The histidine triad protein Hint is not required for murine development or Cdk7 function

Biochemical, crystallographic and biophysical characterization of histidine triad nucleotide-binding protein 2 with different ligands including a non-hydrolyzable analog of Ap4A

BACKGROUND

Human HINT2 is an important mitochondrial enzyme involved in many processes such as apoptosis and bioenergetics, but its endogenous substrates and the three-dimensional structure of the full-length protein have not been identified yet.

METHODS

An HPLC assay was used to test the hydrolytic activity of HINT2 against various adenosine, guanosine, and 2'-deoxyguanosine derivatives containing phosphate bonds of different types and different leaving groups. Data on binding affinity were obtained by microscale thermophoresis (MST). Crystal structures of HINT2, in its apo form and with a dGMP ligand, were resolved to atomic resolution.

RESULTS

HINT2 substrate specificity was similar to that of HINT1, but with the major exception of remarkable discrimination against substrates lacking the 2'-hydroxyl group. The biochemical results were consistent with binding affinity measurements. They showed a similar binding strength of AMP and GMP to HINT2, and much weaker binding of dGMP, in contrast to HINT1. A non-hydrolyzable analog of Ap4A (JB419) interacted with both proteins with similar K_d and Ap4A is the signaling molecule that can interact with hHINT1 and regulate the activity of some transcription factors.

CONCLUSIONS

Several forms of homo- and heterodimers of different lengths of N-terminally truncated polypeptides resulting from degradation of the full-length protein were described. Ser144 in HINT2 appeared to be functionally equivalent to Ser107 in HINT1 by supporting the protonation of the leaving group in the hydrolytic mechanism of HINT2.

SIGNIFICANCE

Our results should be considered in future studies on the natural function of HINT2 and its role in nucleotide prodrug processing.

Clinical significance of down-regulated HINT2 in hepatocellular carcinoma

To study the clinical significance of HINT2 expression in patients with HCC.We investigated HINT2 mRNA expression in tumors and adjacent non-tumor hepatic tissues from 106 HCC patients using quantitative real-time PCR. Appropriate statistical methods were then applied to assess the relationships between the HINT2 mRNA level and clinical parameters.HINT2 was significantly down-regulated in HCC (P < .0001). No significant correlation was found between HINT2 expression and clinicopathological factors in HCC patients. A Kaplan-Meier survival curve showed that HINT2 expression is related to recurrence-free survival (P < .05). Multivariate analyses revealed that tumor size and HINT2 expression are risk factors for HCC recurrence.HINT2 is down-regulated in HCC, and low HINT2 expression predicts earlier tumor recurrence. HINT2 expression may serve as a prognostic indicator of recurrence in HCC.

Crystallographic studies of the complex of human HINT1 protein with a non-hydrolyzable analog of Ap4A

Histidine triad nucleotide-binding protein 1 (HINT1) represents the most ancient and widespread branch in the histidine triad proteins superfamily. HINT1 plays an important role in various biological processes, and it has been found in many species. Here, we report the first structure (at a 2.34Å resolution) of a complex of human HINT1 with a non-hydrolyzable analog of an Ap4A dinucleotide, containing bis-phosphorothioated glycerol mimicking a polyphosphate chain, obtained from a primitive monoclinic space group P21 crystal. In addition, the apo form of hHINT1 at the space group P21 refined to 1.92Å is reported for comparative studies.

Expression, purification, crystallization and preliminary X-ray crystallographic analysis of human histidine triad nucleotide-binding protein 2 (hHINT2)

Histidine triad nucleotide-binding protein 2 (HINT2) is a mitochondrial adenosine phosphoramidase mainly expressed in the pancreas, liver and adrenal gland. HINT2 possibly plays a role in apoptosis, as well as being involved in steroid biosynthesis, hepatic lipid metabolism and regulation of hepatic mitochondria function. The expression level of HINT2 is significantly down-regulated in hepatocellular carcinoma patients. To date, endogenous substrates for this enzyme, as well as the three-dimensional structure of human HINT2, are unknown. In this study, human HINT2 was cloned, overexpressed in Escherichia coli and purified. Crystallization was performed at 278 K using PEG 4000 as the main precipitant; the crystals, which belonged to the tetragonal space group P41212 with unit-cell parameters a = b = 76.38, c = 133.25 Å, diffracted to 2.83 Å resolution. Assuming two molecules in the asymmetric unit, the Matthews coefficient and the solvent content were calculated to be 2.63 Å(3) Da(-1) and 53.27%, respectively.

Kinetic mechanism of human histidine triad nucleotide binding protein 1

Human histidine triad nucleotide binding protein 1 (hHint1) is a member of a ubiquitous and ancient branch of the histidine triad protein superfamily. hHint1 is a homodimeric protein that catalyzes the hydrolysis of model substrates, phosphoramidate and acyl adenylate, with a high efficiency. Recently, catalytically inactive hHint1 has been identified as the cause of inherited peripheral neuropathy [Zimon, M., et al. (2012) Nat. Genet. 44, 1080-1083]. We have conducted the first detailed kinetic mechanistic studies of hHint1 and have found that the reaction mechanism is consistent with a double-displacement mechanism, in which the active site nucleophile His112 is first adenylylated by the substrate, followed by hydrolysis of the AMP-enzyme intermediate. A transient burst phase followed by a linear phase from the stopped-flow fluorescence assay indicated that enzyme adenylylation was faster than the subsequent intermediate hydrolysis and product release. Solvent viscosity experiments suggested that both chemical transformation and diffusion-sensitive events (product release or protein conformational change) limit the overall turnover. The catalytic trapping experiments and data simulation indicated that the true koff rate of the final product AMP is unlikely to control the overall kcat. Therefore, a protein conformational change associated with product release is likely rate-limiting. In addition, the rate of Hint1 adenylylation was found to be dependent on two residues with pKa values of 6.5 and 8, with the former pKa agreeing well with the nuclear magnetic resonance titration results for the pKa of the active site nucleophile His112. In comparison to the uncatalyzed rates, hHint1 was shown to enhance acyl-AMP and AMP phosphoramidate hydrolysis by 10(6)-10(8)-fold. Taken together, our analysis indicates that hHint1 catalyzes the hydrolysis of phosphoramidate and acyl adenylate with high efficiency, through a mechanism that relies on rapid adenylylation of the active residue, His112, while being partially rate-limited by intermediate hydrolysis and product release associated with a conformational change. Given the high degree of sequence homology of Hint proteins across all kingdoms of life, it is likely that their kinetic and catalytic mechanisms will be similar to those elucidated for hHint1.

Synthesis and anti-herpetic activity of phosphoramidate ProTides

Among the many prodrug approaches aimed at delivering nucleoside monophosphates into cells, the phosphoramidate ProTide approach is one that has shown success, which has made it possible for some of the phosphoramidates to enter into clinical trials. Herein, we report the synthesis and antiviral activity of a series of phosphoramidate ProTides designed to bypass the thymidine kinase (TK) dependence of the parent nucleoside analogues. Phosphoramidate derivatives of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) that contain L-alanine or pivaloyloxymethyl iminodiacetate (IDA-POM) exhibit anti-HSV-1 and anti-VZV activity in cell cultures, but they largely lost antiviral potency against TK-deficient virus strains. Among deazapurine nucleosides and their phosphoramidate derivatives, the 7-deazaadenine containing nucleosides and their phosphoramidate triester derivatives showed weak antiviral activity against VZV. Apparently, intracellular nucleotide delivery with these phosphoramidates is partly successful. However, none of the compound prodrugs showed superior activity to their parent drugs.

A new crystal form of human histidine triad nucleotide-binding protein 1 (hHINT1) in complex with adenosine 5'-monophosphate at 1.38 Å resolution

Histidine triad nucleotide-binding protein 1 (HINT1) represents the most ancient and widespread branch of the histidine triad protein superfamily. HINT1 plays an important role in various biological processes and has been found in many species. Here, the structure of the human HINT1-adenosine 5'-monophosphate (AMP) complex at 1.38 Å resolution obtained from a new monoclinic crystal form is reported. The final structure has R(cryst) = 0.1207 (R(free) = 0.1615) and the model exhibits good stereochemical quality. Detailed analysis of the high-resolution data allowed the details of the protein structure to be updated in comparison to the previously published data.

E. coli histidine triad nucleotide binding protein 1 (ecHinT) is a catalytic regulator of D-alanine dehydrogenase (DadA) activity in vivo

Histidine triad nucleotide binding proteins (Hints) are highly conserved members of the histidine triad (HIT) protein superfamily. Hints comprise the most ancient branch of this superfamily and can be found in Archaea, Bacteria, and Eukaryota. Prokaryotic genomes, including a wide diversity of both Gram-negative and Gram-positive bacteria, typically have one Hint gene encoded by hinT (ycfF in E. coli). Despite their ubiquity, the foundational reason for the wide-spread conservation of Hints across all kingdoms of life remains a mystery. In this study, we used a combination of phenotypic screening and complementation analyses with wild-type and hinT knock-out Escherichia coli strains to show that catalytically active ecHinT is required in E. coli for growth on D-alanine as a sole carbon source. We demonstrate that the expression of catalytically active ecHinT is essential for the activity of the enzyme D-alanine dehydrogenase (DadA) (equivalent to D-amino acid oxidase in eukaryotes), a necessary component of the D-alanine catabolic pathway. Site-directed mutagenesis studies revealed that catalytically active C-terminal mutants of ecHinT are unable to activate DadA activity. In addition, we have designed and synthesized the first cell-permeable inhibitor of ecHinT and demonstrated that the wild-type E. coli treated with the inhibitor exhibited the same phenotype observed for the hinT knock-out strain. These results reveal that the catalytic activity and structure of ecHinT is essential for DadA function and therefore alanine metabolism in E. coli. Moreover, they provide the first biochemical evidence linking the catalytic activity of this ubiquitous protein to the biological function of Hints in Escherichia coli.

Hit proteins, mitochondria and cancer

The histidine triad (HIT) superfamily comprises proteins that share the histidine triad motif, His-ϕ-His-ϕ-His-ϕ-ϕ, where ϕ is a hydrophobic amino acid. HIT proteins are ubiquitous in prokaryotes and eukaryotes. HIT proteins bind nucleotides and exert dinucleotidyl hydrolase, nucleotidylyl transferase or phosphoramidate hydrolase enzymatic activity. In humans, 5 families of HIT proteins are recognized. The accumulated epidemiological and experimental evidence indicates that two branches of the superfamily, the HINT (Histidine Triad Nucleotide Binding) members and FHIT (Fragile Histidine Triad), have tumor suppressor properties but a conclusive physiological role can still not be assigned to these proteins. Aprataxin forms another discrete branch of the HIT superfamily, is implicated in DNA repair mechanisms and unlike the HINT and FHIT members, a defective protein can be conclusively linked to a disease, ataxia with oculomotor apraxia type 1. The scavenger mRNA decapping enzyme, DcpS, forms a fourth branch of the HIT superfamily. Finally, the GalT enzymes, which exert specific nucleoside monophosphate transferase activity, form a fifth branch that is not implicated in tumorigenesis. The molecular mechanisms by which the HINT and FHIT proteins participate in bioenergetics of cancer are just beginning to be unraveled. Their purported actions as tumor suppressors are highlighted in this review.

First identification and functional analysis of a histidine triad nucleotide binding protein in an invertebrate species Haliotis diversicolor supertexta

Histidine triad nucleotide binding protein (HINT) represents the most ancient and widespread branches in the histidine triad superfamily. HINT plays an important role in many biological processes especially in cell biology, and it has been found in a wide variety of species. However, the functional attributes of HINT homologues in invertebrates have not yet been reported. Here we identified a HINT homologue in abalone, which we named ab-HINT. The ab-HINT shows significant structural and functional similarities to mammalian HINT. RT-PCR and western blot analysis show that ab-HINT is ubiquitously expressed in abalone tissues and highly expressed in hemocyte and gills. In addition, significant up-regulation of ab-HINT was observed after LPS or Poly I:C challenge. Immunostainings suggest that ab-HINT is expressed predominantly in epithelial cells and mainly localized in the cytoplasmic compartment. Studies of the effect on cell apoptosis indicate that ab-HINT can trigger hemocytes apoptosis and p53 is involved in this process. These results conclude that ab-HINT is involved in the immune response of abalone and may be a potential pro-apoptotic factor. To the best of our knowledge, this is the first identification and characterization of a HINT homologue in invertebrates.

Evidence that human histidine triad nucleotide binding protein 3 (Hint3) is a distinct branch of the histidine triad (HIT) superfamily

Human Hint3 (hHint3) has been classified as a member of the histidine triad nucleotide (Hint) binding protein subfamily. While Hint1 is ubiquitously expressed by both eukaryotes and prokaryotes, Hint3 is found only in eukaryotes. Previously, our laboratory has characterized and compared the aminoacyl-adenylate and nucleoside phosphoramidate hydrolase activity of hHint1 and Escherichia coli hinT. In this study, hHint3-1(Ala36) and its single nucleotide polymorphism, hHint3-2 (A36G variant), were cloned, overexpressed, and purified. Steady-state kinetic studies with a synthetic fluorogenic indolepropinoic acyl-adenylate (AIPA) and with a series of fluorogenic tryptamine nucleoside phosphoramidates revealed that hHint3-1 and hHint3-2 are adenylate and phosphoramidate hydrolases with apparent second-order rate constants (kcat/Km) ranging from 10(2) to 10(6) s(-1) M(-1). Unlike hHint1, hHint3-1 and hHint3-2 prefer AIPA over tryptamine adenosine phosphoramidate by factors of 33- and 16-fold, respectively. In general, hHint3s hydrolyze phosphoramidate 370- to 2000-fold less efficiently than hHint1. Substitution of the potential active-site nucleophile, His145, by Ala was shown to abolish the adenylate and phosphoramidate hydrolase activity for hHint3-1. However, 0.2-0.4% residual activity was observed for the H145A mutant of hHint3-2. Both hHint3-1 and hHint3-2 were found to hydrolyze lysyl-adenylate generated by human lysyl-tRNA synthetase (hLysRS) by proceeding through an adenylated protein intermediate. hLysRS-dependent labeling of hHint3-1 and hHint3-2 was found to depend on His145, which aligns with the His112 of the Hint1 active site. The extent of active-site His145-AMP labeling was shown to be similar to His112-AMP labeling of hHint1. In contrast to all previously characterized members of the histidine triad superfamily, which have been shown to exist exclusively as homodimers, wild type and the H145A of hHint3-1 were found to exist across a range of multimeric states, from dimers to octamers and even larger oligomers, while wild type and the H145A of hHint3-2 exist predominantly in a monomeric state. The differences in oligomeric state may be important in vivo, because unlike tetracysteine-tagged Hint1, which was found along linear arrays exclusively in the cytoplasm in transfected HeLa cells, tagged Hint3-1 and Hint3-2 were found as aggregates both in the cytosol and in the nucleus. Taken together, these results imply that while Hint3 and Hint1 prefer aminoacyl-adenylates as substrates and catalytically interact with aminoacyl-tRNA synthetases, the significant differences in phosphoramidase activity, oligomeric state, and cellular localization suggest that Hint3s should be placed in a distinct branch of the histidine triad superfamily.

The histidine triad protein Hint1 triggers apoptosis independent of its enzymatic activity

Hint1 is a member of the evolutionarily conserved family of histidine triad proteins that acts as a haplo-insufficient tumor suppressor inducing spontaneous tumor formation in Hint+/- and Hint-/- mouse models. However, the molecular mechanisms for the tumor-suppressing activity are poorly defined. In this respect, we have recently shown that Hint1, by interaction with Pontin and Reptin, inhibits T-cell factor/beta-catenin-mediated transcription of Wnt target genes. In this study, we have found that, after transient transfection with Hint1, SW480 and MCF-7 cells undergo apoptosis as analyzed by pro-caspase-3 and poly(ADP-ribose) polymerase cleavage, M30 CytoDEATH staining, cytochrome c release, and DNA fragmentation enzyme-linked immunosorbent assay. Hint1 is involved in the regulation of apoptotic pathways by inducing an up-regulation of p53 expression coinciding with an up-regulation of the proapoptotic factor Bax and a concomitant down-regulation of the apoptosis inhibitor Bcl-2. Bad and Puma levels remained unchanged. Further analyses revealed that Hint1 is associated with the Bax promoter and is a component of the Tip60 histone acetyltransferase complex and, in this context, appears to be involved in the regulation of Bax expression. Knockdown of Hint1 by short hairpin RNA resulted in down-regulation of p53 and Bax but had no effect on Bcl-2 expression. A mutant Hint1 (H112N) protein defective in enzymatic activity as an AMP-NH2 hydrolase was not impaired in induction of apoptosis, suggesting that the Hint1 pro-apoptotic activity is independent of the Hint1 enzymatic activity.

Hint2, a mitochondrial apoptotic sensitizer down-regulated in hepatocellular carcinoma

BACKGROUND & AIMS

Hints, histidine triad nucleotide-binding proteins, are adenosine monophosphate-lysine hydrolases of uncertain biological function. Here we report the characterization of human Hint2.

METHODS

Tissue distribution was determined by real-time quantitative polymerase chain reaction and immunoblotting, cellular localization by immunocytochemistry, and transfection with green fluorescent protein constructs. Enzymatic activities for protein kinase C and adenosine phosphoramidase in the presence of Hint2 were measured. HepG2 cell lines with Hint2 overexpressed or knocked down were established. Apoptosis was assessed by immunoblotting for caspases and by flow cytometry. Tumor growth was measured in SCID mice. Expression in human tumors was investigated by microarrays.

RESULTS

Hint2 was predominantly expressed in liver and pancreas. Hint2 was localized in mitochondria. Hint2 hydrolyzed adenosine monophosphate linked to an amino group (AMP-pNA; k(cat):0.0223 s(-1); Km:128 micromol/L). Exposed to apoptotic stress, fewer HepG2 cells overexpressing Hint2 remained viable (32.2 +/- 0.6% vs 57.7 +/- 4.6%), and more cells displayed changes of the mitochondrial membrane potential (87.8 +/- 2.35 vs 49.7 +/- 1.6%) with more cleaved caspases than control cells. The opposite was observed in HepG2 cells with knockdown expression of Hint2. Subcutaneous injection of HepG2 cells overexpressing Hint2 in SCID mice resulted in smaller tumors (0.32 +/- 0.13 g vs 0.85 +/- 0.35 g). Microarray analyses revealed that HINT2 messenger RNA is downregulated in hepatocellular carcinomas (-0.42 +/- 0.58 log2 vs -0.11 +/- 0.28 log2). Low abundance of HINT2 messenger RNA was associated with poor survival.

CONCLUSION

Hint2 defines a novel class of mitochondrial apoptotic sensitizers down-regulated in hepatocellular carcinoma.

Hint1 is a haplo-insufficient tumor suppressor in mice

The HINT1 protein, a member of the histidine triad (HIT) family, is highly conserved in diverse species and ubiquitously expressed in mammalian tissues. However, its precise function in mammalian cells is not known. As a result of its structural similarity to the tumor-suppressor protein FHIT, we used homozygous-deleted Hint1 mice to study its role in tumorigenesis. We discovered that after 2 to 3 years of age the spontaneous tumor incidence in Hint1 -/- mice was significantly greater than that in wild-type Hint1 +/+ mice (P < 0.05). Using a well-established mouse model of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary carcinogenesis we found a marked and significant (P < 0.05) increase in the incidence of mammary and ovarian tumors in both, Hint1 -/- and +/- mice versus +/+ mice. The Hint1 -/- and +/- mice had similar tumor incidence and similar tumor histologies. Therefore, deletion of Hint1 in mice enhances both spontaneous tumor development and susceptibility to tumor induction by DMBA. In addition, since the Hint1 +/- tumors retained expression of the unmutated wild-type allele, Hint1 is haplo-insufficient with respect to tumor suppression in this model system.

The histidine triad protein Hint1 interacts with Pontin and Reptin and inhibits TCF-beta-catenin-mediated transcription

Pontin and Reptin previously were identified as nuclear beta-catenin interaction partners that antagonistically modulate beta-catenin transcriptional activity. In this study, Hint1/PKCI, a member of the evolutionary conserved family of histidine triad proteins, was characterised as a new interaction partner of Pontin and Reptin. Pull-down assays and co-immunoprecipitation experiments show that Hint1/PKCI directly binds to Pontin and Reptin. The Hint1/PKCI-binding site was mapped to amino acids 214-295 and 218-289 in Pontin and Reptin, respectively. Conversely, Pontin and Reptin bind to the N-terminus of Hint1/PKCI. Moreover, by its interaction with Pontin and Reptin, Hint1/PKCI is associated with the LEF-1/TCF-beta-catenin transcription complex. In this context, Hint1/PKCI acts as a negative regulator of TCF-beta-catenin transcriptional activity in Wnt-transfected cells and in SW480 colon carcinoma cells as shown in reporter gene assays. Consistent with these observations, Hint1/PKCI represses expression of the endogenous target genes cyclin D1 and axin2 whereas knockdown of Hint1/PKCI by RNA interference increases their expression. Disruption of the Pontin/Reptin complex appears to mediate this modulatory effect of Hint1/PKCI on TCF-beta-catenin-mediated transcription. These data now provide a molecular mechanism to explain the tumor suppressor function of Hint1/PKCI recently suggested from the analysis of Hint1/PKCI knockout mice.

Role of a novel dual flavin reductase (NR1) and an associated histidine triad protein (DCS-1) in menadione-induced cytotoxicity

Microsomal cytochrome P450 reductase catalyzes the one-electron transfer from NADPH via FAD and FMN to various electron acceptors, such as cytochrome P450s or to some anti-cancer quinone drugs. This results in generation of free radicals and toxic oxygen metabolites, which can contribute to the cytotoxicity of these compounds. Recently, a cytosolic NADPH-dependent flavin reductase, NR1, has been described which is highly homologous to the microsomal cytochrome P450 reductase. In this study, we show that over-expression of NR1 in human embryonic kidney cells enhances the cytotoxic action of the model quinone, menadione. Furthermore, we show that a novel human histidine triad protein DCS-1, which is expressed together with NR1 in many tissues, can significantly reduce menadione-induced cytotoxicity in these cells. We also show that DCS-1 binds NF1 and directly modulates its activity. These results suggest that NR1 may play a role in carcinogenicity and cell death associated with one-electron reductions.

HinT proteins and their putative interaction partners in Mollicutes and Chlamydiaceae

BACKGROUND

HinT proteins are found in prokaryotes and eukaryotes and belong to the superfamily of HIT proteins, which are characterized by an histidine-triad sequence motif. While the eukaryotic variants hydrolyze AMP derivates and modulate transcription, the function of prokaryotic HinT proteins is less clearly defined. In Mycoplasma hominis, HinT is concomitantly expressed with the proteins P60 and P80, two domains of a surface exposed membrane complex, and in addition interacts with the P80 moiety.

RESULTS

An cluster of hitABL genes, similar to that of M. hominis was found in M. pulmonis, M. mycoides subspecies mycoides SC, M. mobile and Mesoplasma florum. RT-PCR analyses provided evidence that the P80, P60 and HinT homologues of M. pulmonis were polycistronically organized, suggesting a genetic and physical interaction between the proteins encoded by these genes in these species. While the hit loci of M. pneumoniae and M. genitalium encoded, in addition to HinT, a protein with several transmembrane segments, the hit locus of Ureaplasma parvum encoded a pore-forming protein, UU270, a P60 homologue, UU271, HinT, UU272, and a membrane protein of unknown function, UU273. Although a full-length mRNA spanning the four genes was not detected, amplification of all intergenic regions from the center of UU270 to the end of UU273 by RT-PCR may be indicative of a common, but unstable mRNA. In Chlamydiaceae the hit gene is flanked upstream by a gene predicted to encode a metal dependent hydrolase and downstream by a gene putatively encoding a protein with ARM-repeats, which are known to be involved in protein-protein interactions. In RT-PCR analyses of C. pneumoniae, regions comprising only two genes, Cp265/Cp266 and Cp266/Cp267 were able to be amplified. In contrast to this in vivo interaction analysis using the yeast two-hybrid system and in vitro immune co-precipitation revealed an interaction between Cp267, which contains the ARM repeats, Cp265, the predicted hydrolase, and Cp266, the HinT protein.

CONCLUSION

In the Mollicutes HinT proteins were shown to be linked with membrane proteins while in the Chlamydiaceae they were genetically and physically associated with cytoplasmic proteins, one of which is predicted to be a metal-dependent phosphoesterase. Future work will elucidate whether these differing associations indicate that HinT proteins have evolved independently or are indeed two hotspots of a common sphere of action of bacterial HinT proteins.

31P NMR and genetic analysis establish hinT as the only Escherchia coli purine nucleoside phosphoramidase and as essential for growth under high salt conditions

Eukaryotic cells encode AMP-lysine (AMP-N-epsilon-(N-alpha-acetyl lysine methyl ester) 5'-phosphoramidate) hydrolases related to the rabbit histidine triad nucleotide-binding protein 1 (Hint1) sequence. Bacterial and archaeal cells have Hint homologs annotated in a variety of ways, but the enzymes have not been characterized, nor have phenotypes been described due to loss of enzymatic activity. We developed a quantitative (31)P NMR assay to determine whether Escherichia coli possesses an adenosine phosphoramidase activity. Indeed, soluble lysates prepared from wild-type laboratory E. coli exhibited activity on the model substrate adenosine 5'-monophosphoramidate (AMP-NH(2)). The E. coli Hint homolog, which had been comprehensively designated ycfF and is here named hinT, was cloned, overexpressed, purified, and characterized with respect to purine nucleoside phosphoramidate substrates. Bacterial hinT was several times more active than human or rabbit Hint1 on five model substrates. In addition, bacterial and mammalian enzymes preferred guanosine versus adenosine phosphoramidates as substrates. Analysis of the lysates from a constructed hinT knock-out strain of E. coli demonstrated that all of the cellular purine nucleoside phosphoramidase activity is due to hinT. Physiological analysis of this mutant revealed that the loss of hinT results in failure to grow in media containing 0.75 m KCl, 0.9 m NaCl, 0.5 m NaOAc, or 10 mm MnCl(2). Thus, cation-resistant bacterial cell growth may be dependent on the hydrolysis of adenylylated and/or guanylylated phosphoramidate substrates by hinT.

Gene expression of metabolic enzymes and a protease inhibitor in the prefrontal cortex are decreased in schizophrenia

Microarray expression studies have reported decreased mRNA expression of histidine triad nucleotide-binding protein (HINT1) and cytosolic malate dehydrogenase (MDH1) in the dorsolateral prefrontal cortex (DLPFC) of individuals with schizophrenia. Microarray results for neuroserpin (SERPINI1) mRNA in the DLPFC have reported increased and decreased expression in individuals with schizophrenia. The relative abundances of HINT1, MDH1, and SERPINI1 mRNA in the DLPFC in individuals with schizophrenia and controls were measured by real-time quantitative polymerase chain reaction (Q-PCR) and for HINT1 expression by in situ hybridization. The Q-PCR results were compared by analysis of covariance between individuals with schizophrenia and controls. Gene expression levels for HINT1, MDH1, and SERPINI1 were significantly different between the groups. The male individuals with schizophrenia compared to male controls showed reductions by 2.8- to 3.7-fold of HINT1, neuroserpin, and MDH1 by Q-PCR. The decreases in mRNA abundance for MDH1 (P = 0.006), HINT1 (P = 0.050), and neuroserpin (P = 0.005) in DLPFC of male individuals with schizophrenia is consistent with prior reports. HINT1 mRNA was reduced significantly by 34% in layer VI. Though there were no significant interactions with gender, gene expression between female patients and the female control group did not differ. These results confirm earlier reports and suggest abnormalities of specific genes related to metabolic and protease activities in the DLPFC might be considered as part of a molecular pathway in male patients with schizophrenia.

Biochemical, crystallographic, and mutagenic characterization of hint, the AMP-lysine hydrolase, with novel substrates and inhibitors

Hint, histidine triad nucleotide-binding protein, is a universally conserved enzyme that hydrolyzes AMP linked to lysine and, in yeast, functions as a positive regulator of the RNA polymerase II C-terminal domain kinase, Kin28. To explore the biochemical and structural bases for the adenosine phosphoramidate hydrolase activity of rabbit Hint, we synthesized novel substrates linking a p-nitroaniline group to adenylate (AMP-pNA) and inhibitors that consist of an adenosine group and 5'-sulfamoyl (AdoOSO(2)NH(2)) or N-ethylsulfamoyl (AdoOSO(2)NHCH(2)CH(3)) group. AMP-pNA is a suitable substrate for Hint that allowed characterization of the inhibitors; titration of each inhibitor into AMP-pNA assays revealed their K(i) values. The N-ethylsulfamoyl derivative has a 13-fold binding advantage over the sulfamoyl adenosine. The 1.8-A cocrystal structure of rabbit Hint with N-ethylsulfamoyl adenosine revealed a binding site for the ethyl group against Trp-123, a residue that reaches across the Hint dimer interface to interact with the alkyl portion of the inhibitor and, presumably, the alkyl portion of a lysyl substrate. Ser-107 is positioned to donate a hydrogen bond to the leaving group nitrogen. Consistent with a role in acid-base catalysis, the Hint S107A mutant protein displayed depressed catalytic activity.