RalA interacts directly with the Arf-responsive, PIP2-dependent phospholipase D1

RalA GTPase associates with a phospholipase D (PLD) that is activated in v-Src- and v-Ras-transformed cells. Two mammalian PLDs were recently cloned: PLD1, which is activated by Arf family GTPases and dependent upon phosphatidylinositol-4,5-bisphosphate (PIP2), and PLD2, which is also dependent upon PIP2, but not stimulated by Arf. Another PLD has been described that is stimulated by oleate. Evidence is provided that the RalA-assiciated PLD is PLD1. First, the PLD precipitated by RalA from murine fibroblasts was stimulated by Arf, dependent upon PIP2, and inhibited by oleate. Second, immobilized RalA precipitated PLD1 from sf9 insect cells overexpressing PLD1. Third, a series of RalA mutants precipitated PLD activity from both PLD1-expressing insect cells and murine fibroblasts with the same efficiency. And finally, immobilized RalA precipitated PLD1 from a purified PLD1 preparation. These data argue that RalA associates directly with the Arf-responsive, PIP2-dependent PLD1.

Altering the reaction coordinate of the ATP sulfurylase-GTPase reaction

ATP sulfurylase, isolated from Escherichia coli K-12, catalyzes and couples two reactions: the hydrolysis of GTP and the synthesis of APS (adenosine 5'-phosphosulfate). Its GTPase activity is regulated in response to ligand binding at the APS-forming active site. In particular, AMP mimics an intermediate-like form of the enzyme that increases the k(cat) for GTP hydrolysis 180-fold. Using equilibrium and pre-steady-state methods, we have determined the relative Gibbs energies for many of the ground and transition states in the GTPase catalytic cycle, in the presence and absence of AMP. GTP and AMP energetically interact throughout the substrate branch of the reaction coordinate; however, once bond breaking occurs, communication between nucleotides ceases. Stopped-flow experiments, using the fluorescent nucleotides 2'-deoxy-mant-GTP and -GDP, indicate that the binding of AMP fosters a conformation of the enzyme that hinders the addition of 2'-deoxy-mant-GTP into the active site without affecting its escaping tendency. These results explain the effects of AMP on the equilibrium binding of the 2'-deoxy-mant-GTP. The second-order rate constants for the binding of 2'-deoxy-mant-GTP or -GDP, approximately 1 x 10(-6) M(-1) s(-1), are 2-3 orders of magnitude less than expected for simple diffusion models, and the binding progress curves appear biphasic. These findings suggest the presence of an intermediate(s) in the binding reactions. The Gibbs energy changes that occur in the reaction coordinate upon binding of AMP clearly show that the catalytic effect of AMP is due primarily to its -3.1 kcal/mol stabilization of the rate-limiting transition state.

ATPase, GTPase, and RNA binding activities associated with the 206-kilodalton protein of turnip yellow mosaic virus

The 206-kDa protein of turnip yellow mosaic virus belongs to an expanding group of proteins containing a domain which includes the consensus nucleotide binding site GxxxxGKS/T. A portion of this protein (amino acids [aa] 916 to 1259) was expressed in Escherichia coli and purified by affinity chromatography to near homogeneity. In the absence of any other viral factors, it exhibited ATPase and GTPase activities in vitro. A mutant protein with a single amino acid substitution in the consensus nucleotide binding site (Lys-982 to Ser) exhibited only low levels of both activities, implying that Lys-982 is important for nucleoside triphosphatase activity. The protein also possessed nonspecific RNA binding capacity. Deletion mutants revealed that an N-terminal domain (aa 916 to 1061) and a C-terminal domain (aa 1182 to 1259) participate in RNA binding. The results presented here provide the first experimental evidence that turnip yellow mosaic virus encodes nucleoside triphosphatase and RNA binding activities.

Human MxB protein, an interferon-alpha-inducible GTPase, contains a nuclear targeting signal and is localized in the heterochromatin region beneath the nuclear envelope

Interferon-inducible Mx proteins belong to the family of large GTPases and are highly homologous with dynamins within their GTP-binding domain. Cytoplasmically localized human MxA protein mediates resistance to influenza and several other viruses, whereas human MxB protein has not been found to have any antiviral activity. Here we show that MxB protein is found both in the cytoplasm and in the nucleus, where it is localized in a granular pattern in the heterochromatin region beneath the nuclear envelope. Transfection experiments in COS cells of N-terminally deleted MxB constructs revealed a functional nuclear localization signal within the first 24 N-terminal amino acids. Nuclear 78-kDa and cytoplasmic 76-kDa forms of MxB protein were found in all of the cell lines studied and in human peripheral blood mononuclear cells. MxB protein proved to be a functional GTPase with activity comparable to that of MxA protein. N-terminally truncated (delta1-82) MxB protein lacking both the nuclear localization signal and a proline-rich domain had almost completely lost its GTPase activity. Analysis of peripheral blood mononuclear cells suggested that MxB protein expression is strictly regulated by interferon-alpha. This is the first documentation that human Mx protein resides in the nucleus. It also emphasizes that there are considerable differences in the localization and structure of functional domains within Mx proteins.

Deletion of the putative effector region of Era, an essential GTP-binding protein in Escherichia coli, causes a dominant-negative phenotype

Era is an essential gene in E. coli, encoding a GTP-binding protein of unknown function. In the present work, a mutant designated Era-dE, for deletion of effector region is described. This is the first and only known era allele that confers a dominant-negative phenotype. Phenotypic analysis of the mutant showed that overproduction of Era-dE caused a dominant inhibition of growth when TCA cycle intermediates such as succinate, pyruvate, malate, alpha-ketoglutarate, and fumarate were provided as the sole carbon source. Examination of the macromolecular composition of cells overexpressing the mutant showed protein, DNA, and ATP levels expected for cells growing at slow rates. The response of cells expressing Era-dE to different stress conditions was studied by examining the rates of synthesis of stress-inducible proteins. Interestingly, when subjected to succinate starvation, cells expressing Era-dE showed a defective carbon starvation response, whereas response to glucose starvation was similar to that seen in control cells. Taken together with previous results, these studies indicate that Era is perhaps involved in multiple cellular processes and Era-dE disrupts more than one of these functions. Furthermore, it appears that some possible functions of Era include regulation of the TCA cycle and response to carbon starvation.

GTPase properties of the interferon-induced human guanylate-binding protein 2

Guanylate-binding proteins (GBPs) were originally described as proteins that are strongly induced by interferons and are capable of binding to agarose-immobilized guanine nucleotides. hGBP1, the first of two members of this protein family in humans, was recently shown to represent a novel type of GTPase that hydrolyzes GTP predominantly to GMP. We now report that purified recombinant hGBP2 also hydrolyzes GTP very efficiently, although GDP rather than GMP was the major reaction product. The biochemical parameters of this reaction were as follows: Km = 313 microM, turnover number = 22 min-1. Both hGBP1 and hGBP2 failed to hydrolyze GDP, however, GDP was an effective inhibitor of the hGBP2- but not the hGBP1-catalyzed GTP hydrolysis reaction. Thus, hGBP1 and hGBP2 have similar biochemical properties, but show pronounced differences in product specificity.

Activation of yeast protein kinase C by Rho1 GTPase

We have investigated the role of the essential Rho1 GTPase in cell integrity signaling in budding yeast. Conditional rho1 mutants display a cell lysis defect that is similar to that of mutants in the cell integrity signaling pathway mediated by protein kinase C (Pkc1), which is suppressed by overexpression of Pkc1.rho1 mutants are also impaired in pathway activation in response to growth at elevated temperature. Pkc1 co-immunoprecipitates with Rho1 in yeast extracts, and recombinant Rho1 associates with Pkc1 in vitro in a GTP-dependent manner. Recombinant Rho1 confers upon Pkc1 the ability to be stimulated by phosphatidylserine, indicating that Rho1 controls signal transmission through Pkc1.

Identification of yeast Rho1p GTPase as a regulatory subunit of 1,3-beta-glucan synthase

1,3-beta-D-glucan synthase [also known as beta(1-->3) glucan synthase] is a multi-enzyme complex that catalyzes the synthesis of 1,3-beta-linked glucan, a major structural component of the yeast cell wall. Temperature-sensitive mutants in the essential Rho-type guanosine triphosphatase (GTPase), Rho1p, displayed thermolabile glucan synthase activity, which was restored by the addition of recombinant Rho1p. Glucan synthase from mutants expressing constitutively active Rho1p did not require exogenous guanosine triphosphate for activity. Rho1p copurified with beta(1-->3)glucan synthase and associated with the Fks1p subunit of this complex in vivo. Both proteins were localized predominantly at sites of cell wall remodeling. Therefore, it appears that Rho1p is a regulatory subunit of beta(1-->3)glucan synthase.

GTP hydrolysis by human tissue transglutaminase homologue

Human tissue transglutaminase homologue cDNA was expressed in E. coli to analyze the catalytic characteristics. The transglutaminase homologue was purified by immunoaffinity chromatography. Specificity of GTP binding by the homologue was demonstrated by photoaffinity labeling in the absence or presence of GTP-gamma-S. The homologue had GTPase activity with an apparent Km value of 1.8 microM, several-fold lower than the reported Km values for the native tissue transglutaminase. GTPase activity was inhibited by guanine nucleotides in order GTP-gamma-S > GDP > GMP. The higher GTPase activity of the homologue may be related to the signaling events function.

The Neisseria transcriptional regulator PilA has a GTPase activity

The pilA gene of Neisseria gonorrhoeae encodes the response regulator of a two-component regulatory system that controls pilin gene expression. Examination of the primary sequence of PilA indicates that the protein contains at least two functional domains. The N-terminal region has a proposed helix-turn-helix motif thought to be involved in DNA binding. This region also contains the residues that are presumed to form the acidic pocket involved in phosphorylation by PilB, the sensor kinase of the system. The C terminus of the protein has extensive homology to the G (GTP-binding) domains of the eukaryotic signal recognition particle (SRP) 54-kDa protein and the alpha subunit of the SRP receptor, or docking protein. This homology also extends to similar regions of the bacterial SRP homologs Ffh and FtsY. Here, we demonstrate that purified PilA has significant GTPase activity, and that this activity has an absolute requirement for MgCl2 and is sensitive to KCl and low pH. We also show that PilA has a strict specificity for GTP, and that GTP hydrolysis follows first order kinetics, with a maximum velocity (Vmax) of 1900 pmol of Pi produced per min per mg of protein and a Km for GTP of 9.6 microM at 37 degrees C.

Purification of chlorpromazine-sensitive GTPase from rat cerebral cortex

The chlorpromazine-sensitive GTPase from the cell membrane of rat cerebral cortex was purified to homogenity by using DEAE Bio-Gel A agarose, hydroxyapatite and heparin agarose chromatography. The purified chlorpromazine-sensitive GTPase was purified 370-fold to obtain a final specific activity of 40 mumol GTP hydrolyzed2min/mg protein. The purified enzyme was inhibited by chlorpromazine but not by compound 48/80. Magnesium was required for its activity instead of calcium. The purified enzyme had an apparent pH optimum of 8.0, and molecular weight was estimated to be 58,000.

Expression, purification, and characterization of a novel G protein, SGP, from Streptococcus mutans

The sgp gene of Streptococcus mutans was recently detected immediately downstream from the dgk gene within the same operon. In this study, the sgp gene was subcloned into the pMAL-c2 vector and SGP (S. mutans G protein) was overexpressed in Escherichia coli as a fusion protein with the maltose-binding protein at a level of 40% of total cellular protein. One-step amylose affinity chromatography purification of this fusion protein yielded a product of approximately 95% purity. SGP was purified from this fusion protein following cleavage with protease factor Xa and DEAE-Sephacel chromatography. In nucleotide binding assays, recombinant SGP showed specific binding for GTP and GDP, but not ATP, CTP, and UTP, and also catalyzed efficient hydrolysis of GTP to GDP. Kinetic studies revealed that the SGP Km value for GTP in this reaction was approximately 5.9 microM. Mg2+ also served as a cofactor of SGP in this reaction. In vivo subcellular localization by immunogold labelling demonstrated that SGP was associated with both membrane and cytoplasmic fractions. SGP not only had structural similarities with other G proteins but also proved to have high-level intrinsic GTPase activity. Therefore, SGP appears to be a new member of the G protein superfamily and may participate in transmembrane signaling in the responses of S. mutans cells to environmental stimuli.

Biochemical similarity of Schizosaccharomyces pombe ras1 protein with RAS2 protein of Saccharomyces cervisiae

Schizosaccharomyces pombe contains single ras oncogene homologue, ras1, that functions in the signal transduction pathway conducting the cell's mating processes. To understand the biochemical basis of yeast ras proteins, we have purified the ras1 protein and compared the major biochemical constants with those of RAS2 protein from Saccharomyces cerevisiae and mammalian ras proteins. The purified ras1 protein showed a remarkably high Kd value for GDP binding (178 nM) and for binding with ATP. In contrast, the Kd value for GTP binding and the rate of GTPase activity were 64 nM and 77 x 10(-6) s-1 at 37 degrees C, respectively; both were higher than normal p21ras protein, but at the same level as the RAS2 protein. We directly measured rate of GTP binding and GDP binding which were 3.9 x 10(-3) s-1 and 1.8 x 10(-3) s-1 at 30 degrees C, respectively. On the other hand, exchange rates between bound and free nucleotides remained almost constant throughout the tested combination of GTP and GDP, and were several-fold lower than the binding rate. These results suggest that the release of the guanine nucleotide is the rate-limiting step in the ras-GTP/GDP cycle. As a whole, the biochemical properties of the ras1 protein are close to those of the RAS2 protein, although these two proteins function differently in the signal transduction pathway in the cells.

Stimulation of human monocytes with macrophage colony-stimulating factor induces a Grb2-mediated association of the focal adhesion kinase pp125FAK and dynamin

Macrophage colony-stimulating factor (M-CSF) is required for the growth and differentiation of mononuclear phagocytes. In the present studies using human monocytes, we show that M-CSF induces interaction of the Grb2 adaptor protein with the focal adhesion kinase pp125FAK. The results demonstrate that tyrosine-phosphorylated pp125FAK directly interacts with the SH2 domain of Grb2. The findings indicate that a pYENV site at Tyr-925 in pp125FAK is responsible for this interaction. We also demonstrate that the Grb2-FAK complex associates with the GTPase dynamin. Dynamin interacts with the SH3 domains of Grb2 and exhibits M-CSF-dependent tyrosine phosphorylation in association with pp125FAK. These findings suggest that M-CSF-induced signaling involves independent Grb2-mediated pathways, one leading to Ras activation and another involving pp125FAK and a GTPase implicated in receptor internalization.

Quantitative analysis of the interactions between prenyl Rab9, GDP dissociation inhibitor-alpha, and guanine nucleotides

Rab9 is a Ras-like GTPase required for the transport of mannose 6-phosphate receptors between late endosomes and the trans Golgi network. Rab9 occurs in the cytosol as a complex with GDP dissociation inhibitor (GDI), which we have shown delivers prenyl Rab9 to late endosomes in a functional form. We report here basal rate constants for guanine nucleotide dissociation and GTP hydrolysis for prenyl Rab9. Both rate constants were influenced in part by the hydrophobic environment of the prenyl group. Guanine nucleotide dissociation and GTP hydrolysis rates were lower in the presence of lipid; detergent stimulated intrinsic nucleotide exchange. GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 2.4-fold. GDI-alpha associated with prenyl Rab9 with a KD of 60 nM in 0.1% Lubrol and 23 nM in 0.02% Lubrol. In 0.1% Lubrol, GDI-alpha inhibited GDP dissociation half maximally at 72 +/- 18 nM, consistent with the KD determinations. These data suggest that GDI-alpha associates with prenyl Rab9 with a KD of < or = 23 nM under physiological conditions. Finally, a previously uncharacterized minor form of GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 1.9-fold and bound prenyl Rab9 with a KD of 67 nM in 0.1% Lubrol.

Isolation of a murine cDNA clone encoding Rab19, a novel tissue-specific small GTPase

Using a rapid amplification of cDNA ends (RACE) cloning approach, we have isolated a cDNA clone encoding Rab19, a novel small GTPase of the Rab subfamily contained within partial sequences previously described [Chavrier et al., Gene 112 (1992) 261-264]. Northern blot analysis of the distribution of the rab19 mRNA in various adult mouse tissues and NIH 3T3 fibroblasts revealed that rab19 is expressed in a tissue-specific manner. The rab19 transcript was detected at high levels in intestine, lung and spleen, and at a lower level in kidney. In contrast, liver, brain, heart and NIH 3T3 fibroblasts contain only very little or no detectable rab19 mRNA. Therefore, Rab19 is likely to represent a novel tissue- or cell type-specific small GTPase.

Dynamin GTPase is stimulated by crosslinking through the C-terminal proline-rich domain

Dynamin is a 100 kDa GTPase required for endocytic-coated vesicle formation. Recombinant human neuronal dynamin (dynamin-1) was used for monoclonal antibody (mAb) production. Two mAbs, designated hudy-2 (for human dynamin) and hudy-4, were chosen for further study based on their differential ability to recognize dynamin-1 and its non-neuronal isoform, dynamin-2. Both bind to the proline-rich C-terminal domain (PRD) of dynamin and inhibit the ability of microtubules and grb2 to stimulate GTPase activity. Hudy-4 binds to an epitope within the last 20 amino acids of dynamin-1 and has no effect on its intrinsic GTPase activity. Hudy-2 binds to an epitope within amino acids 822-838 that is common to dynamin-1 and dynamin-2. Hudy-2 stimulates dynamin's intrinsic GTPase activity in a manner proportional to the valency of the immunoglobulin (Ig) G. Crosslinking IgGs with secondary antibodies caused a 2-fold increase in GTPase activity, while F(ab)s were inactive. Importantly, our findings suggest that the stimulation of dynamin GTPase activity by multivalent proteins which bind in vitro to the PRD may not be a valid criterion on its own for assessing the in vivo functional significance of these interactions.

Biochemical and functional characterization of a recombinant GTPase, Rab5, and two of its mutants

Biochemical, structural, and functional properties of Rab5 wild-type (WT) protein were compared with those of Q79L and N133I mutants. The detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate increased guanine nucleotide binding to Rab5 WT approximately 10-fold. The single-step catalytic rate of Rab5 WT exceeded that of Q79L 12.2-fold, but the steady-state GTPase rate was only 2.8-fold greater because GDP dissociation was rate-limiting and GDP dissociation was 3.6-fold slower than for Q79L. In contrast, dissociation rates of GTP were indistinguishable. Binding to Rab5 N133I was not detectable. GTP protected Rab5 WT and Q79L from any apparent proteolysis by trypsin. A 20-kDa fragment was the major product of digestion in the presence of GDP, and 12- and 8-kDa fragments were the major products in the absence of added guanine nucleotides. Rab5 N133I underwent no apparent proteolysis with 10 mM GTP or GDP, suggesting a "triphosphate" conformation may be induced in Rab5 N133I by either GTP or GDP. Partially geranylgeranylated Rab5 WT stimulated endosome fusion in vitro, whereas unmodified Rab5 WT did not. Processed Rab5 Q79L failed to inhibit endosome fusion, and Rab5 N133I could not be geranylgeranylated. These findings identify biochemical and structural features of Rab5 proteins, providing data for the interpretation of functional assays.

Quantitative analysis of the complex between p21ras and the Ras-binding domain of the human Raf-1 protein kinase

The Ras-binding domain (RBD) of human Raf-1 was purified from Escherichia coli, and its interaction with Ras was investigated. Its dissociation constant with p21ras.guanyl-5'-yl imidodiphosphate was found to be 18 nM, with a slight preference for H-ras over K- and N-ras. Oncogenic forms bind with slightly lower affinity. The affinity of RBD for effector region mutants or the GDP-bound form of p21ras is in the micromolar range, which means that 100-fold lower affinity is not sufficient for signal transduction. The rate of the GTPase of p21ras is not modified by RBD. Since P(i) release is found not to be rate limiting, the Ras-Raf signal of the cell may be terminated by the intrinsic GTPase of p21ras.

Characterization of breakpoint cluster region kinase and SH2-binding activities

BCR is an interesting signaling protein, whose cellular function is currently unknown. Its biochemical properties include serine kinase activity, SH2-binding activity, and a GTPase-activating activity. The SH2-binding activity is particularly interesting because it may link BCR to signaling pathways involving SH2-containing molecules. Since tyrosine phosphorylation of BCR has been detected in CML-derived cell lines and since tyrosine-phosphorylated BCR shows increased affinity toward certain SH2 domains, it seems particularly important to further characterize this activity. This chapter described a simple purification scheme for partial purification of BCR, which can be used to assess in vitro kinase and SH2-binding activities.