Enhancement of guanine-nucleotide exchange activity of C3G for Rap1 by the expression of Crk, CrkL, and Grb2

Crk is an adaptor protein that consists almost entirely of SH2 and SH3 domains. We have previously demonstrated, by using in vivo and in vitro systems, that C3G, which was identified as a Crk SH3 domain-binding guanine nucleotide exchange factor, specifically activates Rap1. C3G also binds to other adaptor proteins, including CrkL and Grb2. In the present study, we analyzed the effect of Crk, CrkL, and Grb2 on the C3G-Rap1 pathway. Expression of Crk, CrkL, and Grb2 with C3G in Cos1 cells significantly increased the ratio of GTP/GDP bound to Rap1. Both the SH2 and SH3 domains of Crk were required for this activity. However, Crk did not stimulate the guanine nucleotide exchange activity of C3G for Rap1 in vitro, suggesting that Crk does not activate C3G by an allosteric mechanism. The requirement of the SH2 domain of Crk for the enhancement of guanine nucleotide exchange activity for Rap1 could be compensated for by the addition of a farnesylation signal to Crk, indicating that Crk enhanced the guanine nucleotide exchange activity of C3G by membrane recruitment of C3G. These results demonstrate that Crk, CrkL, and Grb2 positively modulate the C3G-Rap1 pathway primarily by recruiting C3G to the cell membrane.

Association of Krev-1/rap1a with Krit1, a novel ankyrin repeat-containing protein encoded by a gene mapping to 7q21-22

Krev-1/rap1A is an evolutionarily conserved Ras-family GTPase whose cellular function remains unclear, but which has been proposed to function as a tumor suppressor gene, and may act as a Ras antagonist. To elucidate Krev-1 activity, we have used LexA-Krev-1 in a two-hybrid screen of a HeLa cell cDNA library. Of the two cDNA classes isolated, one contained a single isolate encoding the known Krev-1 interactor Raf, while the second contained multiple isolates coding for a previously undescribed protein which we have designated Kritl (for Krev Interaction Trapped 1). The full length Krit1 cDNA encodes a protein of 529 amino acids, with an amino-terminal ankyrin repeat domain and a novel carboxy-terminal domain required for association with Krit1. Krit1 interacted strongly with Krev-1 but only weakly with Ras, suggesting it might specifically regulate Krev-1 activities. Krit1 mRNA and protein are expressed endogenously at low levels, with tissue specific variation. Intriguingly, the Krit cDNA has been mapped by FISH to chromosome 7q21-22, a region known to be frequently deleted or amplified in multiple forms of cancer.

Biochemical characterization of C3G: an exchange factor that discriminates between Rap1 and Rap2 and is not inhibited by Rap1A(S17N)

A catalytically active fragment of the Rap-specific guanine-nucleotide exchange factor C3G was expressed in E coli. It was purified and its interaction with GTP-binding proteins was investigated using fluorescence spectroscopy. C3G stimulates GDP dissociation from Rap1, but not from Rap2, neither from Bud1, which is believed to be the yeast homologue of Rap1 nor from all other proteins of the human Ras-subfamily. Like the corresponding fragment from CDC25Mm, the increase in the GDP dissociation rate is linear with increasing concentration of Rap1A x GDP up to 100 microM, indicating an apparent K(M) higher than 100 microM. Unlike the Ras-CDC25Mm system, the Rap1A(S17N) mutant does not inhibit the C3G-activated guanine nucleotide dissociation from wild-type Rap1A in vitro. These data suggest that Rap1A(S17N) is unlikely to titrate away C3G in vivo, the proposed mechanism by which S17N-mutants exert their dominant negative effects.

Ras effectors and their role in mitogenesis and oncogenesis

Ras proteins are membrane-bound GTP-binding proteins that play a critical role in the control of cell growth. Through a large number of genetic and biochemical studies it is becoming increasingly evident that the biological activity of Ras proteins is mediated by multiple signaling pathways. This review provides an account of the target proteins that interact with Ras and the functional consequences of these interactions. The relative contribution of the different Ras effector pathways to the mitogenic and oncogenic effects of Ras are discussed.

Molecular dynamics on complexes of ras-p21 and its inhibitor protein, rap-1A, bound to the ras-binding domain of the raf-p74 protein: identification of effector domains in the raf protein

We have computed the average structures for the ras-p21 protein and its strongly homologous inhibitor protein, rap-1A, bound to the ras-binding domain (RBD) of the raf protein, using molecular dynamics. Our purpose is to determine the differences in structure between these complexes that would result in no mitogenic activity of rap-1A-RBD but full activity of p21-RBD. We find that despite the similarities of the starting structures for both complexes, the average structures differ considerably, indicating that these two proteins do not interact in the same way with this vital target protein. p21 does not undergo major changes in conformation when bound to the RBD, while rap-1A undergoes significant changes in structure on binding to the RBD, especially in the critical region around residue 61. The p21 and rap-1A make substantially different contacts with the RBD. For example, the loop region from residues 55-71 of rap-1a makes extensive hydrogen-bond contacts with the RBD, while the same residues of p21 do not. Comparison of the structures of the RBD in both complexes reveals that it undergoes considerable changes in structure when its structure bond to p21 is compared with that bound to rap-1A. These changes in structure are due to displacements of regular structure (e.g., alpha-helices and beta-sheets) rather than to changes in the specific conformations of the segments themselves. Three regions of the RBD have been found to differ significantly from one another in the two complexes: the binding interface between the two proteins at residues 60 and 70, the region around residues 105-106, and 118-120. These regions may constitute effector domains of the RBD whose conformations determine whether or not mitogenic signal transduction will occur.

Three-dimensional structure of the Ras-interacting domain of RalGDS

The Ras-interacting domains of the the protein-kinase Raf and the Ral guanine nucleotide dissociation stimulator, RalGDS, lack extensive sequence similarity, but their overall three-dimensional structures are very similar to each other. Mutational analysis indicated that three residues in the RalGDS domain are critical for its interaction with Ras.

Dimerization of Cdc25p, the guanine-nucleotide exchange factor for Ras from Saccharomyces cerevisiae, and its interaction with Sdc25p

The oligomerization state of Cdc25p, the guanine nucleotide exchange factor for ras from yeast, was analyzed using different complementary approaches. The two-hybrid system showed that the C-terminal part of Cdc25p (Cdc25-Ct) can interact with itself but also with Sdc25p-Ct, the corresponding part of Sdc25p, the other guanine exchange factor from yeast. The homotropic interaction of Cdc25p-Ct has been confirmed in yeast using immunoprecipitation experiments with epitope-tagged and beta-galactosidase-fused polypeptides. No other component was required for this interaction, since dimerization was shown to occur with material synthesized in vitro. The size of Cdc25-Ct produced in Escherichia coli has been directly measured on gel filtration columns and corresponds to a dimer. The dimerization domain is localized in the same part of the molecule as the catalytic domain and the portion responsible for membrane localization. The biological relevance of dimerization is still an open question, however by allowing heterodimerization with Sdc25p it could permit a more complex combinatorial regulation of ras in yeast.

RAP1-like binding activity in islet cells corresponds to members of the Sp1 family of transcription factors

Deletion and mutational analyses of the gastrin promoter have identified a binding site for the yeast transcription factor RAP1 relevant for transcriptional activation in islet cells. We here report that the mammalian transcription factors binding to this site in islet cells are the Sp transcription factor members Sp1 and Sp3. Furthermore, functional analyses revealed Sp1- and Sp3-mediated transcriptional activation of gastrin. These data reveal that the zinc finger proteins Sp1 and Sp3 do have similar binding specificities as the multifunctional yeast RAP1 protein.

Immunolocalization of rap1 in the rat parotid gland: detection on secretory granule membranes

The objective of this study was to localize rap1 in the rat parotid gland. Rap1 is a small GTP-binding protein that has been linked to phagocytosis in neutrophils and various functions in platelets. In this study, we used [alpha-32P]-GTP-blot overlay analysis, immunoblot analysis, and immunohistochemistry to identify rap1 in rat parotid gland. The immunohistochemical techniques included immunoperoxidase and widefield microscopy with image deconvolution. Rap1 was identified in the secretory granule membrane (SGM), plasma membrane (PM), and cytosolic (CY) fractions, with the largest signal being in the SGM fraction. The tightly bound vs loosely adherent nature of SGM-associated rap1 was determined using sodium carbonate, and its orientation on whole granules was assessed by trypsin digestion. Rap1 was found to be a tightly bound protein rather than a loosely adherent contaminant protein of the SGM. Its orientation on the cytosolic face of the secretory granule (SG) is of significance in postulating a function for rap1 because exocytosis involves the fusion of the cytoplasmic face of the SG with the cytoplasmic face of the PM, with subsequent release of granule contents (CO). Therefore, the localization and high concentration of rap1 on the SGM and its cytosolic orientation suggest that it may play a role in the regulation of secretion.

Cloning and evaluation of RALGDS as a candidate for the tuberous sclerosis gene TSC1

RALGDS is a 115 kDa protein which was identified by its ability to enhance guanine nucleotide exchange for the ras family member ral. It also binds to activated ras and rap1, and appears to function as part of a signalling complex in downstream events following rap1 activation. Here we report the identification of full-length cDNA clones for human RALGDS, isolated from a brain cDNA library. The predicted protein has strong sequence homology to rat and murine isoforms of RALGDS in the N- and C-terminal regions, but an internal region (aa 250-380) shows relatively high divergence with only 42% identical amino acid residues. The human RALGDS gene is contained within a 30 kb region of 9q34, approximately 200 kb proximal to the ABO gene, within the current critical region for the tuberous sclerosis gene TSC1. Partial genomic structure was determined; it consists of at least 11 exons. Based upon analysis of Southern blots from 110 TSC patients, genomic DNA SSCP analysis, and RT-PCR analysis which demonstrated RNA expression of both alleles in patients from 9q34-linked TSC families using intragenic polymorphisms, we conclude that RALGDS is not likely to be TSC1.

Isolation and characterization of the krev-1 gene, a novel member of ras superfamily in Neurospora crassa: involvement in sexual cycle progression

Genes belonging to the ras superfamily encode low-molecular-weight GTP/GDP-binding proteins that are highly conserved in wide variety of organisms. We used the polymerase chain reaction (PCR) to isolate a novel member of the ras superfamily from the filamentous fungus Neurospora crassa and obtained a mammalian Krev-1 homolog. We named the gene krev-1 and analyzed its structure and function. The krev-1 gene encodes a polypeptide of 225 amino acids, which is nearly 60% homologous to the mammalian Krev-1 p21. The krev-1 gene product (KREV1) is functionally analogous to mammalian Krev-1 p21 and Rsr1p/Bud1p, a Krev-1 homolog from the yeast Saccharomyces cerevisiae. GAL1-driven expression of KREV1 in a wild-type yeast strain resulted in a random budding pattern, as did its mammalian counterpart Krev-1 p21. We disrupted the krev-1 gene by RIP (repeat-induced point mutation), but the krev-1 disruptants showed no abnormalities. By in vitro mutagenesis, we constructed several mutant krev-1 genes (G21V, A68T, and D128A) which mimic constitutively active mutants of Ha-ras, and the krev-1 (K25N) mutant which is analogous to a dominant-negative mutant of Ha-ras. Each mutant gene was introduced into the wild-type strain and the phenotypes were analyzed. We could not observe any difference in vegetative growth between these transformants. When each strain was used as the female in mating tests, the development of perithecia from protoperithecia was inhibited in all cases. The results indicate that the krev-1 gene may be involved in sexual cycle progression.

Telomere length regulation: getting the measure of chromosome ends

Telomeres, the protein-DNA complexes that comprise the ends of linear eukaryotic chromosomes, serve to protect the chromosome ends and allow their complete replication. Telomeres also appear to play an essential role in chromosome segregation. In most organisms telomeric DNA consists of a series of short repeats that are variable in length, but regulated at a fixed average value in the germline. The possible involvement of telomere repeat shortening in aging and carcinogenesis has recently attracted attention to the more basic question of how telomere length is sensed and regulated by the cell. Telomere length in the budding yeast Saccharomyces cerevisiae has been known for over a decade now to be under complex genetic control, and this organism has provided a useful model system to address basic mechanistic questions. This review focuses on recent studies in yeast which indicate that the double-strand telomere-repeat binding protein Rap1 may play an important role in a negative-feedback mechanism that senses and controls the length of the telomere repeats. Although the same carboxy-terminal domain of Rap1p is involved in both telomere length regulation and telomeric silencing (telomere position effect), it appears that these two functions are mediated by separate sets of Rap1p-interacting proteins. Results from other systems suggest that negative regulation of telomere elongation by a double-stranded telomere-repeat binding protein may be a highly conserved strategy for telomere length control.

Inhibition of type I and type II geranylgeranyl-protein transferases by the monoterpene perillyl alcohol in NIH3T3 cells

The monoterpene perillyl alcohol has anticancer activities that include both prevention and treatment of a wide variety of cancers in animal models. In purified enzyme studies, perillyl alcohol inhibited farnesyl-protein transferase and type I geranylgeranyl-protein transferase. However, whether and which of the polyprenyl-protein transferases is inhibited by perillyl alcohol in vivo is not known. The previously reported monoterpene-induced inhibition of the incorporation of [14C]mevalonolactone into proteins in cultured cells could be due to an inhibition of one or several enzymes in the mevalonate pathway or to changes in the levels of protein substrates for isoprenylation. In the current study, we first analyzed the levels of individual phosphorylated isoprenoid intermediates between mevalonate and geranylgeranyl pyrophosphate in NIH3T3 cells labeled for 4 hr with [14C]mevalonolactone and found that perillyl alcohol did not inhibit the synthesis of these intermediates. Next, proteins including Ras, RhoA, and Rab6 were immunoprecipitated from NIH3T3 cells. Perillyl alcohol was found to inhibit the incorporation of [14C]mevalonolactone into RhoA and Rab6 but not Ras protein. The cellular levels of these three proteins were constant over the 4-hr treatment period. Finally, the distribution of Ras, Rap1, and Rab6 proteins between the aqueous and the detergent-enriched phases was measured. Rap1 and Rab6 but not Ras from perillyl alcohol-treated NIH3T3 cells accumulated in the aqueous phase. Thus, we conclude that perillyl alcohol can inhibit the in vivo prenylation of specific proteins by type I and type II geranylgeranyl-protein transferases but not farnesyl-protein transferase in NIH3T3 cells.

In search of a function for the Ras-like GTPase Rap1

Rap1 (Krev-1) is a small GTPase first identified as a transformation suppressor of K-ras. This GTPase is very similar to Ras, particularly in the effector region, but its function is still elusive. Recent progress in the search for Rap1 function has come from the development of a novel assay to measure Rap1 activation. Using this assay activation of Rap1 was observed in human platelets and neutrophils after stimulation with various agonists. We speculate that Rap1 plays a role in one of the specialised functions of these cells.

Localization of Sir2p: the nucleolus as a compartment for silent information regulators

In wild-type budding yeast strains, the proteins encoded by SIR3, SIR4 and RAP1 co-localize with telomeric DNA in a limited number of foci in interphase nuclei. Immunostaining of Sir2p shows that in addition to a punctate staining that coincides with Rap1 foci, Sir2p localizes to a subdomain of the nucleolus. The presence of Sir2p at both the spacer of the rDNA repeat and at telomeres is confirmed by formaldehyde cross-linking and immunoprecipitation with anti-Sir2p antibodies. In strains lacking Sir4p, Sir3p becomes concentrated in the nucleolus, by a pathway requiring SIR2 and UTH4, a gene that regulates life span in yeast. The unexpected nucleolar localization of Sir2p and Sir3p correlates with observed effects of sir mutations on rDNA stability and yeast longevity, defining a new site of action for silent information regulatory factors.

Interaction of activated Ras with Raf-1 alone may be sufficient for transformation of rat2 cells

v-H-ras effector mutants have been assessed for transforming activity and for the ability of the encoded proteins to interact with Raf-1-, B-Raf-, byr2-, ralGDS-, and CDC25-encoded proteins in the yeast two-hybrid system. Transformation was assessed in rat2 cells as well as in a mutant cell line, rv68BUR, that affords a more sensitive transformation assay. Selected mutant Ras proteins were also examined for their ability to interact with an amino-terminal fragment of Raf-1 in vitro. Finally, possible cooperation between different v-H-ras effector mutants and between effector mutants and overexpressed Raf-1 was assessed. Ras transforming activity was shown to correlate best with the ability of the encoded protein to interact with Raf-1. No evidence for cooperation between v-H-ras effector mutants was found. Signaling through the Raf1-MEK-mitogen-activated protein kinase cascade may be the only effector pathway contributing to RAS transformation in these cells.

Aluminum fluoride associates with the small guanine nucleotide binding proteins

AlF4- has long been known to associate with and activate the GDP-bound alpha subunits of heterotrimeric G-proteins. Recently the small guanine nucleotide binding protein Ras has also been shown to associate with AlF4- in the presence of stoichiometric amounts of its GTPase activating protein (GAP). Here we present the isolation of a stable Ras x GDP- x AlF4- x GAP ternary complex by gel filtration. In addition, we generalise the association of AlF4- with the small GTP-binding proteins by demonstrating ternary complex formation for the Cdc42, Rap and Ran proteins in the presence of their respective GAP proteins.

Inhibition of protein geranylgeranylation causes a superinduction of nitric-oxide synthase-2 by interleukin-1beta in vascular smooth muscle cells

Recently, we have designed farnesyltransferase and geranylgeranyltransferase I inhibitors (FTI-277 and GGTI-298) that selectively block protein farnesylation and geranylgeranylation, respectively. In this study, we describe the opposing effects of these inhibitors on interleukin-1beta (IL-1beta)-stimulated induction of nitric-oxide synthase-2 (NOS-2) in rat pulmonary artery smooth muscle cells (RPASMC) and rat hepatocytes. Pretreatment of cells with GGTI-298 caused a superinduction of NOS-2 by IL-1beta. RPASMC treated with GGTI-298 (10 microM) prior to IL-1beta (10 ng/ml) expressed levels of NOS-2 protein five times higher than those exposed to IL-1beta alone. This superinduction of NOS-2 protein by pretreatment with GGTI-298 resulted in nitrite concentrations in the medium that were 5-fold higher at 10 ng/ml IL-1beta and 10-fold higher at 1 ng/ml IL-1beta. Furthermore, NOS-2 mRNA levels in RPASMC were also increased 6- and 14-fold (at 10 and 1 ng/ml IL-1beta, respectively) when the cells were pretreated with GGTI-298. In contrast, treatment of cells with the inhibitor of protein farnesylation, FTI-277 (10 microM), blocked IL-1beta-induced NOS-2 expression at mRNA and protein levels. Pretreatment with lovastatin, an inhibitor of protein prenylation, resulted in superinduction of NOS-2. This superinduction was reversed by geranylgeraniol, but not by farnesol, further confirming that inhibition of geranylgeranylation, not farnesylation, is responsible for enhanced NOS-2 expression. The results demonstrate that a farnesylated protein(s) mediates IL-1beta induction of NOS-2, whereas a geranylgeranylated protein(s) represses this induction.

Coassociation of Rap1A and Ha-Ras with Raf-1 N-terminal region interferes with ras-dependent activation of Raf-1

Raf-1 is a major downstream effector of mammalian Ras. Binding of the effector domain of Ras to the Ras-binding domain of Raf-1 is essential for Ras-dependent Raf-1 activation. However, Rap1A, which has an identical effector domain to that of Ras, cannot activate Raf-1 and even antagonizes several Ras functions in vivo. Recently, we identified the cysteine-rich region (CRR) of Raf-1 as another Ras-binding domain. Ha-Ras proteins carrying mutations N26G and V45E, which failed to bind to CRR, also failed to activate Raf-1. Since these mutations replace Ras residues with those of Rap1A, we examined if Rap1A lacks the ability to bind to CRR. Contrary to the expectation, Rap1A exhibited a greatly enhanced binding to CRR compared with Ha-Ras. Enhanced CRR binding was also found with Ha-Ras carrying another Rap1A-type mutation E31K. Both Rap1A and Ha-Ras(E31K) mutant failed to activate Raf-1 and interfered with Ha-Ras-dependent activation of Raf-1 in Sf9 cells. Enhanced binding of Rap1A to CRR led to co-association of Rap1A and Ha-Ras with Raf-1 N-terminal region through binding to CRR and Ras-binding domain, respectively. These results suggest that Rap1A interferes with Ras-dependent Raf-1 activation by inhibiting binding of Ras to Raf-1 CRR.

Coincident induction of K rev-1/rap 1A, rap 1B and H-ras mRNAs in the rat spinal cord by noxious stimulation

Two cDNA fragments, K rev-1/rap 1A and rap 1B, were amplified from total cellular RNA of the rat spinal cord by reverse transcription-polymerase chain reaction with a set of oligonucleotide primers specific for the human rap 1A cDNA. We report here using Northern blot analysis with these cDNA probes that noxious stimulation causes a marked and coincident increase in rap 1A, rap 1B and H-ras mRNAs in the rat spinal cord. This suggests that Rap 1 participates in sensory processing in spinal neurons in parallel with Ras.

Mutational analysis of the role of Rap1 in regulating cytoskeletal function in Dictyostelium

It was shown previously that increased expression of the ras-related rap1 gene in Dictyostelium discoideum altered cell morphology (Rebstein et al., Dev. Genet., 1993, 14, 347-355). Vegetative Rap1 transformants were more flattened and spread than parental Ax2 cells and had increased F-actin near the cell periphery. In addition, Rap1 cells were inhibited in the rapid cell contraction that occurs upon refeeding with nutrient media. In this communication, we show that expression of Rap also markedly reduces the contraction response that occurs upon addition of azide to vegetative cells. The changes in cell morphology, the refeeding contraction response, and the azide contraction response have been used to analyze mutants of Rap1 generated by site-directed mutagenesis. The substitution G12V, predicted to increase the proportion of protein binding GTP, did not alter the effect of Rap on cell morphology or on its ability to inhibit the contraction response to azide, but modestly enhanced the ability of Rap1 to inhibit cell rounding in response to nutrient media. The substitution S17N, predicted to restrict the protein to the GDP-bound state, did not produce the flattened cell morphology and abolished the inhibitory effects of Rap in the two cell contraction assays. These results are consistent with a requirement of GTP binding for the Rap-induced effects. Transformants carrying the Rap-S17N protein had a more polar morphology than the parental Ax2 cells, suggesting the possibility that Rap-S17N interferes with the ability of endogenous Rap to regulate the cytoskeleton. Substitutions at amino acid 38, within the presumptive effector domain, reduced but did not abolish the effects of Rap1 on cell contraction, while the substitution T61Q had no effect on Rap1 activity. Taken together, the results suggest that Rap may have multiple regulatory effects on cytoskeletal function.

A novel Rap1p-interacting factor, Rif2p, cooperates with Rif1p to regulate telomere length in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Rap1 protein binds with high affinity to sites within the poly(C(1-3)A) tracts at telomeres, where it plays a role in both telomere length regulation and the initiation of telomeric silencing. Rap1p initiates silencing at telomeres by interacting through its carboxy-terminal domain with Sir3p and Sir4p, both of which are required for repression. This same domain of Rap1p also negatively regulates telomere elongation, through an unknown mechanism. We have identified a new Rap1-interacting factor (Rif2p) that plays a role in telomere length regulation. Rif2p has considerable functional similarities with a Rap1p-interacting factor (Rif1p) identified previously. Mutations in RIF1 or RIF2 (unlike mutations in the silencing genes SIR3 and SIR4) result in moderate telomere elongation and improved telomeric silencing. However, deletion of both RIF1 and RIF2 in the same cell results in a dramatic increase in telomere length, similar to that seen with a carboxy-terminal truncation of Rap1p. In addition, overexpression of either RIF1 or RIF2 decreases telomere length, and co-overexpression of these proteins can reverse the telomere elongation effect of overexpression of the Rap1p carboxyl terminus. Finally, we show that Rif1p and Rif2p can interact with each other in vivo. These results suggest that telomere length regulation is mediated by a protein complex consisting of Rif1p and Rif2p, each of which has distinct regulatory functions. One role of Rap1p in telomere length regulation is to recruit these proteins to the telomeres.

The role of the metal ion in the p21ras catalysed GTP-hydrolysis: Mn2+ versus Mg2+

GTP and ATP hydrolysing proteins have an absolute requirement for a divalent cation, which is usually Mg2+, as a cofactor in the enzymatic reaction. Other phosphoryl transfer enzymes employ more than one divalent ion for the enzymatic reaction. It is shown here for p21ras, a well studied example of GTP hydrolysing proteins, that the GTP-hydrolysis rate is significantly faster if Mg2+ is replaced by Mn2+, both in the presence or absence of its GTPase-activating protein Ras-GAP. This effect is not due to a different stoichiometry of metal ion binding, since one metal ion is sufficient for full enzymatic activity. To determine the role of the metal ion, the crystal structure of p21(G12P). GppCp complexed with Mn2+ was determined and shown to be very similar to the corresponding p21(G12P). GppCp.Mg2+ structure. Especially the coordination sphere around the metal ions is very similar, and no second metal ion binding site could be detected, consistent with the assumption that one metal ion is sufficient for GTP hydrolysis. In order to explain the biochemical differences, we analysed the GTPase reaction mechanism with a linear free energy relationships approach. The result suggests that the reaction mechanism is not changed with Mn2+ but that the transition metal ion Mn2+ shifts the pKa of the gamma-phosphate by almost half a unit and increases the reaction rate due to an increase in the basicity of GTP acting as the general base. This suggests that the intrinsic GTPase reaction could be an attractive target for anti-cancer drug design. By using Rap1A and Ran, we show that the acceleration of the GTPase by Mn2+ appears to be a general phenomenon of GTP-binding proteins.

Expression of Rap 1 suppresses genomic instability of H-ras transformed mouse fibroblasts

Among the multiple genetic changes that occur during cancer progression are the activation of proto-oncogenes and the inactivation or loss of genes encoding tumor suppressors. The potential roles for these genes in the perturbation of genome stability continues to be of major interest. We have previously shown that conditional expression of H-ras in NIH3T3 cells increases genetic instability in these cells, rendering them more permissive to gene amplification and to the generation of chromosome aberrations which can be induced within a single cell cycle. In the present study we show that genetic instability induced by H-ras expression can be suppressed by co-expression of Rap 1, a Ras-related tumor suppressor gene. An NIH3T3 cell line transformed with activated human H-ras was transfected with Rap 1. Expression of the Rap 1 gene reverted the transformed cells to a flat morphology. The reverted cells reestablished contact inhibition of growth and lost the capacity to form colonies in soft agar. These cells were subsequently studied for the role of Rap 1 on the suppression of genomic instability induced by oncogenic H-ras. Cells transformed with H-ras manifest an increase in methotrexate resistance as measured by an increase in Dhfr gene amplification. Cells which concommitantly express Rap 1 showed reduced levels of methotrexate resistance as well as reduction of gene amplification capacity. Furthermore fluorescent-in-situ hybridization (FISH) with a pancentromeric mouse probe showed that elevated levels of chromosome aberrations in cells expressing H-ras were also suppressed after co-expression of Rap 1.

RalGDS functions in Ras- and cAMP-mediated growth stimulation

Thyroid-stimulating hormone stimulates proliferation through both the cAMP-dependent protein kinase and Ras but not through Raf-1 and mitogen-activated and extracellular signal-related kinase kinase. We now report that thyroid-stimulating hormone represses mitogen-activated protein kinase activity and that microinjection of an effector domain mutant Ha-Ras protein, Ras(12V,37G), defective in Raf-1 binding and mitogen-activated protein kinase activation, stimulates DNA synthesis in quiescent and thyroid-stimulating hormone-treated thyrocytes. A yeast two-hybrid screen identified RalGDS as a Ras(12V,37G) binding protein and therefore a potential effector of Ras in these cells. Associations between Ras and RalGDS were observed in extracts prepared from thyroid cells. Microinjection of a mutant RalA(28N) protein thought to sequester RalGDS family members reduced DNA synthesis stimulated by Ras as well as cAMP-mediated DNA synthesis in two cell lines which respond to cAMP with mitogenesis. These results support the idea that RalGDS may be an effector of Ras in cAMP-mediated growth stimulation.