.

RHO-associated protein kinase alpha potentiates insulin-induced MAP kinase activation in Xenopus oocytes

We recently identified Xenopus Rho-associated protein kinase alpha (xROKalpha) as a Xenopus insulin receptor substrate-1 binding protein and demonstrated that the non-catalytic carboxyl terminus of xROKalpha binds Xenopus insulin receptor substrate-1 and blocks insulin-induced MAP kinase activation and germinal vesicle breakdown in Xenopus oocytes. In the current study we further examined the role of xROKalpha in insulin signal transduction in Xenopus oocytes. We demonstrate that injection of mRNA encoding the xROKalpha kinase domain or full length xROKalpha enhanced insulin-induced MAP kinase activation and germinal vesicle breakdown. In contrast, injection of a kinase-dead mutant of xROKalpha or pre-incubation of oocytes with an xROKalpha inhibitor significantly reduced insulin-induced MAP kinase activation. To further dissect the mechanism by which xROKalpha may participate in insulin signalling, we explored a potential function of xROKalpha in regulating cellular Ras function, since insulin-induced MAP kinase activation and germinal vesicle breakdown is known to be a Ras-dependent process. We demonstrate that whereas injection of mRNA encoding c-H-Ras alone induced xMAP kinase activation and GVBD in a very low percentage (about 10%) of injected oocytes, co-injection of mRNA encoding xROKalpha and c-H-Ras induced xMAP kinase activation and germinal vesicle breakdown in a significantly higher percentage (50-60%) of injected oocytes. These results suggest a novel function for xROKalpha in insulin signal transduction upstream of cellular Ras function.

G protein beta gamma subunit-dependent Rac-guanine nucleotide exchange activity of Ras-GRF1/CDC25(Mm)

Ras-GRF1 has been implicated as a Ras-specific guanine nucleotide exchange factor (GEF), which mediates calcium- and muscarinic receptor-triggered signals in the brain. Although a Dbl homology domain known as a motif conserved among GEFs that target Rho family GTP-binding proteins exists in Ras-GRF1, GEF activity toward Rho family proteins has not been observed. Here we show that Ras-GRF1 exhibits Rac1-specific GEF activity when recovered from cells overexpressing G protein beta gamma subunits (Gbeta gamma). Substitution of conserved amino acids within the Dbl homology domain abolished this activity. Activation of the Rac pathway in the cell was further evidenced by synergistic activation of the stress kinase JNK1 by Ras-GRF1 and Gbeta gamma, which is sensitive to inhibitory action of dominant-negative Rac1(17N). In addition, association of Ras-GRF1 with Rac1(17N) was demonstrated by coimmunoprecipitation. Evidence for the involvement of tyrosine kinase(s) in Gbeta gamma-mediated induction of Rac1-specific GEF activity was provided by the use of specific inhibitors. These results suggest a role of Ras-GRF1 for regulating Rac-dependent as well as Ras-dependent signaling pathways, particularly in the brain functions.

Interaction of heterotrimeric G protein Galphao with Purkinje cell protein-2. Evidence for a novel nucleotide exchange factor

The heterotrimeric G protein Galphao is ubiquitously expressed throughout the central nervous system, but many of its functions remain to be defined. To search for novel proteins that interact with Galphao, a mouse brain library was screened using the yeast two-hybrid interaction system. Pcp2 (Purkinje cell protein-2) was identified as a partner for Galphao in this system. Pcp2 is expressed in cerebellar Purkinje cells and retinal bipolar neurons, two locations where Galphao is also expressed. Pcp2 was first identified as a candidate gene to explain Purkinje cell degeneration in pcd mice (Nordquist, D. T., Kozak, C. A., and Orr, H. T. (1988) J. Neurosci. 8, 4780-4789), but its function remains unknown as Pcp2 knockout mice are normal (Mohn, A. R., Feddersen, R. M., Nguyen, M. S., and Koller, B. H. (1997) Mol. Cell. Neurosci. 9, 63-76). Galphao and Pcp2 binding was confirmed in vitro using glutathione S-transferase-Pcp2 fusion proteins and in vitro translated [35S]methionine-labeled Galphao. In addition, when Galphao and Pcp2 were cotransfected into COS cells, Galphao was detected in immunoprecipitates of Pcp2. To determine whether Pcp2 could modulate Galphao function, kinetic constants kcat and koff of bovine brain Galphao were determined in the presence and absence of Pcp2. Pcp2 stimulates GDP release from Galphao more than 5-fold without affecting kcat. These findings define a novel nucleotide exchange function for Pcp2 and suggest that the interaction between Pcp2 and Galphao is important to Purkinje cell function.

The p21-Ras signal transduction pathway and growth regulation in human high-grade gliomas

Deregulated p21-Ras function, as a result of mutation, overexpression or growth factor-induced overactivation, contributes to at least 30% of human cancer. This article reviews the potential role of the p21-Ras family of GTPases in the regulation of growth of high-grade gliomas and describes how targeting this oncoprotein clinically may provide a novel strategy to counteract glioma proliferation. The application of strategies directed at selectively opposing the deregulated signal transduction pathway of high-grade gliomas may be of potential therapeutic benefit and may offer a whole new arsenal of antineoplastic agents to be included in the multimodal treatment of these challenging neoplasms.

Identification and characterization of potential effector molecules of the Ras-related GTPase Rap2

In search for effectors of the Ras-related GTPase Rap2, we used the yeast two-hybrid method and identified the C-terminal Ras/Rap interaction domain of the Ral exchange factors (RalGEFs) Ral GDP dissociation stimulator (RalGDS), RalGDS-like (RGL), and RalGDS-like factor (Rlf). These proteins, which also interact with activated Ras and Rap1, are effectors of Ras and mediate the activation of Ral in response to the activation of Ras. Here we show that the full-length RalGEFs interact with the GTP-bound form of Rap2 in the two-hybrid system as well as in vitro. When co-transfected in HeLa cells, an activated Rap2 mutant (Rap2Val-12) but not an inactive protein (Rap2Ala-35) co-immunoprecipitates with RalGDS and Rlf; moreover, Rap2-RalGEF complexes can be isolated from the particulate fraction of transfected cells and were localized by confocal microscopy to the resident compartment of Rap2, i.e. the endoplasmic reticulum. However, the overexpression of activated Rap2 neither leads to the activation of the Ral GTPase via RalGEFs nor inhibits Ras-dependent Ral activation in vivo. Several hypotheses that could explain these results, including compartmentalization of proteins involved in signal transduction, are discussed. Our results suggest that in cells, the interaction of Rap2 with RalGEFs might trigger other cellular responses than activation of the Ral GTPase.

Expression of Ras-GRF in the SK-N-BE neuroblastoma accelerates retinoic-acid-induced neuronal differentiation and increases the functional expression of the IRK1 potassium channel

Ras-GRF, a neuron-specific Ras exchange factor of the central nervous system, was transfected in the SK-N-BE neuroblastoma cell line and stable clones were obtained. When exposed to retinoic acid, these clones showed a remarkable enhancement of Ras-GRF expression with a concomitant high increase in the level of active (GTP-bound) Ras already after 24 h of treatment. In the presence of retinoic acid, the transfected cells stopped growing and acquired a differentiated neuronal-like phenotype more rapidly than the parental ones. Cells expressing Ras-GRF also exhibited a more hyperpolarized membrane potential. Moreover, treatment with retinoic acid led to the appearance of an inward rectifying potassium channel with electrophysiological properties similar to IRK1. This current was present in a large number of cells expressing Ras-GRF, while only a small percentage of parental cells exhibited this current. However, Northern analysis with a murine cDNA probe indicated that IRK1 mRNA was induced by retinoic acid at a similar level in both kinds of cells. Brief treatment with a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway reduced the number of transfected cells showing IRK1 activity. These findings suggest that activation of the Ras pathway accelerates neuronal differentiation of this cell line. In addition, our results suggest that Ras-GRF and/or Ras-pathway may have a modulatory effect on IRK1 channel activity.

A novel PDZ domain containing guanine nucleotide exchange factor links heterotrimeric G proteins to Rho

Small GTP-binding proteins of the Rho family play a critical role in signal transduction. However, there is still very limited information on how they are activated by cell surface receptors. Here, we used a consensus sequence for Dbl domains of Rho guanine nucleotide exchange factors (GEFs) to search DNA data bases, and identified a novel human GEF for Rho-related GTPases harboring structural features indicative of its possible regulatory mechanism(s). This protein contained a tandem DH/PH domain closely related to those of Rho-specific GEFs, a PDZ domain, a proline-rich domain, and an area of homology to Lsc, p115-RhoGEF, and a Drosophila RhoGEF that was termed Lsc-homology (LH) domain. This novel molecule, designated PDZ-RhoGEF, activated biological and biochemical pathways specific for Rho, and activation of these pathways required an intact DH and PH domain. However, the PDZ domain was dispensable for these functions, and mutants lacking the LH domain were more active, suggesting a negative regulatory role for the LH domain. A search for additional molecules exhibiting an LH domain revealed a limited homology with the catalytic region of a newly identified GTPase-activating protein for heterotrimeric G proteins, RGS14. This prompted us to investigate whether PDZ-RhoGEF could interact with representative members of each G protein family. We found that PDZ-RhoGEF was able to form, in vivo, stable complexes with two members of the Galpha12 family, Galpha12 and Galpha13, and that this interaction was mediated by the LH domain. Furthermore, we obtained evidence to suggest that PDZ-RhoGEF mediates the activation of Rho by Galpha12 and Galpha13. Together, these findings suggest the existence of a novel mechanism whereby the large family of cell surface receptors that transmit signals through heterotrimeric G proteins activate Rho-dependent pathways: by stimulating the activity of members of the Galpha12 family which, in turn, activate an exchange factor acting on Rho.

Points of convergence between Ca2+ and Ras signalling pathways

p21 Ras proteins play a critical role in the regulation of cellular growth and differentiation. In addition, Ras and proteins which regulate Ras activity have been implicated in long-term memory consolidation and long-term potentiation processes. Over the last few years, much evidence has emerged which indicates that changes in cytoplasmic Ca2+ levels can regulate Ras protein activity and subsequent biological function. Also, Ras proteins themselves can modulate intracellular Ca2+ levels by regulating both Ca2+ release and Ca2+ influx processes. Here we examine the signalling components which regulate Ras activity and, in particular, consider points of convergence between intracellular Ca2+ and p21 Ras signalling processes. In addition, we consider the possible biological consequences resulting from the integration of these signalling pathways and highlight the importance of our understanding protein protein interactions. Finally, we discuss the possibility of protein-protein interactions mediated via Ca2+-responsive structural domains, such as the C2 and IQ domains, playing important roles in Ca2+-dependent Ras functions yet to be established.

Mutations at position 1122 in the catalytic domain of the mouse ras-specific guanine nucleotide exchange factor CDC25Mm originate both loss-of-function and gain-of-function proteins

The role of two residues within the catalytic domain of CDC25Mm, a mouse ras-specific guanine nucleotide exchange factor (GEF), was investigated by site-directed mutagenesis. The function of the mutant proteins was tested in vivo in both a Saccharomyces cerevisiae cdc25 complementation assay and in a mammalian fos-luciferase assay, and in in vitro assays on human and yeast Ras proteins. Mutants CDC25Mm(E1048K) and CDC25Mm(S1122V) were shown to be (partly) inactive proteins, similar to their yeast homologs. Mutant CDC25Mm(S1122A) showed higher nucleotide exchange activity than the wild type protein on the basis of both in vitro and in vivo assays. Thus, alanine and valine substitutions at position 1122 within the GEF catalytic domain originate mutations with opposite biological properties, indicating an important role for position 1122 in GEF function.

Regulation of RasGRP via a phorbol ester-responsive C1 domain

As part of a cDNA library screen for clones that induce transformation of NIH 3T3 fibroblasts, we have isolated a cDNA encoding the murine homolog of the guanine nucleotide exchange factor RasGRP. A point mutation predicted to prevent interaction with Ras abolished the ability of murine RasGRP (mRasGRP) to transform fibroblasts and to activate mitogen-activated protein kinases (MAP kinases). MAP kinase activation via mRasGRP was enhanced by coexpression of H-, K-, and N-Ras and was partially suppressed by coexpression of dominant negative forms of H- and K-Ras. The C terminus of mRasGRP contains a pair of EF hands and a C1 domain which is very similar to the phorbol ester- and diacylglycerol-binding C1 domains of protein kinase Cs. The EF hands could be deleted without affecting the ability of mRasGRP to transform NIH 3T3 cells. In contrast, deletion of the C1 domain or an adjacent cluster of basic amino acids eliminated the transforming activity of mRasGRP. Transformation and MAP kinase activation via mRasGRP were restored if the deleted C1 domain was replaced either by a membrane-localizing prenylation signal or by a diacylglycerol- and phorbol ester-binding C1 domain of protein kinase C. The transforming activity of mRasGRP could be regulated by phorbol ester when serum concentrations were low, and this effect of phorbol ester was dependent on the C1 domain of mRasGRP. The C1 domain could also confer phorbol myristate acetate-regulated transforming activity on a prenylation-defective mutant of K-Ras. The C1 domain mediated the translocation of mRasGRP to cell membranes in response to either phorbol ester or serum stimulation. These results suggest that the primary mechanism of activation of mRasGRP in fibroblasts is through its recruitment to diacylglycerol-enriched membranes. mRasGRP is expressed in lymphoid tissues and the brain, as well as in some lymphoid cell lines. In these cells, RasGRP has the potential to serve as a direct link between receptors which stimulate diacylglycerol-generating phospholipase Cs and the activation of Ras.

Ras- and Raf-induced down-modulation of non-muscle tropomyosin are MEK-independent

Transformation is accompanied by the down-regulation of the high molecular weight isoforms of non-muscle tropomyosin. Several lines of evidence suggest that tropomyosin down-regulation may be essential for ras-induced tumorigenicity. It is unclear which of the many signaling pathways downstream of Ras are involved in tropomyosin down-regulation. Here we demonstrate that Raf activation induces tropomyosin down-regulation comparable to that induced by Ras. Expression of the effector-domain mutant Ras-G12V,Y40C, which is unable to bind Raf, induced only modest down-modulation of tropomyosin. Treatment with the MEK-specific inhibitor PD98059 had little effect on tropomyosin levels in ras- or raf-transformed cells. In contrast, a mutant form of MEK-1, MEK-1-S218A,S222A, restored tropomyosin levels in ras-transformed NIH3T3 cells almost to the levels observed in non-transformed cells. MEK-1-S218A,S222A does not inhibit MEK phosphorylation and is a poor inhibitor of ERK phosphorylation. These data suggest that this mutant form of MEK-1 interferes with a yet uncharacterized pathway controlled by Raf. We conclude that the ras-induced down-modulation of tropomyosin is predominantly Raf-mediated, but MEK-independent, and that a novel pathway exists downstream of Raf which may play an important role in regulation of the cytoskeleton.

Ca2+ dependent purinergic regulation of p42 and p44 MAP kinases in astroglial cultured cells

Adenosine triphosphate (ATP) is a signaling molecule for brain cells including astrocytes. In these cells, it has been shown that ATP stimulates myelin basic protein (MBP) kinase activity which is believed to represent the Erk family of MAP kinases. Indeed, we show that ATP activates simultaneously MBP kinase activity and phosphotyrosine incorporation in p42 Erk2 and p44 Erk1. Maximal effect of ATP is obtained at 50 microM after 5 min and disappears after 60 min. Effect of ATP is mimicked by 2-methylthio-ATP whereas alpha beta-methyleneadenosine 5' triphosphate (AMP-CPP) and adenosine do not promote any effect. Uridine triphosphate (UTP) activates also p42 and p44 MAP kinases. These observations indicate that p42-p44 MAP kinases activation can be obtained through P2v and P2u receptors. Purinergic stimulation of Erk is insensitive to pertussis toxin which inactivates heterotrimeric Gi protein. It is not inhibited by a PLA2 inhibitor (4 bromophenacyl bromide [B phi B]) and the PI3 kinase inhibitor, wortmannin. In contrast, purinergic stimulation of Erk is partially inhibited by the PKC inhibitor. GF109203X, at 5 microM and suppressed when extracellular calcium is complexed by ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA).

Calmodulin inhibitor W13 induces sustained activation of ERK2 and expression of p21(cip1)

One of the major signaling pathways by which extracellular signals induce cell proliferation and differentiation involves the activation of extracellular signal-regulated kinases (ERKs). Because calmodulin is essential for quiescent cells to enter cell cycle, the role of calmodulin on ERK2 activation was studied in cultured fibroblasts. Serum, phorbol esters, or active Ras induced ERK2 activation in NIH 3T3 fibroblasts. This activation was not inhibited by calmodulin blockade. Surprisingly, inhibition of calmodulin prior to fetal bovine serum addition prolonged activation of ERK2. Furthermore, inactivation of calmodulin in serum-starved cells induced ERK2 phosphorylation that was dependent on MAP kinase kinase (MEK). Inactivation of calmodulin in serum-starved cells also induced activation of Ras, Raf, and MEK. On the contrary, tyrosine phosphorylation of tyrosine kinase receptors was not observed. These results indicate that calmodulin inhibits ERK2 activation pathway at the level of Ras. Calmodulin inhibition induced overexpression of p21(cip1) which was dependent on MEK activity. We propose that inhibition of Ras by calmodulin prevents the activation of ERK2 at low serum concentration. Thus, entering into the cell cycle after serum addition would imply the overcoming of the inhibitory effect of calmodulin and consequently ERK2 activation. Furthermore, down-regulation of Ras by calmodulin may be also important to determine the duration of ERK2 activation and to prevent a high p21(cip1) expression that would lead to an inhibition of cell proliferation.

Involvement of Galphai2 in the maintenance and biogenesis of epithelial cell tight junctions

Polarized epithelial cells have highly developed tight junctions (TJ) to maintain an impermeant barrier and segregate plasma membrane functions, but the mechanisms that promote TJ formation and maintain its integrity are only partially defined. Treatment of confluent monolayers of Madin-Darby canine kidney (MDCK) cells with AlF4- (activator of heterotrimeric G protein alpha subunits) results in a 3-4-fold increase in transepithelial resistances (TER), a reliable indicator of TJ integrity. MOCK cells transfected with activated Galpha0 (Q205L) have acclerated TJ formation (Denker, B. M., Saha, C. , Khawaja, S., and Nigam, S. J. (1996) J. Biol. Chem. 271, 25750-25753). Galphai2 has been localized within the tight junction, and a role for Galphai2 in the formation and/or maintenance of the tight junction was studied by transfection of MDCK cells with vector without insert (PC), wild type Galphai2, or a GTPase-deficient mutant (constitutively activated), Q205Lalphai2. Tryptic conformational analysis confirmed expression of a constitutively active Galphai2 in Q205Lalphai2-MDCK cells, and confocal microscopy showed a similar pattern of Galphai2 localization in the three cell lines. Q205Lalphai2-MDCK cells had significantly higher base-line TER values than wild type Galphai2- or PC-MDCK cells (1187 +/- 150 versus 576 +/- 89 (Galphai2); 377 +/- 52 Omega.cm2 (PC)), and both Galphai2- and Q205Lalphai2-transfected cell lines more rapidly develop TER in the Ca2+ switch, a model widely used to study the mechanisms of junctional assembly. Treatment of cells with AlF4- during the Ca2+ switch had little effect on the kinetics of TER development in Galphai2- or Q205Lalphai2-MDCK cells, but PC cells reached half-maximal TER significantly sooner in the presence of AlF4- (similar times to Galphai2-transfected cells). Base-line TER values obtained after the switch were significantly higher for all three cell lines in the presence of AlF4-. These findings indicate that Galphai2 is important for both the maintenance and development of the TJ, although additional Galpha subunits are likely to play a role.

Localization of GTPase-activating protein-(GAP) like immunoreactivity in mouse cerebral regions

GTPase-activating protein is known to regulate the conversion between ras-GTP and ras-GDP. We studied the basal expression of GTPase-activating protein-like immunoreactivity in mouse cerebral regions using a polyclonal anti-GTPase-activating protein antibody. Cells with GTPase-activating protein-like immunoreactivity were distributed in frontal cortical layers IV and V, and in the parietal cortex, piriform cortex, amygdaloid area, septum, lateral thalamus, and hypothalamus. The GTPase-activating protein-like immunoreactivity was also observed in fiber-like structures in the caudate putamen, stria terminalis, internal capsule, and medial forebrain bundle, and around CA2 pyramidal cells in Ammon's horn. These results imply that GTPase-activating protein is constitutively expressed in mouse brain regions and may have physiological functions in specific neuronal pathways in the brain.

Transforming potential of Dbl family proteins correlates with transcription from the cyclin D1 promoter but not with activation of Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun

The dbl family of oncogenes encodes a large, structurally related, family of growth-regulatory molecules that possess guanine nucleotide exchange factor activity for specific members of the Rho family of Ras-related GTPases. We have evaluated matched sets of weakly and strongly transforming versions of five Dbl family proteins (Lfc, Lsc, Ect2, Dbl, and Dbs) to determine their ability to stimulate signaling pathways that are activated by Rho family proteins. We found that the transforming potential of this panel did not correlate directly with their ability to activate Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun. In contrast, transient stimulation of transcription from the cyclin D1 promoter provided a strong correlation with transforming potential, and we found constitutive up-regulation of cyclin D1 protein in Dbl family protein-transformed cells. In addition, we observed that at least two Dbl family members (Lfc and Ect2) induced changes in the actin cytoskeleton and exhibited nuclear signaling profiles that are consistent with a broader range of in vivo substrate utilization than is predicted from their in vitro exchange specificities. In summary, although Dbl family proteins exhibit signaling profiles that are consistent with their in vivo activation of Rho proteins, stimulation of cyclin D1 transcription is the only activity that correlates with transforming potential, thus suggesting that deregulated cell cycle progression may be important for Dbl family protein transformation.

RasGRP, a Ras guanyl nucleotide- releasing protein with calcium- and diacylglycerol-binding motifs

RasGRP, a guanyl nucleotide-releasing protein for the small guanosine triphosphatase Ras, was characterized. Besides the catalytic domain, RasGRP has an atypical pair of "EF hands" that bind calcium and a diacylglycerol (DAG)-binding domain. RasGRP activated Ras and caused transformation in fibroblasts. A DAG analog caused sustained activation of Ras-Erk signaling and changes in cell morphology. Signaling was associated with partitioning of RasGRP protein into the membrane fraction. Sustained ligand-induced signaling and membrane partitioning were absent when the DAG-binding domain was deleted. RasGRP is expressed in the nervous system, where it may couple changes in DAG and possibly calcium concentrations to Ras activation.

Analysis of the secondary structure of the catalytic domain of mouse Ras exchange factor CDC25Mm

The minimal active domain (GEF domain) of the mouse Ras exchange factor CDC25Mm was purified to homogeneity from recombinant Escherichia coli culture. The 256 amino acids polypeptide shows high activity in vitro and forms a stable complex with H-ras p21 in absence of guanine nucleotides. Circular dichroism (CD) spectra in the far UV region indicate that this domain is highly structured with a high content of alpha-helix (42%). Near UV CD spectra evidenced good signal due to phenylalanine and tyrosine while a poor contribution was elicited by the three tryptophan residues contained in this domain. The tryptophan fluorescence signal was scarcely affected by denaturation of the protein or by formation of the binary complex with H-ras p21, suggesting that the Trp residues, which are well conserved in the GEF domain of several Ras-exchange factors, were exposed to the surface of the protein and they are not most probably directly involved in the interaction with Ras proteins.

Ras-GRF activates Ha-Ras, but not N-Ras or K-Ras 4B, protein in vivo

Human cells contain four homologous Ras proteins, but it is unknown whether these homologues have different biological functions. As a first step in determining if Ras homologues might participate in distinct signaling cascades, we assessed whether a given Ras guanine nucleotide exchange factor could selectively activate a single Ras homologue in vivo. We found that Ras-GRF/Cdc25Mm activates Ha-Ras, but does not activate N-Ras or K-Ras 4B, protein in vivo. Moreover, our results suggested that residues within the C-terminal hypervariable domains of Ras proteins may dictate, at least in part, the specificity of Ras-GRF/CDC25Mm for Ha-Ras protein. Our studies represent the first biochemical evidence that a Ras GEF can selectively activate a single Ras homologue in vivo. Selective activation of a single Ras homologue by Ras-GRF/Cdc25Mm or other Ras guanine nucleotide exchange factors could potentially enable each of the Ras homologues to participate in different signal transduction pathways.

The Ras-specific exchange factors mouse Sos1 (mSos1) and mSos2 are regulated differently: mSos2 contains ubiquitination signals absent in mSos1

We have compared aspects of the mouse sos1 (msos1) and msos2 genes, which encode widely expressed, closely related Ras-specific exchange factors. Although an msos1 plasmid did not induce phenotypic changes in NIH 3T3 cells, addition of a 15-codon myristoylation signal to its 5' end enabled the resulting plasmid, myr-sos1, to induce approximately one-half as many foci of transformed cells as a v-H-ras control. By contrast, an isogenic myr-sos2 plasmid, which was made by fusing the first 102 codons from myr-sos1 at homologous sequences to an intact msos2 cDNA, did not induce focal transformation directly, although it could form foci in cooperation with c-H-ras. Pulse-chase experiments indicated that the half-life of Sos1 in NIH 3T3 cells was greater than 18 h, while that of Sos2 was less than 3 h. While in vitro-translated Sos1 was stable in a rabbit reticulocyte lysate, Sos2 was degraded in the lysate, as were each of two reciprocal chimeric Sos1-Sos2 proteins, albeit at a slower rate. In the lysate, Sos2 and the two chimeric proteins could be stabilized by ATPgammaS. Unlike Sos1, Sos2 was specifically immunoprecipitated by antiubiquitin antibodies. In a myristoylated version, the chimeric gene encoding Sos2 at its C terminus made a stable protein in NIH 3T3 cells and induced focal transformation almost as efficiently as myr-msos1, while the myristoylated protein encoded by the other chimera was unstable and defective in the transformation assay. We conclude that mSos2 is much less stable than mSos1 and is degraded by a ubiquitin-dependent process. A second mSos2 degradation signal, mapped to the C terminus in the reticulocyte lysate, does not seem to function under the growth conditions of the NIH 3T3 cells.

A role for the Ras signalling pathway in synaptic transmission and long-term memory

Members of the Ras subfamily of small guanine-nucleotide-binding proteins are essential for controlling normal and malignant cell proliferation as well as cell differentiation. The neuronal-specific guanine-nucleotide-exchange factor, Ras-GRF/CDC25Mm, induces Ras signalling in response to Ca2+ influx and activation of G-protein-coupled receptors in vitro, suggesting that it plays a role in neurotransmission and plasticity in vivo. Here we report that mice lacking Ras-GRF are impaired in the process of memory consolidation, as revealed by emotional conditioning tasks that require the function of the amygdala; learning and short-term memory are intact. Electrophysiological measurements in the basolateral amygdala reveal that long-term plasticity is abnormal in mutant mice. In contrast, Ras-GRF mutants do not reveal major deficits in spatial learning tasks such as the Morris water maze, a test that requires hippocampal function. Consistent with apparently normal hippocampal functions, Ras-GRF mutants show normal NMDA (N-methyl-D-aspartate) receptor-dependent long-term potentiation in this structure. These results implicate Ras-GRF signalling via the Ras/MAP kinase pathway in synaptic events leading to formation of long-term memories.

The significance of Ras guanine nucleotide exchange factor, son of sevenless protein, in renal cell carcinoma cell lines

PURPOSE

The aim of the present study is to clarify the significance of the Ras guanine-nucleotide exchange reaction in the proliferation of human renal cell carcinoma cell lines.

MATERIALS AND METHODS

We examined the expression of human son of sevenless-1 (hSos-1) protein and the epidermal growth factor (EGF) receptor in human renal cell carcinoma cell lines by Western blot analysis. Additionally, a dominant negative H-ras mutant, N116Y, which is known to inhibit the Ras guanine-nucleotide exchange reaction, was transfected into these cell lines by lipofection.

RESULTS

Human renal cell carcinoma cell lines expressed much higher amounts of the EGF receptor and hSos-1 protein than normal kidney tissue. Moreover, the N116Y ras mutant could strongly suppress cellular proliferation in these cell lines.

CONCLUSIONS

Augmentation of the Ras guanine-nucleotide exchange reaction might be essential to the proliferation of human renal cell carcinoma cells.

The Ras Guanine nucleotide Exchange Factor CDC25Mm is present at the synaptic junction

CDC25Mm, a mouse Ras-Guanine nucleotide Exchange Factor, is specifically expressed as a product of 140 kDa (p140) in the postnatal and adult brain. Immunohistochemical analysis indicates that it is present throughout the brain particularly concentrated in discrete punctate structures. Subcellular fractionation of the mouse brain shows that p140 is present in synaptosomes but not in highly purified synaptic vesicles. Moreover, isolated postsynaptic densities (PSDs) are largely enriched in CDC25Mm. This protein can be phosphorylated by calcium/calmodulin kinase II, the most abundant protein in PSDs. Altogether these results suggest that CDC25Mm is present at synaptic junctions and that it may be involved in synaptic signal transduction leading to Ras activation.

T cell receptor-induced phosphorylation of Sos requires activity of CD45, Lck, and protein kinase C, but not ERK

Stimulation of the T cell antigen receptor (TCR) activates signaling pathways involving protein kinases, phospholipase Cgamma1, and Ras. How these second messengers interact to initiate distal activation events is an area of intense scrutiny. In this report, we confirm that TCR ligation results in phosphorylation of Sos, a guanine nucleotide exchange factor for Ras. This requires expression of both the CD45 tyrosine phosphatase and the Lck protein tyrosine kinase and depends upon signaling via protein kinase C. In contrast to previous studies examining requirements for Sos phosphorylation following insulin and epidermal growth factor receptor engagement, we show that TCR-induced phosphorylation of Sos does not require activation of the mitogen-activated protein kinase/extracellular-signal regulated kinase (MEK/ERK) pathway. However, the basal phosphorylation of Sos in T cells is affected by either MEK or MEK-dependent kinases. Although Sos phosphorylation results in its dissociation from Grb2 following insulin stimulation in Chinese hamster ovary cells, TCR engagement on the Jurkat T cell line fails to elicit a similar effect. These data demonstrate that the kinases responsible for Sos phosphorylation differ following ligation of various cell surface receptors and that the consequences of Sos phosphorylation relies, at least in part, on sites of its phosphorylation.

Ras-GRF, the activator of Ras, is expressed preferentially in mature neurons of the central nervous system

In rodents, the Ras-specific guanine-nucleotide exchange factor (Ras-GRF) is expressed in different areas of the brain and, at a reduced level, also in the spinal cord. No expression of the 140 kDa Ras-GRF was detected in dorsal root ganglia and all other tissues tested. Analysis of primary cultures derived from brain reveals that this exchange factor is only present in neurons of the central nervous system. In primary hippocampal cultures, the expression of Ras-GRF increases in parallel with the onset of a neuronal network and in the whole brain it increases sharply after birth.

Activation of R-Ras by Ras-guanine nucleotide-releasing factor

Ras-GRF/CDC25(Mm), mSos, and C3G have been identified as guanine nucleotide-releasing factors for Ras family proteins. We investigated in this study the guanine nucleotide-releasing activities of Ras-GRF, mSos, and C3G toward R-Ras, which shows high sequence similarity to Ras. Ras-GRF markedly stimulated the dissociation of GDP from R-Ras, and C3G also promoted the release of R-Ras-bound GDP. Under the same conditions, mSos little affected the reaction. When Ras-GRF and R-Ras were coexpressed in COS7 cells, the remarkable accumulation of the active GTP-bound form of R-Ras was observed. C3G also increased active R-Ras in COS7 cells, while mSos did not give any effect. These results indicated that Ras-GRF and C3G could activate R-Ras. Furthermore, the activation of R-Ras by Ras-GRF was enhanced when cells were treated with ionomycin, which is known to increase the intracellular calcium concentration. The examination of tissue distribution of R-Ras, Ras-GRF, and mSos by the reverse transcription-polymerase chain reaction revealed that Ras-GRF was expressed only in brain and testis, whereas R-Ras, C3G, and mSos were expressed rather ubiquitously. These findings raise the possibility that R-Ras is activated by Ras-GRF in brain and testis, and by C3G in other tissues, respectively.

Activation of the exchange factor Ras-GRF by calcium requires an intact Dbl homology domain

Ras-GRF is a guanine nucleotide exchange factor that activates Ras proteins. Its activity on Ras in cells is enhanced upon calcium influx. Activation follows calcium-induced binding of calmodulin to an IQ motif near the N-terminus of Ras-GRF. Ras-GRF also contains a Dbl homology (DH) domain C-terminal to the IQ motif. In many proteins, DH domains act as exchange factors for Rho-GTPase family members. However, we failed to detect exchange activity of this domain on well characterized Rho family members. Instead, we found that mutations analogous to those that block exchange activity of Dbl prevented Ras-GRF activation by calcium/ calmodulin in vivo. All DH domains are followed immediately by a pleckstrin homology (PH) domain. We found that a mutation at a conserved site within the PH domain following the DH domain also prevented Ras-GRF activation by calcium in vivo. These results suggest that in addition to playing a role as activators of Rho proteins, DH domains can also contribute to the coupling of cellular signals to Ras activation.

The N-terminal moiety of CDC25(Mm), a GDP/GTP exchange factor of Ras proteins, controls the activity of the catalytic domain. Modulation by calmodulin and calpain

This work describes the in vitro properties of full-length CDC25(Mm) (1262 amino acid residues), a GDP/GTP exchange factor (GEF) of H-ras p21. CDC25(Mm), isolated as a recombinant protein in Escherichia coli and purified by various chromatographic methods, could stimulate the H-ras p21.GDP dissociation rate; however, its specific activity was 25 times lower than that of the isolated catalytic domain comprising the last C-terminal 285 residues (C-CDC25(Mm285)) and 5 times lower than the activity of the C-terminal half-molecule (631 residues). This reveals a negative regulation of the catalytic domain by other domains of the molecule. Accordingly, the GEF activity of CDC25(Mm) was increased severalfold by the Ca2+-dependent protease calpain that cleaves around a PEST-like region (residues 798-853), producing C-terminal fragments of 43-56 kDa. In agreement with the presence of an IQ motif on CDC25(Mm) (residues 202-229), calmodulin interacted functionally with the exchange factor. Depending on the calmodulin concentration an inhibition up to 50% of the CDC25(Mm)-induced nucleotide exchange activity on H-ras p21 was observed, an effect requiring Ca2+ ions. Calmodulin also inhibited C-CDC25(Mm285) but with a approximately 100 times higher IC50 than in the case of CDC25(Mm) ( approximately 10 microM versus 0.1 microM, respectively). Together, these results emphasize the role of the other domains of CDC25(Mm) in controlling the activity of the catalytic domain and support the involvement of calmodulin and calpain in the in vivo regulation of the CDC25(Mm) activity.

Cloning and characterization of Ras-GRF2, a novel guanine nucleotide exchange factor for Ras

Conversion of Ras proteins into an activated GTP-bound state able to bind effector proteins is catalyzed by specific guanine nucleotide exchange factors in response to a large number of extracellular stimuli. Here we report the isolation of mouse cDNAs encoding Ras-GRF2, a multidomain 135-kDa protein containing a COOH-terminal Cdc25-related domain that stimulates release of GDP from Ras but not other GTPases in vitro. Ras-GRF2 bound specifically to immobilized Ras lacking bound nucleotides, suggesting stabilization of the nucleotide-free form of Ras as a mechanism of catalyzing nucleotide exchange. The NH2-terminal region of Ras-GRF2 is predicted to contain features common to various signaling proteins including two pleckstrin homology domains and a Dbl homology region. Ras-GRF2 also contains an IQ motif which was required for its apparent constitutive association with calmodulin in epithelial cells ectopically expressing Ras-GRF2. Transient expression of Ras-GRF2 in kidney epithelial cells stimulated GTP binding by Ras and potentiated calcium ionophore-induced activation of mitogen-activated protein kinase (ERK1) dependent upon the IQ motif. Calcium influx caused Ras-GRF2 subcellular localization to change from cytosolic to peripheral, suggesting a possible mechanism for controlling Ras-GRF2 interactions with Ras at the plasma membrane. Epithelial cells overexpressing Ras-GRF2 are morphologically transformed and grow in a disorganized manner with minimal intercellular contacts. Northern analysis indicated a 9-kb GRF2 transcript in brain and lung, where p135 Ras-GRF2 is known to be expressed, and RNAs of 12 kb and 2.2 kb were detected in several tissues. Thus, Ras-GRF2 proteins with different domain structures may be widely expressed and couple diverse extracellular signals to Ras activation.

Phosphotyrosine-dependent activation of Rac-1 GDP/GTP exchange by the vav proto-oncogene product

The oncogenic protein Vav harbours a complex array of structural motifs, including leucine-rich, Dbl-homology, pleckstrin-homology, zinc-finger, SH2 and SH3 domains. Upon stimulation by antigens or mitogens, Vav becomes phosphorylated on key tyrosine residues and associates with other signalling proteins, including the mitogen receptors Zap-70 (ref. 6), Vap-1 (ref. 5) and Slp-76 (ref. 7). Disruption of the vav locus by homologous recombination causes severe defects in signalling by primary antigen receptors, leading to abnormal lymphocyte proliferation and lymphopenia. Despite the importance of Vav cell signalling, the function of this protein remains unknown. Here we show that tyrosine-phosphorylated Vav, but not the non-phosphorylated protein, catalyses GDP/GTP exchange on Rac-1, a protein implicated in cell proliferation and cytoskeletal organization, causing this GTPase to switch from its inactive to its active state. Transfection experiments also show that phosphorylation of Vav on tyrosine residues leads to nucleotide exchange on Rac-1 in vivo and stimulates c-Jun kinase, a downstream element in the signalling pathway involving this GTPase. Our results have identified a function for Vav and define a mechanism in which engaged membrane receptors activate its signalling pathway.

Signaling mechanisms in growth factor-stimulated cell motility

Most mammalian cells have the capacity to migrate. When placed into culture, cells will generally display a set rate of basal, unstimulated locomotion. The cells will begin to move in one direction and, after some time, change directions resulting in a random walk. External stimuli can influence cell motility in several ways to either enhance or retard the rate of migration (chemokinesis), to change the average amount of cell migration observed before the cell turns (persistence), or to increase the directionality of movement by limiting the number of turns made by the cells. Several factors have been identified that stimulate cell movement, but the signaling mechanisms that mediate this induced cell movement have only recently begun to be studied. In this review, we discuss the signals that support the directional movement of fibroblasts and epithelial cells in response to chemoattractant gradients. The work will emphasize studies carried out by our laboratory and others on the stimulation of cell motility by the PDGF. These results indicate that at least two sets of signaling molecules cooperate to regulate cell motility in vivo. These include phospholipase C-gamma, phosphoinositide-3' kinase and the Ras-GTPase activating protein Ras-GAP. The first set are those which bind to the intracellular domain of the receptor tyrosine kinase and bring about the phosphorylation and/or activation of intracellular effectors proximal to the receptor. The second is a set of down-stream effectors that regulate either the rate of cell movement or the directionality of that movement depending on the cell type. These include Ras and the Ras-related GTPase Rac along with free phosphoinositides and calcium ions that regulate the actin polymerization machinery. Signals that mediate nuclear changes leading to cell proliferation, such as elements of the MAP kinase pathway, do not appear to play a role in PDGF-stimulated cell migration. Current work thus suggests that a coordinated spatial regulation of signaling elements that interact with the cell membrane and cytoskeleton but not necessarily with nuclear elements is the controlling mediator of directional cell motility.

Identification of a novel RalGDS-related protein as a candidate effector for Ras and Rap1

Although Ras and Rap1 share interaction with common candidate effector proteins, Rap1 lacks the transforming activity exhibited by Ras proteins. It has been speculated that Rap antagonizes Ras transformation through the formation of nonproductive complexes with critical Ras effector targets. To understand further the distinct biological functions of these two closely related proteins, we searched for Rap1b-binding proteins by yeast two-hybrid screening. We identified multiple clones that encode the COOH-terminal sequences of a protein that shares sequence identity with RalGDS and RGL, which we have designated RGL2. A 158-amino acid COOH-terminal fragment of RGL2 (RGL2 C-158) bound to Ras superfamily proteins which shared identical effector domain sequences with Rap1 (Ha-Ras, R-Ras, and TC21). RGL2 C-158 binding was impaired by effector domain mutations in Rap1b and Ha-Ras. Furthermore, RGL2 C-158 bound exclusively to the GTP-, but not the GDP-bound form of Ha-Ras. Finally, coexpression of RGL2 C-158 impaired oncogenic Ras activation of transcription from a Ras-responsive promoter element and focus-forming activity in NIH 3T3 cells. We conclude that RGL2 may be an effector for Ras and/or Rap proteins.

Lfc and Lsc oncoproteins represent two new guanine nucleotide exchange factors for the Rho GTP-binding protein

Lfc and Lsc are two recently identified oncoproteins that contain a Dbl homology domain in tandem with a pleckstrin homology domain and thus share sequence similarity with a number of other growth regulatory proteins including Dbl, Tiam-1, and Lbc. We show here that Lfc and Lsc, like their closest relative Lbc, are highly specific guanine nucleotide exchange factors (GEFs) for Rho, causing a >10-fold stimulation of [3H]GDP dissociation from Rho and a marked stimulation of GDP-[35S]GTPgammas (guanosine 5'-O-(3-thiotriphosphate) exchange. All three proteins (Lbc, Lfc, and Lsc) are able to act catalytically in stimulating the guanine nucleotide exchange activity, such that a single molecule of each of these oncoproteins can activate a number of molecules of Rho. Neither Lfc nor Lsc shows any ability to stimulate GDP dissociation from other related GTP-binding proteins such as Rac, Cdc42, or Ras. Thus Lbc, Lfc, and Lsc appear to represent a subgroup of Dbl-related proteins that function as highly specific GEFs toward Rho and can be distinguished from Dbl, Ost, and Dbs which are less specific and show GEF activity toward both Rho and Cdc42. Consistent with these results, Lbc, Lfc, and Lsc each form tight complexes with the guanine nucleotide-depleted form of Rho and bind weakly to the GDP- and GTPgammaS-bound states. None of these oncoproteins are able to form complexes with Cdc42 or Ras. However, Lfc (but not Lbc nor Lsc) can bind to Rac, and this binding occurs equally well when Rac is nucleotide-depleted or is in the GDP- or GTPgammaS-bound state. These findings raise the possibility that in addition to acting directly as a GEF for Rho, Lfc may play other roles that influence the signaling activities of Rac and/or coordinate the activities of the Rac and Rho proteins.

The Ca2+-dependent lipid binding domain of P120GAP mediates protein-protein interactions with Ca2+-dependent membrane-binding proteins. Evidence for a direct interaction between annexin VI and P120GAP

The CaLB domain is a 43-amino acid sequence motif found in a number of functionally diverse signaling proteins including three Ras-specific GTPase activating proteins (GAPs). In the Ras GTPase activating protein, P120(GAP), this domain has the ability to confer membrane association in response to intracellular Ca2+ elevation. Here we have isolated three proteins, p55, p70, and p120, which interact with the P120(GAP) CaLB domain in vitro. We identify p70 as the Ca2+-dependent phospholipid-binding protein annexin VI. Using co-immunoprecipitation studies, we have shown that the interaction between P120(GAP) and annexin VI is also detectable in rat fibroblasts, suggesting that this interaction may have a physiological role in vivo. Thus, the CaLB domain in P120(GAP) appears to have the ability to direct specific protein-protein interactions with Ca2+-dependent membrane-associated proteins. In addition, annexin VI is known to have tumor suppressor activity. Therefore, it is possible that the interaction of annexin VI with P120(GAP) may be important in the subsequent modulation of p21(ras) activity.

Phosphatidylinositol 4,5-bisphosphate provides an alternative to guanine nucleotide exchange factors by stimulating the dissociation of GDP from Cdc42Hs

Members of the Rho subfamily of Ras-related GTP-binding proteins play important roles in the organization of the actin cytoskeleton and in the regulation of cell growth. We have shown previously that the dbl oncogene product, which represents a prototype for a family of growth regulatory proteins, activates Rho subfamily GTP-binding proteins by catalyzing the dissociation of GDP from their nucleotide binding site. In the present study, we demonstrate that the acidic phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), provides an alternative mechanism for the activation of Cdc42Hs. Among a variety of lipids tested, only PIP2 was able to stimulate GDP release from Cdc42Hs in a dose-dependent manner, with a half-maximum effect at approximately 50 microM. Unlike the Dbl oncoprotein, which requires the presence of (free) guanine nucleotide in the medium to replace the GDP bound to Cdc42Hs, PIP2 stimulates GDP release from Cdc42Hs in the absence of free guanine nucleotide. PIP2, when incorporated into phosphatidylcholine carrier vesicles, binds tightly to the guanine nucleotide-depleted form of Cdc42Hs and weakly to the GDP-bound form of the GTP-binding protein but does not bind to GTP-bound Cdc42Hs, similar to what was observed for the Dbl oncoprotein. However, mutational analysis of Cdc42Hs indicates that the site that is essential for the functional interaction between PIP2 and Cdc42Hs is distinct from the Dbl-binding site and is located at the positively charged carboxyl-terminal end of the GTP-binding protein. The GDP-releasing activity of PIP2 is highly effective toward Cdc42Hs and Rho (and is similar to the reported effects of PIP2 on Arf (Terui, T., Kahn, R. A., and Randazzo, P. A., (1994) J. Biol. Chem. 269, 28130-28135)), is less effective with Rac, and is not observed with Ras, Rap1a, or Ran. The ability of PIP2 to activate Cdc42Hs (or Rho) and Arf provides a possible point of convergence for the biological pathways regulated by these different GTP-binding proteins and may be related to the synergism observed between Arf and Rho-subtype proteins in the stimulation of phospholipase D activity (Singer, W. D., Brown, H. A., Bokoch, G. M., and Sternweis, P. C. (1995) J. Biol. Chem. 270, 14944-14950).