The GTPase superfamily: a conserved switch for diverse cell functions

The structure of Ras protein: a model for a universal molecular switch

X-ray crystallography has revealed the molecular architecture of the cellular and oncogenic forms of p21Ha-ras, the protein encoded by the human Ha-ras gene, in both its active (GTP-bound) and in its inactive (GDP-bound) forms. From comparison of these two structures, a mechanism is suggested for the GTPase hydrolysis reaction that triggers the conformational change necessary for signal transduction. The structures have also allowed identification of the structural consequences of point mutations and the way in which they interfere with the intrinsic GTPase activity of p21ras. The p21ras structure is similar to that of the G-domain of elongation factor Tu (EF-Tu) from Escherichia coli, suggesting that p21ras can serve as a good model for other guanine nucleotide binding proteins.

Identification of a functionally conserved surface region of rat cytochromes P450IA

A region of rat cytochrome P450IA1 at residues 294-301 (Gln-Asp-Arg-Arg-Leu-Asp-Glu-Asn), equivalent to a proinhibitory region of cytochrome P450IA2, was identified by sequence alignment. Anti-peptide antibodies were successfully raised when the peptide was coupled through either its N- or its C-terminus to carrier protein, but no antibodies were produced against the so-called multiple peptide antigen, which consisted of eight copies of the peptide attached through its C-terminus to a synthetic base. Both of the anti-peptide antibodies bound specifically to cytochrome P450IA1 in the rat, as shown by e.l.i.s.a. and immunoblotting. They inhibited microsomal aryl hydrocarbon hydroxylase activity and the mutagenic activation of 2-acetylaminofluorene (these reactions are catalysed by cytochrome P450IA1), but not high-affinity phenacetin O-de-ethylation activity, which is catalysed by cytochrome P450IA2. However, there was differences in the properties of the two antisera in their binding to cytochromes P450IA1 in species other than the rat, their relative binding to the multiple peptide antigen, the yield of antibody following affinity purification using peptide coupled through its N-terminus to CNBr-activated Sepharose, and the binding of the purified preparations to N- and C-terminal-coupled peptide conjugates. These observations indicated that the antibodies were directed to the region of the peptide opposite to the end which was coupled to the carrier protein. Nevertheless, both of the antibody preparations bound equally well to the target cytochrome P450, thus indicating that, in the native protein, the whole of the peptide region is exposed on the surface of cytochrome P450IA1 and is available for binding by the antibodies. The role of this region appears to be the same in both cytochromes P450IA1 and P450IA2, despite the difference in its primary structure in the two cytochromes P450.

Oncogenes: a review of their clinical application

One objective of this review is to sort through and collate the recent data that suggest that human cellular oncogenes, which have been implicated as the etiologic agents in both animal and human malignancies, have also the potential to be employed as clinical tools in the struggle against cancer. For nearly 10 years, reports have been suggesting that advantage can be taken of cellular oncogenes as to their use as diagnostic and prognostic indicators of cancer and eventually as therapeutic cancer agents. It is also the purpose of this review to give an objective evaluation of these predictions. Moreover, this review will try to highlight some of the significant advances in this most rapidly evolving field of biology. Although the enormity of what has been learned about cellular oncogenes is nothing less than impressive, it is the view here that the routine implementation of oncogenes into the clinical setting will not become evident as early as the many predictions had purported.

Effect of Thermus thermophilus elongation factor Ts on the conformation of elongation factor Tu

Affinity labeling in situ of the Thermus thermophilus elongation factor Tu (EF-Tu) nucleotide binding site was achieved with periodate-oxidized GDP (GDPoxi) or GTP (GTPoxi) in the absence and presence of elongation factor Ts (EF-Ts). Lys52 and Lys137, both reacting with GDPoxi and GTPoxi, are located in the nucleotide binding region. In the absence of EF-Ts Lys137 and to a lesser extent Lys52 were accessible to the reaction with GTPoxi. GDPoxi reacted much more efficiently with Lys52 than with Lys137 under these conditions [Peter, M. E., Wittman-Liebold, B. & Sprinzl, M. (1988) Biochemistry 27, 9132-9138]. In the presence of EF-Ts, GDPoxi reacted more efficiently with Lys137 than with Lys52, indicating that the interaction of EF-Ts with EF-Tu.GDPoxi induces a conformation resembling that of the EF-Tu.GDPoxi complex in the absence of EF-Ts. Binding of EF-Ts to EF-Tu.GDP enhances the accessibility of the Arg59-Gly60 peptide bond of EF-Tu to trypsin cleavage. Hydrolysis of this peptide bond does not interfere with the ability of EF-Ts to bind to EF-Tu. EF-Ts is protected against trypsin cleavage by interaction with EF-Tu.GDP. High concentrations of EF-Ts did not interfere significantly with aminoacyl-tRNA.EF-Tu.GTP complex formation.

Effects of adenosine 3':5'-cyclic monophosphate and guanine nucleotides on calcium-evoked ACTH release from electrically permeabilized AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the investigation of adenosine 3':5'-cyclic monophosphate (cyclic AMP)-mediated enhancement of calcium-evoked adrenocorticotrophin (ACTH) secretion. 2. AtT-20 cells were permeabilized by subjecting the cells to intense electric fields. Exposure of permeabilized cells to calcium (1 mM) in the external medium significantly stimulated ACTH secretion over the first 20 min of exposure. This calcium-stimulated ACTH secretion was dependent upon the presence of MgATP (5 mM). 3. The amount of ACTH secreted, in a 20 min incubation at 37 degrees C, from permeabilized cells depended upon the free calcium concentration in the permeabilization medium. Calcium stimulated ACTH secretion from permeabilized cells in the concentration range of 10(-7)-10(-5) M (half maximal = 7 x 10(-7) M). Cyclic AMP(10(-4) M) increased the amount of ACTH secreted at each effective concentration of calcium. However, cyclic AMP did not alter the potency of calcium as a stimulant of ACTH secretion. 4. Guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S, 10(-4) M) stimulated ACTH secretion from permeabilized cells in the absence of calcium and was additive with calcium-evoked ACTH secretion up to a maximum which could be stimulated by calcium acting singly. Guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S, 10(-4) M) inhibited calcium-evoked ACTH secretion from permeabilized cells. 5. GTP-gamma-S stimulated ACTH secretion from permeabilized cells in a concentration-dependent manner. The nucleotide significantly stimulated ACTH secretion at concentrations of 10-'M and above. Cyclic AMP (10-4 M) increased the amount of ACTH secretion evoked by effective concentrations of GTP-y-S.6. The results of the present study support the hypothesis that, in AtT-20 cells, cyclic AMP is acting at some site, distal to calcium entry, which modulates the ability of an increase in cytosolic calcium concentration to stimulate ACTH secretion. One such site may be a GTP-binding protein which the present study suggests may mediate the effects of calcium upon the secretory apparatus. These GTP-binding proteins may be a target for regulation by cyclic AMP.

The G protein connection: molecular basis of membrane association

Two distinct types of lipid modification, myristoylation and isoprenylation, are critical for membrane association of heterotrimeric G proteins. Elucidation of the molecular basis for G protein membrane association has important implications for understanding G protein structure and function, and is relevant to potential therapeutic approaches to AIDS and cancer.

Overexpression of RPI1, a novel inhibitor of the yeast Ras-cyclic AMP pathway, down-regulates normal but not mutationally activated ras function

A high-copy-number plasmid genomic library was screened for genes that when overexpressed down-regulate Ras protein activity in Saccharomyces cerevisiae. We report on the structure and characterization of one such gene, RPI1, which potentially encodes a novel 46-kDa negative regulator of the Ras-cyclic AMP pathway. Three lines of evidence suggest that the RPI1 gene product operates upstream to negatively regulate the activity of normal but not mutationally activated Ras proteins: (i) overexpressed RPI1 lowers cyclic AMP levels in wild-type yeast cells but not in yeast cells carrying the RAS2Val-19 mutation, (ii) overexpressed RPI1 suppresses the heat shock sensitivity phenotype induced by overexpression of normal RAS2 but does not suppress the same phenotype induced by RAS2Val-19, and (iii) disruption of RPI1 results in a heat shock sensitivity phenotype which can be suppressed by mutations that lower normal Ras activity. Thus, RPI1 appears to encode an inhibitor of Ras activity that shares a common feature with Ras GTPase-activating proteins in that it fails to down-regulate activated RAS2Val-19 function. We present evidence that the down-regulatory effect of RPI1 requires the presence of one of the two Ras GTPase activators, IRA1 and IRA2.

Identification of FAP locus genes from chromosome 5q21

Recent studies suggest that one or more genes on chromosome 5q21 are important for the development of colorectal cancers, particularly those associated with familial adenomatous polyposis (FAP). To facilitate the identification of genes from this locus, a portion of the region that is tightly linked to FAP was cloned. Six contiguous stretches of sequence (contigs) containing approximately 5.5 Mb of DNA were isolated. Subclones from these contigs were used to identify and position six genes, all of which were expressed in normal colonic mucosa. Two of these genes (APC and MCC) are likely to contribute to colorectal tumorigenesis. The MCC gene had previously been identified by virtue of its mutation in human colorectal tumors. The APC gene was identified in a contig initiated from the MCC gene and was found to encode an unusually large protein. These two closely spaced genes encode proteins predicted to contain coiled-coil regions. Both genes were also expressed in a wide variety of tissues. Further studies of MCC and APC and their potential interaction should prove useful for understanding colorectal neoplasia.

Reconstitution of GTP-binding Sar1 protein function in ER to Golgi transport

In the yeast secretory pathway, two genes SEC12 and SAR1, which encode a 70-kD integral membrane protein and a 21-kD GTP-binding protein, respectively, cooperate in protein transport from the ER to the Golgi apparatus. In vivo, the elevation of the SAR1 dosage suppresses temperature sensitivity of the sec12 mutant. In this paper, we show cell-free reconstitution of the ER-to-Golgi transport that depends on both of these gene products. First, the membranes from the sec12 mutant cells reproduce temperature sensitivity in the in vitro ER-to-Golgi transport reaction. Furthermore, the addition of the Sar1 protein completely suppresses this temperature-sensitive defect of the sec12 membranes. The analysis of Sar1p partially purified by E. coli expression suggests that GTP hydrolysis is essential for Sar1p to execute its function.

Double-pulse calcium channel current facilitation in adult rat sympathetic neurones

1. Double-pulse facilitation of Ca2+ channel currents in enzymatically dispersed adult rat superior cervical ganglion neurones was investigated using the whole-cell variant of the patch-clamp technique. Voltage-clamp recordings were performed at room temperature (21-24 degrees C) in solutions designed to isolate Ca2+ channel currents. 2. Ba2+ currents, elicited by a 0 mV test pulse, were increased in amplitude when preceded by a 40 ms pulse to voltages greater than 0 mV. The magnitude of facilitation was dependent on pre-pulse voltage and reached a maximum of 50% (i.e. 1.5 x the current amplitude elicited without a pre-pulse) at a pre-pulse voltage of +80 mV. Half-maximal facilitation occurred at about +25 mV. A small decrease (-6%) in test pulse amplitude was present at pre-pulse voltages between -40 and 0 mV. The magnitude of facilitation was also dependent on test pulse voltage. Facilitation was greatest between test pulse voltages of -10 and 0 mV. 3. Facilitation slowly decreased during prolonged (1 h) dialysis of the neurone even though the Ba2+ current amplitude was well maintained. 4. Increasing the pre-pulse duration over the range 0-120 ms produced an exponential increase in facilitation with a time constant of 17.3 ms. Conversely, lengthening the interpulse duration over the range 5-915 ms, while maintaining a constant pre-pulse amplitude and duration, resulted in an exponential decrease in facilitation with a time constant of 197 ms. 5. At a test potential of 0 mV, the decay of the facilitated Ba2+ current component was fitted to a double exponential function with time constants of about 25 and 150 ms. The time constants had little pre-pulse voltage dependence between +30 to +80 mV. 6. The initial rising phase of both the control and facilitated Ba2+ current were reasonably well described by a single exponential (tau rise) after a delay of 300 microseconds. The tau rise versus test pulse potential relationship was 'bell shaped' over the test pulse voltage of -20 to +30 mV reaching a maximum near -5 mV. tau rise was similar for control and facilitated currents except at potentials greater than +10 mV where the rise of the facilitated current was accelerated. 7. Control and facilitated activation curves, as derived from tail current amplitudes, were described by the sum of two Boltzmann functions. A facilitating pre-pulse produced an increase in the proportion of the current contributed by the component activated at more hyperpolarized test potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

A stimulatory GDP/GTP exchange protein for smg p21 is active on the post-translationally processed form of c-Ki-ras p21 and rhoA p21

We have purified a stimulatory GDP/GTP exchange protein for smg p21A and -B, ras p21-like small GTP-binding proteins (G proteins), cloned its cDNA, and named it GDP dissociation stimulator (smg p21 GDS). We show here that smg p21 GDS is active not only on smg p21A and -B but also on c-Ki-ras p21 and rhoA p21, all of which are post-translationally processed. Furthermore, we show that smg p21 GDS is inactive on the post-translationally unprocessed form of these proteins and on the post-translationally unprocessed form of c-Ha-ras p21 and smg p25A. All of the small G proteins recognized by smg p21 GDS have a cDNA-predicted C-terminal "CAAX" motif (where C is cysteine, A is an aliphatic amino acid, and X is any amino acid) and a polybasic region upstream of this motif. These results suggest that smg p21 GDS is at least active on a group of small G proteins having these unique C-terminal structures. Moreover, they suggest that the C-terminal post-translational processing of these small G proteins, by farnesylation or geranylgeranylation of the C-terminal cysteine residue, removal of amino acids in positions denoted "AAX", and carboxyl methylation of the exposed cysteine residue, is important for the smg p21 GDS action.

Overexpression of RPI1, a novel inhibitor of the yeast Ras-cyclic AMP pathway, down-regulates normal but not mutationally activated ras function

A high-copy-number plasmid genomic library was screened for genes that when overexpressed down-regulate Ras protein activity in Saccharomyces cerevisiae. We report on the structure and characterization of one such gene, RPI1, which potentially encodes a novel 46-kDa negative regulator of the Ras-cyclic AMP pathway. Three lines of evidence suggest that the RPI1 gene product operates upstream to negatively regulate the activity of normal but not mutationally activated Ras proteins: (i) overexpressed RPI1 lowers cyclic AMP levels in wild-type yeast cells but not in yeast cells carrying the RAS2Val-19 mutation, (ii) overexpressed RPI1 suppresses the heat shock sensitivity phenotype induced by overexpression of normal RAS2 but does not suppress the same phenotype induced by RAS2Val-19, and (iii) disruption of RPI1 results in a heat shock sensitivity phenotype which can be suppressed by mutations that lower normal Ras activity. Thus, RPI1 appears to encode an inhibitor of Ras activity that shares a common feature with Ras GTPase-activating proteins in that it fails to down-regulate activated RAS2Val-19 function. We present evidence that the down-regulatory effect of RPI1 requires the presence of one of the two Ras GTPase activators, IRA1 and IRA2.

Biogenesis of secretory granules. Implications arising from the immature secretory granule in the regulated pathway of secretion

In endocrine cells the regulated secretion of hormones, peptides, enzymes and neurotransmitters into the external medium occurs when mature secretory granules fuse with the plasma membrane. Secretory granules form at the trans-Golgi network (TGN) by envelopment of the dense-core aggregate of regulated secretory proteins by a specific membrane. The secretory granules initially formed at the TGN, referred to here as immature secretory granules, are morphologically and biochemically distinct from mature secretory granules. The functional similarities and differences between the immature secretory granule and the mature secretory granule, and the events involved in the maturation of the secretory granules are briefly discussed.

A severely truncated form of translational initiation factor 2 supports growth of Escherichia coli

We have constructed strains carrying null mutations in the chromosomal copy of the gene for translational initiation factor (IF) 2 (infB). A functional copy of the infB gene is supplied in trans by a thermosensitive lysogenic lambda phage integrated at att lambda. These strains enabled us to test in vivo the importance of different structural elements of IF2 expressed from genetically engineered plasmid constructs. We found that, as expected, the gene for IF2 is essential. However, a protein consisting of the C-terminal 55,000 Mr fragment of the wild-type IF2 protein is sufficient to allow growth when supplied in excess. This result suggests that the catalytic properties are localized in the C-terminal half of the protein, which includes the G-domain, and that this fragment is sufficient to complement the IF2 deficiency in the infB deletion strain.

Bacterial lipopolysaccharide-stimulated GTPase activity in RAW 264.7 macrophage membranes

The molecular mechanisms surrounding the toxicity and high mortality rate that accompany the release of bacterial lipopolysaccharide (LPS) are unclear, although its potent activity suggests that an amplification system is involved. Because previous studies suggest that a guanine-nucleotide-binding protein (G-protein) may participate in LPS action, we have evaluated the effects of LPS on GTPase activity in membranes isolated from macrophage (RAW 264.7) and fibroblast (B82L) cell lines. LPS induced substantial GTPase activation (200-300% above basal), and kinetic analyses indicated that the maximal LPS-stimulated increase in velocity is observed within 15 min, that it is a low-Km (for GTP) activity, that it can be enhanced by ammonium sulphate, and that it appears to be pertussis toxin-insensitive. Moreover, the LPS-enhanced GTPase activity was not antagonized by phosphatase/ATPase inhibitors such as p-nitrophenyl phosphate, ouabain, bafilomycin or N-ethylmaleimide, and in fact was potentiated by the addition of ATP or ADP. Conversely, the LPS precursor, lipid X, which can decrease the lethal effects of LPS, was found to dose-dependently inhibit the LPS-mediated stimulation of GTPase activity. Half-maximal inhibition was seen at the same lipid X/LPS ratio known to be effective in vivo, i.e. 1:1(w/w). These effects appear to be specific because other phospholipids, detergents and glycosides neither stimulated basal, nor inhibited LPS-induced, GTPase activity. These data suggest the involvement of a GTPase in LPS action, and indicate that lipid X may act to directly antagonize LPS at this level.

GTP-binding membrane proteins in activated and differentiating T cells

We have earlier reported changes in the GTP binding of several membrane proteins including Gs alpha and Gi alpha during thymic differentiation of T cells. Using an [alpha-32P]GTP-photoaffinity labeling technique we have studied the pattern of GTP binding proteins in activated and resting T lymphocytes and in T cells induced to differentiate by TPA. The GTP binding proteins in mitogen-activated T cells resembled those seen in leukemia T cell lines. Treatment of Jurkat, but not of CCRF-CEM, T cells with TPA caused increased GTP-labeling of a 34 kDa protein and Gi alpha. The GTP labeling pattern in TPA-treated Jurkat cells resembled that in resting T lymphocytes. TPA induced de novo expression of functional TCR/CD3 on CCRF-CEM and downregulation of TCR/CD3 on Jurkat cells but these changes did not correlate with the altered GTP-labeling patterns.

A ras effector domain mutant which is temperature sensitive for cellular transformation: interactions with GTPase-activating protein and NF-1

A series of v-rasH effector domain mutants were analyzed for their ability to transform rat 2 cells at either low or high temperatures. Three mutants were found to be significantly temperature sensitive: Ile-36 changed to Leu, Ser-39 changed to Cys (S39C), and Arg-41 changed to Leu. Of these, the codon 39 mutant (S39C) showed the greatest degree of temperature sensitivity. When the same mutation was analyzed in the proto-oncogene form of ras(c-rasH), this gene was also found to be temperature sensitive for transformation. Biochemical analysis of the proteins encoded by v-rasH(S39C) and c-rasH(S39C) demonstrated that the encoded p21ras proteins were stable and bound guanine nucleotides in vivo at permissive and nonpermissive temperatures. On the basis of these findings, it is likely that the temperature-sensitive phenotype results from an inability of the mutant (S39C) p21ras to interact properly with the ras target effector molecule(s) at the nonpermissive temperature. We therefore analyzed the interaction between the c-rasH(S39C) protein and the potential target molecules GTPase-activating protein (GAP) and the GAP-related domain of NF-1, on the basis of stimulation of the mutant p21ras GTPase activity by these molecules in vitro. Assays conducted across a range of temperatures revealed no temperature sensitivity for stimulation of the mutant protein, compared with that of authentic c-rasH protein. We conclude that for this mutant, there is a dissociation between the stimulation of p21ras GTPase activity by GAP and the GAP-related domain NF-1 and their potential target function. Our results are also consistent with the existence of a distinct, as-yet-unidentified effector for mammalian ras proteins.

Identification of a GTPase-activating protein homolog in Schizosaccharomyces pombe

Loss of function of the Schizosaccharomyces pombe gap1 gene results in the same phenotypes as those caused by an activated ras1 mutation, i.e., hypersensitivity to the mating factor and inability to perform efficient mating. Sequence analysis of gap1 indicates that it encodes a homolog of the mammalian Ras GTPase-activating protein (GAP). The predicted gap1 gene product has 766 amino acids with relatively short N- and C-terminal regions flanking the conserved core sequence of GAP. Genetic analysis suggests that S. pombe Gap1 functions primarily as a negative regulator of Ras1, like S. cerevisiae GAP homologs encoded by IRA1 and IRA2, but is unlikely to be a downstream effector of the Ras protein, a role proposed for mammalian GAP. Thus, Gap1 and Ste6, a putative GDP-GTP-exchanging protein for Ras1 previously identified, appear to play antagonistic roles in the Ras-GTPase cycle in S. pombe. Furthermore, we suggest that this Ras-GTPase cycle involves the ra12 gene product, another positive regulator of Ras1 whose homologs have not been identified in other organisms, which could function either as a second GDP-GTP-exchanging protein or as a factor that negatively regulates Gap1 activity.

Structural heterogeneity of membrane receptors and GTP-binding proteins and its functional consequences for signal transduction

Recent information obtained, mainly by recombinant cDNA technology, on structural heterogeneity of hormone and transmitter receptors, of GTP-binding proteins (G-proteins) and, especially, of G-protein-linked receptors is reviewed and the implications of structural heterogeneity for diversity of hormone and transmitter actions is discussed. For the future, three-dimensional structural analysis of membrane proteins participating in signal transmission and transduction pathways is needed in order to understand the molecular basis of allosteric regulatory mechanisms governing the interactions between these proteins including hysteretic properties and cell-cybernetic aspects.

Inhibition of the action of a stimulatory GDP/GTP exchange protein for smg p21 by acidic membrane phospholipids

A stimulatory GDP/GTP exchange protein for smg p21 (smg p21 GDS) stimulated the dissociation of GDP from smg p21B. This reaction was inhibited by acidic membrane phospholipids such as phosphatidylinositol, phosphatidylinositol-4-monophosphate, phosphatidylinositol-4,5-bisphosphate, phosphatidic acid, and phosphatidylserine but not by phosphatidylcholine or phosphatidylethanolamine. These acidic phospholipids inhibited the smg p21 GDS action in a manner competitive with both smg p21 GDS and smg p21B. smg p21 GDS has other actions to inhibit the binding of smg p21B to membranes and to induce the dissociation of prebound smg p21B from the membranes. The acidic phospholipids also inhibited these two actions of smg p21 GDS. smg p21B has a polybasic region and an isoprenoid moiety in its C-terminal region which are necessary for its membrane-binding activity and its sensitivity to the smg p21 GDS actions. Therefore, it is possible that acidic membrane phospholipids interact with this polybasic region and thereby inhibit the smg p21 GDS actions.

G alpha 12 and G alpha 13 subunits define a fourth class of G protein alpha subunits

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) are central to the signaling processes of multicellular organisms. We have explored the diversity of the G protein subunits in mammals and found evidence for a large family of genes that encode the alpha subunits. Amino acid sequence comparisons show that the different alpha subunits fall into at least three classes. These classes have been conserved in animals separated by considerable evolutionary distances; they are present in mammals, Drosophila, and nematodes. We have now obtained cDNA clones encoding two murine alpha subunits, G alpha 12 and G alpha 13, that define a fourth class. The translation products are predicted to have molecular masses of 44 kDa and to be insensitive to ADP-ribosylation by pertussis toxin. They share 67% amino acid sequence identity with each other and less than 45% identity with other alpha subunits. Their transcripts can be detected in every tissue examined, although the relative levels of the G alpha 13 message appear somewhat variable.

Yeast BUD5, encoding a putative GDP-GTP exchange factor, is necessary for bud site selection and interacts with bud formation gene BEM1

Cells of the yeast S. cerevisiae choose bud sites in an axial or bipolar spatial pattern depending on their cell type. We have identified a gene, BUD5, that resembles BUD1 and BUD2 in being required for both patterns; bud5- mutants also exhibit random budding in all cell types. The BUD5 nucleotide sequence predicts a protein of 538 amino acids that has similarity to the S. cerevisiae CDC25 product, an activator of RAS proteins that catalyzes GDP-GTP exchange. Two potential targets of BUD5 are known: BUD1 (RSR1) and CDC42, proteins involved in bud site selection and bud formation, respectively, that have extensive similarity to RAS. We also show that BUD5 interacts functionally with a gene, BEM1, that is required for bud formation. This interaction provides further support for the view that products involved in bud site selection guide the positioning of a complex necessary for bud formation.

The smg GDS-induced activation of smg p21 is initiated by cyclic AMP-dependent protein kinase-catalyzed phosphorylation of smg p21

We have shown that smg p21B is phosphorylated by cyclic AMP-dependent protein kinase (protein kinase A) and that membrane acidic phospholipids such as phosphatidic acid and phosphatidylinositol markedly inhibit the smg GDS-induced activation of smg p21B. However, we show here that phosphatidic acid and phosphatidylinositol exhibit a less inhibitory effect on the smg GDS-induced activation of the phosphorylated form of smg p21B. Thus, in the presence of membrane acidic phospholipids, the smg GDS-induced activation of smg p21B is totally dependent on the protein kinase A-catalyzed phosphorylation of smg p21B. Since smg p21B is located on the membranes in resting cells, it is likely that the smg GDS-induced activation of smg p21B is initiated by the protein kinase A activation.

Functional cloning of BUD5, a CDC25-related gene from S. cerevisiae that can suppress a dominant-negative RAS2 mutant

By searching for genes that behave like CDC25 of S. cerevisiae in their ability to counteract a dominant-negative RAS2 mutant in a wild-type RAS-dependent manner, we have isolated a CDC25-like homolog, BUD5. BUD5 is tightly linked to the MAT locus. Although overexpressed BUD5 cannot substitute for CDC25 function, we present evidence that its gene product can bind to the guanine nucleotide binding-deficient RAS2val19ala22 gene product and thereby counteract its dominant-negative effect. We propose that BUD5 is a member of a family of CDC25-related genes that encode activators of RAS and RAS-like proteins.

Yeast alpha-mating factor receptor-linked G-protein signal transduction suppresses Ras-dependent activity

Homologues of mammalian Ras conserved in Saccharomyces cerevisiae mediate glucose-stimulated cyclic AMP formation and we used this response to test for regulation of yeast Ras activity by the alpha-mating factor signal transduction pathway. alpha-Mating factor suppresses glucose-stimulated cyclic AMP formation by up to 57 +/- 12.6% (n = 5) and similar inhibition was observed in four different yeast strains (MATa cells). Moreover, this response is potent (IC50 = 0.14 +/- 0.19 microM (n = 4)), rapid (maximal within 1-2 min), and displays an absolute requirement for both the alpha-mating factor receptor (STE2) and associated G-protein beta-subunit (STE4). Inhibition appears independent of both phosphodiesterase activation and alpha-mating factor-stimulated cytoplasmic alkalinization. Also, basal cyclic AMP levels are unaffected by pheromone. This is the first demonstration that a cell-surface receptor linked to a heterotrimeric G-protein can suppress Ras-dependent activity and could provide important insight into mechanisms controlling p21ras in man. Inhibition of Ras-dependent cyclic AMP formation could also be a key event facilitating responses characteristic of yeast mating.

GTPase domains of ras p21 oncogene protein and elongation factor Tu: analysis of three-dimensional structures, sequence families, and functional sites

GTPase domains are functional and structural units employed as molecular switches in a variety of important cellular functions, such as growth control, protein biosynthesis, and membrane traffic. Amino acid sequences of more than 100 members of different subfamilies are known, but crystal structures of only mammalian ras p21 and bacterial elongation factor Tu have been determined. After optimal superposition of these remarkably similar structures, careful multiple sequence alignment, and calculation of residue-residue interactions, we analyzed the two subfamilies in terms of structural conservation, sequence conservation, and residue contact strength. There are three main results. (i) A structure-based alignment of p21 and elongation factor Tu. (ii) The definition of a common conserved structural core that may be useful as the basis of model building by homology of the three-dimensional structure of any GTPase domain. (iii) Identification of sequence regions, other than the effector loop and the nucleotide binding site, that may be involved in the functional cycle: they are loop L4, known to change conformation after GTP hydrolysis; helix alpha 2, especially Arg-73 and Met-67 in ras p21; loops L8 and L10, including ras p21 Arg-123, Lys-147, and Leu-120; and residues located spatially near the N and C termini. These regions are candidate sites for interaction either with the GTP/GDP exchange factor, with a GTPase-affected function, or with a molecule delivered to a destination site with the aid of the GTPase domain.

Molecular cloning of a GTPase activating protein specific for the Krev-1 protein p21rap1

The rap1/Krev-1 gene encodes a ras-related protein that suppresses transformation by ras oncogenes. We have purified an 88 kd GTPase activating protein (GAP), specific for the rap1/Krev-1 gene product, from bovine brain. Based on partial amino acid sequences obtained from this protein, a 3.3 kb cDNA was isolated from a human brain library. Expression of the cDNA in insect Sf9 cells resulted in high level production of an 85-95 kd rap1GAP that specifically stimulated the GTPase activity of p21rap1. The complete deduced amino acid sequence is not homologous to any known protein sequences, including GAPs specific for p21ras. Northern and Western blotting analysis indicate that rap1GAP is not ubiquitously expressed and appears most abundant in fetal tissues and certain tumor cell lines, particularly the Wilms' kidney tumor, SK-NEP-1, and the melanoma, SK-MEL-3, cell lines.

Multiple intercellular signalling systems control the development of the Caenorhabditis elegans vulva

Developmental, genetic and molecular studies indicate that multiple intercellular signalling systems interact to specify the types and spatial patterns of cells generated during the formation of the vulva of the nematode Caenorhabditis elegans. Two classes of evolutionarily conserved transmembrane receptors and a Ras protein function in these signalling systems. The biology of vulval development provides a framework for understanding how cell interactions control the development of animals as diverse as nematodes, insects and mammals.

sar1, a gene from Schizosaccharomyces pombe encoding a protein that regulates ras1

Proper ras1 function is required for normal sexual function in the yeast Schizosaccharomyces pombe. We have found a gene in S. pombe, sar1, that encodes a product capable of regulating ras1 function. sar1 is a member of an expanding family of RAS GTPase-activating proteins (GAPs) that includes mammalian GAP, the yeast Saccharomyces cerevisiae IRA proteins, and the product of the human neurofibromatosis locus, NF1 sar1, like these other proteins, can complement the loss of IRA function in S. cerevisiae. Computer analysis shows that the highest degree of sequence conservation is restricted to a very small number of diagnostic residues represented by the motif Phe-Leu-Arg-X-X-X-Pro-Ala-X-X-X-Pro. We find no evidence that sar1 is required for the effector function of ras1.

G proteins and G-protein-coupled receptors: structure, function and interactions

The G protein family continues to grow and at least 15 heterotrimeric G proteins have now been identified. This review deals with the nature of the functional domains of the members of the G-protein-coupled receptor family as well as the associated G proteins.

Ionic channels: modulation by G proteins and by phosphorylation

The gating of ion channels may be modulated by G proteins or by phosphorylation. Direct coupling between G proteins and ion channels has been shown in excised patches of membrane. Steps must now be taken to study the protein domains of G proteins and ion channels involved in the mutual interaction. The concept of channel modulation by protein kinases has recently been extended to include additional types of ion channel.

Identification of a GTPase-activating protein homolog in Schizosaccharomyces pombe

Loss of function of the Schizosaccharomyces pombe gap1 gene results in the same phenotypes as those caused by an activated ras1 mutation, i.e., hypersensitivity to the mating factor and inability to perform efficient mating. Sequence analysis of gap1 indicates that it encodes a homolog of the mammalian Ras GTPase-activating protein (GAP). The predicted gap1 gene product has 766 amino acids with relatively short N- and C-terminal regions flanking the conserved core sequence of GAP. Genetic analysis suggests that S. pombe Gap1 functions primarily as a negative regulator of Ras1, like S. cerevisiae GAP homologs encoded by IRA1 and IRA2, but is unlikely to be a downstream effector of the Ras protein, a role proposed for mammalian GAP. Thus, Gap1 and Ste6, a putative GDP-GTP-exchanging protein for Ras1 previously identified, appear to play antagonistic roles in the Ras-GTPase cycle in S. pombe. Furthermore, we suggest that this Ras-GTPase cycle involves the ra12 gene product, another positive regulator of Ras1 whose homologs have not been identified in other organisms, which could function either as a second GDP-GTP-exchanging protein or as a factor that negatively regulates Gap1 activity.

Molecular mechanisms in neurotransmitter release

The vesicle hypothesis of neurotransmitter release was first formulated in the 1950s, but only recently have the molecular mechanisms involved in neurotransmitter release begun to be elucidated. This short review summarizes current concepts on neurosecretion and the available information on synaptic vesicle exocytosis.

A ras effector domain mutant which is temperature sensitive for cellular transformation: interactions with GTPase-activating protein and NF-1

A series of v-rasH effector domain mutants were analyzed for their ability to transform rat 2 cells at either low or high temperatures. Three mutants were found to be significantly temperature sensitive: Ile-36 changed to Leu, Ser-39 changed to Cys (S39C), and Arg-41 changed to Leu. Of these, the codon 39 mutant (S39C) showed the greatest degree of temperature sensitivity. When the same mutation was analyzed in the proto-oncogene form of ras(c-rasH), this gene was also found to be temperature sensitive for transformation. Biochemical analysis of the proteins encoded by v-rasH(S39C) and c-rasH(S39C) demonstrated that the encoded p21ras proteins were stable and bound guanine nucleotides in vivo at permissive and nonpermissive temperatures. On the basis of these findings, it is likely that the temperature-sensitive phenotype results from an inability of the mutant (S39C) p21ras to interact properly with the ras target effector molecule(s) at the nonpermissive temperature. We therefore analyzed the interaction between the c-rasH(S39C) protein and the potential target molecules GTPase-activating protein (GAP) and the GAP-related domain of NF-1, on the basis of stimulation of the mutant p21ras GTPase activity by these molecules in vitro. Assays conducted across a range of temperatures revealed no temperature sensitivity for stimulation of the mutant protein, compared with that of authentic c-rasH protein. We conclude that for this mutant, there is a dissociation between the stimulation of p21ras GTPase activity by GAP and the GAP-related domain NF-1 and their potential target function. Our results are also consistent with the existence of a distinct, as-yet-unidentified effector for mammalian ras proteins.

Kinesin-related gene unc-104 is required for axonal transport of synaptic vesicles in C. elegans

unc-104 encodes a novel kinesin paralog that may act as a microtubule-based motor in the nervous system. Neuronal cell lineages and axonogenesis are normal in unc-104 null mutants, but axons have few synaptic vesicles and make only a few small synapses. By contrast, neuron cell bodies have surfeits of similar vesicles tethered together within the cytoplasm. Based on behavioral and cellular phenotypes, we suggest that UNC-104 is a neuron-specific motor used for anterograde translocation of synaptic vesicles along axonal microtubules. Other membrane-bounded organelles are transported normally.

Evolutionary grouping of the RAS-protein family

Over 50 proteins related to the mammalian H-, K-, and N-RAS GTP binding and hydrolyzing proteins are known. These relatively low molecular weight proteins are usually grouped into four subfamilies, termed true RAS, RAS-like, RHO, and RAB/YPT, based on the presence of shared amino acid sequence motifs in addition to those involved in guanine nucleotide binding. Here, we apply parsimony analysis to the overall amino acid sequences of these proteins to infer possible phylogenetic relationships among them.

Diversity of G proteins in signal transduction

The heterotrimeric guanine nucleotide-binding proteins (G proteins) act as switches that regulate information processing circuits connecting cell surface receptors to a variety of effectors. The G proteins are present in all eukaryotic cells, and they control metabolic, humoral, neural, and developmental functions. More than a hundred different kinds of receptors and many different effectors have been described. The G proteins that coordinate receptor-effector activity are derived from a large gene family. At present, the family is known to contain at least sixteen different genes that encode the alpha subunit of the heterotrimer, four that encode beta subunits, and multiple genes encoding gamma subunits. Specific transient interactions between these components generate the pathways that modulate cellular responses to complex chemical signals.

Mutant alpha subunits of Gi2 inhibit cyclic AMP accumulation

One or more of three Gi proteins, Gi1-3, mediates hormonal inhibition of adenylyl cyclase. Whether this inhibition is mediated by the alpha or by the beta gamma subunits of Gi proteins is unclear. Mutations inhibiting the intrinsic GTPase activity of another G protein, the stimulatory regulator of adenylyl cyclase (Gs), constitutively activate it by replacing either of two conserved amino acids in its alpha subunit (alpha s). These mutations create the gsp oncogene which is found in human pituitary and thyroid tumours. In a second group of human endocrine tumours, somatic mutations in the alpha subunit of Gi2 replace a residue cognate to one of those affected by gsp mutations. This implies that the mutations convert the alpha i2 gene into a dominantly acting oncogene, called gip2, and that the mutant alpha i2 subunits are constitutively active. We have therefore assessed cyclic AMP accumulation in cultured cells which stably or transiently express exogenous wild-type alpha i2 complementary DNA or either of two mutant alpha i2 cDNAs. The results show that putatively oncogenic mutations in alpha i2 constitutively activate the protein's ability to inhibit cAMP accumulation.