Low concentration of inducer favors production of active form of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Escherichia coli

Enzymatic preparation of high-specific-activity beta-D-[6,6'-3H]fructose-2,6-bisphosphate: Application to a sensitive assay for fructose-2,6-bisphosphatase

beta-D-Fructose-2,6-bisphosphate (Fru-2,6-P(2)) is an important regulator of eukaryotic glucose homeostasis, functioning as a potent activator of 6-phosphofructo-1-kinase and inhibitor of fructose-1,6-bisphosphatase. Pharmaceutical manipulation of intracellular Fru-2,6-P(2) levels, therefore, is of interest for the treatment of certain diseases, including diabetes and cancer. [2-(32)P]Fru-2,6-P(2) has been the reagent of choice for studying the metabolism of this effector molecule; however, its short half-life necessitates frequent preparation. Here we describe a convenient, economical, one-pot enzymatic preparation of high-specific-activity tritium-labeled Fru-2,6-P(2). The preparation involves conversion of readily available, carrier-free d-[6,6'-(3)H]glucose to [6,6'-(3)H]Fru-2,6-P(2) using hexokinase, glucose-6-phosphate isomerase, and 6-phosphofructo-2-kinase. The key reagent in this preparation, bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from human liver, was produced recombinantly in Escherichia coli and purified in a single step using an appendant C-terminal hexa-His affinity tag. Following purification by anion exchange chromatography using triethylammonium bicarbonate as eluant, radiochemically pure [6,6'-(3)H]Fru-2,6-P(2) having a specific activity of 50 Ci/mmol was obtained in yields averaging 35%. [6,6'-(3)H]Fru-2,6-P(2) serves as a stable, high-specific-activity substrate in a facile assay capable of detecting fructose-2,6-bisphosphatase in the range of 10(-14) to 10(-15) mol, and it should prove to be useful in many studies of the metabolism of this important biofactor.

The first alpha-1,3-glucosidase from bacterial origin belonging to glycoside hydrolase family 31

Genome analysis of Lactobacillus johnsonii NCC533 has been recently completed. One of its annotated genes, lj0569, encodes the protein having the conserved domain of glycoside hydrolase family 31. Its homolog gene (ljag31) in L. johnsonii NBRC13952 was cloned and expressed using an Escherichia coli expression system, resulting in poor production of recombinant LJAG31 protein due to inclusion body formation. Production of soluble recombinant LJAG31 was improved with high concentration of NaCl in medium, possible endogenous chaperone induction by benzyl alcohol, and over-expression of GroES-GroEL chaperones. Recombinant LJAG31 was an alpha-glucosidase with broad substrate specificity toward both homogeneous and heterogeneous substrates. This enzyme displayed higher specificity (in terms of k(cat)/K(m)) toward nigerose, maltulose, and kojibiose than other natural substrates having an alpha-glucosidic linkage at the non-reducing end, which suggests that these sugars are candidates for prebiotics contributing to the growth of L. johnsonii. To our knowledge, LJAG31 is the first bacterial alpha-1,3-glucosidase to be characterized with a high k(cat)/K(m) value for nigerose [alpha-d-Glcp-(1 --> 3)-d-Glcp]. Transglucosylation of 4-nitrophenyl alpha-d-glucopyranoside produced two 4-nitrophenyl disaccharides (4-nitrophenyl alpha-nigeroside and 4-nitrophenyl alpha-isomaltoside). These hydrolysis and transglucosylation properties of LJAG31 are different from those of mold (Acremonium implicatum) alpha-1,3-glucosidase of glycoside hydrolase family 31.

Periplasmic production of native human proinsulin as a fusion to E. coli ecotin

Native proinsulin belongs to the class of the difficult-to-express proteins in Escherichia coli. Problems mainly arise due to its small size, a high proteolytic decay, and the necessity to form a native disulfide pattern. In the present study, human proinsulin was produced in the periplasm of E. coli as a fusion to ecotin, which is a small periplasmic protein of 16 kDa encoded by the host, containing one disulfide bond. The fusion protein was secreted to the periplasm and native proinsulin was determined by ELISA. Cultivation parameters were studied in parallel batch mode fermentations using E. coli BL21(DE3)Gold as a host. After improvement of fed-batch high density fermentation conditions, 153 mg fusion protein corresponding to 51.5mg native proinsulin was obtained per L. Proteins were extracted from the periplasm by osmotic shock treatment. The fusion protein was purified in one step by ecotin affinity chromatography on immobilized trypsinogen. After thrombin cleavage of the fusion protein, the products were separated by Ni-NTA chromatography. Proinsulin was quantified by ELISA and characterized by mass spectrometry. To evaluate the influence of periplasmic proteases, the amount of ecotin-proinsulin was determined in E. coli BL21(DE3)Gold and in a periplasmic protease deficient strain, E. coli SF120.

Involvement of the C-terminal proline-rich motif of G protein-coupled receptor kinases in recognition of activated rhodopsin

G protein-coupled receptor kinases (GRKs) are a family of serine/threonine kinases that phosphorylate many activated G protein-coupled receptors (GPCRs) and play an important role in GPCR desensitization. Our previous work has demonstrated that the C-terminal conserved region (CC) of GRK-2 participates in interaction with rhodopsin and that this interaction is necessary for GRK-2-mediated receptor phosphorylation (Gan, X. Q., Wang, J. Y., Yang, Q. H., Li, Z., Liu, F., Pei, G., and Li, L. (2000) J. Biol. Chem. 275, 8469-8474). In this report, we further investigated whether the CC of other GRKs had the same functions and defined the specific sequences in CC that are required for the functions. The CC regions of GRK-1, GRK-2, and GRK-5, representatives of the three subfamilies of GRKs, could bind rhodopsin in vitro and inhibit GRK-2-mediated phosphorylation of rhodopsin, but not a peptide GRK substrate. Through a series of mutagenesis analyses, a proline-rich motif in the CC was identified as the key element involved in the interaction between the CC region and rhodopsin. Point mutations of this motif not only disrupted the interaction of GRK-2 with rhodopsin but also abolished the ability of GRK-2 to phosphorylate rhodopsin. The findings that the CC region of GRKs interact only with the light-activated but not the non-activated rhodopsin and that the N-terminal domain of GRK-2 interacts with rhodopsin in a light-independent manner suggest that the CC region is responsible for the recognition of activated GPCRs in the canonical model.

Involvement of the chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase sequence His444-Arg-Glu-Arg in modulation of the bisphosphatase activity by its kinase domain

The bisphosphatase activity of the hepatic bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is repressed by its kinase domain, and regulated by cAMP-dependent protein kinase (PKA)-catalysed phosphorylation. In the present study, the mechanism by which the bisphosphatase activity is repressed by the kinase domain and regulated by phosphorylation was investigated. We found that truncation of the C-terminus of the enzyme by 25, but not 20, amino acids dramatically enhanced the catalytic rate of the bisphosphatase, abrogated the inhibition by the kinase domain, and eliminated the effect of PKA-mediated phosphorylation on activity. In addition, mutation of His444-Arg-Glu-Arg to Ala-Ala-Glu-Ala had similar effects as the deletion. Moreover, the mutations also significantly affected the phosphorylation-mediated regulation of the kinase activity of the enzyme. Furthermore, the mutations altered the pH-dependence of the bisphosphatase, and the mutant bisphosphatases were more sensitive to modification by diethyl pyrocarbonate and guanidine-induced inactivation than the wild-type enzyme. Taken together, these results demonstrate that the sequence His444-Arg-Glu-Arg plays a critical role in repression of the bisphosphatase activity by both the N-terminal kinase domain and the C-terminal tail itself. These results also explain the activation of the bisphosphatase activity by PKA-catalysed phosphorylation, by suggesting that phosphorylation may relieve the inhibitory effect of the kinase domain that is mediated by the three basic residues in this sequence.

Binding of ATP to the fructose-2,6-bisphosphatase domain of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase leads to activation of its 6-phosphofructo-2-kinase

To understand the mechanism by which the activity of the 6-phosphofructo-2-kinase (6PF-2K) of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is stimulated by its substrate ATP, we studied two mutants of the enzyme. Mutation of either Arg-279, the penultimate basic residue within the Walker A nucleotide-binding fold in the bisphosphatase domain, or Arg-359 to Ala eliminated the activation of the chicken 6PF-2K by ATP. Binding analysis by fluorescence spectroscopy using 2'(3')-O-(N-methylanthraniloyl)-ATP revealed that the kinase domains of these two mutants, unlike that of the wild type enzyme, showed no cooperativity in ATP binding and that the mutant enzymes possess only the high affinity ATP binding site, suggesting that the ATP binding site on the bisphosphatase domain represents the low affinity site. This conclusion was supported by the result that the affinity of ATP for the isolated bisphosphatase domain is similar to that for the low affinity site in the wild type enzyme. In addition, we found that the 6PF-2K of a chimeric enzyme, in which the last 25 residues of chicken enzyme were replaced with those of the rat enzyme, could not be activated by ATP, despite the fact that the ATP-binding properties of this chimeric enzyme were not different from those of the wild type chicken enzyme. These results demonstrate that activation of the chicken 6PF-2K by ATP may result from allosteric binding of ATP to the bisphosphatase domain where residues Arg-279 and Arg-359 are critically involved and require specific C-terminal sequences.

The difference in the carboxy-terminal sequence is responsible for the difference in the activity of chicken and rat liver fructose-2,6-bisphosphatase

The fructose-2,6-bisphosphatase domain of the bifunctional chicken liver enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase shares approximately 95% amino acid sequence homology with that of the rat enzyme. However, these two enzymes are significantly different in their phosphatase activities. In this report, we show that the COOH-terminal 25 amino acids of the two enzymes are responsible for the different enzymatic activities. Although these 25 amino acids are not required for the phosphatase activity, their removal diminishes the differences in the activities between the two enzymes. In addition, two chimeric molecules (one consisting of the catalytic core of the chicken bisphosphatase domain and the rat COOH-terminal 25 amino acids, and the other consisting of most of the intact chicken enzyme and the rat COOH-terminal 25 amino acids) showed the same kinetic properties as the rat enzyme. Furthermore, substitution of the residues Pro456Pro457Ala458 of the chicken enzyme with GluAlaGlu, the corresponding sequence in the rat liver enzyme, yields a chicken enzyme that behaves like the rat enzyme. These results demonstrate that the different bisphosphatase activities of the chicken and rat liver bifunctional enzymes can be attributed to the differences in their COOH-terminal amino acid sequences, particularly the three residues.

Interaction between the conserved region in the C-terminal domain of GRK2 and rhodopsin is necessary for GRK2 to catalyze receptor phosphorylation

The C-terminal domain of G protein-coupled receptor kinases (GRKs) consists of a conserved region and a variable region, and the variable region has been shown to direct the membrane translocation of cytosolic enzymes. The present work has revealed that the C-terminal domain may also be involved in kinase-receptor interaction that is primarily mediated by the conserved region. Truncation of the C-terminal domain or deletion of the conserved region in this domain of GRK2 resulted in a complete loss of its ability to phosphorylate rhodopsin and in an obvious decrease in its sensitivity to receptor-mediated phosphorylation of a peptide substrate. On the contrary, deletion of the betagamma subunit binding region in the C-terminal domain of GRK2 did not significantly alter the ability of the enzyme to phosphorylate rhodopsin. In addition, the recombinant proteins that represent the C-terminal domain and the conserved region of GRK2 could inhibit GRK2-mediated phosphorylation of rhodopsin and receptor-mediated activation of GRK2 but not GRK2-mediated phosphorylation of the peptide substrate. Furthermore, the conserved region as well as the C-terminal domain could directly bind rhodopsin in vitro. These results indicate that the C-terminal domain, or more precisely, the conserved region of this domain, is important for enzyme-receptor interaction and that this interaction is required for GRK2 to catalyze receptor phosphorylation.

An artificial gene for human porphobilinogen synthase allows comparison of an allelic variation implicated in susceptibility to lead poisoning

Porphobilinogen synthase (PBGS) is an ancient enzyme essential to tetrapyrrole biosynthesis (e.g. heme, chlorophyll, and vitamin B(12)). Two common alleles encoding human PBGS, K59 and N59, have been correlated with differential susceptibility of humans to lead poisoning. However, a model for human PBGS based on homologous crystal structures shows the location of the allelic variation to be distant from the active site with its two Zn(II). Previous microbial expression systems for human PBGS have resulted in a poor yield. Here, an artificial gene encoding human PBGS was constructed by recursive polymerase chain reaction from synthetic oligonucleotides to rectify this problem. The artificial gene was made to resemble the highly expressed homologous Escherichia coli hemB gene and to remove rare codons that can confound heterologous protein expression in E. coli. We have expressed and purified recombinant human PBGS variants K59 and N59 in 100-mg quantities. Both human PBGS proteins purified with eight Zn(II)/octamer; Zn(II) binding was shown to be pH-dependent; and Pb(II) could displace some of the Zn(II). However, there was no differential displacement of Zn(II) by Pb(II) between K59 and N59, and simple Pb(II) inhibition studies revealed no allelic difference.

Structures of human dihydroorotate dehydrogenase in complex with antiproliferative agents

BACKGROUND

Dihydroorotate dehydrogenase (DHODH) catalyzes the fourth committed step in the de novo biosynthesis of pyrimidines. As rapidly proliferating human T cells have an exceptional requirement for de novo pyrimidine biosynthesis, small molecule DHODH inhibitors constitute an attractive therapeutic approach to autoimmune diseases, immunosuppression, and cancer. Neither the structure of human DHODH nor any member of its family was known.

RESULTS

The high-resolution crystal structures of human DHODH in complex with two different inhibitors have been solved. The initial set of phases was obtained using multiwavelength anomalous diffraction phasing with selenomethionine-containing DHODH. The structures have been refined to crystallographic R factors of 16.8% and 16.2% at resolutions of 1. 6 A and 1.8 A for inhibitors related to brequinar and leflunomide, respectively.

CONCLUSIONS

Human DHODH has two domains: an alpha/beta-barrel domain containing the active site and an alpha-helical domain that forms the opening of a tunnel leading to the active site. Both inhibitors share a common binding site in this tunnel, and differences in the binding region govern drug sensitivity or resistance. The active site of human DHODH is generally similar to that of the previously reported bacterial active site. The greatest differences are that the catalytic base removing the proton from dihydroorotate is a serine rather than a cysteine, and that packing of the flavin mononucleotide in its binding site is tighter.

Characterization of human T-cell leukemia virus type I integrase expressed in Escherichia coli

The C-terminal part of the pol gene of the human T-cell leukemia virus type I (HTLV-I) is predicted to encode the integrase (IN) of the virus; however, this protein has not yet been detected in virions or infected cells. We expressed the putative IN from an infectious molecular clone of HTLV-I in Escherichia coli. Comparison with protein resulting from coexpression of HTLV-I protease (PR) and Pol in insect cells indicated that the bacterially expressed protein is identical with or very similar to IN released from a PR-Pol precursor by proteolytic cleavage. HTLV-I IN was purified from E. coli under native conditions. The protein behaved like a dimer in size-exclusion chromatography. It carried out activities characteristic of retroviral IN with high efficiency, displaying a strong preference for U5-derived vs. U3-derived sequences in the processing and strand-transfer reactions. In the disintegration reaction, HTLV-I IN not only accepted the double-stranded branched substrate corresponding to the product of a strand-transfer reaction, but was also able to carry out a phosphoryl transfer on a branched molecule with a single-stranded or a single adenosine overhang.