Isolation, cloning and characterization of a low-molecular-mass purine nucleoside- and nucleotide-binding protein

HINT1 Gene Polymorphisms, Smoking Behaviour, and Personality Traits: A Haplotype Case-Control Study

The factors influencing the development and maintenance of nicotine dependence are numerous and complex. Recent studies indicate that smokers exhibit distinct genetic predispositions to nicotine dependence. We aimed to analyse (1) the association between rs2551038 and cigarette smoking, (2) the association of between the rs3864236-rs2526303-rs2551038 haplotype and cigarette smoking, and (3) the personality traits measured by the NEO Five-Factor Inventory in cigarette users and never-smokers. No significant differences were present in the frequency of rs2551038 genotypes and alleles in the studied cigarette users compared to the control group. Cigarette users, compared to the control group, had higher scores on the NEO-FFI Extraversion scale (p = 0.0011), and lower scores were obtained by the cigarette users for the NEO-FFI Openness (p = 0.0060), Agreeability (p ≤ 0.000), and Conscientiousness (p ≤ 0.000) scales. There was a significant positive Pearson's linear correlation between the age and the Fagestrom test (r = 0.346; p < 0.0001) and the NEO-FFI Openness scale (r = 0.180; p < 0.0001) in the group of cigarette users. We observed significant linkage disequilibrium between rs2526303 and rs3864236 (D' = 0.3581; p < 2.2204 × 10-16) and between rs2526303 and rs2551038 (D' = 0.9993; p < 2.2204 × 10-16) in the tested sample. The sex-stratified haplotype analysis revealed that in the group of male never-smokers, the GTC haplotype was significantly more frequent than in the group of cigarette users (38% vs. 22%; p = 0.0039). The presented study reveals significant differences in personality trait scores between cases and controls. Moreover, the sex-stratified analysis showed significant differences in haplotype distribution. These results underscore the interplay between genetic predisposition, sex, and personality in nicotine-using individuals.

Histidine trinucleotide binding protein 2: from basic science to clinical implications

Histidine triad nucleotide-binding protein 2 (HINT2) is a dimeric protein that belongs to the histidine triad protein superfamily, predominantly expressed in the liver, pancreas, and adrenal gland, and localised to the mitochondrion. HINT2 binds nucleotides and catalyses the hydrolysis of nucleotidyl substrates. Moreover, HINT2 has been identified as a key regulator of multiple biological processes, including mitochondria-dependent apoptosis, mitochondrial protein acetylation, and steroidogenesis. Genetic manipulation has provided new insights into the physiological roles of HINT2 in several processes, such as inhibition of cancer progression, regulation of hepatic lipid metabolism, and protective effects on the cardiovascular system. The current review outlines the background and functions of HINT2. In addition, it summarises research progress on the correlation between HINT2 and human malignancies, hepatic metabolic diseases, and cardiovascular diseases, with an attempt to provide new research directions emerging in this field and to unveil the therapeutic value of HINT2 as a target in the combat of human diseases.

A novel mutation in HINT1 gene causes autosomal recessive axonal neuropathy with neuromyotonia, effective treatment with carbamazepine and review of the literature

INTRODUCTION

Autosomal recessive axonal neuropathy with neuromyotonia (ARAN-NM) is a rare disease entity linked to mutations in the histidine triad nucleotide binding protein 1 (HINT1) gene. The diagnosis and treatment of ARAN-NM are challenging. There have been few reports of ARAN-NM in East Asia.

METHODS

A 15-year-old Chinese ARAN-NM patient developed muscle weakness, cramps and atrophy in the lower limbs at the age of 12. Electromyography (EMG) showed motor axonal degeneration and neuromyotonic discharges. Whole exome sequencing was performed. Bioinformatic methods and computational 3D structure modeling were used to analyze the identified variant. According to literature review, carbamazepine was prescribed to the patient.

RESULTS

Genetic tests identified a homozygous mutation c.356G > T (p.R119L) in the HINT1 gene, which has never been reported before according to HGMD database. Several bioinformatic approaches predicted the variant was damaging. Computational 3D modeling indicated the variant changed the structure of HINT1 protein. Notably, we demonstrated the positive effects of carbamazepine in treating muscle stiffness and cramps of ARAN-NM.

DISCUSSION

22 variants have been reported in the HINT1 gene, and we identified a novel c.356G > T (p.R119L) variant. Our study expands the genetic spectrum of ARAN-NM. Moreover, large clinical trials are required to further demonstrate the role of carbamazepine in ARAN-NM.

HINT1 in Neuropsychiatric Diseases: A Potential Neuroplastic Mediator

Although many studies have investigated the functions of histidine triad nucleotide-binding protein 1 (HINT1), its roles in neurobiological processes remain to be fully elucidated. As a member of the histidine triad (HIT) enzyme superfamily, HINT1 is distributed in almost every organ and has both enzymatic and nonenzymatic activity. Accumulating clinical and preclinical evidence suggests that HINT1 may play an important role as a neuroplastic mediator in neuropsychiatric diseases, such as schizophrenia, inherited peripheral neuropathies, mood disorders, and drug addiction. Though our knowledge of HINT1 is limited, it is believed that further research on the neuropathological functions of HINT1 would eventually benefit patients with neuropsychiatric and even psychosomatic diseases.

Axonal neuropathy with neuromyotonia: there is a HINT

Recessive mutations in the gene encoding the histidine triad nucleotide binding protein 1 (HINT1) were recently shown to cause a motor-predominant Charcot–Marie–Tooth neuropathy. About 80% of the patients exhibit neuromyotonia, a striking clinical and electrophysiological hallmark that can help to distinguish this disease and to guide diagnostic screening. HINT1 neuropathy has worldwide distribution and is particularly prevalent in populations inhabiting central and south-eastern Europe. With 12 different mutations identified in more than 60 families, it ranks among the most common subtypes of axonal Charcot–Marie–Tooth neuropathy. This article provides an overview of the present knowledge on HINT1 neuropathy with the aim to increase awareness and spur interest among clinicians and researchers in the field. We propose diagnostic guidelines to recognize and differentiate this entity and suggest treatment strategies to manage common symptoms. As a recent player in the field of hereditary neuropathies, the role of HINT1 in peripheral nerves is unknown and the underlying disease mechanisms are unexplored. We provide a comprehensive overview of the structural and functional characteristics of the HINT1 protein that may guide further studies into the molecular aetiology and treatment strategies of this peculiar Charcot–Marie–Tooth subtype.

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.

N-Acyl-phosphoramidates as potential novel form of gemcitabine prodrugs

Gemcitabine (dFdC) is a cytidine analog remarkably active against a wide range of solid tumors. Inside a cell, gemcitabine is phosphorylated by deoxycytidine kinase to yield gemcitabine monophosphate, further converted to gemcitabine di- and triphosphate. The most frequent form of acquired resistance to gemcitabine in vitro is the deoxycytidine kinase deficiency. Thus, proper prodrugs carrying the 5'-pdFdC moiety may help to overcome this problem. A series of new derivatives of gemcitabine possessing N-acyl(thio)phosphoramidate moieties were prepared and their cytotoxic properties were determined. N-Acyl-phosphoramidate derivatives of gemcitabine have similar cytotoxicity as gemcitabine itself, and have been found accessible to the cellular enzymes. The nicotinic carboxamide derivative of gemcitabine 5'-O-phosphorothioate occurred to be the best inhibitor of bacterial DNA polymerase I and human DNA polymerase α.

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.

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.

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.

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.

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.

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.

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

The histidine triad (HIT) protein Hint has been found to associate with mammalian Cdk7, as well as to interact both physically and genetically with the budding yeast Cdk7 homologue Kin28. To study the function of Hint and to explore its possible role in modulating Cdk7 activity in vivo, we have characterized the expression pattern of murine Hint and generated Hint-deficient (Hint(-/-)) mice. Hint was widely expressed during mouse development, with pronounced expression in several neuronal ganglia, epithelia, hearts, and testes from embryonic day 15 onward. Despite this widespread expression, disruption of Hint did not impair murine development. Moreover, Hint-deficient mice had a normal life span and were apparently healthy. Histological examination of tissues with high Hint expression in wild-type animals did not show signs of abnormal pathology in Hint(-/-) mice. Functional redundancy within the HIT family was addressed by crossing Hint(-/-) mice with mice lacking the related HIT protein, Fhit, and by assaying the expression levels of the HIT protein gene family members Hint2 and Hint3 in Hint(+/+) and Hint(-/-) tissues. Finally, Cdk7 kinase activity and cell cycle kinetics were found to be comparable in wild-type and Hint(-/-) mouse embryonic fibroblasts, suggesting that Hint may not be a key regulator of Cdk7 activity.

Feminizing chicks: a model for avian sex determination based on titration of Hint enzyme activity and the predicted structure of an Asw-Hint heterodimer

BACKGROUND

In birds and some lizards, females are heterogametic with a ZW karyotype, while males are ZZ homogametes. The molecular basis for sexual differentiation in birds is unknown: arguments exist for doses of Z masculinizing chicks and for W information feminizing. ASW was identified as a tandemly repeated gene conserved on avian W chromosomes that is expressed in early female development and appears to be an inactive form of avian Z-encoded HINT. Hint is a dimeric enzyme that hydrolyzes AMP linked to lysine, whose enzyme activity is required for regulation of the Cdk7 homologous Kin28 kinase in yeast. Of 16 residues most conserved across all life forms for AMP interactions, 15 are sexually dimorphic in birds, that is, altered in the female-specific Asw protein. Genomic and expression data suggest that Asw may feminize chicks, dominantly interfering with Hint function by heterodimerization.

RESULTS

We consider whether positive cooperativity could explain how Hint heterodimerization with an inert enzyme might reduce specific activity by more than 50% and provide data sufficient to reject this model. Instead, we hypothesize that Asw carries a signal for mislocalization and/or proteolysis, and/or dominantly suppresses the remaining Hint active site to function as a dominant negative.

CONCLUSIONS

Molecular modeling suggests that Asw and Hint can heterodimerize and that Gln 127, an Asw-specific alteration for Trp123, dominantly interferes with the Hint active site. An extra dose of HINT in ZZW chicks, and thus more Hint homodimer, may partially overcome the feminizing influence of ASW and lead to the observed intersexual characteristics of ZZW triploids.

Hint, Fhit, and GalT: function, structure, evolution, and mechanism of three branches of the histidine triad superfamily of nucleotide hydrolases and transferases

HIT (histidine triad) proteins, named for a motif related to the sequence HphiHphiHphiphi (phi, a hydrophobic amino acid), are a superfamily of nucleotide hydrolases and transferases, which act on the alpha-phosphate of ribonucleotides, and contain a approximately 30 kDa domain that is typically either a homodimer of approximately 15 kDa polypeptides with two active-sites or an internally, imperfectly repeated polypeptide that retains a single HIT active site. On the basis of sequence, substrate specificity, structure, evolution, and mechanism, HIT proteins can be classified into the Hint branch, which consists of adenosine 5'-monophosphoramide hydrolases, the Fhit branch, which consists of diadenosine polyphosphate hydrolases, and the GalT branch, which consists of specific nucleoside monophosphate transferases, including galactose-1-phosphate uridylyltransferase, diadenosine tetraphosphate phosphorylase, and adenylyl sulfate:phosphate adenylytransferase. At least one human representative of each branch is lost in human diseases. Aprataxin, a Hint branch hydrolase, is mutated in ataxia-oculomotor apraxia syndrome. Fhit is lost early in the development of many epithelially derived tumors. GalT is deficient in galactosemia. Additionally, ASW is an avian Hint family member that has evolved to have unusual gene expression properties and the complete loss of its nucleotide binding site. The potential roles of ASW and Hint in avian sexual development are discussed elsewhere. Here we review what is known about biological activities of HIT proteins, the structural and biochemical bases for their functions, and propose a new enzyme mechanism for Hint and Fhit that may account for the differences between HIT hydrolases and transferases.

Cloning and characterisation of the Hint homologue of the thermophile Thermus thermophilus

Screening of a genomic library of the thermophile Thermus thermophilus revealed a novel thermophilic hint gene, homologues of which are highly conserved in genera from archaea to mammals. Hint belongs to the HIT protein super-family, which contains two broad groups, Fhit, associated with tumour suppression in eukaryotes and Hint with putatitively protein kinase C inhibitory activity. In T. thermophilus the 321 bp gene has a GC content of 67% overall and 94.4% in the third nucleotide position, with unusually no thymine as a wobble base. The gene product, a small highly conserved 11,996 Da predicted soluble cytoplasmic protein, offers an ideal opportunity to investigate thermostabilising amino acid substitutions. Here we report on the characterisation of the novel hint sequence.

Adenosine monophosphoramidase activity of Hint and Hnt1 supports function of Kin28, Ccl1, and Tfb3

The histidine triad superfamily of nucleotide hydrolases and nucleotide transferases consists of a branch of proteins related to Hint and Aprataxin, a branch of Fhit-related hydrolases, and a branch of galactose-1-phosphate uridylyltransferase (GalT)-related transferases. Although substrates of Fhit and GalT are known and consequences of mutations in Aprataxin, Fhit, and GalT are known, good substrates had not been reported for any member of the Hint branch, and mutational consequences were unknown for Hint orthologs, which are the most ancient and widespread proteins in the Hint branch and in the histidine triad superfamily. Here we show that rabbit and yeast Hint hydrolyze the natural product adenosine-5'-monophosphoramidate (AMPNH(2)) in an active-site-dependent manner at second order rates exceeding 1,000,000 m(-1) s(-1). Yeast strains constructed with specific loss of the Hnt1 active site fail to grow on galactose at elevated temperatures. Loss of Hnt1 enzyme activity also leads to hypersensitivity to mutations in Ccl1, Tfb3, and Kin28, which constitute the TFIIK kinase subcomplex of general transcription factor TFIIH and to mutations in Cak1, which phosphorylates Kin28. The target of Hnt1 regulation in this pathway was shown to be downstream of Cak1 and not to affect stability of Kin28 monomers. Functional complementation of all Hnt1 phenotypes was provided by rabbit Hint, which is only 22% identical to yeast Hnt1 but has very similar adenosine monophosphoramidase activity.

Interactions of Cdk7 and Kin28 with Hint/PKCI-1 and Hnt1 histidine triad proteins

Cyclin-dependent kinase 7 (Cdk7) forms a trimeric complex with cyclin H and Mat1 to form the mammalian Cdk-activating kinase, CAK, as well as a part of the basal transcription factor TFIIH, where Cdk7 phosphorylates the C-terminal domain (CTD) of the large subunit of RNA polymerase II. Here, we report a novel interaction between Cdk7 and a histidine triad (HIT) family protein, Hint/PKCI-1. This interaction was initially observed in a yeast two-hybrid study and subsequently verified by co-immunoprecipitation and subcellular localization studies, where overexpression of Cdk7 leads to partial relocalization of Hint to the nucleus. The physical association is independent of cyclin H binding or Cdk7 kinase activity and is conserved between the related Sacharomyces cerevisiae CTD kinase Kin28 and the HIT protein Hnt1. Furthermore, combination of a disruption of HNT1 and a KIN28 temperature-sensitive allele in S. cerevisiae led to highly elongated cell morphology and reduced colony formation, indicating a genetic interaction between KIN28 and HNT1. The physical and genetic interactions of Hint and Hnt1 with Cdk7 and Kin28 suggest a role for this class of histidine triad proteins in the regulation of Cdk7 and Kin28 functions.

The histidine triad superfamily of nucleotide-binding proteins

Histidine triad (HIT) proteins were until recently a superfamily of proteins that shared only sequence motifs. Crystal structures of nucleotide-bound forms of histidine triad nucleotide-binding protein (Hint) demonstrated that the conserved residues in HIT proteins are responsible for their distinctive, dimeric, 10-stranded half-barrel structures that form two identical purine nucleotide-binding sites. Hint-related proteins, found in all forms of life, and fragile histidine triad (Fhit)-related proteins, found in animals and fungi, represent the two main branches of the HIT superfamily. Hint homologs are intracellular receptors for purine mononucleotides whose cellular function remains elusive. Fhit homologs bind and cleave diadenosine polyphosphates (Ap(n)A) such as ApppA and AppppA. Fhit-Ap(n)A complexes appear to function in a proapoptotic tumor suppression pathway in epithelial tissues. In invertebrates, Fhit homologs are encoded as fusion proteins with proteins related to plant and bacterial nitrilases that are candidate signaling partners in tumor suppression.

Metal-dependent nucleotide binding to the Escherichia coli rotamase SlyD

Upon expression and purification of the first catalytic domain of mammalian adenylate cyclase type 1 (IC1), a 27 kDa contaminant was observed, which was labelled by three radioactive ATP analogues (8-azido-ATP, 3'-O-(4-benzoyl)benzoyl-ATP and 2',3'-dialdehyde-ATP); the protein was purified separately and identified as Escherichia coli SlyD by N-terminal amino acid sequence determination. SlyD is the host protein required for lysis of E. coli upon infection with bacteriophage PhiX174 and has recently been shown to display rotamase (peptidylproline cis-trans-isomerase) activity. The covalent incorporation of ATP analogues into SlyD was promoted by bivalent transition metal ions (Zn(2+)>/=Ni(2+)>Co(2+)>Cu(2+)) but not by Mg(2+) or Ca(2+); this is consistent with the known metal ion specificity of SlyD. ATP, ADP, GTP and UTP suppressed labelling of SlyD with comparable potencies. Similarly, SlyD bound 2',3'-O-(-2,4, 6-trinitrophenyl)-ATP with an affinity in the range of 10 microM, as determined by fluorescence enhancement. This interaction was further augmented in the presence of Zn(2+) (K(d)= approximately 2 microM at saturating Zn(2+)) but not of Mg(2+). Irrespective of the assay conditions, hydrolysis of nucleotides by SlyD was not detected. Upon gel filtration on a Superose HR12 column, SlyD (predicted molecular mass=21 kDa) migrated with an apparent molecular mass of 44 kDa, indicating that the protein was a dimer. However, the migration of SlyD was not affected by the presence of Zn(2+) or of Zn(2+) and ATP. Thus we concluded that SlyD binds nucleotides in the presence of metal ions. These findings suggest that SlyD serves a physiological role that goes beyond that accounted for by its intrinsic rotamase activity, which is observed in the absence of metal ions.

The role of the FHIT/FRA3B locus in cancer

Common fragile sites form gaps at characteristic chromosome bands in metaphases from normal cells after aphidicolin induction. The distribution of common fragile sites parallels the positions of neoplasia-associated chromosomal rearrangements, prompting the proposal that fragility disposes to chromosomal rearrangements. Implicit in this hypothesis is that genes at fragile sites are altered by chromosome rearrangement and thus contribute to neoplastic growth. Chromosome band 3p14.2, encompassing the most inducible common fragile region, FRA3B, has been cloned and the FHIT gene, straddling FRA3B, characterized. The gene is inactivated by deletions in cancer-derived cell lines and primary tumors and Fhit protein is absent or reduced in lung, stomach, kidney, and cervical carcinomas, consistent with function as a tumor suppressor. FRA3B thus fulfills the prophecy that fragile site alterations contribute to the neoplastic process through inactivation of a tumor suppressor gene.