Purification and Characterization from Bovine Brain Cytosol of Two Gtpase-Activating Proteins Specific for smg p21, a GTP-binding Protein Having the Same Effector Domain as c-ras p21s

IκB kinase γ/nuclear factor-κB-essential modulator (IKKγ/NEMO) facilitates RhoA GTPase activation, which, in turn, activates Rho-associated KINASE (ROCK) to phosphorylate IKKβ in response to transforming growth factor (TGF)-β1

Transforming growth factor (TGF)-β1 plays several roles in a variety of cellular functions. TGF-β1 transmits its signal through Smad transcription factor-dependent and -independent pathways. It was reported that TGF-β1 activates NF-κB and RhoA, and RhoA activates NF-κB in several kinds of cells in a Smad-independent pathway. However, the activation molecular mechanism of NF-κB by RhoA upon TGF-β1 has not been clearly elucidated. We observed that RhoA-GTP level was increased by TGF-β1 in RAW264.7 cells. RhoA-GDP and RhoGDI were bound to N- and C-terminal domains of IKKγ, respectively. Purified IKKγ facilitated GTP binding to RhoA complexed with RhoGDI. Furthermore, Dbs, a guanine nucletotide exchange factor of RhoA much more enhanced GTP binding to RhoA complexed with RhoGDI in the presence of IKKγ. Indeed, si-IKKγ abolished RhoA activation in response to TGF-β1 in cells. However, TGF-β1 stimulated the release of RhoA-GTP from IKKγ and Rho-associated kinase (ROCK), an active RhoA effector protein, directly phosphorylated IKKβ in vitro, whereas TGF-β1-activated kinase 1 activated RhoA upon TGF-β1 stimulation. Taken together, our data indicate that IKKγ facilitates RhoA activation via a guanine nucletotide exchange factor, which in turn activates ROCK to phosphorylate IKKβ, leading to NF-κB activation that induced the chemokine expression and cell migration upon TGF-β1.

Neurofibromatosis type I tumor suppressor neurofibromin regulates neuronal differentiation via its GTPase-activating protein function toward Ras

Neurofibromin, the neurofibromatosis type 1 (NF1) gene product, contains a central domain homologous to a family of proteins known as Ras-GTPase-activating proteins (Ras-GAPs), which function as negative regulators of Ras. The loss of neurofibromin function has been thought to be implicated in the abnormal regulation of Ras in NF1-related pathogenesis. In this study, we found a novel role of neurofibromin in neuronal differentiation in conjunction with the regulation of Ras activity via its GAP-related domain (GRD) in neuronal cells. In PC12 cells, time-dependent increases in the GAP activity of cellular neurofibromin (NF1-GAP) were detected after NGF stimulation, which were correlated with the down-regulation of Ras activity during neurite elongation. Interestingly, the NF1-GAP increase was due to the induction of alternative splicing of NF1-GRD type I triggered by the NGF-induced Ras activation. Dominant-negative (DN) forms of NF1-GRD type I significantly inhibited the neurite extension of PC12 cells via regulation of the Ras state. NF1-GRD-DN also reduced axonal and dendritic branching/extension of rat embryonic hippocampal neurons. These results demonstrate that the mutual regulation of Ras and NF1-GAP is essential for normal neuronal differentiation and that abnormal regulation in neuronal cells may be implicated in NF1-related learning and memory disturbance.

Activation of rho through a cross-link with polyamines catalyzed by Bordetella dermonecrotizing toxin

The small GTPase Rho, which regulates a variety of cell functions, also serves as a specific substrate for bacterial toxins. Here we demonstrate that Bordetella dermonecrotizing toxin (DNT) catalyzes cross-linking of Rho with ubiquitous polyamines such as putrescine, spermidine and spermine. Mass spectrometric analyses revealed that the cross-link occurred at Gln63, which had been reported to be deamidated by DNT in the absence of polyamines. Rac1 and Cdc42, other members of the Rho family GTPases, were also polyaminated by DNT. The polyamination, like the deamidation, markedly reduced the GTPase activity of Rho without affecting its GTP-binding activity, indicating that polyaminated Rho behaves as a constitutively active analog. Moreover, polyamine-linked Rho, even in the GDP-bound form, associated more effectively with its effector ROCK than deamidated Rho in the GTP-bound form and, when microinjected into cells, induced the anomalous formation of stress fibers indistinguishable from those seen in DNT-treated cells. The results imply that the polyamine-linked Rho, transducing signals to downstream ROCK in a novel GTP-independent manner, plays an important role in DNT cell toxicity.

Human SPA-1 gene product selectively expressed in lymphoid tissues is a specific GTPase-activating protein for Rap1 and Rap2. Segregate expression profiles from a rap1GAP gene product

Mouse Spa-1 gene with a region homologous to the human rap1GAP gene is transcriptionally induced in the lymphocytes by mitogenic stimulation. Herein we have cloned a cDNA for its human counterpart. SPA-1 cDNA encodes a 130-kDa protein (p130(SPA-1)) consisting of proline-rich regions and rap1GAP-related domain followed by a coiled-coil stretch. Baculovirally expressed p130(SPA-1) exhibited GTPase-activating protein (GAP) activity for Rap1 and Rap2, but not for Ras, Rho, Cdc42, Rac, and Ran, with comparable specific activity to the rap1GAP gene product (p85/95(rap1GAP)). In the cells, p130(SPA-1) was mostly localized at the perinuclear membranous region co-localizing with Rap1 and Rap2. Expression of SPA-1 and rap1GAP genes tended to be segregate in various tissues, lymphoid tissues expressing abundant SPA-1 transcript without rap1GAP, while those such as brain, kidney, and pancreas exhibiting rap1GAP mRNA with little SPA-1. Promyelocytic HL-60 cells, which expressed p130(SPA-1) with little p85/95(rap1GAP) in uninduced state, showed progressive decline in p130(SPA-1) and conversely drastic increase in p85/95(rap1GAP) as they ceased from proliferation and differentiated into macrophages by 12-O-tetradecanoylphorbol-13-acetate. These results suggested that products of SPA-1 and rap1GAP genes, albeit comparable GAP activity for Rap1 and Rap2, functioned in the distinct contexts depending on cell types and/or states.

The tuberous sclerosis 2 gene product, tuberin, functions as a Rab5 GTPase activating protein (GAP) in modulating endocytosis

The tuberous sclerosis complex 2 (TSC2) is a tumor suppressor gene that plays a causative role in the autosomal dominant syndrome of tuberous sclerosis. The latter is characterized by the development of hamartomas and occasional malignancies. Expression of the wild-type gene in TSC2 mutant tumor cells inhibits proliferation and tumorigenicity. This "suppressor" activity is encoded by functional domain(s) in the C terminus that contains homology to Rap1GAP. Using a yeast two-hybrid assay to identify proteins that interact with the C-terminal domain of tuberin, the product of TSC2, a cytosolic factor, rabaptin-5, was found to associate with a distinct domain lying adjacent to the TSC2 GAP homology region. Rabaptin-5 also binds the active form of GTPase Rab5. Immune complexes of native tuberin, as well as recombinant protein, possessed activity to stimulate GTP hydrolysis of Rab5. Tuberin GAP activity was specific for Rab5 and showed no cross-reactivity with Rab3a or Rab6. Cells lacking tuberin possessed minimal Rab5GAP activity and were associated with an increased uptake of horseradish peroxidase. Re-expression of tuberin in TSC2 mutant cells reduced the rate of fluid-phase endocytosis. These findings suggest that tuberin functions as a Rab5GAP in vivo to negatively regulate Rab5-GTP activity in endocytosis.

Characterization of human platelet GTPase activating protein for the Ral GTP-binding protein

RalA, a ras p21 related 27 kDa GTP-binding protein, was expressed as a fusion protein in Escherichia coli and purified to homogeneity using an immunoaffinity column. The purified protein was capable of binding and hydrolyzing GTP. Addition of platelet cytosolic or detergent solubilized particulate proteins stimulated the intrinsic GTPase activity of ralA by at least six-fold with maximal effect observed at pH 6.5. Addition of platelet proteins denatured by boiling had no effect on ralA GTPase activity. Analysis of GTPase reaction products by thin layer chromatography demonstrated that in samples containing ralA, 78.5 +/- 6.3% of the radioactivity was recovered in the GTP form while samples containing ralA plus platelet cytosol or particulate proteins, only 7.5 +/- 0.2% and 9.0 +/- 1.4% of the radioactivity was in the GTP form respectively. The GTPase activating protein(s) in the cytosolic and particulate fraction was further characterized by measuring GAP activity in proteins eluted from gel slices after sodium dodecyl sulfate polyacrylamide gel electrophoresis. The ralA GTPase activating protein present in the cytosol and particulate fractions was recovered in a single gel slice of identical apparent molecular weight. The molecular mass of the ral specific GTPase activating protein was estimated to be 34 +/- 2 kDa. This protein did not stimulate the intrinsic GTPase activity of ras p21, G25K/CDC42Hs or rab3A GTP-binding proteins. Results demonstrate that in human platelets, the activity/function of ral-related GTP-binding protein(s) is under the regulation of a specific GTPase activating protein of molecular mass of 34 +/- 2 kDa that is distributed equally in the cytosol and particulate fraction.

Cyclic AMP-dependent activation of Rap1b

Rap1 proteins belong to the Ras superfamily of small molecular weight GTP-binding proteins. Although Rap1 and Ras share approximately 50% overall amino acid sequence identity, the effector domains of the two proteins are identical, suggesting either similar or antagonistic signaling roles. Several pathways leading to Ras activation have been defined, including those initiated by agonist binding to tyrosine kinase or Gi-coupled receptors. Nothing is known about such events for Rap1 proteins. The cAMP-mediated inhibition of Ras-dependent MAP kinase activation is well documented and resembles that caused by expression of GTPase-deficient Rap1. We have developed a system whereby signals leading to Rap1b activation, i.e. an increase in Rap1b-bound GTP/GDP ratio, can be measured. We report here that treatment of cells with agents that elevate intracellular cAMP levels result in Rap1b activation. These results demonstrate for the first time agonist-dependent activation of Rap1 proteins.

Up-regulation of small GTP-binding proteins smg P21A and ras P21S during TPA-induced differentiation of human leukemia cell lines

The protein smg p21A/Krev-1/rap 1A was identified as a ras p21-like small G-protein, having the ability to revert v-Ki-ras transformed NIH 3T3 fibroblasts. The expression level of smg p21A and ras p21s during phorbol ester-induced differentiation of HL-60 and MEG-01 cell lines was analyzed by immuno- and Northern blotting. In both cell lines, levels of smg p21 and ras p21s increased quickly in early phase of differentiation along with the appearance of differentiation phenotypes. They increased 3-4 fold on days 1-2, then decreased gradually. The increasing smg 21 mRNA levels also corresponded with that of products. Among ras mRNAs, Ha-ras and N-ras transcripts increased somewhat faster than smg 21. These small G-proteins may play closely related roles in the differentiation of these leukemia cell lines.

Regulation of the GTPase activity of the ras-related rap2 protein

The small GTP-binding protein rap2A exhibits a high level of identity with rap1 and ras proteins (60% and 46%, respectively). Nevertheless, its intrinsic GTPase activity is not stimulated by ras-GAP, and unlike the rap1A protein, it cannot compete with ras proteins for their interaction with ras-GAP. In addition, rap1-GAPm that is highly active on the GTPase activity of the rap1A product, also stimulates the GTPase activity of the rap2A protein but with a 30-40-fold lower efficiency. An activity that greatly stimulated the GTPase activity of the rap2 protein (rap2-GAP) was found in bovine brain cytosol and purified. However, it copurified with the cytosolic form of rap1-GAP and was more efficient at stimulating the GTPase activity of the rap1 protein; this 55 kD polypeptide, that is recognized by an antibody raised against rap1-GAPm, likely represents a degraded and soluble form of the full size 89 kD molecule. In bovine brain membranes, a weak GAP activity toward the rap2A protein was also detected; however, it was also attributable to the membrane-associated rap1-GAPm. Thus, it appears that a single rap-GAP protein, complete or degraded, is able to stimulate the GTPase activity of both rap1 and rap2 proteins.

Purification and characterization of a low M(r) GTP-binding protein, ram p25, expressed by baculovirus expression system

The ram gene was isolated from rat megakaryocyte cDNA library with an oligonucleotide probe which is specific for a low M(r) GTP-binding proteins c25KG purified from human platelets. Its gene product (ram p25) is a monomeric 25-kDa guanine nucleotide-binding protein. The protein was expressed by using baculovirus transfer vector, pAcYM1, which allowed the production at a high level of soluble recombinant ram p25 in Spodoptera frugiperda (Sf9) cells under the control of polyhedrin promoter. The expressed protein in cytosol of Sf9 cells was purified to near homogeneity by a combination of DEAE-Toyopearl 650(S) and hydroxyapatite HCA-100S column chromatography. The purified ram p25 bound approx. 0.8 +/- 0.02 mol of guanosine 5'-O-1-thiotriphosphate (GTP gamma S)/mol of protein with a Kd value of 340 +/- 4.91 nM in a reaction mixture containing 10 microM of free magnesium ions. In the presence of 5 mM Mg2+, [3H]GDP was dissociated from ram p25 at the rate of 0.015 +/- 0.0010 min-1 and the dissociation was greatly enhanced by addition of 250 mM (NH4)2SO4. The rate of [gamma-32P]GTP-hydrolysis for ram p25 was 0.010 +/- 0.0012 min-1. Thus, it was indicated that the GTP-hydrolysis reaction is a rate-limiting step in the guanine nucleotide turnover of ram p25. ram p25 shares 23 and 80% amino-acid homology with the Ha-ras p21 and c25KG protein, respectively, and is similar to them in GTP gamma S binding activity in a time- and dose-dependent manner. But it differs from ras p21 in the rate-limiting step of the guanine nucleotide turnover.

Evidence of the presence of GTPase inhibiting proteins for a low M(r) GTP-binding protein, ram p25, in rat spleen and pheochromocytoma PC12 cells

1. GTPase inhibiting activity for a low M(r) GTP-binding protein, ram p25, was detected in the cytosolic fractions of rat spleen and PC12 cells. 2. The inhibitors were heat-labile and trypsin-sensitive, indicating that they were of proteinous nature. 3. The molecular mass of the inhibitor in the spleen appeared to be about 65 kDa on the elution profile from the gel filtration column.

Distinct and specific GAP activities in rat pancreas act on the yeast GTP-binding proteins Ypt1 and Sec4

Previous studies have demonstrated that Sec4, a 23.5 kDa guanine nucleotide-binding protein of the ras superfamily is required for exocytosis in the budding yeast Saccharomyces cerevisiae. Ypt1, another ras-like 23 kDa guanine nucleotide-binding protein in yeast has been found to be involved in ER-Golgi transport. A mammalian homologue of Ypt1 called rab1 has also been identified. Recent studies using purified Sec4 protein have identified a component of yeast lystate that specifically stimulates the hydrolysis of GTP bound to Sec4. In the present study, purified recombinant Sec4 and Ypt1 proteins expressed in E. coli have been used as substrates to determine if GTPase activating proteins (GAPs) directed toward these proteins are present in rat pancreas. Our studies showed that 65% of Sec4-GAP activity was associated with the 150,000 x g pancreatic particulate fraction with approximately 35% being found in the cytosol. On the other hand, more than 95% of Ypt1-GAP activity was found to associate with the particulate fraction. Sec4 and Ypt1 competition assays further demonstrated the specificity of the Sec4 and Ypt1 GAPs. The results from the present study suggest the presence of a distinct GAP in the pancreas that interacts with Sec4, and another that interacts with Ypt1.

Generation of specific antibodies against the rap1A, rap1B and rap2 small GTP-binding proteins. Analysis of rap and ras proteins in membranes from mammalian cells

Specific antibodies against rap1A and rap1B small GTP-binding proteins were generated by immunization of rabbits with peptides derived from the C-terminus of the processed proteins. Immunoblot analysis of membranes from several mammalian cell lines and human thrombocytes with affinity-purified antibodies against rap1A or rap1B demonstrated the presence of multiple immunoreactive proteins in the 22-23 kDa range, although at strongly varying levels. Whereas both proteins were present in substantial amounts in membranes from myelocytic HL-60, K-562 and HEL cells, they were hardly detectable in membranes from lymphoma U-937 and S49.1 cyc- cells. Membranes from human thrombocytes and 3T3-Swiss Albino fibroblasts showed strong rap1B immunoreactivity, whereas rap1A protein was present in much lower amounts. In the cytosol of HL-60 cells, only small amounts of rap1A and rap1B proteins were detected, unless the cells were treated with lovastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase, suggesting that both proteins are isoprenylated. By comparison with recombinant proteins, the ratio of rap1A/ras proteins in membranes from HL-60 cells was estimated to be about 4:1. An antiserum directed against the C-terminus of rap2 reacted strongly with recombinant rap2, but not with membranes from tested mammalian cells. In conclusion, rap1A and rap1B proteins are distributed differentially among membranes from various mammalian cell types and are isoprenylated in HL-60 cells.

Presence and some characterization of GDP dissociation inhibitors for a low Mr GTP-binding protein, ram p25, in rat spleen cytosol

Two proteinous factors, designated here as ram p25 GDP dissociation inhibitor (I) and (II) (ram-GDI (I) and (II)), were detected in the cytosolic fraction of rat spleen, which inhibited the initial dissociation of GDP from ram p25 produced by E. coli by causing characteristic lag. They had very weak effects on the rate of dissociation of GDP after the lag, and did not affect the mode of the dissociation of 5'-(3-O-thio)triphosphate (GTP gamma S) from ram p25. By gel filtration, the molecular masses of ram-GDI (I) and (II) were calculated to be 90 KDa and 40 KDa, respectively. These ram-GDIs did not affect the GDP-dissociation of Ha-ras protein produced in E. coli.

Partial suppression of tumorigenicity in a human lung cancer cell line transfected with Krev-1

A human non-small-cell lung carcinoma cell line, Calu-6 (from an anaplastic carcinoma), was transfected with the Ki-ras-related anti-oncogene Krev-1. Several transfectant lines were obtained that showed a reduced tumorigenicity in nude mice with respect to the parental and control transfected cell lines. This decrease was approximately 50% in tumor incidence at 4 wk after subcutaneous inoculation of the transfected cells. In addition, the volume of the Calu-6 revertant-derived tumors was three to 10 times smaller than that of the equivalent tumors produced by inoculation of the control cell line transfected with the neomycin-resistance gene. Krev-1--transfected cells that exhibited reduced tumorigenicity expressed Krev-1 mRNA and had variable numbers of copies of the Krev-1 gene. Moreover, Krev-1--transfected cells exhibited a more differentiated squamous epithelial morphology than the parental and control cell lines did. Moderately elevated levels of protein kinase C activity were detected in some revertant clones. Such activity correlated with the level of expression of Krev-1 mRNA in most cases. In summary, Krev-1 induced important morphological and biological changes in transfected Calu-6 cells that we interpreted as partial reversion of the malignant phenotype.

Heterogeneous amino acids in Ras and Rap1A specifying sensitivity to GAP proteins

Guanosine triphosphatase (GTPase) activity of Ras is increased by interaction with Ras-GAP (GTPase-activating protein) or with the GAP-related domain of the type 1 neurofibromatosis protein (NF1-GRD), but Ras is not affected by interaction with cytoplasmic and membrane forms of Rap-GAP; Rap1A, whose effector function can suppress transformation by Ras, is sensitive to both forms of Rap-GAP and resistant to Ras-GAP and NF1-GRD. A series of chimeric proteins composed of portions of Ras and Rap were constructed; some were sensitive to Ras-GAP but resistant to NF1-GRD, and others were sensitive to cytoplasmic Rap-GAP but resistant to membrane Rap-GAP. Sensitivity of chimeras to Ras-GAP and cytoplasmic Rap-GAP was mediated by amino acids that are carboxyl-terminal to the effector region. Residues 61 to 65 of Ras conferred Ras-GAP sensitivity, but a larger number of Rap1A residues were required for sensitivity to cytoplasmic Rap-GAP. Chimeras carrying the Ras effector region that were sensitive only to Ras-GAP or only to cytoplasmic Rap-GAP transformed NIH 3T3 cells poorly. Thus, distinct amino acids of Ras and Rap1A mediate sensitivity to each of the proteins with GAP activity, and transforming potential of Ras and sensitivity of Ras to Ras-GAP are at least partially independent properties.

Identification and partial purification of GTPase-activating proteins from yeast and mammalian cells that preferentially act on Ypt1/Rab1 proteins

Two GTPase-activating proteins of apparent molecular mass of 100 kDa and 30 kDa have been partially purified from porcine liver cytosol using mammalian Ypt1/Rab1 protein as substrate. Both proteins act most efficiently on Ypt1/Rab1p, but are inactive with H-Ras p21. From the budding yeast Saccharomyces cerevisiae, a cytosolic 40 kDa yptGAP was partially purified. It accelerates the intrinsic GTPase activity of wild-type Ypt1p but not of H-Ras p21 or a mutant ypt1p with an amino acid substitution of the effector domain which renders the protein functionally inactive in yeast cells.

Effects of the ras-related rap2 protein on cellular proliferation

Ras oncogenes encode 21-kDa (p21s) GTP binding proteins that are capable of transforming immortalized cells in culture. The ras-related rap1A/Krev-1/smgp21A protein, that exhibits a similar structural organization and contains the same effector domain as ras proteins, antagonizes ras-transformation. In order to investigate whether the closely related (61% identical) rap2 protein had similar capacities, the corresponding cDNA was inserted into constitutive as well as inducible mammalian expression vectors. Neither the wild-type, nor an "activated" mutant carrying a glycine-to-valine substitution at position 12, had any transforming activity. Several independent lines of evidence demonstrate that the rap2 protein exhibits neither growth-promoting nor growth-inhibitory effects, and that its over-expression does not interfere with ras-induced transformation. Thus, in spite of their great similarities, the rap1A/Krev-1/smgp21A and rap2 proteins have distinct physiological properties.

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.

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.

The Ras superfamilies: regulatory proteins and post-translational modifications

The Ras-like GTP-binding proteins comprise a large superfamily of proteins that play key roles in a wide variety of cellular activities, including cell growth, differentiation, secretion, and protein trafficking. During the past few years, it has become clear that these GTP-binding proteins are regulated by a variety of manners, including interactions with specific types of regulatory proteins and post-translational modification events.

Stoichiometric interaction of smg p21 with its GDP/GTP exchange protein and its novel action to regulate the translocation of smg p21 between membrane and cytoplasm

We have previously purified a GDP/GTP exchange protein for smg p21A and -B, members of a ras p21/ras p21-like small GTP-binding protein superfamily. This regulatory protein, named smg p21 GDP dissociation stimulator (GDS), stimulates the dissociation of both GDP and GTP from and the subsequent binding of both GDP and GTP to smg p21s. We show here that smg p21 GDS forms a complex with both the GDP- and GTP-bound forms of smg p21B at a molar ratio of about 1:1. Both the GDP- and GTP-bound forms of smg p21B bound to membranes. smg p21 GDS inhibited this binding and moreover induced the dissociation of the prebound smg p21B from the membranes. These results indicate that smg p21 GDS stoichiometrically interacts with smg p21B and thereby regulates its GDP/GTP exchange reaction and its translocation between membranes and cytoplasm.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

GTP binding proteins and growth factor signal transduction

There is a large body of evidence supporting a role for GTP-binding proteins in signal transduction by growth factors. In certain cells, ligands which activate or inhibit the production of cAMP via heterotrimeric G proteins promote replication of the target cell. These mechanisms play an important role in a limited number of tumours. Ligands which activate PI hydrolysis through heterotrimeric G proteins may also promote growth in certain systems, but the precise role for PI hydrolysis remains to be determined. Receptors with intrinsic tyrosine kinases may also interact with the heterotrimeric G proteins, but it is not known if these interactions represent side reactions, or whether they are central in the responses of certain cell types. Lastly, p21ras and other small molecular weight G proteins appear to be profoundly important in growth control. The tyrosine kinase growth factor receptors may interact indirectly with these GTP binding proteins via GAP proteins. The molecular detail of this process is emerging rapidly and is likely to be worked out in the near future.