Altered specificity of Hint-W123Q supports a role for Hint inhibition by ASW in avian sex determination

Role and function of the Hintw in early sex differentiation in chicken (Gallus gallus) embryo

Mono-Sex culturing is an important methodology for intensive livestock and poultry production. Here, Hintw was identified as a potential key gene in sex-determination process in chickens via RNA-seq. Then we developed an effective method to interfere or overexpress Hintw in chicken embryos through the intravascular injection. QRT-PCR, ELISA and H&E staining were used to detect the effects of Hintw on gonadal development of chicken embryos. Results showed that Hintw exhibited a female-biased expression pattern in the early stage of PGCs (primordial germ cells) in embryonic gonads. The qRT-PCR analysis showed that Foxl2, Cyp19a1 in females were upregulated under the overexpression of Hintw, while Sox9 and Dmrt1 were downregulated Hintw. Overexpression of Hintw can promote the development of gonadal cortex, while interference with Hintw show the opposite result. Additionally, we found that overexpression of the Hintw in male chicken embryos could inhibit androgen levels and increase estrogen levels. On the other hand, interfering with Hintw in female chicken embryos decreased estrogen levels and increased androgen levels. In conclusion, this work sets the basis for the understanding of the molecular regulatory network for the sex-determination process in chicken embryos as well as providing the theoretical basis for mono-sex culturing of poultry.

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.

Multi-Copy Gene Family Evolution on the Avian W Chromosome

The sex chromosomes often follow unusual evolutionary trajectories. In particular, the sex-limited chromosomes frequently exhibit a small but unusual gene content in numerous species, where many genes have undergone massive gene amplification. The reasons for this remain elusive with a number of recent studies implicating meiotic drive, sperm competition, genetic drift, and gene conversion in the expansion of gene families. However, our understanding is primarily based on Y chromosome studies as few studies have systematically tested for copy number variation on W chromosomes. Here, we conduct a comprehensive investigation into the abundance, variability, and evolution of ampliconic genes on the avian W. First, we quantified gene copy number and variability across the duck W chromosome. We find a limited number of gene families as well as conservation in W-linked gene copy number across duck breeds, indicating that gene amplification may not be such a general feature of sex chromosome evolution as Y studies would initially suggest. Next, we investigated the evolution of HINTW, a prominent ampliconic gene family hypothesized to play a role in female reproduction and oogenesis. In particular, we investigated the factors driving the expansion of HINTW using contrasts between modern chicken and duck breeds selected for different female-specific selection regimes and their wild ancestors. Although we find the potential for selection related to fecundity in explaining small-scale gene amplification of HINTW in the chicken, purifying selection seems to be the dominant mode of evolution in the duck. Together, this challenges the assumption that HINTW is key for female fecundity across the avian phylogeny.

Sex Reversal and Comparative Data Undermine the W Chromosome and Support Z-linked DMRT1 as the Regulator of Gonadal Sex Differentiation in Birds

The exact genetic mechanism regulating avian gonadal sex differentiation has not been completely resolved. The most likely scenario involves a dosage mechanism, whereby the Z-linked DMRT1 gene triggers testis development. However, the possibility still exists that the female-specific W chromosome may harbor an ovarian determining factor. In this study, we provide evidence that the universal gene regulating gonadal sex differentiation in birds is Z-linked DMRT1 and not a W-linked (ovarian) factor. Three candidate W-linked ovarian determinants are HINTW, female-expressed transcript 1 (FET1), and female-associated factor (FAF). To test the association of these genes with ovarian differentiation in the chicken, we examined their expression following experimentally induced female-to-male sex reversal using the aromatase inhibitor fadrozole (FAD). Administration of FAD on day 3 of embryogenesis induced a significant loss of aromatase enzyme activity in female gonads and masculinization. However, expression levels of HINTW, FAF, and FET1 were unaltered after experimental masculinization. Furthermore, comparative analysis showed that FAF and FET1 expression could not be detected in zebra finch gonads. Additionally, an antibody raised against the predicted HINTW protein failed to detect it endogenously. These data do not support a universal role for these genes or for the W sex chromosome in ovarian development in birds. We found that DMRT1 (but not the recently identified Z-linked HEMGN gene) is male upregulated in embryonic zebra finch and emu gonads, as in the chicken. As chicken, zebra finch, and emu exemplify the major evolutionary clades of birds, we propose that Z-linked DMRT1, and not the W sex chromosome, regulates gonadal sex differentiation in birds.

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.

Loss-of-function mutations in HINT1 cause axonal neuropathy with neuromyotonia

Inherited peripheral neuropathies are frequent neuromuscular disorders known for their clinical and genetic heterogeneity. In 33 families, we identified 8 mutations in HINT1 (encoding histidine triad nucleotide-binding protein 1) by combining linkage analyses with next-generation sequencing and subsequent cohort screening of affected individuals. Our study provides evidence that loss of functional HINT1 protein results in a distinct phenotype of autosomal recessive axonal neuropathy with neuromyotonia.

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.

Side chain independent recognition of aminoacyl adenylates by the Hint1 transcription suppressor

Human Hint1 suppresses specific gene transcription by interacting with the transcription factor MITF in mast cells. Hint1 activity is connected to lysyl-tRNA synthetase (LysRS), a member of the universal aminoacyl tRNA synthetase family that catalyzes specific aminoacylation of their cognate tRNAs, through an aminoacyl adenylate (aa-AMP) intermediate. During immune activation, LysRS produces a side-product diadenosine tetraphosphate (Ap₄A) from the condensation of Lys-AMP with ATP. The pleiotropic signaling molecule Ap₄A then binds Hint1 to promote activation of MITF-target gene transcription. Earlier work showed that Hint1 can also bind and hydrolyze Lys-AMP, possibly to constrain Ap₄A production. Because Ap₄A can result from condensation of other aa-AMP's with ATP, the specificity of the Hint1 aa-AMP–hydrolysis activity is of interest. Here we show that Hint1 has broad specificity for adenylate hydrolysis, whose structural basis we revealed through high-resolution structures of Hint1 in complex with three different aa-AMP analogues. Hint1 recognizes only the common main chain of the aminoacyl moiety, and has no contact with the aa side chain. The α-amino group is anchored by a cation-pi interaction with Trp123 at the C-terminus of Hint1. These results reveal the structural basis for the remarkable adenylate surveillance activity of Hint1, to potentially control Ap₄A levels in the cell.

Phosphoramidate pronucleotides: a comparison of the phosphoramidase substrate specificity of human and Escherichia coli histidine triad nucleotide binding proteins

To facilitate the delivery of nucleotide-based therapeutics to cells and tissues, a variety of pronucleotide approaches have been developed. Our laboratory and others have demonstrated that nucleoside phosphoramidates can be activated intracellularly to the corresponding 5'-monophosphate nucleotide and that histidine triad nucleotide binding proteins (Hints) are potentially responsible for their bioactivation. Hints are conserved and ubiquitous enzymes that hydrolyze phosphoramidate bonds between nucleoside 5'-monophosphate and an amine leaving group. On the basis of the ability of nucleosides to quench the fluorescence of covalently linked amines containing indole, a sensitive, continuous fluorescence-based assay was developed. A series of substrates linking the naturally fluorogenic indole derivatives to nucleoside 5'-monophosphates were synthesized, and their steady state kinetic parameters of hydrolysis by human Hint1 and Escherichia coli hinT were evaluated. To characterize the elemental and stereochemical effect on the reaction, two P-diastereoisomers of adenosine or guanosine phosphoramidothioates were synthesized and studied to reveal a 15-200-fold decrease in the specificity constant (kcat/Km) when the phosphoryl oxygen is replaced with sulfur. While a stereochemical preference was not observed for E. coli hinT, hHint1 exhibited a 300-fold preference for d-tryptophan phosphoramidates over l-isomers. The most efficient substrates evaluated to date are those that contain the less sterically hindering amine leaving group, tryptamine, with kcat and Km values comparable to those found for adenosine kinase. The apparent second-order rate constants (kcat/Km) for adenosine tryptamine phosphoramidate monoester were found to be 107 M-1 s-1 for hHint1 and 106 M-1 s-1 for E. coli hinT. Both the human and E. coli enzymes preferred purine over pyrimidine analogues. Consistent with observed hydrogen bonding between the 2'-OH group of adenosine monophosphate and the active site residue, Asp43, the second-order rate constant (kcat/Km) for thymidine tryptamine phosphoramidate was found to be 3-4 orders of magnitude smaller than that for uridine tryptamine phosphoramidate for hHint1 and 2 orders of magnitude smaller than that for E. coli hinT. Ara-A tryptamine phosphoramidate was, however, shown to be a good substrate with a specificity constant (kcat/Km) only 10-fold lower than the value for adenosine tryptamine phosphoramidate. Consequently, nucleoside phosphoramidates containing unhindered primary amines and either an alpha or beta 2'-OH group should be easily bioactivated by Hints with efficiencies rivaling those for the 5'-monophosphorylation of nucleosides by nucleoside kinases. The differential substrate specificity observed for human and E. coli enzymes represents a potential therapeutic rationale for the development of selective antibiotic phosphoramidate pronucleotides.

Lysyl-tRNA synthetase-generated lysyl-adenylate is a substrate for histidine triad nucleotide binding proteins

Histidine triad nucleotide binding proteins (Hints) are the most ancient members of the histidine triad protein superfamily of nucleotidyltransferases and hydrolyases. Protein-protein interaction studies have found that complexes of the transcription factors MITF or USF2 and lysyl-tRNA synthetase (LysRS) are associated with human Hint1. Therefore, we hypothesized that lysyl-AMP or the LysRS.lysyl-AMP may be a native substrate for Hints. To explore the biochemical relationship between Hint1 and LysRS, a series of catalytic radiolabeling, mutagenesis, and kinetic experiments was conducted with purified LysRSs and Hints from human and Escherichia coli. After incubation of the E. coli or human LysRS with Hints and [alpha-(32)P]ATP, but not [alpha-(32)P]GTP, (32)P-labeled Hints were observed. By varying time and the concentrations of lysine, Mg(2+), or LysRS, the adenylation of Hint was found to be dependent on the formation of lysyl-AMP. Site-directed mutagenesis studies of the active site histidine triad revealed that Hint labeling could be abolished by substitution of either His-101 of E. coli hinT or His-112 of human Hint1 by either alanine or glycine. Ap(4)A, believed to be synthesized by LysRS in vivo, and Zn(2+) were shown to inhibit the formation of Hint-AMP with an IC(50) value in the low micromolar range. Consistent with pyrophosphate being an inhibitor for aminoacyl-tRNA synthetase, incubations in the presence of pyrophosphatase resulted in enhanced formation of Hint-AMP. These results demonstrate that the lysyl-AMP intermediate formed by LysRS is a natural substrate for Hints and suggests a potential highly conserved regulatory role for Hints on LysRS and possibly other aminoacyl-tRNA synthetases.

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.

PKCI-W Forms a Heterodimer with PKCI-Z and Inhibits the Biological Activities of PKCI-Z In Vitro, Supporting the Predicted Role of PKCI-W in Sex Determination in Birds

The two chicken genes, PKCI-W on the W chromosome and PKCI-Z on the Z chromosome, belong to the gene family encoding the Hint (histidine triad nucleotide-binding protein)-branch proteins in the widely conserved HIT (histidine triad)-family. It has been speculated that PKCI-W is involved in the sex determination of birds by forming a heterodimer with PKCI-Z and inhibiting the function of PKCI-Z in female embryos. In this study, both PKCI-W and PKCI-Z were expressed in fusion [maltose-binding protein (MBP) or glutathione-S-transferase (GST)] and tagged [(His)(6) or FLAG] forms (FT-forms) in Escherichia coli and purified. Formation of homodimers of PKCI-W-containing or the PKCI-Z-containing FT-protein and the formation of a heterodimer between the PKCI-W-containing and the PKCI-Z-containing FT-proteins were demonstrated by Western blotting after GST-pulldown or binding to and elution from the Co(2+)-resin. The homodimer of PKCI-Z, but not PKCI-W, bound to an N(6)-(3- aminopropyl) adenosine affinity column and hydrolyzed adenosine 5'-monophosphoramidate. Both of these activities were inhibited in vitro in a dominant-negative manner by the formation of a heterodimer containing PKCI-W. These in vitro experimental results support the predicted role of PKCI-W in the process of sex determination in birds.

Gene conversion drives the evolution of HINTW, an ampliconic gene on the female-specific avian W chromosome

The HINTW gene on the female-specific W chromosome of chicken and other birds is amplified and present in numerous copies. Moreover, as HINTW is distinctly different from its homolog on the Z chromosome (HINTZ), is a candidate gene in avian sex determination, and evolves rapidly under positive selection, it shows several common features to ampliconic and testis-specific genes on the mammalian Y chromosome. A phylogenetic analysis within galliform birds (chicken, turkey, quail, and pheasant) shows that individual HINTW copies within each species are more similar to each other than to gene copies of related species. Such convergent evolution is most easily explained by recurrent events of gene conversion, the rate of which we estimated at 10(-6)-10(-5) per site and generation. A significantly higher GC content of HINTW than of other W-linked genes is consistent with biased gene conversion increasing the fixation probability of mutations involving G and C nucleotides. Furthermore, and as a likely consequence, the neutral substitution rate is almost twice as high in HINTW as in other W-linked genes. The region on W encompassing the HINTW gene cluster is not covered in the initial assembly of the chicken genome, but analysis of raw sequence reads indicates that gene copy number is significantly higher than a previous estimate of 40. While sexual selection is one of several factors that potentially affect the evolution of ampliconic, male-specific genes on the mammalian Y chromosome, data from HINTW provide evidence that gene amplification followed by gene conversion can evolve in female-specific chromosomes in the absence of sexual selection. The presence of multiple and highly similar copies of HINTW may be related to protein function, but, more generally, amplification and conversion offers a means to the avoidance of accumulation of deleterious mutations in nonrecombining chromosomes.

Disease-associated mutations inactivate AMP-lysine hydrolase activity of Aprataxin

Ataxia-oculomotor apraxia syndrome 1 is an early onset cerebellar ataxia that results from loss of function mutations in the APTX gene, encoding Aprataxin, which contains three conserved domains. The forkhead-associated domain of Aprataxin mediates protein-protein interactions with molecules that respond to DNA damage, but the cellular phenotype of the disease does not appear to be consistent with a major loss in DNA damage responses. Disease-associated mutations in Aprataxin target a histidine triad domain that is similar to Hint, a universally conserved AMP-lysine hydrolase, or truncate the protein NH2-terminal to a zinc finger. With novel fluorigenic substrates, we demonstrate that Aprataxin possesses an active-site-dependent AMP-lysine and GMP-lysine hydrolase activity that depends additionally on the zinc finger for protein stability and on the forkhead associated domain for enzymatic activity. Alleles carrying any of eight recessive mutations associated with ataxia and oculomotor apraxia encode proteins with huge losses in protein stability and enzymatic activity, consistent with a null phenotype. The mild presentation allele, APTX-K197Q, associated with ataxia but not oculomotor apraxia, encodes a protein with a mild defect in stability and activity, while enzyme encoded by the atypical presentation allele, APTX-R199H, retained substantial function, consistent with altered and not loss of activity. The data suggest that the essential function of Aprataxin is reversal of nucleotidylylated protein modifications, that all three domains contribute to formation of a stable enzyme, and that the in vitro behavior of cloned APTX alleles can score disease-associated mutations.

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.