Impact of labile and recalcitrant carbon treatments on available nitrogen and plant communities in a semiarid ecosystem

In a 10-year study, we assessed the influence of five carbon (C) treatments on the labile C and nitrogen (N) pools of historically N-enriched plots on the Shortgrass Steppe Long Term Ecological Research site located in northeastern Colorado. For eight years, we applied sawdust, sugar, industrial lignin, sawdust + sugar, and lignin + sugar to plots that had received N and water additions in the early 1970s. Previous work showed that past water and N additions altered plant species composition and enhanced rates of nutrient cycling; these effects were still apparent 25 years later. We hypothesized that labile C amendments would stimulate microbial activity and suppress rates of N mineralization, whereas complex forms of carbon (sawdust and lignin) could enhance humification and lead to longer-term reductions in N availability. Results indicated that, of the five carbon treatments, sugar, sawdust, and sawdust + sugar suppressed N availability, with sawdust + sugar being the most effective treatment to reduce N availability. The year after treatments stopped, N availability remained less in the sawdust + sugar treatment plots than in the high-N control plots. Three years after treatments ended, reductions in N availability were smaller (40-60%). Our results suggest that highly labile forms of carbon generate strong short-term N sinks, but these effects dissipate within one year of application, and that more recalcitrant forms reduce N longer. Sawdust + sugar was the most effective treatment to decrease exotic species canopy cover and increase native species density over the long term. Labile carbon had neither short- nor long-term effects on exotic species. Even though the organic amendments did not contribute to recovery of the dominant native species Bouteloua gracilis, they were effective in increasing another native species, Carex eleocharis. These results indicate that organic amendments may be a useful tool for restoring some native species in the shortgrass steppe, though not all.

Application of beta-galactosidase enzyme complementation technology as a high throughput screening format for antagonists of the epidermal growth factor receptor

We have applied enzyme complementation technology to develop a screen for antagonists of the epidermal growth factor (EGF) receptor. Chimeric proteins containing two weakly complementing deletion mutants of Escherichia coli beta-galactosidase (beta-gal), each fused to the EGF receptor extracellular and transmembrane domains, have been stably expressed in C2C12 cells. In this cell line, formation of active beta-gal is dependent on agonist-stimulated dimerization of the EGF receptor. We have developed a homogenous 384-well assay protocol and have applied this to characterize the pharmacology of the receptor and to develop a high throughput screen (HTS) for EGF receptor antagonists. The assay is tolerant to DMSO concentrations of up to 2% and, across 21 passages in culture, exhibits an EC(50) for EGF of 5.4 +/- 3.6 ng/ml (n = 11) and a Z' of 0.55 +/- 0.13 (n = 11). A random set of 1,280 compounds was screened in duplicate at 11 microM to examine the robustness of enzyme complementation technology and to characterize the false-positive hit rate in the assay. Using a cutoff of 40% inhibition of EGF-promoted beta-gal activity, the hit rate on day 1 was 2.5% and on day 2 was 1.9%. After retesting the active compounds, the hit rate was reduced to 0.4%, of which one of the compounds was identified as a beta-gal inhibitor and the remainder appeared to be nonspecific inhibitors in the assay. This technology is amenable to automated screen workstations, there are highly sensitive chemiluminescent and fluorescent beta-gal assay reagents amenable to detection in miniaturized plate formats, and the assay benefits from a low false-positive hit rate. Enzyme complementation technology may have wide application within the HTS environment for the detection of modulators of receptor activation or inhibitors of protein-protein interactions in mammalian cells.

A method to measure the interaction of Rac/Cdc42 with their binding partners using fluorescence resonance energy transfer between mutants of green fluorescent protein

Enhanced blue fluorescent protein (EBFP) and enhanced green fluorescent protein (EGFP) mutants of GFP in close proximity to one another can act as a fluorescence resonance energy transfer (FRET) pair. Unstructured amino acid linkers of varying length were inserted between EBFP and EGFP, revealing that linkers even as long as 50 amino acids can be accommodated and still allow FRET to occur. This led to the development of a novel biosensor for Rac/Cdc42 binding to their effector proteins based on the insertion of amino acids 75-118 of p21-activated kinase (PAK) between the GFP mutants. We demonstrate that this protein construct allows significant FRET between EBFP and EGFP and retains the ability to bind to Rac in its GTP-bound form with a binding affinity similar to the uncomplexed PAK fragment, and furthermore, on binding to Rac or Cdc42 a marked change in FRET takes place. This forms the basis for a simple, sensitive, and rapid method to measure binding of Rac/Cdc42 to their effector proteins. Since the signal is dependent upon the interaction with active GTP-bound forms it acts as a biosensor for the activation of Rac/Cdc42. It has the potential for use in live cells and for identifying localization of Rac/Cdc42 within subcellular compartments.

Structure of Cdc42 bound to the GTPase binding domain of PAK

The Rho family GTPases, Cdc42, Rac and Rho, regulate signal transduction pathways via interactions with downstream effector proteins. We report here the solution structure of Cdc42 bound to the GTPase binding domain of alphaPAK, an effector of both Cdc42 and Rac. The structure is compared with those of Cdc42 bound to similar fragments of ACK and WASP, two effector proteins that bind only to Cdc42. The N-termini of all three effector fragments bind in an extended conformation to strand beta2 of Cdc42, and contact helices alpha1 and alpha5. The remaining residues bind to switches I and II of Cdc42, but in a significantly different manner. The structure, together with mutagenesis data, suggests reasons for the specificity of these interactions and provides insight into the mechanism of PAK activation.

Residues in Cdc42 that specify binding to individual CRIB effector proteins

Cdc42 is a member of the Rho family of small G proteins. Signal transduction events emanating from Cdc42 lead to cytoskeletal rearrangements, cell proliferation, and cell differentiation. Many effector proteins have been identified for Cdc42; however, it is not clear how certain effectors specifically recognize and bind to Cdc42, as opposed to Rac or Rho, or in many cases, which effector controls what cellular events. Mutations were introduced into Cdc42 at residues: Met1, Val8, Phe28, Tyr32, Val33, Thr35, Val36, Phe37, Asp38, Tyr40, Val42, Met45, Ile46, Glu127, Ala130, Asn132, Gln134, Lys135, and Leu174. Measurements were made of their equilibrium binding constants to the Cdc42 binding domains of the CRIB effectors ACK, PAK, and WASP and to the GTPase-activating protein Rho GAP. Generally, mutations in the effector loop have an equally deleterious effect on binding to all CRIB proteins tested, though the F37A mutation resulted in significant selectivity. Residues outside the effector loop were found to be important for binding of Cdc42 to CRIB containing proteins and also to contribute to selectivity. Mutations such as V42A and L174A resulted in large, selective changes in binding to specific CRIB effectors. Neither mutation resulted in alteration in PAK binding, whereas both severely disrupt binding to ACK and only L174A disrupted binding to WASP. These mutations are interpreted using the structures of the Cdc42/ACK and Cdc42/WASP complexes to give insight into how effectors can specifically recognize Cdc42. Those mutations in Cdc42 that inhibit certain interactions, while retaining others, should aid investigations of the role of specific effectors in Cdc42 signaling in vivo.

Magnesium fluoride-dependent binding of small G proteins to their GTPase-activating proteins

GTPase-activating proteins (GAPs) enhance the intrinsic GTPase activity of small G proteins, such as Ras and Rho, by contributing a catalytic arginine to the active site. An intramolecular arginine plays a similar role in heterotrimeric G proteins. Aluminum fluoride activates the GDP form of heterotrimeric G proteins, and enhances binding of the GDP form of small G proteins to their GAPs. The resultant complexes have been interpreted as analogues of the transition state of the hydrolytic reaction. Here, equilibrium binding has been measured using scintillation proximity assays to provide quantitative information on the fluoride-mediated interaction of Ras and Rho proteins with their respective GAPs, neurofibromin (NF1) and RhoGAP. High-affinity fluoride-mediated complex formation between Rho.GDP and RhoGAP occurred in the absence of aluminum; however, under these conditions, magnesium was required. Additionally, the novel observation was made of magnesium-dependent, fluoride-mediated binding of Ras.GDP to NF1 in the absence of aluminum. Aluminum was required for complex formation when the concentration of magnesium was low. Thus, either aluminum fluoride or magnesium fluoride can mediate the high-affinity binding of Rho. GDP or Ras.GDP to GAPs. It has been reported that magnesium fluoride can activate heterotrimeric G proteins. Thus, magnesium-dependent fluoride effects might be a general phenomenon with G proteins. Moreover, these data suggest that some protein.nucleotide complexes previously reported to contain aluminum fluoride may in fact contain magnesium fluoride.

Structure of the small G protein Cdc42 bound to the GTPase-binding domain of ACK

The proteins Cdc42 and Rac are members of the Rho family of small GTPases (G proteins), which control signal-transduction pathways that lead to rearrangements of the cell cytoskeleton, cell differentiation and cell proliferation. They do so by binding to downstream effector proteins. Some of these, known as CRIB (for Cdc42/Rac interactive-binding) proteins, bind to both Cdc42 and Rac, such as the PAK1-3 serine/threonine kinases, whereas others are specific for Cdc42, such as the ACK tyrosine kinases and the Wiscott-Aldrich-syndrome proteins (WASPs). The effector loop of Cdc42 and Rac (comprising residues 30-40, also called switch I), is one of two regions which change conformation on exchange of GDP for GTP. This region is almost identical in Cdc42 and Racs, indicating that it does not determine the specificity of these G proteins. Here we report the solution structure of the complex of Cdc42 with the GTPase-binding domain ofACK. Both proteins undergo significant conformational changes on binding, to form a new type of G-protein/effector complex. The interaction extends the beta-sheet in Cdc42 by binding an extended strand from ACK, as seen in Ras/effector interactions, but it also involves other regions of the G protein that are important for determining the specificity of effector binding.

The conserved arginine in rho-GTPase-activating protein is essential for efficient catalysis but not for complex formation with Rho.GDP and aluminum fluoride

The Rho family of small GTP-binding proteins are downregulated by an intrinsic GTPase, which is enhanced by GTPase-activating proteins (GAPs). RhoGAPs contain a single conserved arginine residue that has been proposed to be involved in catalysis. Here, the role of this arginine has been elucidated by mutagenesis followed by determination of catalytic and equilibrium binding constants using single-turnover kinetics, isothermal titration calorimetry, and scintillation proximity assays. The turnover numbers for wild-type, R282A, and R282K RhoGAPs were 5.4, 0.023, and 0.010 s-1, respectively. Thus, the function of this arginine could not be replaced by lysine or alanine. Nevertheless, the R282A mutation had a minimal effect on the binding affinity of RhoGAP for either Rho. GTP or Rho.GMPPNP, which confirms the importance of the arginine residue for catalysis as opposed to formation of the protein-protein complex. The R282A mutant RhoGAP still increased the hydrolysis rate of Rho.GTP by 160-fold, whereas the wild-type enzyme increased it by 38000-fold. We conclude that this arginine contributes half of the total reduction of activation energy of catalysis. In the presence of aluminum fluoride, the R282A mutant RhoGAP binds almost as well as the wild type to Rho.GDP, demonstrating that the conserved arginine is not required for this interaction. The affinity of wild-type RhoGAP for the triphosphate form of Rho is similar to that for Rho.GDP with aluminum fluoride. These last two observations show that this complex is not associated with the free energy changes expected for the transition state, although the Rho.GDP.AlF4-.RhoGAP complex might well be a close structural approximation.

Delineation of the Cdc42/Rac-binding domain of p21-activated kinase

p21-activated kinases (PAKs) serve as effector proteins for the GTP-binding proteins Cdc42 and Rac. They are serine/threonine kinases containing the Cdc42/Rac interactive binding (CRIB) motif. The main aim of this study was to define the minimal domain of alphaPAK required for Cdc42/Rac binding. Eight stable PAK fragments of varying lengths, each containing the CRIB motif (residues 75-88), were expressed in Escherichia coli, and their ability to interact with Cdc42 and Rac was assessed using scintillation proximity assays, isothermal titration calorimetry, and fluorescence techniques. The shortest fragments examined (residues 70-94 and 75-94) bound only weakly to either Cdc42 or Rac. A longer fragment starting at residue 75 and ending at residue 105 showed binding to Q61L Rac.GTP with Kd = 1.9 microM. Highest affinity binding (Kd approximately 0.05 microM) was seen with longer fragments ending at residue 118 or 132. A small increase in affinity was seen with those fragments starting at residue 70 rather than residue 75. PAK fragments bound with approximately 3-10-fold higher affinity to Cdc42 than to Rac and bound Q61L variants with 5-10-fold higher affinity than wild type. The dissociation rates of Q61L Rac.mant-GTP and of Q61L Cdc42. mant-GTP from PAK fragment residues 70-132 were measured to be 0.66 and 0.25 min-1, respectively, which are 100-fold lower than dissociation rates for Ras:Ras-effector domains, although their affinities are similar. Calorimetric measurements revealed that binding was associated with a relatively slow heat change. It is suggested that these PAK fragments (in the absence of Cdc42 or Rac) might exist predominantly in an inactive conformation that slowly interconverts with an active conformation and/or a slow conformational change may occur upon binding to Cdc42/Rac. In conclusion, the PAK CRIB motif itself is insufficient for high-affinity binding to Cdc42/Rac, but a 30 amino acid region of PAK (residues 75-105), containing this motif, is sufficient.

Nitroarginine and tetrahydrobiopterin binding to the haem domain of neuronal nitric oxide synthase using a scintillation proximity assay

Nitric oxide synthases (NOS) have a bidomain structure comprised of an N-terminal oxygenase domain and a C-terminal reductase domain. The oxygenase domain binds haem, (6R)-5,6,7,8-tetrahydro-l-biopterin (tetrahydrobiopterin) and arginine, is the site where nitric oxide synthesis takes place and contains determinants for dimeric interactions. A novel scintillation proximity assay has been established for equilibrium and kinetic measurements of substrate, inhibitor and cofactor binding to a recombinant N-terminal haem-binding domain of rat neuronal NOS (nNOS). Apparent Kd values for nNOS haem-domain-binding of arginine and Nomega-nitro-L-arginine (nitroarginine) were measured as 1.6 microM and 25 nM respectively. The kinetics of [3H]nitroarginine binding and dissociation yielded an association rate constant of 1.3x10(4) s-1.M-1 and a dissociation rate constant of 1.2x10(-4) s-1. These values are comparable to literature values obtained for full-length nNOS, suggesting that many characteristics of the arginine binding site of NOS are conserved in the haem-binding domain. Additionally, apparent Kd values were compared and were found to be similar for the inhibitors, L-NG-monomethylarginine, S-ethylisothiourea, N-iminoethyl-L-ornithine, imidazole, 7-nitroindazole and 1400W (N-[3-(aminomethyl) benzyl] acetamidine). [3H]Tetrahydrobiopterin bound to the nNOS haem domain with an apparent Kd of 20 nM. Binding was inhibited by 7-nitroindazole and stimulated by S-ethylisothiourea. The kinetics of interaction with tetrahydrobiopterin were complex, showing a triphasic binding process and a single off rate. An alternating catalytic site mechanism for NOS is proposed.

The importance of two conserved arginine residues for catalysis by the ras GTPase-activating protein, neurofibromin

Ras proteins are guanine-nucleotide binding proteins that have a low intrinsic GTPase activity that is enhanced 10(5)-fold by the GTPase-activating proteins (GAPs) p120-GAP and neurofibromin. Comparison of the primary sequences of RasGAPs shows two invariant arginine residues (Arg1276 and Arg1391 of neurofibromin). In this study, site-directed mutagenesis was used to change each of these residues in the catalytic domain of neurofibromin (NF1-334) to alanine. The ability of the mutant proteins to bind to Ras.GTP and to stimulate their intrinsic GTPase rate was then determined by kinetic methods under single turnover conditions using a fluorescent analogue of GTP. The separate contributions of each of these residues to catalysis and binding affinity to Ras were measured. Both the R1276A and the R1391A mutant NF1-334 proteins were 1000-fold less active than wild-type NF1-334 in activating the GTPase when measured at saturating concentrations. In contrast, there was only a minor effect of either mutation on NF1-334 affinity for wild-type Ha-Ras. These data are consistent with both arginines being required for efficient catalysis. Neither arginine is absolutely essential, because the mutant NF1-334 proteins increase the intrinsic Ras.GTPase by at least 100-fold. The roles of Arg1276 and Arg1391 in neurofibromin are consistent with proposals based on the recently published x-ray structure of p120-GAP complexed with Ras.

Cysteine-200 of human inducible nitric oxide synthase is essential for dimerization of haem domains and for binding of haem, nitroarginine and tetrahydrobiopterin

Nitric oxide synthase (EC 1.14.13.39) is a homodimer. Limited proteolysis has previously shown that it consists of two major domains. The C-terminal or reductase domain binds FMN, FAD and NADPH. The N-terminal or oxygenase domain is known to bind arginine, (6R)-5,6,7,8-tetrahydro-l-biopterin (tetrahydrobiopterin) and haem. The exact residues of the inducible nitric oxide synthase (iNOS) protein involved in binding to these molecules have yet to be identified, although the haem moiety is known to be co-ordinated through a cysteine thiolate ligand. We have expressed two forms of the haem-binding domain of human iNOS (residues 1-504 and 59-504) in Escherichia coli as glutathione S-transferase (GST) fusion proteins. The iNOS 1-504 and 59-504 fusion proteins bound similar amounts of haem, Nomega-nitro-l-arginine (nitroarginine) and tetrahydrobiopterin, showing that the first 58 residues are not required for binding these factors. Using site-directed mutagenesis we have mutated Cys-200, Cys-217, Cys-228, Cys-290, Cys-384 and Cys-457 to alanine residues within the iNOS 59-504 haem-binding domain. Mutation of Cys-200 resulted in a complete loss of haem, nitroarginine and tetrahydrobiopterin binding. Mutants of Cys-217, Cys-228, Cys-290, Cys-384 or Cys-457 showed no effect on the haem content of the fusion protein, no effect on the reduced CO spectral peak (444 nm) and were able to bind nitroarginine and tetrahydrobiopterin at levels equivalent to the wild-type fusion protein. After removal of the GST polypeptide, the wild-type iNOS 59-504 domain was dimeric, whereas the C200A mutant form was monomeric. When the mutated domains were incorporated into a reconstructed full-length iNOS protein expressed in Xenopus oocytes, only the Cys-200 mutant showed a loss of catalytic activity: all the other mutant iNOS proteins showed near wild-type enzymic activity. From this systematic approach we conclude that although Cys-217, Cys-228, Cys-290, Cys-384 and Cys-457 are conserved in all three NOS isoforms they are not essential for cofactor or substrate binding or for enzymic activity of iNOS, and that Cys-200 provides the proximal thiolate ligand for haem binding in human iNOS.

Delineation of the arginine- and tetrahydrobiopterin-binding sites of neuronal nitric oxide synthase

Nitric oxide synthase (EC 1.14.13.39) catalyses the conversion of arginine, NADPH and oxygen to nitric oxide and citrulline, using haem, (6R)-5,6,7,8-tetrahydro-l-biopterin (tetrahydrobiopterin), calmodulin, FAD and FMN as cofactors. The enzyme consists of a central calmodulin-binding sequence flanked on the N-terminal side by a haem-binding region that contains the arginine and tetrahydrobiopterin sites and on the C-terminal side by a region homologous with NADPH:cytochrome P-450 reductase. By using domain boundaries defined by limited proteolysis of full-length enzyme, recombinant haem-binding regions of rat brain neuronal nitric oxide synthase were expressed and purified. Two proteins were made in high yield: one, corresponding to residues 221-724, contained bound haem and tetrahydrobiopterin and was able to bind Nomega-nitro-l-arginine (nitroarginine) or arginine; the other, containing residues 350-724, contained bound haem but was unable to bind tetrahydrobiopterin, nitroarginine or arginine. These results showed that rat brain neuronal nitric oxide synthase contains a critical determinant for arginine/tetrahydrobiopterin binding between residues 221 and 350. Limited proteolysis with chymotrypsin of the former protein resulted in a new species with an N-terminal residue 275 that retained the ability to bind nitroarginine, further defining the critical region for arginine binding as being between 275 and 350. Comparison of the sequences of nitric oxide synthase and the tetrahydrobiopterin-requiring amino acid hydroxylases revealed a similarity in the region between residues 470 and 600, suggesting that this might represent the core region of the pterin-binding site. The stoichiometries of binding of substrate and cofactors to the recombinant domains were not more than 0.5 mol/mol of monomer, suggesting that there might be a single high-affinity site per dimer.

The kinetic mechanism of the GAP-activated GTPase of p21ras

Guanine nucleotides modified by acetylation of the ribose moiety with the small fluorophore N -methylanthranilic acid(mant) have been shown to bind to p21ras with similar equilibrium and kinetic rate constants as the parent nucleotides. Hydrolysis of p21.mantGTP to p21.mantGDP results in a 10% decrease in fluorescence intensity occurring at the same rate as the cleavage step. A similar process occurs with the non-hydrolysable analogue mantGMP.PNP, and this has led to the proposal that a conformational change of p21.mantGTP precedes and controls the rate of the cleavage step. The fluorescence change with p21.mantGMP.PNP is accelerated in the presence of the C-terminal catalytic domain of GAP, which is consistent with this mechanism. The same conformational change does not occur with oncogenic mutants of p21ras, Asp-12 and Val-12, but does occur with the weakly oncogenic Pro-12 mutant. Stopped flow measurements of the interaction of GAP with p21.mantGTP show an exponential decrease in fluorescence, the rate of which does not vary linearly with GAP concentration. These data imply a rapidly reversible formation of the p21.mantGTP complex with GAP followed by the isomerization of this complex. This is at least 10 5 -fold faster than the same process in the absence of GAP.

Equilibrium and kinetic measurements reveal rapidly reversible binding of Ras to Raf

Raf is a serine/threonine kinase that binds through its amino-terminal regulatory domain to the GTP form of Ras and thereby activates the mitogen-activated protein kinase pathway. In this study, we have characterized the interaction of the Ras-binding domain of Raf with Ras using equilibrium binding methods (scintillation proximity assay and fluorescence anisotropy), rather than with more widely used nonequilibrium procedures (such as enzyme-linked immunosorbent assay and affinity precipitation). Initial studies using glutathione S-transferase fusion proteins with either residues 1-257 or 1-190 of Raf showed that although it was possible to detect Ras binding using an enzyme-linked immunosorbent assay or affinity precipitation, it was substoichiometric; under equilibrium conditions with only a small excess of Raf almost no binding was detected. This difference was probably due to the presence of a high percentage of inactive Raf protein. Further studies used protein containing residues 51-131 of Raf, which expressed in Escherichia coli as a stable glutathione S-transferase fusion. With this protein, binding with Ras could readily be measured under equilibrium conditions. The catalytic domain of neurofibromin inhibited binding of Ras to Raf, and Raf inhibited the binding of Ras to neurofibromin showing that Raf and neurofibromin cannot be bound simultaneously to Ras. The affinities of interaction of neurofibromin and Raf with Harvey-RasLeu-61 were similar. The rate constant for dissociation of Raf from Ras was estimated to be >1 min-1, suggesting that Ras, Raf, and neurofibromin may be in rapid equilibrium in the cell. In contrast to previous reports, under equilibrium conditions there was no evidence for a difference in affinity between the minimal Ras binding domain of Raf (residues 51-131) and a region containing an additional 16 carboxyl-terminal amino acids, suggesting that residues 132-147 do not form a critical binding determinant.

Identification of the domains of neuronal nitric oxide synthase by limited proteolysis

Nitric oxide synthase (EC 1.14.13.39) binds arginine and NADPH as substrates, and FAD, FMN, tetrahydrobiopterin, haem and calmodulin as cofactors. The protein consists of a central calmodulin-binding sequence flanked on the N-terminal side by a haem-binding region, analogous to cytochrome P-450, and on the C-terminal side by a region homologous with NADPH:cytochrome P-450 reductase. The structure of recombinant rat brain nitric oxide synthase was analysed by limited proteolyis. The products were identified by using antibodies to defined sequences, and by N-terminal sequencing. Low concentrations of trypsin produced three fragments, similar to those in a previous report [Sheta, McMillan and Masters (1994) J. Biol. Chem. 269, 15147-15153]: that of Mr approx. 135000 (N-terminus Gly-221) resulted from loss of the N-terminal extension (residues 1-220) unique to neuronal nitric oxide synthase. The fragments of Mr 90000 (haem region) and 80000 (reductase region, N-terminus Ala-728) were produced by cleavage within the calmodulin-binding region. With more extensive trypsin treatment, these species were shown to be transient, and three smaller, highly stable fragments of Mr 14000 (N-terminus Leu-744 within the calmodulin region), 60000 (N-terminus Gly-221) and 63000 (N-terminus Lys-856 within the FMN domain) were formed. The species of Mr approx. 60000 represents a domain retaining haem and nitroarginine binding. The two species of Mr 63000 and 14000 remain associated as a complex. This complex retains cytochrome c reductase activity, and thus is the complete reductase region, yet cleaved at Lys-856. This cleavage occurs within a sequence insertion relative to the FMN domain present in inducible nitric oxide synthase. Prolonged proteolysis treatment led to the production of a protein of Mr approx. 53000 (N-terminus Ala-953), corresponding to a cleavage between the FMN and FAD domains. The major products after chymotryptic digestion were similar to those with trypsin, although the pathway of intermediates differed. The haem domain was smaller, starting at residue 275, yet still retained the arginine binding site. These data have allowed us to identify stable domains representing both the arginine/haem-binding and the reductase regions.

Mechanism of inhibition by arachidonic acid of the catalytic activity of Ras GTPase-activating proteins

Ras is a guanine nucleotide-binding protein that acts as a molecular switch controlling cell growth. The Ras GTPase-activating proteins (GAPs) p120-GAP and neurofibromin are candidates as Ras effectors. The GTPase-activating activity of both proteins is inhibited by mitogenic lipids, such as arachidonic acid and phosphatidic acid, and differential inhibition of the two GAPs led to the hypothesis that both were effectors in a Ras-controlled mitogenic pathway (Bollag, G., and McCormick, F. (1991) Nature 351, 576-579). We have studied the mechanism of inhibition by arachidonic acid in three ways: first, by measurements of catalytic activity under multiple turnover conditions; second, using p-((6-phenyl)-1,3,5-hexatrienyl)benzoic acid as a fluorescent probe for ligands binding to GAPs; and third, by using a scintillation proximity assay to measure direct binding of Ras to neurofibromin. We found no significant differential inhibition between p120-GAP and neurofibromin by arachidonic acid. The inhibition by arachidonic acid included a major component that is competitive with Ras GTP. These data suggest that insomuch as the mitogenic effects of lipids are mediated via inhibition of GAPs, GAPs are not Ras effector proteins. Additionally, lipids can exert a non-competitive type effect, consistent with a protein denaturing activity, making difficult extrapolations from in vitro data to the situation within cells, and possibly explaining the variability of literature data on inhibition by lipids.

Kinetics of inorganic phosphate release during the interaction of p21ras with the GTPase-activating proteins, p120-GAP and neurofibromin

The rate of GTP hydrolysis on p21ras is accelerated by approximately 10(5) times by the catalytic domains of GTPase-activating proteins (GAPs), p120-GAP (GAP-344) or neurofibromin (NF1-334). The kinetic mechanism of this activation has been investigated by following the release of inorganic phosphate (Pi), using a fluorescent probe that is sensitive to Pi [Brune, M., Hunter, J., Corrie, J. E. T., & Webb, M. R. (1994) Biochemistry 33, 8262-8271]. Measurements were made in real time with a stopped-flow apparatus, in which the p21ras complex with the 2',3'-methanthraniloyl analogue of GTP (mantGTP) was mixed with the GAP in the presence of this Pi probe. The results show that Pi release is fast and that the overall hydrolysis is controlled by the cleavage itself or a conformational change preceding the cleavage. The time courses were single exponentials over a range of [GAP-344] and were modeled to show that a single step controlled Pi release. The maximum rate constant was 15 s-1 (all data at 30 degrees C, pH 7.6, low ionic strength) in experiments in which GAP-344 underwent a single turnover, compared with 5 s-1 for multiple-turnover experiments, and possible causes of this discrepancy were investigated and discussed. With NF1-334 the time courses were more complex, showing a lag prior to rapid release of Pi. The results were consistent with a Kd of 0.04 microM for NF1-344 affinity is some 3 orders of magnitude tighter than that of GAP-344.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct measurement of the binding of RAS to neurofibromin using a scintillation proximity assay

Protein-protein interactions are of major importance in many cellular processes. When no enzymic activity is involved, assays for direct binding are required. One such example is the relatively weak interaction between oncogenic Ras and the GTPase-activating protein neurofibromin (NF1). The complex between the catalytic domain of NF1 and the GTP-form of oncogenic Ras protein dissociates rapidly; hence, equilibrium binding must be quantitated. Scintillation proximity assay (SPA) technology, a radioisotopic technique that requires no separation step, was used to characterize this interaction. Leu-61 Ras complexed with [3H]GTP was generated by nucleotide exchange in the presence of a GTP-regenerating system. A SPA signal was obtained when radiolabeled Ras was mixed with NF1 fused with glutathione S-transferase (GST), anti-GST, and protein A-coated SPA beads. This signal was abolished when any of the components were omitted and also by the addition of NaCl, which potently reduces the affinity of interaction between Ras and NF1. The neutralizing anti-Ras monoclonal antibody Y13-259 and the detergent n-dodecyl maltoside, a specific inhibitor of NF1 catalytic activity, both abolished the SPA signal from the NF1/Ras assay but neither affected a control SPA signal in which a [3H]GTP.GST-Ras fusion protein was bound to protein A-coated SPA beads. This technology could be readily extended to the measurement of other protein-protein interactions and could form the basis for high-throughput screens for the discovery of novel therapeutic agents.

Cloning and characterisation of cDNAs encoding a novel non-receptor tyrosine kinase, brk, expressed in human breast tumours

Using a polymerase chain reaction based differential screening approach, we have isolated and characterised a cDNA from a human metastatic breast tumour representing a novel protein tyrosine kinase (brk). Sequencing of brk cDNAs isolated from T-47D and MCF-7 human breast tumour cell lines indicate that they encode a protein with the features of a novel nonreceptor tyrosine kinase, including amino terminal SH3 and SH2 domains. When synthesised in recombinant baculovirus and bacterial expression systems, brk protein products are capable of autophosphorylation on tyrosine residues. Initial expression studies have detected low levels of brk transcripts in some human breast tumours and breast tumour cell lines, but not in normal breast tissue.

Kinetics of interaction between normal and proline 12 Ras and the GTPase-activating proteins, p120-GAP and neurofibromin. The significance of the intrinsic GTPase rate in determining the transforming ability of ras

Single turnover and equilibrium binding measurements on the interaction of Gly-12 and Pro-12 Ras.GTP with the catalytic domains of the GTPase-activating proteins, p120-GAP and neurofibromin, have been made utilizing fluorescent 2'(3')O-(N-methylanthraniloyl)-nucleotides. These have enabled the equilibrium dissociation constants (Kd) for their initial binding and the rate constants of the hydrolysis step to be measured. p120-GAP binds to both Ras proteins with a Kd of 17 microM, whereas neurofibromin binds to both Ras proteins with a Kd of 1 microM. Both p120-GAP and neurofibromin increased the rate constant of the GTP hydrolysis step of Pro-12 Ras, but the maximal activation at 30 degrees C was 120-fold and 560-fold, as compared with 70,000- and 52,000-fold, with Gly-12 Ras. The affinity with which p120-GAP and neurofibromin binds to either Gly-12 or Pro-12 Ras protein was decreased dramatically by increasing ionic strength caused by addition of NaCl. The rate constant of the cleavage step of hydrolysis catalyzed by neurofibromin increases with increasing ionic strength, whereas that catalyzed by p120-GAP appears to be unaffected. The high ionic strength within the cell might result in a much lower overall GTPase-activating protein activity than is measured under conditions of low ionic strength in vitro, with p120-GAP being more severely inhibited. The GTP hydrolysis rate of Pro-12 Ras is 2-fold faster than that of normal Ras. The low oncogenicity of Pro-12 ras is explained by a model in which the intrinsic rates of hydrolysis and exchange, as well as GTPase-activating protein- and exchange factor-stimulated rates, are determinants of the biological activity of Ras proteins in fibroblasts.

Solution dynamics of p21ras proteins bound with fluorescent nucleotides: a time-resolved fluorescence study

The solution dynamics of normal and transforming p21ras proteins in both the GTP- and GDP-bound forms were examined with time-resolved fluorescence spectroscopy. The fluorescent 2'(3')-O-(N-methylanthraniloyl) derivatives (mant derivatives) of GTP, dGTP, and GDP and the aminocoumarin and fluorescein derivatives of GTP and GDP were synthesized and used as reporter groups. The fluorescence lifetimes at 5 degrees C of the mant nucleotide derivatives increased from approximately 4 ns in solution to approximately 9 ns when bound to p21ras. At 30 degrees C, there was a 7.8% difference in lifetime between normal p21ras.mantGTP and p21ras.mantGDP, but no difference between similar complexes of the [Asp-12]p21ras protein. These data are consistent with steady-state fluorescence intensity differences among p21ras.mantGTP, p21ras.mantGDP, and the free nucleotides. Rotational correlation times for the mantGTP- and mantGDP-bound p21 proteins, N-ras, K-ras, and H-ras, were similar at 26 ns (5 degrees C), which is significantly longer than the 15-ns rotational correlation time predicted for a globular 21,000-Da protein. The p21-bound fluorescein and aminocoumarin nucleotide derivatives reported correlation times of 19 and 29 ns, respectively. Global analysis of the three fluorophore.p21 complexes with linked protein rotational correlation functions were best fit with a common rotational correlation time of 28 ns. Gel permeation chromatography of the GDP and mantGDP complexes of normal p21N-ras also showed greater apparent molecular weights than were expected in both cases, demonstrating that the high rotational correlation times obtained from time-resolved fluorescence measurements were not a result of the introduction of the fluorophore.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of a rat neuronal nitric oxide synthase coding sequence in a baculovirus/insect cell system

A DNA sequence encoding rat neuronal NO synthase (nNOS) was isolated and cloned into the baculovirus expression vector pVL1393 to generate pVLRBNOS. Transfection of Spodoptera frugiperda Sf-21 cells with the construct pVLRBNOS resulted in the synthesis of high levels of neuronal NO synthase. Analysis of the expression pattern revealed soluble, enzymatically active NO synthase in the cytoplasm of cell extracts. Active enzyme could also be purified from culture supernatants using 2'-5' ADP sepharose affinity chromatography. This enzyme was recognised by antibodies to the native nNOS and showed a similar degree of inhibition by arginine analogs as the native nNOS. The majority of the NOS synthesised had accumulated as insoluble "inclusion-body" material. The purification of recombinant nNOS from insect cells should facilitate characterisation of neuronal NO synthase.

Interaction of GTPase activating proteins (GAPs) with p21ras measured by a novel fluorescence anisotropy method. Essential role of Arg-903 of GAP in activation of GTP hydrolysis on p21ras

Ras GTPase activating proteins (GAPs) contain an invariant motif, -FLR-, within the most conserved region of their catalytic domains. Certain mutations in this motif have greatly reduced activity (Skinner, R. H., Bradley, S., Brown, A. L., Johnson, N. J., Rhodes, S., Stammers, D. K., and Lowe, P. N. (1991) J. Biol. Chem. 266, 14163-14166), but it was not determined whether the reduced activity was due to loss of binding or impaired catalysis. In order to address this question, we have developed a simple physical method to study formation of GAP.p21ras complexes. This utilizes the increase of fluorescence anisotropy upon binding of GAP to p21ras complexed with 2'(3')-O-(N-methylanthraniloyl) (mant) derivatives of guanine nucleotides. Dissociation constants obtained for the catalytic domains of either p120-GAP (GAP-344) or neurofibromin (NF1-GRD) with normal and Leu-61 p21ras proteins are comparable with those obtained by kinetic methods. In the course of these studies, we found, in contrast to previous observations, that both GAP and NF1-GRD can weakly activate the GTPase of Leu-61 mutant p21, showing that Gln-61 is not absolutely required for the stimulation of GTPase activity by GAPs. The fluorescence anisotropy method allowed us to show that mutation of Arg-903, within the FLR motif of GAP, can result in protein defective in catalysis but not in binding to p21ras. These data suggest a direct role for this residue in catalyzing GTP hydrolysis on p21ras, possibly by contributing a catalytic group to the p21 active site. This method is independent of the catalytic activity of the proteins, and so it could be extended generally to the measurement of binding of effector molecules, exchange factors, or other macromolecules to guanine nucleotide-binding proteins.

Use of the Glu-Glu-Phe C-terminal epitope for rapid purification of the catalytic domain of normal and mutant ras GTPase-activating proteins

The C-terminal catalytic domain (residues 704-1047) of the human ras GTPase-activating protein (GAP) has been engineered so as to incorporate the tripeptide, Glu-Glu-Phe, at its C terminus. This motif is recognized by the commercially available YL1/2 monoclonal antibody to alpha-tubulin and has previously been used for the immunoaffinity purification of HIV enzymes engineered to contain this epitope (Stammers, D. K., Tisdale, M., Court, S., Parmar, V., Bradley, C., and Ross, C. K. (1991) FEBS Lett. 283, 298-302). The engineered GAP catalytic domain (GAP-344) was obtained in high yield and purity from Escherichia coli extracts by means of a single affinity column of immobilized YL1/2, eluted under mild conditions with the dipeptide, Asp-Phe. The protein had similar activity to that previously described for full-length GAP, suggesting that the addition of the epitope did not grossly affect the activity. R903K and L902I mutants of GAP-344 were constructed, and the immunoaffinity purification procedure allowed their rapid characterization. The R903K mutant had less than 3% the activity of the normal protein, whereas the L902I substitution had less than 0.5% of normal activity, suggesting an important role for Leu-902 and Arg-903, residues absolutely conserved among GAP-related proteins. This work exemplifies the general utility of the C-terminal Glu-Glu-Phe motif for the rapid purification of proteins whose function is not altered by C-terminal modification.

Fluorescence approaches to the study of the p21ras GTPase mechanism

The use of ribose-modified guanine nucleotides and tryptophan mutants of p21ras, neither of which have significant effect on the kinetic mechanism of the p21ras GTPase and the GAP-activated p21ras GTPase, will now allow a detailed kinetic study of how GAP and other regulatory proteins interact with p21ras. This will lead to a better understanding of how the relative concentrations of 'active' p21ras. GTP and 'inactive' p21ras. GDP are regulated in the cell.

Characterization of recombinant human Kirsten-ras (4B) p21 produced at high levels in Escherichia coli and insect baculovirus expression systems

Kirsten-ras is the oncogene most frequently activated in human tumors. Studies of its biological function have been limited by the nonavailability of significant amounts of the major protein product, Kirsten-ras (4B) p21. When expressed in Escherichia coli K12, the recombinant protein was rapidly cleaved upon cell lysis in the lysine-rich C terminus region, probably by the ompT protease. However, soluble full-length protein was obtained when the Kirsten-ras gene was expressed in an E. coli strain lacking the ompT gene, and also in a baculovirus/insect cell expression system. Additionally, the baculovirus/insect cell system produced about half of the Kirsten-ras protein in a membrane-associated form, which was post-translationally modified by polyisoprenylation and carboxyl-methylation. A C-terminally truncated form (residues 1-166) was also expressed at high levels in E. coli for x-ray crystallographic studies. The kinetics of GDP release and of GTP hydrolysis of the purified proteins are similar to those of the corresponding Harvey-ras proteins, though there are small differences in the relative affinities for GDP and GTP. Biological activity of full-length Kirsten Val-12 p21 was demonstrated by microinjection into Swiss 3T3 cells, resulting in morphological transformation, with a lower potency than that of Harvey Val-12 protein.

The Ha-ras protein, p21, is modified by a derivative of mevalonate and methyl-esterified when expressed in the insect/baculovirus system

Using the insect/baculovirus expression system, we demonstrate the incorporation of [3H]mevalonate and [3H]methyl groups into recombinant c-Ha-ras protein (p21). Unlike the post-translational palmitoylation of p21 expressed in this system, the modification by mevalonate is not removed by hydroxylamine suggesting the absence of a thioester linkage. It is highly likely that the insect expression system recognizes the C-terminal CAAX Motif in p21, incorporates the mevalonate into the recently described polyisoprenylation modification and carboxyl-methylates the protein.

Inhibition of pyruvate:ferredoxin oxidoreductase from Trichomonas vaginalis by pyruvate and its analogues. Comparison with the pyruvate decarboxylase component of the pyruvate dehydrogenase complex

Pyruvate:ferredoxin oxidoreductase and the pyruvate dehydrogenase multi-enzyme complex both catalyse the CoA-dependent oxidative decarboxylation of pyruvate but differ in size, subunit composition and mechanism. Comparison of the pyruvate:ferredoxin oxidoreductase from the protozoon Trichomonas vaginalis and the pyruvate dehydrogenase component of the Escherichia coli pyruvate dehydrogenase complex shows that both are inactivated by incubation with pyruvate under aerobic conditions in the absence of co-substrates. However, only the former is irreversibly inhibited by incubation with hydroxypyruvate, and only the latter by incubation with bromopyruvate. Pyruvate:ferredoxin oxidoreductase activity is potently, but reversibly, inhibited by addition of bromopyruvate in the presence of CoA, and it is suggested that the mechanism involves formation of an adduct between CoA and bromopyruvate in the active site of the enzyme. It is proposed that both enzymes are inactivated by pyruvate through a mechanism involving oxidation of an enzyme-bound thiamin pyrophosphate/substrate adduct to form a tightly bound inhibitory species, possibly thiamin thiazolone pyrophosphate as hypothesized by Sumegi & Alkonyi.

Expression and characterization of the Ha-ras p21 protein produced at high levels in the insect/baculovirus system

Normal and mutated cDNAs of Ha-ras have each been cloned into a standard (pAc373) and a novel (p36C) baculovirus transfer vector and introduced via homologous recombination into the genome of Autographa californica nuclear polyhedrosis virus immediately downstream of the polyhedrin promoter. Spodoptera frugiperda cells infected with recombinant virus containing the normal Ha-ras gene express very high levels of ras p21 protein (approximately 20% of total cell protein), whereas the mutant protein was expressed at considerably lower levels. Molecular analysis showed that this was most likely due to a post-transcriptional event. The expression vector p36C produced considerably higher levels of recombinant p21 compared to the more commonly used pAc373. The majority of the normal ras p21 protein is soluble, cytoplasmic, and appears to be nonacylated. However, about 10% of the p21 associates with the membrane fraction of infected cells and migrates as a slightly faster band on gels. Furthermore, this band is sensitive to hydroxylamine treatment and shows specific incorporation of [3H]palmitate, strongly suggesting that it is the palmitoylated form of p21, which is the biologically active form of the protein. Both the soluble and membrane-associated p21 have been purified to homogeneity under nondenaturing conditions, the latter in the presence of detergents. The isolation of native palmitoylated p21 has not been reported previously. The difference in hydrophobicity between these two proteins has been demonstrated by Triton X-114 partitioning. The use of the insect/baculovirus expression system to express relatively high levels (20 mg/liter) of palmitoylated p21 should aid experiments to resolve the structural and functional properties of this molecule.

Purification and characterization of [acyl-carrier-protein] acetyltransferase from Escherichia coli

A multi-step procedure has been developed for the purification of [acyl-carrier-protein] acetyltransferase from Escherichia coli, which allows the production of small amounts of homogeneous enzyme. The subunit Mr was estimated to be 29,000 and the native Mr was estimated to be 61,000, suggesting a homodimeric structure. The catalytic properties of the enzyme are consistent with a Bi Bi Ping Pong mechanism and the existence of an acetyl-enzyme intermediate in the catalytic cycle. The enzyme was inhibited by N-ethylmaleimide and more slowly by iodoacetamide in reactions protected by the substrate, acetyl-CoA. However, the enzyme was apparently only weakly inhibited by the thiol-specific reagent methyl methanethiosulphonate. The nature of the acetyl-enzyme intermediate is discussed in relationship to that found in other similar enzymes from E. coli, yeast and vertebrates.

Purification and characterization of pyruvate: ferredoxin oxidoreductase from the anaerobic protozoon Trichomonas vaginalis

The pyruvate: ferredoxin oxidoreductase from the anaerobic protozoon Trichomonas vaginalis is an extrinsic protein bound to the hydrogenosomal membrane. It has been solubilized and purified to homogeneity, principally by salting-out chromatography on Sepharose 4B. Low recoveries of active enzyme were caused by inactivation by O2 and the irreversible loss of thiamin pyrophosphate. It is a dimeric enzyme of overall Mr 240,000 and subunit Mr 120,000. The enzyme contains, per mol of dimer, 7.3 +/- 0.3 mol of iron and 5.9 +/- 0.9 mol of acid-labile sulphur, suggesting the presence of two [4Fe-4S] centres, and 0.47 mol of thiamin pyrophosphate. The absorption spectrum of the enzyme is characteristic of a non-haem iron protein. The pyruvate: ferredoxin oxidoreductase from T. vaginalis is therefore broadly similar to the 2-oxo acid: ferredoxin (flavodoxin) oxidoreductases purified from bacterial sources, except that it is membrane-bound.

Probing the active sites of aspartate: 2-oxoglutarate aminotransferases from Trichomonas vaginalis and pig heart cytoplasm using substrate analogues

1. Series of structural analogues of the substrates and products of the aspartate: 2-oxoglutarate aminotransferase reaction have been tested as reversible inhibitors of the purified aspartate aminotransferases from the protozoon Trichomonas vaginalis and from pig heart cytoplasm. 2. The results highlight differences and similarities between the active site regions of the two enzymes which are relevant to a better understanding of the nature of the enzyme/substrate interactions which influence substrate specificity.

Aminotransferase activities in Trichomonas vaginalis

A survey of aminotransferase activities present in a cell-free extract of the anaerobic protozoan, Trichomonas vaginalis was performed. 2-Oxoglutarate, oxaloacetate or phenylpyruvate acted as effective amino acceptors with tyrosine, phenylalanine, tryptophan, leucine, valine, isoleucine, aspartate, alanine, ornithine or lysine. Arginine, serine, glutamine, glycine, beta-alanine and gamma-aminobutyrate were not active as amino donors. With pyruvate as acceptor, significant, yet low, activity was seen only with glutamate, lysine or phenylalanine. Partial purification of enzymes catalysing transamination of leucine, valine, isoleucine, alanine, ornithine and lysine were carried out. A single enzyme catalysed the transamination of ornithine and lysine. The substrate specificity of this enzyme is novel. A separate enzyme catalysed the transamination of all three branched chain amino acids. A third enzyme catalysed the alanine aminotransferase reaction. A fourth enzyme catalysing the transamination both of aromatic amino acids and aspartate has previously been purified [Lowe, P.N. and Rowe, A.F. (1985) Biochem. J. 232, 689-695].

Modulation of amino acid and 2-oxo acid pools in Trichomonas vaginalis by aspartate aminotransferase inhibitors

The amino acid pool sizes of Trichomonas vaginalis are reported. Alanine, glutamic acid, proline and leucine account for 72% of the measured amino acids. Growth of T. vaginalis was unaffected by gostatin, an irreversible inhibitor of aspartate aminotransferase, when the enzyme activity within the cell had been completely inhibited and a specific elevation of the aspartate pool had occurred. In media lacking aspartate and glutamate, the amino acid substrates of the aspartate aminotransferase reaction, gostatin caused a larger increase in the aspartate pool. During incubation of cells with or without gostatin, aspartate and glutamate were produced in the medium, presumably by proteolysis of medium proteins. Hence any requirement for the aspartate aminotransferase reaction might have been bypassed. Glutamate-gamma-hydroxamate and aminooxyacetate inhibited growth of T. vaginalis but caused large changes in the pool-sizes of aspartate, glutamate, pyruvate plus oxaloacetate and 2-oxoglutarate, suggesting a more general interference with amino acid metabolism.

Aspartate:2-oxoglutarate aminotransferase from Trichomonas vaginalis: comparison with pig heart cytoplasmic enzyme

The aspartate:2-oxoglutarate aminotransferase from the protozoon Trichomonas vaginalis exists as a mixture of sub-forms of identical Mr and amino acid composition, and of similar catalytic properties. The amino acid composition closely resembles that of aspartate aminotransferase from prokaryotic and vertebrate sources. Some molecular and catalytic properties of the T. vaginalis aspartate aminotransferase are compared with those of the cytoplasmic pig heart enzyme. A major difference is in the ability of the trichomonal enzyme to transaminate aromatic amino acids and 2-oxo acids. A range of inhibitors have been used to compare the active-site regions of the T. vaginalis and cytoplasmic pig heart aspartate aminotransferases.

Aspartate: 2-oxoglutarate aminotransferase from trichomonas vaginalis. Identity of aspartate aminotransferase and aromatic amino acid aminotransferase

Aspartate: 2-oxoglutarate aminotransferase from the anaerobic protozoon Trichomonas vaginalis was purified to homogeneity and characterized. It is a dimeric protein of overall Mr approx. 100000. Only a single isoenzyme was found in T. vaginalis. The overall molecular and catalytic properties have features in common with both the vertebrate cytoplasmic and mitochondrial isoenzymes. The purified aspartate aminotransferase from T. vaginalis showed very high rates of activity with aromatic amino acids as donors and 2-oxoglutarate as acceptor. This broad-spectrum activity was restricted to aromatic amino acids and aromatic 2-oxo acids, and no significant activity was seen with other common amino acids, other than with the substrates and products of the aspartate: 2-oxoglutarate aminotransferase reaction. Co-purification and co-inhibition, by the irreversible inhibitor gostatin, of the aromatic amino acid aminotransferase and aspartate aminotransferase activities, in conjunction with competitive substrate experiments, strongly suggest that a single enzyme is responsible for both activities. Such high rates of aromatic amino acid aminotransferase activity have not been reported before in eukaryotic aspartate aminotransferase.

Limited proteolysis of the pyruvate dehydrogenase multienzyme complex of Bacillus subtilis

Limited digestion of the pyruvate dehydrogenase complex of Bacillus subtilis with either trypsin or chymotrypsin at 0 degrees C inhibited its ability to decarboxylate pyruvate and 2-oxoisovalerate oxidatively, without causing disassembly of the complex. The proteinases selectively cleaved the E1 alpha subunits to form two fragments of Mr 31500 and approx. 9500, as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, both fragments remaining bound to the complex. Trypsin also caused a much slower cleavage of the E2 subunits, to form a fragment of apparent Mr 34000. The inhibition of overall dehydrogenase-complex activity was accompanied by the apparent loss of the pyruvate-driven and 2-oxoisovalerate-driven E1 activities, which was found to be due to a large increase in the Km for the 2-oxo acids: this change was correlated with the cleavage of the E1 alpha subunit.

Bromopyruvate as an active-site-directed inhibitor of the pyruvate dehydrogenase multienzyme complex from Escherichia coli

Bromopyruvate behaves as an active-site-directed inhibitor of the pyruvate decarboxylase (E1) component of the pyruvate dehydrogenase complex of Escherichia coli. It requires the cofactor thiamin pyrophosphate (TPP) and acts initially as an inhibitor competitive with pyruvate (Ki ca. 90 microM) but then proceeds to react irreversibly with the enzyme, probably with the thiol group of a cysteine residue. E1 catalyzes the decomposition of bromopyruvate, the enzyme becoming inactivated once every 40-60 turnovers. Bromopyruvate also inactivates the intact pyruvate dehydrogenase complex in a TPP-dependent process, but the inhibition is more rapid and is mechanistically different. Under these conditions, bromopyruvate is decarboxylated, and the lipoic acid residues in the lipoate acetyltransferase (E2) component become reductively bromoacetylated. Further bromopyruvate then reacts with the new thiol groups thus generated in the lipoic acid residues, inactivating the complex. If reaction with the lipoic acid residues is prevented by prior treatment of the complex with N-ethylmaleimide in the presence of pyruvate, the mode of inhibition reverts to irreversible reaction with the E1 component. In both types of inhibition of E1, reaction of 1 mol of bromopyruvate/mol of E1 chain is required for complete inactivation, and all the evidence is consistent with reaction taking place at or near the pyruvate binding site.

Dual role of a single multienzyme complex in the oxidative decarboxylation of pyruvate and branched-chain 2-oxo acids in Bacillus subtilis

The pyruvate dehydrogenase and branched-chain 2-oxo acid dehydrogenase activities of Bacillus subtilis were found to co-purify as a single multienzyme complex. Mutants of B. subtilis with defects in the pyruvate decarboxylase (E1) and dihydrolipoamide dehydrogenase (E3) components of the pyruvate dehydrogenase complex were correspondingly affected in branched-chain 2-oxo acid dehydrogenase complex activity. Selective inhibition of the E1 or lipoate acetyltransferase (E2) components in vitro led to parallel losses in pyruvate dehydrogenase and branched-chain 2-oxo acid dehydrogenase complex activity. The pyruvate dehydrogenase and branched-chain 2-oxo acid dehydrogenase complexes of B. subtilis at the very least share many structural components, and are probably one and the same. The E3 component appeared to be identical for the pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase and branched-chain 2-oxo acid dehydrogenase complexes in this organism and to be the product of a single structural gene. Long-chain branched fatty acids are thought to be essential for maintaining membrane fluidity in B. subtilis, and it was observed that the ace (pyruvate dehydrogenase complex) mutant 61142 was unable rapidly to take up acetoacetate, unlike the wild-type, indicative of a defect in membrane permeability. A single pyruvate dehydrogenase and branched-chain 2-oxo acid dehydrogenase complex can be seen as an economical means of supplying two different sets of essential metabolites.