The post-translational processing of ras p21 is critical for its stimulation of mitogen-activated protein kinase

Regulators of small GTPases

Small GTPases are converted from the GDP-bound inactive form to the GTP-bound active form by a GDP/GTP exchange reaction which is regulated by GDP/GTP exchange proteins (GEPs). We have found both stimulatory and inhibitory GEPs, which we have named GDP dissociation stimulators (GDSs) and GDP dissociation inhibitors (GDIs) respectively. We have isolated Smg GDS, Rho GDI and Rab GDI, cloned them, and determined their primary structures. These GEPs are active on a group of small GTPases: Smg GDS on at least K-Ras, Rap1/Smg21, Rho and Rac; Rho GDI on at least Rho, Rac and Cdc42; Rab GDI on most of the Rab family members. These GEPs have an additional function, regulating the translocation of their substrate small GTPases between the membrane and the cytosol. The GEPs interact only with the post-translationally modified form of their substrate small GTPases.

Recent advances in the molecular genetics of malignant gliomas disclose targets for antitumor agent perillyl alcohol

Tumors of glial origin such as glioblastoma multiforme (GBM) comprise the majority of human brain tumors. Despite advances in surgery, radiation, and chemotherapy, the prognosis for patients with malignant glioma has not improved, emphasizing the need for a search for new chemotherapeutic drugs. Deregulated p21-Ras function, as a result of mutation, overexpression, or growth factor-induced overactivation, contributes to the growth of GBM. The monoterpene perillyl alcohol (POH) has preventive and therapeutic effects in a wide variety of preclinical tumor models and is currently under phase I and phase II clinical trials. As inhibition of posttranslational isoprenylation of Ras, a family of proteins that are involved in signal transduction is among the drug-related activities observed in this compound; POH may be a potential chemotherapeutic agent for GBM. Intranasal delivery is a practical and noninvasive approach that allows therapeutic agents that do not cross the blood-brain barrier to enter the central nervous system, reducing unwanted systemic side effects. This article describes the effect of intranasal delivery of POH in a patient with relapsed GBM.

NMR characterization of full-length farnesylated and non-farnesylated H-Ras and its implications for Raf activation

The C terminus, also known as the hypervariable region (residues 166-189), of H-, N-, and K-Ras proteins has sequence determinants necessary for full activation of downstream effectors such as Raf kinase and PI-3 kinase as well as for the correct targeting of Ras proteins to lipid rafts and non-raft membranes. There is considerable interest in understanding how residues in the extreme C terminus of the different Ras proteins and farnesylation of the CaaX box cysteine affect Ras membrane localization and allosteric activation of Raf kinase. To provide insights into the structural and dynamic changes that occur in Ras upon farnesylation, we have used NMR spectroscopy to compare the properties of truncated H-Ras (1-166), to non-processed full-length H-Ras (residues 1-185) and full-length (1-189) farnesylated H-Ras. We report that the C-terminal helix alpha-5 extends to residue N172, and the remaining 17 amino acid residues in the C terminus are conformationally averaged in solution. Removal of either 23 or 18 amino acid residues from the C terminus of full length H-Ras generates truncated H-Ras (1-166) and H-Ras (1-171) proteins, respectively, that have been structurally characterized and are biochemical active. Here we report that C-terminal truncation of H-Ras results in minor structural and dynamic perturbations that are propagated throughout the H-Ras protein including increased flexibility of the central beta-sheet and the C-terminal helix alpha-5. Ordering of residues in loop-2, which is involved in Raf CRD binding is also observed. Farnesylation of full-length H-Ras at C186 does not result in detectable conformational changes in H-Ras. Chemical shift mapping studies of farnesylated and non-farnesylated forms of H-Ras with the Raf-CRD show that the farnesyl moiety, the extreme H-Ras C terminus and residues 23-30, contribute to H-Ras:Raf-CRD interactions, thereby increasing the affinity of H-Ras for the Raf-CRD.

A review of tobacco BY-2 cells as an excellent system to study the synthesis and function of sterols and other isoprenoids

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate (IPP), the universal precursor for isoprenoid biosynthesis. In this paper we review findings and observations made primarily with tobacco BY-2 cells (TBY-2), which have proven to be an excellent system in which to study the two biosynthetic pathways. A major advantage of these cells as an experimental system is their ability to readily take up specific inhibitors and stably- and/or radiolabeled precursors. This permits the functional elucidation of the role of isoprenoid end products and intermediates. Because TBY-2 cells undergo rapid cell division and can be synchronized within the cell cycle, they constitute a highly suitable test system for determination of those isoprenoids and intermediates that act as cell cycle inhibitors, thus giving an indication of which branches of the isoprenoid pathway are essential. Through chemical complementation; and use of precursors, intracellular compartmentation can be elucidated, as well as the extent to which the plastidial and cytosolic pathways contribute to the syntheses of specific groups of isoprenoids (e.g., sterols) via exchange of intermediates across membranes. These topics are discussed in the context of the pertinent literature.

n-6 and n-3 polyunsaturated fatty acids differentially modulate oncogenic Ras activation in colonocytes

Ras proteins are critical regulators of cell function, including growth, differentiation, and apoptosis, with membrane localization of the protein being a prerequisite for malignant transformation. We have recently demonstrated that feeding fish oil, compared with corn oil, decreases colonic Ras membrane localization and reduces tumor formation in rats injected with a colon carcinogen. Because the biological activity of Ras is regulated by posttranslational lipid attachment and its interaction with stimulatory lipids, we investigated whether docosahexaenoic acid (DHA), found in fish oil, compared with linoleic acid (LA), found in corn oil, alters Ras posttranslational processing, activation, and effector protein function in young adult mouse colon cells overexpressing H-ras (YAMC-ras). We show here that the major n-3 polyunsaturated fatty acid (PUFA) constituent of fish oil, DHA, compared with LA (an n-6 PUFA), reduces Ras localization to the plasma membrane without affecting posttranslational lipidation and lowers GTP binding and downstream p42/44(ERK)-dependent signaling. In view of the central role of oncogenic Ras in the development of colon cancer, the finding that n-3 and n-6 PUFA differentially modulate Ras activation may partly explain why dietary fish oil protects against colon cancer development.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

Activation of H-ras61L-specific signaling pathways does not require posttranslational processing of H-ras

We have previously demonstrated that H-ras61L retained transforming activity when lacking C-terminal lipid modifications, provided that plasma membrane localization was restored by an N-terminal transmembrane domain. Since several ras-activated pathways contribute to the transformed phenotype, we utilized a novel set of transmembrane domain-anchored H-ras derivatives to examine if lipids are required for activation of any specific signaling pathways. We demonstrate here that H-ras61L-induced activation of the Raf/MEK/MAPK pathway, including recruitment of Raf to the plasma membrane and activation of Raf and MAPK, does not require C-terminal processing of H-ras61L. Biochemical fractionation experiments confirm the localization of TM-ras derivatives to the plasma membrane, as well as the ras-mediated recruitment of c-Raf-1. Changes in the actin cytoskeleton, controlled by H-ras61L-mediated activation of the Rac/ Rho pathway, as well as PI 3-kinase activation, can also occur in the absence of C-terminal lipid modifications. Finally, downstream events, such as the induction of the immediate-early gene c-fos or neurite outgrowth in PC12 cells, are stimulated by the expression of plasma membrane-anchored, nonlipidated H-ras6lL. These results demonstrate that H-ras can be functionally targeted to the plasma membrane using a transmembrane domain sequence and that several signal transduction pathways downstream of H-ras can be activated without the presence of normal lipid modifications.

Farnesylation of Ras is important for the interaction with phosphoinositide 3-kinase gamma

The correct functioning of Ras proteins requires post-translational modification of the GTP hydrolases (GTPases). These modifications provide hydrophobic moieties that lead to the attachment of Ras to the inner side of the plasma membrane. In this study we investigated the role of Ras processing in the interaction with various putative Ras-effector proteins. We describe a specific, GTP-independent interaction between post-translationally modified Ha- and Ki-Ras4B and the G-protein responsive phosphoinositide 3-kinase p110gamma. Our data demonstrate that post-translational processing increases markedly the binding of Ras to p110gamma in vitro and in Sf9 cells, whereas the interaction with p110alpha is unaffected under the same conditions. Using in vitro farnesylated Ras, we show that farnesylation of Ras is sufficient to produce this effect. The complex of p110gamma and farnesylated RasGTP exhibits a reduced dissociation rate leading to the efficient shielding of the GTPase from GTPase activating protein (GAP) action. Moreover, Ras processing affects the dissociation rate of the RasGTP complex with the Ras binding domain (RBD) of Raf-1, indicating that processing induces alterations in the conformation of RasGTP. The results suggest a direct interaction between a moiety present only on fully processed or farnesylated Ras and the putative target protein p110gamma.

Alpha-cyanocinnamide derivatives: a new family of non-peptide, non-sulfhydryl inhibitors of Ras farnesylation

Farnesylation of Ras and other proteins is required for their membrane attachment and normal function. Here we report on the synthesis of alpha-cyanocinnamide derivatives, a new family of farnesyltransferase inhibitors. These compounds are nonpeptidic and do not contain sulfhydryl groups. The most potent compound is a pure competitive inhibitor with respect to the Ras protein and mixed competitive with respect to farnesyl diphosphate. Selectivity studies against geranylgeranyltransferase and biological activities of selected compounds are described.

Human geranylgeranyl diphosphate synthase. cDNA cloning and expression

Geranylgeranyl diphosphate (GGPP) synthase (GGPPSase) catalyzes the synthesis of GGPP, which is an important molecule responsible for the C20-prenylated protein biosynthesis and for the regulation of a nuclear hormone receptor (LXR.RXR). The human GGPPSase cDNA encodes a protein of 300 amino acids which shows 16% sequence identity with the known human farnesyl diphosphate (FPP) synthase (FPPSase). The GGPPSase expressed in Escherichia coli catalyzes the GGPP formation (240 nmol/min/mg) from FPP and isopentenyl diphosphate. The human GGPPSase behaves as an oligomeric molecule with 280 kDa on a gel filtration column and cross-reacts with an antibody directed against bovine brain GGPPSase, which differs immunochemically from bovine brain FPPSase. Northern blot analysis indicates the presence of two forms of the mRNA.

Molecular cloning of cDNA for BRab from the brain of Bombyx mori and biochemical properties of BRab expressed in Escherichia coli

From a brain cDNA library of Bombyx mori, we cloned cDNA for BRab, which encoded a 202-amino-acid polypeptide sharing 60-80% similarity with rab1 family members. To characterize its biochemical properties, cDNA for BRab was inserted into an expression vector (pGEX2T) and expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein. The recombinant protein was purified to homogeneity with glutathione S-Sepharose. The purified GST-BRab bound [35S]-GTP gamma S and [3H]-GDP with association constants of 1.5 x 10(6) M-1 and 0.58 x 10(6) M-1, respectively. The binding of [35S]-GTP gamma S was inhibited with GTP and GDP, but with no other nucleotides. The GTP-hydrolysis activity was evaluated to be 5 m mole/min/mole of BRab. In the presence of 6 mM MgCl2, bound [35S]-GTP gamma S and [3H]-GDP were exchanged with GTP gamma S most efficiently. These results suggest that BRab, having a higher affinity for GTP than GDP, converts from the GTP-bound state into the GDP-bound state by intrinsic GTP hydrolysis activity and returns to the GTP-bound state with the exchange of GDP with GTP.

MKP-1 induced in rat brain after electroconvulsive shock is independent of regulation of 42- and 44-kDa MAPK activity

Electroconvulsive shock (ECS) activates MAPKs in rat brain and also induces immediate early genes. We investigated whether ECS induces MKP-1, a specific MAPK phosphatase and an immediate early gene, for feedback regulation of MAPK activity. ECS induced MKP-1 in the cortex, but MAPK activity returned to its basal level before MKP-1 protein increased, within 10 min of ECS. MKP-1 protein amount peaked 1 hr after ECS. MKP-1 induced did not lower the basal level of MAPK activity or attenuate MAPK activation by second ECS. MAPK activation in cerebellum was very weak, but the MKP-1 induction was faster and more prominent than in the cortex. These results suggest that ECS induces MKP-1 in various rat brain regions, however, the induction may not be related to the activation of MAPK and the MKP-1 induced may be independent of the regulation of MAPK activity after ECS.

Replacement of the H-Ras farnesyl group by lipid analogues: implications for downstream processing and effector activation in Xenopus oocytes

Ras proteins must undergo a series of posttranslational lipidation steps before they become biologically functional. While the fact that farnesylation is required for subsequent processing steps and indispensable for Ras function has been established, the significance of the isoprenoid structure per se in the context of fully processed Ras is unknown. Here, we describe a novel approach for studying the isoprenoid structure-function relationship in vivo by replacing the H-Ras farnesyl group with synthetic analogues and analyzing their biological functions following microinjection into Xenopus oocytes. We show that the H-Ras farnesyl group can be stripped of most of its isoprenoid features that distinguish it from a fatty acid without any apparent effect on its ability to induce oocyte maturation and activation of mitogen-activated protein kinase. In contrast, replacement by the less hydrophobic isoprenoid geranyl causes severely delayed oocyte activation. Analysis of posttranslational processing reveals a striking correlation between the kinetics of processing, membrane binding, and the onset of biological activity regardless of lipid structure and suggests that slow C-terminal proteolysis and/or methylation can become rate-limiting for H-Ras function. Thus, while our results suggest no stringent requirement for the H-Ras farnesyl structure for effector activation in Xenopus oocytes, they reveal an important role for the lipid present at the farnesylation site in promoting efficient proteolysis and/or methylation which allows rapid palmitoylation, membrane localization, and biological activity. Xenopus oocytes provide a useful in vivo system for the kinetic analysis of the function of the protein of interest present at the physiological dose, which is required for accurate determination of structure-function relationships.

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.

Different effects of various phospholipids on Ki-Ras-, Ha-Ras-, and Rap1B-induced B-Raf activation

We have recently purified a Ki-Ras- and Ha-Ras-dependent extracellular signal-regulated kinase kinase from bovine brain and identified it as B-Raf protein kinase complexed with 14-3-3 proteins (Yamamori, B., Kuroda, S., Shimizu, K., Fukui, K., Ohtsuka, T., and Takai, Y. (1995) J. Biol. Chem. 270, 11723-11726). Moreover, we found that Rap1B as well as Ki-Ras and Ha-Ras stimulate the B-Raf activity. Since B-Raf contains a cysteine-rich domain originally found in protein kinase C as a domain responsible for interaction with phosphatidylserine (PS) and diacylglycerol or 12-O-tetradecanoylphorbol-13-acetate, we have examined here the effect of these compounds on the Ki-Ras-, Ha-Ras-, and Rap1B-induced activation of bovine brain B-Raf. Bovine brain PS enhanced Ki-Ras-stimulated B-Raf activity. Phosphatidic acid was slightly active, but other phospholipids, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol (PI), PI-4-monophosphate, PI-4,5-bisphosphate, and PI-3,4,5-trisphosphate, were inactive. However, none of the above phospholipids affected the Ha-Ras-stimulated B-Raf activity, whereas PI, PS, phosphatidylethanolamine, and phosphatidic acid inhibited the Rap1B-stimulated B-Raf activity. Phosphatidylcholine or PI-4-monophosphate did not show any effect on the Rap1B-stimulated B-Raf activity. Synthetic PS with two unsaturated fatty acids, such as 1,2-dioleoyl-PS or 1,2-dilinoleoyl-PS, showed the same effect toward the Ki-Ras- and Rap1B-stimulated B-Raf activities, but synthetic PS with two saturated fatty acids, such as 1, 2-distearoyl-PS, was inactive. 12-O-Tetradecanoylphorbol-13-acetate did not affect the stimulatory or inhibitory effect of PS on the Ki-Ras- and Rap1B-stimulated B-Raf activities, respectively. PS did not affect the Ki-Ras-, Ha-Ras-, or Rap1B-independent basal B-Raf activity or the mitogen-activated protein kinase kinase or extracellular signal-regulated kinase activity. These results indicate that various phospholipids differently affect Ki-Ras-, Ha-Ras, and Rap1B-induced B-Raf activation.

Palmitoylation of Ha-Ras facilitates membrane binding, activation of downstream effectors, and meiotic maturation in Xenopus oocytes

Ras proteins serve as critical relays in signal transduction pathways that control growth and differentiation and must undergo posttranslational modifications before they become functional. While it is established that farnesylation is necessary for membrane binding and cellular functions of all Ras proteins, the significance of palmitoylation is unclear. We have studied the contribution of Ha-Ras palmitoylation for biological activity in Xenopus oocytes. In contrast to wild-type Ha-Ras, which binds to membranes and induces meiosis when microinjected into oocytes, a nonpalmitoylated but farnesylated and methylated mutant mislocalizes to the cytosol and fails to promote maturation. This lack of responsiveness correlates with the inability of the mutant to induce phosphorylation and activation of mitogen-activated protein kinase and maturation promoting factor, which are both strongly activated by wild-type Ha-Ras. Costimulation of oocytes with insulin increases their responsiveness to Ras and partially rescues the biological activity of the palmitoylation-resistant mutant. However, 25-50 times higher doses of mutant were required to elicit responses equivalent to wild-type Ha-Ras. These results suggest that palmitoylation and membrane association of Ha-Ras is necessary for efficient activation of the mitogen-activated protein kinase cascade in vivo and are consistent with a biochemical function for Ras as a membrane targeting signal for downstream effectors in this pathway.

Ras effectors

The search for proteins which interact with the active GTP-bound form of Ras in order to transmit signals for proliferation, differentiation and oncogenesis has been a long one. Now there are several strong candidates for Ras effectors that include protein kinases, lipid kinases and guanine nucleotide exchange factors. Structural information on how one Ras-binding domain in an effector interacts with Ras.GTP has recently been obtained. Recent data show that transformation by Ras oncoproteins requires the activation of several signal transduction pathways, including those which transmit signals via members of the Rho family of GTPases.

Post-translational modification of H-Ras is required for activation of, but not for association with, B-Raf

B-Raf is regulated by Ras protein and acts as a mitogen-activated protein (MAP) kinase kinase kinase in PC12 cells and brain. Ras protein undergoes a series of post-translational modifications on its C-terminal CAAX motif, and the modifications are critical for its function. To elucidate the role of the post-translational modifications in interaction with, and activation of, B-Raf, we have analyzed a direct association between H-Ras and B-Raf, and constructed an in vitro system for B-Raf activation by H-Ras. By using methods based on inhibition of yeast adenylyl cyclase or RasGAP activity and by in vitro binding assays, we have shown that the segment of B-Raf corresponding to amino acid 1-326 binds directly to H-Ras with a dissociation constant (Kd) comparable to that of Raf-1 and that the binding is not significantly affected by the post-translational modifications. However, when the activity of B-Raf to stimulate MAP kinase was measured by using a cell-free system derived from rat brain cytosol, we observed that the unmodified form of H-Ras possesses an almost negligible activity to activate B-Raf in vitro compared to the fully modified form. H-RasSer-181,184 mutant, which was farnesylated but not palmitoylated, was equally active as the fully modified form. These results indicate that the post-translational modifications, especially farnesylation, are required for H-Ras to activate B-Raf even though they have no apparent effect on the binding properties of H-Ras to B-Raf.

Activation of brain B-Raf protein kinase by Rap1B small GTP-binding protein

Rap1 small GTP-binding protein has the same amino acid sequence at its effector domain as that of Ras. Rap1 has been shown to antagonize the Ras functions, such as the Ras-induced transformation of NIH 3T3 cells and the Ras-induced activation of the c-Raf-1 protein kinase-dependent mitogen-activated protein (MAP) kinase cascade in Rat-1 cells, whereas we have shown that Rap1 as well as Ras stimulates DNA synthesis in Swiss 3T3 cells. We have established a cell-free assay system in which Ras activates bovine brain B-Raf protein kinase. Here we have used this assay system and examined the effect of Rap1 on the B-Raf activity to phosphorylate recombinant MAP kinase kinase (MEK). Recombinant Rap1B stimulated the activity of B-Raf, which was partially purified from bovine brain and immunoprecipitated by an anti-B-Raf antibody. The GTP-bound form was active, but the GDP-bound form was inactive. The fully post-translationally lipid-modified form was active, but the unmodified form was nearly inactive. The maximum B-Raf activity stimulated by Rap1B was nearly the same as that stimulated by Ki-Ras. Rap1B enhanced the Ki-Ras-stimulated B-Raf activity in an additive manner. These results indicate that not only Ras but also Rap1 is involved in the activation of the B-Raf-dependent MAP kinase cascade.

The farnesyl group of H-Ras facilitates the activation of a soluble upstream activator of mitogen-activated protein kinase

To study the function of the farnesyl modification of Ras, the farnesyl group and a variety of its structural analogs, which lack one or more double bonds and/or the methyl groups, were enzymatically incorporated into recombinant H-Ras in vitro. These proteins were used in a cell- and membrane-free, Ras-dependent mitogen-activated protein kinase (MAP kinase) activation system derived from Xenopus laevis eggs to examine the contribution of the farnesyl group toward the activation of the kinase. Whereas non-farnesylated H-Ras is unable to activate MAP kinase, farnesylation of H-Ras alone, in the absence of further processing, is sufficient to cause the activation of MAP kinase in this system. All of the analogs of the farnesyl group, when incorporated into H-Ras, support the activation of the kinase to variable extents. These results suggest a direct but fairly nonspecific interaction of the farnesyl moiety of H-Ras with a soluble upstream activator of MAP kinase.

Prenylcysteine analogs mimicking the C-terminus of GTP-binding proteins stimulate exocytosis from permeabilized HIT-T15 cells: comparison with the effect of Rab3AL peptide

Most guanine nucleotide binding proteins (G-proteins) possess an S-prenylated C-terminal cysteine whose carboxyl group can be reversibly methylated. The prenylcysteine analog N-acetyl-S-geranylgeranyl-cysteine (AGGC) (50 microM), a competitive inhibitor of prenylcysteine methyl transferases, introduced into streptolysin-O permeabilized HIT-T15 cells doubled the rate of basal (0.1 microM Ca2+) and of stimulated (10 microM Ca2+ or 100 microM GTP gamma S) insulin secretion in a reversible and ATP-dependent manner. N-acetyl-S-farnesylcysteine (AFC) was less potent while N-acetyl-S-geranyl-cysteine was inactive. Prenylcysteine action on exocytosis did not involve inhibition of G-protein methylation, since (1) the methyl ester derivative of AFC, an inefficient inhibitor of methyltransferases in HIT-T15 cell fractions, was as potent as AGGC in stimulating exocytosis; (2) S-adenosyl-homocysteine, a general inhibitor of methylation reactions, did not alter basal or GTP gamma S-triggered secretion while inhibiting Ca(2+)-induced insulin release. The binding of G-proteins to Rab/GDP-dissociation inhibitor, Rab3A/GTPase activating protein or rabphilin-3A was not affected by the prenylcysteine analogs. AGGC or AFC had the same effect on insulin release as a synthetic peptide mimicking the amino acid residues 52-67 of the G-protein Rab3A (Rab3AL). Moreover, the action on secretion of the combination of Rab3AL and prenylcysteines was not additive. We propose that the prenylcysteines and the Rab3AL peptide influence exocytosis by affecting the association of Rab3A with different proteins of the exocytotic machinery of insulin-secreting cells.

Purification of a Ras-dependent mitogen-activated protein kinase kinase kinase from bovine brain cytosol and its identification as a complex of B-Raf and 14-3-3 proteins

We previously purified a protein factor, named REKS (Ras-dependent Extracellular Signal-regulated Kinase (ERK)/mitogen-activated protein kinase Kinase (MEK) Stimulator), from Xenopus eggs by use of a cell-free assay system in which recombinant GTP gamma S (guanosine 5'-(3-O-thio)triphosphate)-Ki-Ras activates recombinant MEK. By use of this assay system, we purified here bovine REKS to near homogeneity from the cytosol fraction of bovine brain by successive chromatographies of Mono S, Mono Q, GTP gamma S-glutathione S-transferase-Ha-Ras-coupled glutathione-agarose, and Mono Q columns. It was composed of three proteins with masses of about 95, 32, and 30 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 95-, 32-, and 30-kDa proteins were identified by immunoblot analysis to be B-Raf protein kinase, 14-3-3 protein, and 14-3-3 protein, respectively. Moreover, the REKS activity was specifically immunoprecipitated by an anti-B-Raf antibody. Bovine REKS was activated by lipid-modified GTP gamma S-Ki-Ras far more effectively than by a lipid-unmodified one. Lipid-modified GDP-Ki-Ras was inactive. Exogenous addition of 14-3-3 proteins stimulated further the REKS activity both in the presence and absence of GTP gamma S-Ki-Ras. These results indicate that at least one of the direct targets of Ras is B-Raf complexed with 14-3-3 proteins in bovine brain.

S-prenylated cysteine analogues inhibit receptor-mediated G protein activation in native human granulocyte and reconstituted bovine retinal rod outer segment membranes

We have previously shown that the S-prenylated cysteine analogue N-acetyl-S-trans,trans-farnesyl-L-cysteine (L-AFC) inhibits basal and formyl peptide receptor-stimulated binding of guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) to and hydrolysis of GTP by membranes of HL-60 granulocytes and have presented evidence suggesting that this inhibition was not caused by reduced protein carboxyl methylation [Scheer, A., & Gierschik, P. (1993) FEBS Lett. 319, 110-114]. We now report a detailed analysis of the structural properties of S-prenylated cysteine analogues required for this inhibition and demonstrate that S-prenylcysteines also suppress basal and receptor-stimulated GTP[S] binding to human peripheral neutrophil and HL-60 granulocyte membranes when stimulated by formyl peptide and complement C5a, respectively. S-Prenylcysteines did not affect pertussis toxin-mediated [32P]ADP-ribosylation of Gi proteins. The inhibitory effect of L-AFC was reversible and was not mimicked by farnesylic acid. L-AFC also interfered with GTP[S] binding to retinal transducin when stimulated by light-activated rhodopsin in a reconstituted system. This inhibitory effect was fully reversed upon increasing the concentration of either the G protein beta gamma dimer or the activated receptor. On the basis of these results, we suggest that S-prenylated cysteine analogues like L-AFC inhibit receptor-mediated G protein activation by specifically and reversibly interfering with the interaction of activated receptors with G proteins, most likely with their beta gamma dimers, rather than by inhibiting alpha.beta gamma heterotrimer formation.

Purification and characterization of REKS from Xenopus eggs. Identification of REKS as a Ras-dependent mitogen-activated protein kinase kinase kinase

We have previously identified a protein factor, named REKS (Ras-dependent Extracellular signal-regulated kinase/Mitogen-activated protein kinase kinase (MEK) Stimulator), which is necessary for Ras-dependent MEK activation. In this study, we attempted to highly purify and characterize REKS. We have highly purified REKS by successive column chromatographies using a cell-free assay system in which REKS activates recombinant extracellular signal-regulated kinase 2 through recombinant MEK in a guanosine 5'-O-(thiotriphosphate) (GTP gamma S)-Ki-Ras-dependent manner. REKS formed a stable complex with GTP gamma S-Ras; REKS was coimmunoprecipitated with GTP gamma S-Ki-Ras or GTP gamma S-Ha-Ras, but not with GDP-Ki-Ras or GDP-Ha-Ras by an anti-Ras antibody. REKS was absorbed to a GTP gamma S-glutathione S-transferase (GST)-Ha-Ras-coupled glutathione-agarose column but not to a GDP-GST-Ha-Ras-coupled glutathione-agarose column and was coeluted with GTP gamma S-GST-Ha-Ras by reduced glutathione. The minimum molecular mass of REKS was estimated to be about 98 kDa on SDS-polyacrylamide gel electrophoresis. REKS phosphorylated this 98-kDa protein as well as recombinant MEK. REKS was not recognized by any of the anti-c-Raf-1, anti-Mos, and anti-mSte11 antibodies. These results indicate that REKS is a Ras-dependent MEK kinase.

Roles of lipid modifications of transducin subunits in their GDP-dependent association and membrane binding

Transducin is an unusually soluble and dissociable heterotrimeric G-protein, although its T alpha and T beta gamma subunits are N-acylated and farnesylated, respectively. These lipid modifications have been suggested to contribute directly to the GDP-dependent T alpha-T beta gamma association, through specific lipid recognition sites on both protein subunits. We studied the dependence of subunit association on their bound lipids and on the presence of different lipidic environments. Association of native N-acylated (nT alpha) or acyl-free recombinant (rT alpha) T alpha with farnesylated and carboxymethylated (fcT beta gamma), farnesylated (fT beta gamma), or farnesyl-free (dfT beta gamma) T beta gamma was analyzed by gradient centrifugation and gel filtration in the presence of detergent or phospholipid-cholate micelles and by cosedimentation with phospholipid vesicles. Without detergent, nT alpha GDP and fcT beta gamma associate only weakly in solution. The loss of T alpha acyl or T beta gamma farnesyl residues induces total dissociation. With detergent or lipids, isolated fcT beta gamma binds tightly to micelles or vesicles, while dfT beta gamma does not; nT alpha GDP binds weakly, while deacylated rT alpha GDP does not bind at all; and nT alpha GDP binds cooperatively with fcT beta gamma, while rT alpha GDP does not. Thus (i) the T alpha acyl chain binds weakly, whereas the T beta gamma farnesyl chain binds strongly to membrane lipids; (ii) there is no evidence for binding of the T alpha acyl chain to a polypeptide site in T beta gamma, nor for binding of the T beta gamma farnesyl chain to a polypeptidic site in T alpha, but the T alpha acyl chain seems to bind cooperatively with the T beta gamma farnesyl chain in the membrane lipids; (iii) the insertion of the two protein-attached lipids into the same membrane could contribute to the association of both subunits by favoring collision coupling of the properly oriented protein moieties on the membrane surface.

Structure-activity relationships among monoterpene inhibitors of protein isoprenylation and cell proliferation

The monoterpene d-limonene inhibits the post-translational isoprenylation of p21ras and other small G proteins, a mechanism that may contribute to its efficacy in the chemoprevention and therapy of chemically induced rodent cancers. In the present study, the relative abilities of 26 limonene-like monoterpenes to inhibit protein isoprenylation and cell proliferation were determined. Many monoterpenes were found to be more potent than limonene as inhibitors of small G protein isoprenylation and cell proliferation. The relative potency of limonene-derived monoterpenes was found to be: monohydroxyl = ester = aldehyde > thiol > acid = diol = epoxide > triol = unsubstituted. All monoterpenes that inhibited protein isoprenylation did so in a selective manner, such that 21-26 kDa proteins were preferentially affected. Perillyl alcohol, one of the most potent terpenes, reduced 21-26 kDa protein isoprenylation to 50% of the control level at a concentration of 1 mM, but had no effect on the isoprenylation of 67, 47 or 17 kDa proteins. In particular, p21ras farnesylation was inhibited 40% by 1 mM perillyl alcohol. At the same concentration, perillyl alcohol completely inhibited the proliferation of human HT-29 colon carcinoma cells. The structure-activity relationships observed among the monoterpene isoprenylation inhibitors support a role for small G proteins in cell proliferation, and suggest that many limonene-derived monoterpenes warrant further investigation as antitumor agents.

Lipid modifications of G proteins

Covalent attachment of lipids is a near-universal mechanism through which eukaryotic cells direct and, in some cases, control membrane localization of G proteins. Studies conducted over the past year have substantially advanced our understanding of both the molecular mechanisms and the functional consequences of these modifications. Of particular note are the processes of palmitoylation of the alpha-subunits of heterotrimeric G proteins, and prenylation of members of the Ras superfamily of monomeric G proteins, where recent findings point to unexpected roles for lipid modifications in signaling through these proteins.

Substrate characterization of the Saccharomyces cerevisiae protein farnesyltransferase and type-I protein geranylgeranyltransferase

The in vitro substrate preferences of recombinant S. cerevisiae protein farnesyltransferase and type-I protein geranylgeranyl-transferase were determined. Proteins ending in 16 different CaaX sequences (C = cysteine, a = aliphatic amino acid, X = variable amino acids) were used to determine the protein substrate preferences of these S. cerevisiae prenyltransferases. The identities of the attached prenyl groups were confirmed by iodomethane treatment of prenylated substrates and reverse-phase HPLC. The CaaX preference of each recombinant yeast enzyme was found to be nearly identical to the reported preferences of purified mammalian protein farnesyltransferase and type-I protein geranylgeranyltransferase. S. cerevisiae farnesyltransferase preferentially farnesylated CaaX sequences ending in methionine, cysteine or serine. The farnesyltransferase also attached geranylgeranyl to some CaaX sequences ending in methionine, leucine and cysteine. The S. cerevisiae type-I geranylgeranyltransferase preferentially geranylgeranylated CaaX sequences ending in leucine and to a lesser degree methionine. Yeast extracts were found to contain prenylating activities identical to those observed with the recombinant enzymes. Farnesyltransferase activity in yeast extracts exceeded type-I geranylgeranyltransferase activity by at least 3-fold, resulting in prenylation of leucine-ending CaaX substrates with a mixture of 75% geranylgeranyl and 25% farnesyl. These results suggest that some substrate overlap may occur between the S. cerevisiae protein farnesyltransferase and the type-I protein geranylgeranyltransferase in vivo.

MAP kinase kinase kinase, MAP kinase kinase and MAP kinase

Signal transduction pathways that respond to external signals through the MAP kinase family of protein kinases are involved in diverse responses in eukaryotic cells. MAP kinases are one element in a series of kinases that serve to connect the plasma membrane with cytoplasmic and nuclear events. MAP kinases have the unusual feature that their activation requires threonine and tyrosine phosphorylation carried out by a dual specificity protein kinase. Recent advances have shown that in two MAP kinase pathways (the mating response pathway in the fission yeast Schizosaccharomyces pombe, and receptor tyrosine kinase signalling), the small GTP binding protein ras p21 links membrane events to kinase pathway activation.

Stimulation of phospholipase C-beta 2 by recombinant guanine-nucleotide-binding protein beta gamma dimers produced in a baculovirus/insect cell expression system. Requirement of gamma-subunit isoprenylation for stimulation of phospholipase C

Recombinant wild-type beta 1 gamma 1 dimers of signal-transducing guanine nucleotide-binding proteins (G proteins) and beta 1 gamma 1 dimers carrying a mutation known to block gamma-subunit isoprenylation (beta 1 gamma 1 C71S) were expressed in baculovirus-infected insect cells. Both wild-type and mutant beta 1 gamma 1 dimers were found in soluble fractions of infected cells upon subcellular fractionation. Anion exchange chromatographic and metabolic-radiolabeling studies revealed that the soluble beta 1 gamma 1 preparation contained approximately equal amounts of non-isoprenylated and isoprenylated beta 1 gamma 1 dimers. Soluble wild-type and mutant beta 1 gamma 1 dimers and native beta 1 gamma 1 dimers purified from bovine retina were reconstituted with recombinant phospholipase C-beta 2. Only isoprenylated beta 1 gamma 1 dimers were capable of stimulating phospholipase C-beta 2. The results show that gamma-subunit isoprenylation and/or additional post-translational processing of the protein are required for beta gamma subunit stimulation of phospholipase C.

Networking with mitogen-activated protein kinases

Mitogen activated protein (MAP) kinases and their target ribosomal protein S6 (RSK) kinases have been recognized as shared components in the intracellular signaling pathways of many diverse cytokines. Recent studies have extended this protein kinase cascade by identifying the major activator of vertebrate MAP kinases as a serine/threonine/tyrosine-protein kinase called MEK, which is related to yeast mating factor-regulated protein kinases encoded by the STE7 and byr1 genes. MEK, in turn, may be activated following its phosphorylation on serine by either of the kinases encoded by proto-oncogenes raf1 or mos, as well as by p78mekk, which is related to the yeast STE11 and byr2 gene products. Isoforms of all of these protein kinases may specifically combine to assemble distinct modules for intracellular signal transmission. However, the fundamental architecture of these protein kinase cascades has been highly conserved during eukaryotic evolution.

Normal and oncogenic p21ras proteins bind to the amino-terminal regulatory domain of c-Raf-1

In higher eukaryotes, the Ras and Raf-1 proto-oncoproteins transduce growth and differentiation signals initiated by tyrosine kinases. The Ras polypeptide and the amino-terminal regulatory domain of Raf-1 (residues 1-257) are shown to interact, directly in vitro and in a yeast expression system. Raf-1 (1-257) binds GTP-Ras in preference to GDP-Ras, and inhibits Ras-GAP activity. Mutations in and around the Ras effector domain impair Ras binding to Raf-1 (1-257) and Ras transforming activity in parallel.