Ral and Rab3a are major GTP-binding proteins of axonal rapid transport and synaptic vesicles and do not redistribute following depolarization stimulated synaptosomal exocytosis

Ral signaling pathway in health and cancer

The Ral (Ras-Like) signaling pathway plays an important role in the biology of cells. A plethora of effects is regulated by this signaling pathway and its prooncogenic effectors. Our team has demonstrated the overactivation of the RalA signaling pathway in a number of human malignancies including cancers of the liver, ovary, lung, brain, and malignant peripheral nerve sheath tumors. Additionally, we have shown that the activation of RalA in cancer stem cells is higher in comparison with differentiated cancer cells. In this article, we review the role of Ral signaling in health and disease with a focus on the role of this multifunctional protein in the generation of therapies for cancer. An improved understanding of this pathway can lead to development of a novel class of anticancer therapies that functions on the basis of intervention with RalA or its downstream effectors.

Exorcising the exocyst complex

The exocyst complex is an evolutionarily conserved multisubunit protein complex implicated in tethering secretory vesicles to the plasma membrane. Originally identified two decades ago in budding yeast, investigations using several different eukaryotic systems have since made great progress toward determination of the overall structure and organization of the eight exocyst subunits. Studies point to a critical role for the complex as a spatiotemporal regulator through the numerous protein and lipid interactions of its subunits, although a molecular understanding of exocyst function has been challenging to elucidate. Recent progress demonstrates that the exocyst is also important for additional trafficking steps and cellular processes beyond exocytosis, with links to development and disease. In this review, we discuss current knowledge of exocyst architecture, assembly, regulation and its roles in a variety of cellular trafficking pathways.

Ral GTPases regulate cell-mediated cytotoxicity in NK cells

NK cells are key components of the immune response to virally infected and tumor cells. Recognition of target cells initiates a series of events in NK cells that culminates in target destruction via directed secretion of lytic granules. Ral proteins are members of the Ras superfamily of small GTPases; they regulate vesicular trafficking and polarized granule secretion in several cell types. In this study, we address the role of Ral GTPases in cell-mediated cytotoxicity. Using a human NK cell line and human primary NK cells, we show that both Ral isoforms, RalA and RalB, are activated rapidly after target cell recognition. Furthermore, silencing of RalA and RalB impaired NK cell cytotoxicity. RalA regulated granule polarization toward the immunological synapse and the subsequent process of degranulation, whereas RalB regulated degranulation but not polarization of lytic granules. Analysis of the molecular mechanism indicated that Ral activation in NK cells leads to assembly of the exocyst, a protein complex involved in polarized secretion. This assembly is required for degranulation, as interference with expression of the exocyst component Sec5 led to reduced degranulation and impaired cytotoxicity in NK cells. Our results thus identify a role for Ral in cell-mediated cytotoxicity, implicating these GTPases in lymphocyte function.

Noradrenaline inhibits exocytosis via the G protein βγ subunit and refilling of the readily releasable granule pool via the α(i1/2) subunit

The molecular mechanisms responsible for the 'distal' effect by which noradrenaline (NA) blocks exocytosis in the β-cell were examined by whole-cell and cell-attached patch clamp capacitance measurements in INS 832/13 β-cells. NA inhibited Ca(2+)-evoked exocytosis by reducing the number of exocytotic events, without modifying vesicle size. Fusion pore properties also were unaffected. NA-induced inhibition of exocytosis was abolished by a high level of Ca(2+) influx, by intracellular application of antibodies against the G protein subunit Gβ and was mimicked by the myristoylated βγ-binding/activating peptide mSIRK. NA-induced inhibition was also abolished by treatment with BoNT/A, which cleaves the C-terminal nine amino acids of SNAP-25, and also by a SNAP-25 C-terminal-blocking peptide containing the BoNT/A cleavage site. These data indicate that inhibition of exocytosis by NA is downstream of increased [Ca(2+)](i) and is mediated by an interaction between Gβγ and the C-terminus of SNAP-25, as is the case for inhibition of neurotransmitter release. Remarkably, in the course of this work, a novel effect of NA was discovered. NA induced a marked retardation of the rate of refilling of the readily releasable pool (RRP) of secretory granules. This retardation was specifically abolished by a Gα(i1/2) blocking peptide demonstrating that the effect is mediated via activation of Gα(i1) and/or Gα(i2).

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Role for Rab3a in oligodendrocyte morphological differentiation

Rab3a, a small GTPase important for exocytosis, is uniquely up-regulated as oligodendrocytes enter terminal differentiation and initiate myelin biosynthesis. In this study, we analyze the role of this protein in oligodendrocyte morphological differentiation by using Rab3a overexpression and siRNAi-mediated Rab3a silencing. We found that Rab3a silencing delayed mature oligodendrocyte morphological differentiation but did not interfere with lineage progression of OL progenitors; this is consistent with the high levels of Rab3a expressed by mature oligodendrocytes compared with progenitor cells. Overexpression of GTP-bound, but not that of wild-type, Rab3a delayed OL morphological differentiation; this suggests that expression of a GTP-bound Rab3a mutant interferes with the normal function of endogenous Rab3a. We have also identified in oligodendrocytes two other exocytic small GTPases, Rab27B and RalA. Together, these findings indicate that Rab3a specifically stimulates morphological differentiation of mature oligodendrocytes and thus may be part of the necessary machinery for myelin membrane biogenesis.

A serine kinase associates with the RAL GTPase and phosphorylates RAL-interacting protein 1

A kinase activity that phosphorylated myelin basic protein in vitro was detected in RalA and RalB immunoprecipitates from human platelets. Protein-protein interaction studies using recombinant GST-RalA, GST-RalB and GST-cH-Ras confirmed that the kinase specifically associates with the Ral GTPase. The Ral Interacting Protein 1 (RIP1), a GTPase Activating Protein (GAP) for Cdc42 and Rac1, was found to be the preferred substrate for the Ral Interacting Kinase (RIK). Phosphoamino acid analysis demonstrated that RIK phosphorylated serine residue in RIP1. The Ral-RIK interaction was not dependent on the guanine nucleotide status of Ral. RIK was detected in a variety of rat tissues with testis containing the highest and skeletal muscle the lowest activity. In-gel kinase renaturation assay using RIP1 as the substrate demonstrated that the kinase activity was associated with polypeptides of molecular mass of approximately 36-40 kDa and was detected in most rat tissues with a prominent 38 kDa band in testis and a 40 kDa band in brain. Human platelets contained a single band of approximately 36 kDa. RIK was distinct from MAPKs, CDKs, cyclic AMP dependent protein kinase and Ca2+/calmodulin dependent kinases. To demonstrate in vivo interaction, the endogenous Ral-RIK complex was isolated using a calmodulin affinity column. The Ral-RIK complex co-eluted from this column upon washing with a 13 residue peptide that encompasses the calmodulin-binding domain in RalA. The data suggest that RIK is a serine specific kinase that phosphorylates RIP1 and is constitutively associated with Ral. The current study provides additional support for a link between Ral and the Cdc42/Rac1 signalling pathways in the cell.

Inhibition of hippocampal synaptic transmission by impairment of Ral function

Large clostridial cytotoxins and protein overexpression were used to probe for involvement of Ras-related GTPases (guanosine triphosphate) in synaptic transmission in cultured rat hippocampal neurons. The toxins TcdA-10463 (inactivates Rho, Rac, Cdc42, Rap) and TcsL-1522 (inactivates Ral, Rac, Ras, R-Ras, Rap) both inhibited autaptic responses. In a proportion of the neurons (25%, TcdA-10463; 54%, TcsL-1522), the inhibition was associated with a shift from activity-dependent depression to facilitation, indicating that the synaptic release probability was reduced. Overexpression of a dominant negative Ral mutant, Ral A28N, caused a strong inhibition of autaptic responses, which was associated with a shift to facilitation in a majority (80%) of the neurons. These results indicate that Ral, along with at least one other non-Rab GTPase, participates in presynaptic regulation in hippocampal neurons.

The Small GTPase RalA controls exocytosis of large dense core secretory granules by interacting with ARF6-dependent phospholipase D1

RalA and RalB constitute a family of highly similar Ras-related GTPases widely distributed in different tissues. Recently, active forms of Ral proteins have been shown to bind to the exocyst complex, implicating them in the regulation of cellular secretion. Since RalA is present on the plasma membrane in neuroendocrine chromaffin and PC12 cells, we investigated the potential role of RalA in calcium-regulated exocytotic secretion. We show here that endogenous RalA is activated during exocytosis. Expression of the constitutively active RalA (G23V) mutant enhances secretagogue-evoked secretion from PC12 cells. Conversely, expression of the constitutively inactive GDP-bound RalA (G26A) or silencing of the RalA gene by RNA interference led to a strong impairment of the exocytotic response. RalA was found to co-localize with phospholipase D1 (PLD1) at the plasma membrane in PC12 cells. We demonstrate that cell stimulation triggers a direct interaction between RalA and ARF6-activated PLD1. Moreover, reduction of endogenous RalA expression level interfered with the activation of PLD1 observed in secretagogue-stimulated cells. Finally, using various RalA mutants selectively impaired in their ability to activate downstream effectors, we show that PLD1 activation is essential for the activation of secretion by GTP-loaded RalA. Together, these results provide evidence that RalA is a positive regulator of calcium-evoked exocytosis of large dense core secretory granules and suggest that stimulation of PLD1 and consequent changes in plasma membrane phospholipid composition is the major function RalA undertakes in calcium-regulated exocytosis.

RalGDS is required for tumor formation in a model of skin carcinogenesis

To investigate the role of signaling by the small GTPase Ral, we have generated mice deficient for RalGDS, a guanine nucleotide exchange factor that activates Ral. We show that RalGDS is dispensable for mouse development but plays a substantial role in Ras-induced oncogenesis. Lack of RalGDS results in reduced tumor incidence, size, and progression to malignancy in multistage skin carcinogenesis, and reduced transformation by Ras in tissue culture. RalGDS does not appear to participate in the regulation of cell proliferation, but instead controls survival of transformed cells. Experiments performed in cells isolated from skin tumors suggest that RalGDS mediates cell survival through the activation of the JNK/SAPK pathway. These studies identify RalGDS as a key component in Ras-dependent carcinogenesis in vivo.

RalA interacts with ZONAB in a cell density-dependent manner and regulates its transcriptional activity

Ral proteins are members of the Ras superfamily of small GTPases and are involved in signalling pathways for actin cytoskeleton remodelling, cell cycle control, cellular transformation and vesicle transport. To identify novel RalA effector proteins, we used the reverse Ras recruitment system and found that RalA interacts with a Y-box transcription factor, ZO-1-associated nucleic acid-binding protein (ZONAB), in a GTP-dependent manner. The amount of the RalA-ZONAB complex increases as epithelial cells become more dense and increase cell contacts. The RalA-ZONAB interaction results in a relief of transcriptional repression of a ZONAB-regulated promoter. Additionally, expression of oncogenic Ras alleviates transcriptional repression by ZONAB in a RalA-dependent manner. The data presented here implicate the RalA/ZONAB interaction in the regulation of ZONAB function.

Vesicles carry most exocyst subunits to exocytic sites marked by the remaining two subunits, Sec3p and Exo70p

Exocytosis in the budding yeast Saccharomyces cerevisiae occurs at discrete domains of the plasma membrane. The protein complex that tethers incoming vesicles to sites of secretion is known as the exocyst. We have used photobleaching recovery experiments to characterize the dynamic behavior of the eight subunits that make up the exocyst. One subset (Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, and Exo84p) exhibits mobility similar to that of the vesicle-bound Rab family protein Sec4p, whereas Sec3p and Exo70p exhibit substantially more stability. Disruption of actin assembly abolishes the ability of the first subset of subunits to recover after photobleaching, whereas Sec3p and Exo70p are resistant. Immunogold electron microscopy and epifluorescence video microscopy indicate that all exocyst subunits, except for Sec3p, are associated with secretory vesicles as they arrive at exocytic sites. Assembly of the exocyst occurs when the first subset of subunits, delivered on vesicles, joins Sec3p and Exo70p on the plasma membrane. Exocyst assembly serves to both target and tether vesicles to sites of exocytosis.

RalA but not RalB enhances polarized delivery of membrane proteins to the basolateral surface of epithelial cells

RalA and RalB constitute a family of highly similar (85% identity) Ras-related GTPases. Recently, active forms of both RalA and RalB have been shown to bind to the exocyst complex, implicating them in the regulation of cellular secretion. However, we show here that only active RalA enhances the rate of delivery of E-cadherin and other proteins to their site in the basolateral membrane of MDCK cells, consistent with RalA being a regulator of exocyst function. One reason for this difference is that RalA binds more effectively to the exocyst complex than active RalB does both in vivo and in vitro. Another reason is that active RalA localizes to perinuclear recycling endosomes, where regulation of vesicle sorting is thought to take place, while active RalB does not. Strikingly, analysis of chimeras made between RalA and RalB reveals that high-affinity exocyst binding by RalA is due to unique amino acid sequences in RalA that are distal to the common effector-binding domains shared by RalA and RalB. Moreover, these chimeras show that the perinuclear localization of active RalA is due in part to its unique variable domain near the C terminus. This distinct localization appears to be important for RalA effects on secretion because all RalA mutants tested that failed to localize to the perinuclear region also failed to promote basolateral delivery of E-cadherin. Interestingly, one of these inactive mutants maintained binding to the exocyst complex, suggesting that RalA binding to the exocyst is necessary but not sufficient for RalA to promote basolateral delivery of membrane proteins.

Transport of glutathione conjugates and chemotherapeutic drugs by RLIP76 (RALBP1): a novel link between G-protein and tyrosine kinase signaling and drug resistance

Our studies have shown that RLIP76 (RALBP1), a 76 kDa Ral-binding, Rho/Rac-GAP and Ral effector protein, is a novel multispecific transporter of xenobiotics as well as GS-Es. Like previously characterized ABC transporters, it mediates ATP-dependent transport of structurally unrelated amphiphilic xenobiotics and displays inherent ATPase activity, which is stimulated by its substrate allocrites. It does not have significant sequence homology with ABC transporters and differs from the ABC transporters in several other important aspects, including (i) lack of any close homologs in humans, (ii) lack of a classical Walker domain, (iii) integral membrane association without clearly defined transmembrane domains and (iv) its role as a direct link to Ras/Ral/Rho and EGF-R signaling through its multifunctional nature, including GAP activity, regulation of exocytosis as well as clathrin-coated pit-mediated receptor endocytosis. Its multifunctional nature derives from the presence of multiple motifs, including a Rho/Rac GAP domain, a Ral effector domain binding motif, 2 distinct ATP-binding domains, a H(+)-ATPase domain, PKC and tyrosine kinase phosphorylation sites and the ability to undergo fragmentation into multiple smaller peptides which participate as components of macromolecular functional complexes. One of the physiologic functions of RLIP76 is regulation of intracellular concentration of the electrophilic intermediates of oxidative lipid metabolism by mediating efflux of GS-E formed from oxidative degradation of arachidonic acid, including leukotrienes and the 4HNE-GSH conjugate. RLIP76-mediated transport of amphiphilic chemotherapeutic agents such as anthracyclines and vinca alkaloids as well as GS-E produced during oxidative metabolism places this multifunctional protein in a central role as a resistance mechanism for preventing apoptosis caused by chemotherapeutic agents and a variety of external/internal stressors, including oxidative stress, heat shock and radiation.

Ral-GTPases: approaching their 15 minutes of fame

Andy Warhol, the famous pop artist, once claimed that "in the future everyone will be famous for 15 minutes". The same, it seems, can be said of proteins, because at any given time some proteins become more "fashionable" to study than others. But most proteins have been highly conserved throughout millions of years of evolution, which implies that they all have essential roles in cell biology. Thus, each one will no doubt enter the limelight if the right experiment in the right cell type is done. A good example of this is the Ras-like GTPases (Ral-GTPases), which until recently existed in the shadow of their close cousins--the Ras proto-oncogenes. Recent studies have yielded insights into previously unappreciated roles for Ral-GTPases in intensively investigated disciplines such as vesicle trafficking, cell morphology, transcription and possibly even human oncogenesis.

HSF-1 interacts with Ral-binding protein 1 in a stress-responsive, multiprotein complex with HSP90 in vivo

Heat shock factor 1 (HSF1) regulates the rapid and transient expression of heat shock genes in response to stress. The transcriptional activity of HSF1 is tightly controlled, and under physiological growth conditions, the HSF1 monomer is in a heterocomplex with the molecular chaperone HSP90. Through unknown mechanisms, transcriptionally repressed HSF1.HSP90 heterocomplexes dissociate following stress, which triggers HSF1 activation and heat shock gene transcription. Using a yeast two-hybrid screening system, we have identified Ral-binding protein 1 (RalBP1) as an additional HSF1-interacting protein. We show that RalBP1 and HSF1 interact in vivo, and transient cotransfection of HSF1 and RalBP1 into hsf1(-/-) mouse embryo fibroblasts represses HSP70 expression. Furthermore, transient cotransfection of HSF1 and the constitutively active form of RalA (RalA23V), an upstream activator of the RalBP1 signaling pathway, increases the heat-inducible expression of HSP70, whereas the dominant negative form (RalA28N) suppresses HSP70 expression. We further find that alpha-tubulin and HSP90 are also present in the RalBP1.HSF1 heterocomplexes in unstressed cells. Upon heat shock, the Ral signaling pathway is activated, and the resulting RalGTP binds RalBP1. Concurrently, HSF1 is activated, leaves the RalBP1 x HSF1 x HSP90 x alpha-tubulin heterocomplexes, and translocates into the nucleus, where it then activates transcription. In conclusion, these observations reveal that the RalGTP signal transduction pathway is critical for activation of the stress-responsive HSF1 and perhaps HSP90 molecular chaperone system.

Rab11-FIP2, an adaptor protein connecting cellular components involved in internalization and recycling of epidermal growth factor receptors

Rab11-FIP2 is a member of a newly identified family of Rab11-binding proteins that have been implicated in the function of recycling endosomes. Here we show that Rab11-FIP2 may also be involved with the process of receptor-mediated endocytosis. First we demonstrate that Rab11-FIP2 contains an NPF motif that allows it to bind Reps1, a member of a family of EH domain proteins involved in endocytosis. We also show that Rab11-FIP2 associates with the alpha-adaptin subunit of AP-2 complexes, which are known to recruit receptors into clathrin-coated vesicles. Finally, we find that overexpression of Rab11-FIP2 suppresses the internalization of epidermal growth factor receptors, but not transferrin receptors, through binding sites that promote complex formation with Rab11, Reps1, and alpha-adaptin. These findings suggest that Rab11-FIP2 may participate in the coupling of receptor-mediated endocytosis to the subsequent sorting of receptor-containing vesicles in endosomes.

Involvement of R-Ras and Ral GTPases in estrogen-independent proliferation of breast cancer cells

A key step in the progression of breast cancer is the conversion of cells from an estrogen-dependent to an estrogen-independent state. Yet the molecular mechanisms underlying this transition in the control of cell proliferation of breast cancer cells remain poorly understood. A potential role for Ras-related GTPases in this process was suggested by the finding that BCAR3/AND-34, a protein isolated on the basis of its ability to convert MCF-7 and ZR-75 breast cancer cell lines to estrogen independence and tamoxifen resistance, is a guanine nucleotide exchange factor with the potential to activate the Ras-related Ral, R-Ras and Rap GTPases. In this study we investigated the potential contribution of these GTPases to the generation of estrogen-independence in MCF-7 cells. We found that elevated R-Ras but not Ral or Rap activity was sufficient to induce estrogen-independent proliferation of MCF-7 cells. The effect of R-Ras was dependent upon its ability to constitutively activate the AKT kinase. Interestingly, although AKT was also constitutively activated when estrogen-independent proliferation was induced by over-expression of EGF receptors, this mechanism of hormone independence did not require AKT activation. In contrast, EGF receptors did require Ral activation to induce estrogen-independent proliferation, while Ral activation was not required for estrogen-induced proliferation of MCF-7 cells. These findings suggest that Ral activity takes on a significant role in controlling cell proliferation of breast cancer cells when progression to estrogen-independence is associated with over-expression of EGF receptor family members. Moreover, because R-Ras promotes hormone-independent growth in a manner distinct from EGF receptors, it may participate in the conversion of breast cancer cells to estrogen independence when over-expression of EGF receptor family members is not involved.

Rac GTPase plays an essential role in exocytosis by controlling the fusion competence of release sites

The role of small GTPases of the Rho family in synaptic functions has been addressed by analyzing the effects of lethal toxin (LT) from Clostridium sordellii strain IP82 (LT82) on neurotransmitter release at evoked identified synapses in the buccal ganglion of Aplysia. LT82 is a large monoglucosyltranferase that uses UDP-glucose as cofactor and glucosylates Rac (a small GTPase related to Rho), and Ras, Ral, and Rap (three GTPases of the Ras family). Intraneuronal application of LT (50 nm) rapidly inhibits evoked acetylcholine (ACh) release as monitored electrophysiologically. Injection of the catalytic domain of the toxin similarly blocked ACh release, but not when key amino acids needed for glucosylation were mutated. Intraneuronal application of competitive nucleotide sugars that differentially prevent glucosylation of Rac- and Ras-related GTPases, and the use of a toxin variant that affects a different spectrum of small GTPases, established that glucosylation of Rac is responsible for the reduction in ACh release. To determine the quantal release parameters affected by Rac glucosylation, we developed a nonstationary analysis of the fluctuations in postsynaptic response amplitudes that was performed before and after the toxin had acted or during toxin action. The results indicate that neither the quantal size nor the average probability for release were affected by lethal toxin action. ACh release blockage by LT82 was only caused by a reduction in the number of functional release sites. This reveals that after docking of synaptic vesicles, vesicular Rac stimulates a membrane effector (or effectors) essential for the fusion competence of the exocytotic sites.

Ral-GTPase influences the regulation of the readily releasable pool of synaptic vesicles

The Ral proteins are members of the Ras superfamily of GTPases. Because they reside in synaptic vesicles, we used transgenic mice expressing a dominant inhibitory form of Ral to investigate the role of Ral in neurosecretion. Using a synaptosomal secretion assay, we found that while K(+)-evoked secretion of glutamate was normal, protein kinase C-mediated enhancement of glutamate secretion was suppressed in the mutant mice. Since protein kinase C effects on secretion have been shown to be due to enhancement of the size of the readily releasable pool of synaptic vesicles docked at the plasma membrane, we directly measured the refilling of this readily releasable pool of synaptic vesicles after Ca(2+)-triggered exocytosis. Refilling of the readily releasable pool was suppressed in synaptosomes from mice expressing dominant inhibitory Ral. Moreover, we found that protein kinase C and calcium-induced phosphorylation of proteins thought to influence synaptic vesicle function, such as MARCKS, synapsin, and SNAP-25, were all reduced in synaptosomes from these transgenic mice. Concomitant with these studies, we searched for new functions of Ral by detecting proteins that specifically bind to it in cells. Consistent with the phenotype of the transgenic mice described above, we found that active but not inactive RalA binds to the Sec6/8 (exocyst) complex, whose yeast counterpart is essential for targeting exocytic vesicles to specific docking sites on the plasma membrane. These findings demonstrate a role for Ral-GTPase signaling in the modulation of the readily releasable pool of synaptic vesicles and suggest the possible involvement of Ral-Sec6/8 (exocyst) binding in modulation of synaptic strength.

The exocyst is a Ral effector complex

Delivery of cytoplasmic vesicles to discrete plasma-membrane domains is critical for establishing and maintaining cell polarity, neurite differentiation and regulated exocytosis. The exocyst is a multisubunit complex required for vectorial targeting of a subset of secretory vesicles. Mechanisms that regulate the activity of this complex in mammals are unknown. Here we show that Sec5, an integral component of the exocyst, is a direct target for activated Ral GTPases. Ral GTPases regulate targeting of basolateral proteins in epithelial cells, secretagogue-dependent exocytosis in neuroendocrine cells and assembly of exocyst complexes. These observations define Ral GTPases as critical regulators of vesicle trafficking.

The brain exocyst complex interacts with RalA in a GTP-dependent manner: identification of a novel mammalian Sec3 gene and a second Sec15 gene

Ral is a small GTPase involved in critical cellular signaling pathways. The two isoforms, RalA and RalB, are widely distributed in different tissues, with RalA being enriched in brain. The best characterized RalA signaling pathways involve RalBP1 and phospholipase D. To investigate RalA signaling in neuronal cells we searched for RalA-binding proteins in brain. We found at least eight proteins that bound RalA in a GTP-dependent manner. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) identified these as the components of the exocyst complex. The yeast exocyst is a regulator of polarized secretion, docking vesicles to regions of the plasma membrane involved in active exocytosis. We identified the human FLJ10893 protein as the mammalian homologue of the yeast exocyst protein Sec3p. The exocyst complex did not contain the previously identified exocyst component rSec15, but a new homologue of both yeast Sec15p and rSec15, called KIAA0919. Western blots confirmed that two rat exocyst proteins, rSec6 and rSec8, bound active RalA in nerve terminals, as did RalBP1. Phospholipase D bound RalA in a nucleotide-independent manner. This places the RalA signaling system in mammalian nerve terminals, where the exocyst may act as an effector for activated RalA in directing sites of exocytosis.

Small GTP-binding protein Ral modulates regulated exocytosis of von Willebrand factor by endothelial cells

Weibel-Palade bodies are endothelial cell-specific organelles, which contain von Willebrand factor (vWF), P-selectin, and several other proteins. Recently, we found that the small GTP-binding protein Ral is present in a subcellular fraction containing Weibel-Palade bodies. In the present study, we investigated whether Ral is involved in the regulated exocytosis of Weibel-Palade bodies. Activation of endothelial cells by thrombin resulted in transient cycling of Ral from its inactive GDP-bound to its active GTP-bound state, which coincided with release of vWF. Ral activation and exocytosis of Weibel-Palade bodies were inhibited by incubation with trifluoperazine, an inhibitor of calmodulin, before thrombin stimulation. Functional involvement of Ral in exocytosis was further investigated by the expression of constitutively active and dominant-negative Ral variants in primary endothelial cells. Introduction of active Ral G23V resulted in the disappearance of Weibel-Palade bodies from endothelial cells. In contrast, the expression of the dominant-negative Ral S28N did not affect the amount of Weibel-Palade bodies in transfected cells. These results indicate that Ral is involved in regulated exocytosis of Weibel-Palade bodies by endothelial cells.

An EGF receptor/Ral-GTPase signaling cascade regulates c-Src activity and substrate specificity

c-Src is a membrane-associated tyrosine kinase that can be activated by many types of extracellular signals, and can regulate the function of a variety of cellular protein substrates. We demonstrate that epidermal growth factor (EGF) and beta-adrenergic receptors activate c-Src by different mechanisms leading to the phosphorylation of distinct sets of c-Src substrates. In particular, we found that EGF receptors, but not beta(2)-adrenergic receptors, activated c-Src by a Ral-GTPase-dependent mechanism. Also, c-Src activated by EGF treatment or expression of constitutively activated Ral-GTPase led to tyrosine phosphorylation of Stat3 and cortactin, but not Shc or subsequent Erk activation. In contrast, c-Src activated by isoproterenol led to tyrosine phosphorylation of Shc and subsequent Erk activation, but not tyrosine phosphorylation of cortactin or Stat3. These results identify a role for Ral-GTPases in the activation of c-Src by EGF receptors and the coupling of EGF to transcription through Stat3 and the actin cytoskeleton through cortactin. They also show that c-Src kinase activity can be used differently by individual extracellular stimuli, possibly contributing to their ability to generate unique cellular responses.

Dissociation of RalA from synaptic membranes by Ca2+/calmodulin

Ras-related small GTP-binding proteins execute many cellular functions, such as cell growth, differentiation, cytoskeletal reorganization, membrane trafficking, and membrane fusion. RalA belongs to the superfamily of Ras-related small GTP-binding proteins. Synaptic vesicles (SV) contain small GTP-binding proteins, where RalA, Rab3A, and Rab5A are the major GTP-binding proteins. It has been postulated that a cycling of these proteins between membrane-bound and soluble states is required for regulating cellular functions. Calmodulin (CaM) was found to dissociate Rab3A from SV membranes by forming a 1:1 complex with Ca2+/CaM. RalA was also found to be a Ca2+/CaM-binding protein. Therefore, we examined if Ca2+/CaM can also cause the RalA to dissociate from SV membranes. In this study, we identified that Ca2+/CaM dissociates RalA as well as Rab3A from synaptic vesicles.

Effector recognition by the small GTP-binding proteins Ras and Ral

The Ral effector protein RLIP76 (also called RIP/RalBP1) binds to Ral.GTP via a region that shares no sequence homology with the Ras-binding domains of the Ser/Thr kinase c-Raf-1 and the Ral-specific guanine nucleotide exchange factors. Whereas the Ras-binding domains have a similar ubiquitin-like structure, the Ral-binding domain of RLIP was predicted to comprise a coiled-coil region. In order to obtain more information about the specificity and the structural mode of the interaction between Ral and RLIP, we have performed a sequence space and a mutational analysis. The sequence space analysis of a comprehensive nonredundant assembly of Ras-like proteins strongly indicated that positions 36 and 37 in the core of the effector region are tree-determinant positions for all subfamilies of Ras-like proteins and dictate the specificity of the interaction of these GTPases with their effector proteins. Indeed, we could convert the specific interaction with Ras effectors and RLIP by mutating these residues in Ras and Ral. We therefore conclude that positions 36 and 37 are critical for the discrimination between Ras and Ral effectors and that, despite the absence of sequence homology between the Ral-binding and the Ras-binding domains, their mode of interaction is most probably similar.

Ca2+/calmodulin stimulates GTP binding to the ras-related protein ral-A

Ral-A is a Ras-related GTP-binding protein that has been suggested to be the downstream target of Ras proteins and is involved in the tyrosine kinase-mediated, Ras-dependent activation of phospholipase D. We reported recently that Ral-A purified from human erythrocyte membrane binds to calmodulin in a Ca2+-dependent manner at a calmodulin binding domain identified near its C-terminal region (Wang, K. L., Khan, M. T., and Roufogalis, B. D. (1997) J. Biol. Chem. 272, 16002-16009). In this study we show the enhancement of GTP binding to Ral-A by Ca2+/calmodulin. The stimulation up to 3-fold by calmodulin was Ca2+-dependent, with half-maximum activation occurring at 180 nM calmodulin and 80 nM free Ca2+ concentration. The present work supports a regulatory role of Ca2+/calmodulin for the activation of Ral-A and suggests a possible direct link between signal transduction pathways of Ca2+/calmodulin and Ral-A proteins.

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.

Ral-specific guanine nucleotide exchange factor activity opposes other Ras effectors in PC12 cells by inhibiting neurite outgrowth

Ras proteins can activate at least three classes of downstream target proteins: Raf kinases, phosphatidylinositol-3 phosphate (PI3) kinase, and Ral-specific guanine nucleotide exchange factors (Ral-GEFs). In NIH 3T3 cells, activated Ral-GEFs contribute to Ras-induced cell proliferation and oncogenic transformation by complementing the activities of Raf and PI3 kinases. In PC12 cells, activated Raf and PI3 kinases mediate Ras-induced cell cycle arrest and differentiation into a neuronal phenotype. Here, we show that in PC12 cells, Ral-GEF activity acts opposite to other Ras effectors. Elevation of Ral-GEF activity induced by transfection of a mutant Ras protein that preferentially activates Ral-GEFs, or by transfection of the catalytic domain of the Ral-GEF Rgr, suppressed cell cycle arrest and neurite outgrowth induced by nerve growth factor (NGF) treatment. In addition, Rgr reduced neurite outgrowth induced by a mutant Ras protein that preferentially activates Raf kinases. Furthermore, inhibition of Ral-GEF activity by expression of a dominant negative Ral mutant accelerated cell cycle arrest and enhanced neurite outgrowth in response to NGF treatment. Ral-GEF activity may function, at least in part, through inhibition of the Rho family GTPases, CDC42 and Rac. In contrast to Ras, which was activated for hours by NGF treatment, Ral was activated for only approximately 20 min. These findings suggest that one function of Ral-GEF signaling induced by NGF is to delay the onset of cell cycle arrest and neurite outgrowth induced by other Ras effectors. They also demonstrate that Ras has the potential to promote both antidifferentiation and prodifferentiation signaling pathways through activation of distinct effector proteins. Thus, in some cell types the ratio of activities among Ras effectors and their temporal regulation may be important determinants for cell fate decisions between proliferation and differentiation.

Heterotrimeric and small molecular mass GTP-binding proteins of rat brain neuronal and glial nuclei

Heterotrimeric and small molecular mass guanine nucleotide binding (GTP-binding) proteins were found in neuronal and glial nuclei isolated from rat brain. Neuronal nuclei bound 0.213 +/- 0.055 pmoles of GTP/microg protein (n = 8) and glial nuclei bound 0.145 +/- 0.038 pmoles of GTP/microg protein (n = 8). The intrinsic GTPase activity of neuronal and glial nuclei was 0.0019 +/- 0.0005 pmoles GTP hydrolyzed/min/microg protein (n = 10) and 0.0022 +/- 0.0006 pmoles GTP hydrolyzed/min/microg protein (n = 10), respectively. Western blot analysis was carried out using a peptide-specific antibody that recognizes a common sequence in the alpha-subunit of the various heterotrimeric G-proteins. The antibody revealed the presence of a polypeptide of molecular mass of 40 kDa only in neuronal nuclei. Small molecular mass GTP-binding proteins were detected by incubating nitrocellulose blots with [alpha-32P]GTP. The results demonstrated the presence of 25-26 kDa GTP-binding proteins in both populations of nuclei. However, the binding of [alpha-32P]GTP to neuronal nuclei was approximately 3-fold greater than to the glial nuclei. Further analysis by two-dimensional polyacrylamide gel electrophoresis resolved the neuronal nuclei 26 kDa protein into three forms (a-c) with the most acidic form (c) being the major species. The neuronal 25 kDa protein was resolved into two forms that were present in approximately equal concentration. In glial nuclei, only the 26 kDa (c) and a small amount of the 25 kDa proteins were detected. However, both populations of nuclei contained the small molecular mass GTP-binding protein, ran. Differential association of non-ran small molecular mass GTP-binding proteins and heterotrimeric G-proteins with neuronal nuclei suggests a potential role for these guanine nucleotide binding proteins in the function of this cell type.

Rab11a redistributes to apical secretory canaliculus during stimulation of gastric parietal cells

Previous investigations in several systems have demonstrated that Rab3 family members redistribute to soluble fractions on fusion of secretory granules with target plasma membranes. Rab proteins are then recycled back onto mature secretory vesicles after reinternalization of the membrane. Although this cycle is well established for Rab3, far less is known about redistribution of other Rab proteins during vesicle fusion and recycling. In the gastric parietal cell, Rab11a is associated with H-K-ATPase-containing tubulovesicles, which fuse with the apical plasma membrane (secretory canaliculus) in response to agonists such as histamine. We have analyzed distribution of Rab11a and other tubulovesicle proteins in resting and histamine-stimulated rabbit parietal cells. Stimulation of isolated gastric glands in the presence of 100 microM histamine and 100 microM 3-isobutyl-1-methylxanthine did not cause a significant increase in soluble Rab11a. H-K-ATPase, Rab11a, Rab25, syntaxin 3, and SCAMPs increased immunoreactivity in stimulus-associated vesicles prepared from rabbits treated with histamine compared with those from ranitidine-treated animals. The large GTPase dynamin was found in both vesicle preparations, but there was no change in amount of immunoreactivity. Immunofluorescence staining of resting and histamine-stimulated primary cultures of parietal cells demonstrated redistribution of H-K-ATPase and Rab11a to F-actin-rich canalicular membranes. Dynamin was present on canalicular membranes in resting and stimulated cells. These results indicate that Rab11a does not cycle off the membrane during the process of tubulovesicle fusion with the secretory canaliculus. Thus Rab11a may remain associated with recycling apical membrane vesicle populations.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Immunodetection of ralA and ralB GTP-binding proteins in various rat tissues and platelets

Polyclonal antibodies were generated against a synthetic peptide corresponding to the C-terminal (amino acids 192-204) region of ralA and ralB GTP-binding proteins. The ralA and ralB antibodies recognized a 27 kDa protein in the human platelet particulate fraction. Incubation of ralA antibodies with ralB immunizing peptide and ralB antibodies with ralA immunizing peptide prior to Western blotting did not abolish the ability of antibodies to recognize the 27 kDa protein in human platelet particulate fraction. However, when antibodies were incubated with the respective immunizing peptide prior to Western blotting, the 27 kDa human platelet protein was no longer recognized by the antibodies. Incubation of nitrocellulose blots containing polypeptides separated using SDS-PAGE with [alpha-32P]GTP demonstrated the presence of GTP-binding proteins of molecular mass between 23-27 kDa in rat platelets and the various tissues tested. Analysis using subtype specific antibodies demonstrated that both ralA and ralB GTP-binding proteins were expressed in rat platelets and the various tissues tested. The protein recognized by the ralA and ralB antibodies in rat tissues and platelets had mobility on SDS-PAGE identical to that of the human platelet ral protein. Varying amounts of these proteins were detected in all the tissues tested except white muscle which contained very low level of ralB protein. The widespread distribution of ralA and ralB GTP-binding proteins suggests that they may participate in a common pathway in mammalian cells and tissues.

An Eps homology (EH) domain protein that binds to the Ral-GTPase target, RalBP1

Ral proteins constitute a family of small GTPases that can be activated by Ras in cells. In the GTP-bound state, Ral proteins bind to RalBP1, a GTPase-activating protein for CDC42 and Rac GTPases. We have used the two-hybrid system in yeast to clone a cDNA for a novel approximately 85-kDa protein that can bind to an additional site on RalBP1. This newly identified protein contains an Eps homology (EH) domain, which was first detected in the epidermal growth factor (EGF) receptor substrate Eps15. Recently, the EH domain of Eps15 has been shown to bind to proteins containing an asparagine-proline-phenylalanine motif. Moreover, EH domains have been found in proteins involved in endocytosis and/or actin cytoskeleton regulation. The RalBP1 associated Eps-homology domain protein, Reps1, is tyrosine-phosphorylated in response to EGF stimulation of cells. In addition, Reps1 has the capacity to form a complex with the SH3 domains of the adapter proteins Crk and Grb2, which may link Reps1 to an EGF-responsive tyrosine kinase. Thus, Reps1 may coordinate the cellular actions of activated EGF receptors and Ral-GTPases.

Ca2+/calmodulin causes Rab3A to dissociate from synaptic membranes

The GTPase Rab3A has been postulated to cycle on and off synaptic membranes during the course of neurotransmission. Moreover, a Rab guanine nucleotide dissociation inhibitor has been shown to cause Rab3A to dissociate from synaptic membranes in vitro. We demonstrate here that Ca2+/calmodulin also can cause Rab3A to dissociate from synaptic membranes in vitro. Like Rab guanine nucleotide dissociation inhibitor, it forms a 1:1 complex with Rab3A that requires both the lipidated C terminus of Rab3A and the presence of bound guanine nucleotide. In addition, a synthetic peptide corresponding to the Lys62-Arg85 sequence of Rab3A can prevent the dissociating effect of each protein and disrupt complexes between each protein and Rab3A. However, Ca2+/calmodulin's effect differs from that of Rab guanine nucleotide dissociation inhibitor not only in being Ca2+-dependent but also in having a less stringent requirement for GDP as opposed to GTP and in involving a less complete dissociation of Rab3A. The functional significance in vivo of Ca2+/calmodulin's effect remains to be determined; it may depend in part on the relative amounts of Ca2+/calmodulin and Rab guanine nucleotide dissociation inhibitor that are available for binding to Rab3A in individual, activated nerve termini.

Identification and characterization of a calmodulin-binding domain in Ral-A, a Ras-related GTP-binding protein purified from human erythrocyte membrane

A 28-kDa protein (p28) has been purified from Triton X-100 extracts of human erythrocyte plasma membrane by calmodulin affinity chromatography. Based on internal peptide sequencing and its protein amino acid composition, this protein has been shown to be highly related, if not identical to, Ral-A, a Ras-related GTP-binding protein. This protein assignment is consistent with the findings that p28 binds [32P]GTP specifically and has low GTPase activity. In this study we describe the identification and characterization of a calmodulin-binding domain in Ral-A. The Ca2+-dependent interaction of p28 with calmodulin was first detected by a calmodulin affinity column. Gel overlay experiments of both p28 and recombinant Ral-A with biotinylated calmodulin provided strong evidence that Ral-A is a calmodulin-binding protein. A peptide of 18 residues (P18) with the sequence SKEKNGKKKRKSLAKRIR has been identified as a putative calmodulin-binding domain in Ral-A, because it comprises a basic/hydrophobic composition with the propensity to form an amphiphilic helix. P18 was synthesized, and its interaction with calmodulin by gel overlay was shown to be Ca2+-dependent. Circular dichroism analysis demonstrated that this interaction results in less alpha-helical content upon calmodulin complex formation. These results indicate that Ral-A is a calmodulin-binding protein, raising the possibility that it may be associated with Ca2+-dependent intracellular signaling pathways.

Differential synaptic vesicle protein expression in the barrel field of developing cortex

The distribution of four proteins associated with synaptic vesicles, SV2, synaptophysin, synapsin I, and rab3a, was investigated during postnatal development of the posteromedial barrel subfield (PMBSF) in the rat somatosensory cortex. A distinct progression in the appearance of the different synaptic vesicle proteins within the PMBSF was observed. SV2, synapsin I, and synaptophysin revealed the organization of the barrel field in the neonate. This early demarcation of the cortical representation of the vibrissal array coincides with the earliest known age for the emergence of the cytoarchitectonic organization of this region. In contrast, rab3a did not delimit the barrels until the end of the 1st postnatal week, coincident with the known onset of adult-like physiological activity and the loss of plasticity in afferents to this region. In addition, the appearance of the different synaptic vesicle proteins occurred earlier within the PMBSF than in the adjacent extra-barrel regions of the cortex. These results show that the molecular differentiation of synaptic fields across the cortex is not a homogeneous and synchronous process in terms of synaptic vesicle protein expression. Because these proteins act together in mature synapses to ensure the regulated release of neurotransmitters, our results suggest that this temporo-spatial asynchrony may underlie different potentials for synaptic activity and thus contribute to the development of cortical maps.

Generation of antibodies specific for the RalA and RalB GTP-binding proteins and determination of their concentration and distribution in human platelets

Peptide specific polyclonal antibodies directed against C-termini of ras p21 related GTP-binding proteins, ralA and ralB, were generated. To assess antibody specificity, cDNAs coding for full length ralA and ralB were expressed in Escherichia coli as GST fusion proteins. Western blotting analysis using enhanced chemiluminescence technique confirmed that ralA and ralB antibodies were specific for their respective protein. To determine the concentration and distribution, varying amounts of GST-ralA and GST-ralB and, human platelet particulate and cytosolic proteins were loaded during Western blotting. The amount of ralA and ralB proteins in the platelet particulate fraction was determined to be 0.16 +/- 0.017 microgram/mg protein (n = 3) and 0.15 +/- 0.009 microgram/mg protein (n = 3) respectively. In the cytosol, only ralB protein was detected and its concentration was estimated to be 0.03 +/- 0.009 microgram/mg protein (n = 3). Both ralA and ralB proteins were isoprenylated in the presence of [3H] mevalonolactone plus rabbit reticulocyte lysate although radioactivity incorporated into ralA was three times higher than that associated with the ralB protein. Addition of geranylgeranyl pyrophosphate to the reaction mixture inhibited incorporation of radioactivity into ralA and ralB but not cH-ras suggesting that both ralA and ralB proteins are geranylgeranylated. Differential distribution of ralA and ralB GTP-binding proteins in human platelets suggests a distinct role for each of these proteins in platelet function.

Evidence for a Ras/Ral signaling cascade

It is becoming clear that Ras proteins mediate their diverse biological functions by binding to, and participating in, the activation of multiple downstream targets. Recent work has identified nucleotide-exchange factors for Ral-GTPases as the newest members of the set of putative Ras 'effector molecules'. This new work has also detected two potential downstream targets of Ral proteins, a novel CDC42/Rac GTPase-activating protein and a phospholipase D.

Post-translational modifications of Ras and Ral are important for the action of Ral GDP dissociation stimulator

Ral GDP dissociation stimulator (RalGDS) is a GDP/GTP exchange protein of Ral and a new effector protein of Ras. Therefore, there may be a new signaling pathway from Ras to Ral. In this paper, we examined the roles of the post-translational modifications of Ras and Ral on this new signal transduction pathway. The post-translationally modified form of Ras bound to RalGDS more effectively than the unmodified form. The modification of Ras was required to regulate the distribution of RalGDS between the cytosol and membrane fractions in COS cells. The post-translational modification of Ral enhanced the activities of RalGDS to stimulate the dissociation of GDP from and the binding of GTP to Ral. Furthermore, the modified form of Ral bound to Ral-binding protein 1 (RalBP1), a putative effector protein of Ral, more effectively than the unmodified form. Taken together with the observations that Ras and Ral are localized to the membranes, these results suggest that the post-translational modifications of Ras and Ral play a role for transmitting the signal effectively on the membranes in the signal transduction pathway of Ras/RalGDS/Ral/RalBP1.

Cell type-specific roles for Cdc42, Rac, and RhoL in Drosophila oogenesis

The Rho subfamily of GTPases has been shown to regulate cellular morphology. We report the discovery of a new member of the Rho family, named RhoL, which is equally similar to Rac, Rho, and Cdc42. Expression of a dominant-negative RhoL transgene in the Drosophila ovary caused nurse cells to collapse and fuse together. Mutant forms of Cdc42 mimicked this effect. Expression of constitutively active RhoL led to nurse cell subcortical actin breakdown and disruption of nurse cell-follicle cell contacts, followed by germ cell apoptosis. In contrast, Rac activity was specifically required for migration of a subset of follicle cells called border cells. All three activities were necessary for normal transfer of nurse cell cytoplasm to the oocyte. These results suggest that Rho protein activities have cell type-specific effects on morphogenesis.

rap1 p21 regulates the interaction of ras p21 with RGL, a new effector protein of ras p21

We have recently found that ralGDS family members (RGL and ralGDS) are putative effector proteins of ras p21. rap1 p21 is a small GTP-binding protein which has the same amino acid sequence as the effector loop of ras p21. We examined the effect of rap1 p21 on the interaction of ras p21 with RGL. The GTP-bound form of rap1 p21 interacted with RGL as well as did ras p21. rap1 p21 inhibited the interaction of ras p21 with RGL. RGL was phosphorylated by cyclic AMP-dependent protein kinase (protein kinase A). Phosphorylation of RGL did not affect its binding to ras p21 and rap1 p21 under the conditions that phosphorylation of Raf-1 reduced its affinity for ras p21. These results demonstrate that rap1 p21 but not protein kinase A regulates the interaction of ras p21 with RGL and suggest that rap1 p21 and protein kinase A may cooperate to distinguish the signal or ras p21 to RGL from that to Raf-1.

Identification and characterization of Ral-binding protein 1, a potential downstream target of Ral GTPases

Ral proteins constitute a distinct family of Ras-related GTPases. Although similar to Ras in amino acid sequence, Ral proteins are activated by a unique nucleotide exchange factor and inactivated by a distinct GTPase-activating protein. Unlike Ras, they fail to promote transformed foci when activated versions are expressed in cells. To identify downstream targets that might mediate a Ral-specific function, we used a Saccharomyces cerevisiae-based interaction assay to clone a novel cDNA that encodes a Ral-binding protein (RalBP1). RalBP1 binds specifically to the active GTP-bound form of RalA and not to a mutant Ral with a point mutation in its putative effector domain. In addition to a Ral-binding domain, RalBP1 also contains a Rho-GTPase-activating protein domain that interacts preferentially with Rho family member CDC42. Since CDC42 has been implicated in bud site selection in S. cerevisiae and filopodium formation in mammalian cells, Ral may function to modulate the actin cytoskeleton through its interactions with RalBP1.

The synaptic vesicle and its targets

Synaptic vesicles play the central role in synaptic transmission. They are regarded as key organelles involved in synaptic functions such as uptake, storage and stimulus-dependent release of neurotransmitter. In the last few years our knowledge concerning the molecular components involved in the functioning of synaptic vesicles has grown impressively. Combined biochemical and molecular genetic approaches characterize many constituents of synaptic vesicles in molecular detail and contribute to an elaborate understanding of the organelle responsible for fast neuronal signalling. By studying synaptic vesicles from the electric organ of electric rays and from the mammalian cerebral cortex several proteins have been characterized as functional carriers of vesicle function, including proteins involved in the molecular cascade of exocytosis. The synaptic vesicle specific proteins, their presumptive function and targets of synaptic vesicle proteins will be discussed. This paper focuses on the small synaptic vesicles responsible for fast neuronal transmission. Comparing synaptic vesicles from the peripheral and central nervous systems strengthens the view of a high conservation in the overall composition of synaptic vesicles with a unique set of proteins attributed to this cellular compartment. Synaptic vesicle proteins belong to gene families encoding multiple isoforms present in subpopulations of neurons. The overall architecture of synaptic vesicle proteins is highly conserved during evolution and homologues of these proteins govern the constitutive secretion in yeast. Neurotoxins from different sources helped to identify target proteins of synaptic vesicles and to elucidate the molecular machinery of docking and fusion. Synaptic vesicle proteins and their markers are useful tools for the understanding of the complex life cycle of synaptic vesicles.

Activated Ras interacts with the Ral guanine nucleotide dissociation stimulator

The yeast two-hybrid system was used to identify proteins that interact with Ras. The H-Ras protein was found to interact with a guanine nucleotide dissociation stimulator (GDS) that has been previously shown to regulate guanine nucleotide exchange on another member of the Ras protein family, Ral. The interaction is mediated by the C-terminal, noncatalytic segment of the RalGDS and can be detected both in vivo, using the two-hybrid system, and in vitro, with purified recombinant proteins. The interaction of the RalGDS C-terminal segment with Ras is specific, dependent on activation of Ras by GTP, and blocked by a mutation that affects Ras effector function. These characteristics are similar to those previously demonstrated for the interaction between Ras and its putative effector, Raf, suggesting that the RalGDS may also be a Ras effector. Consistent with this idea, the RalGDS was found to inhibit the binding of Raf to Ras.