The histidine triad superfamily of nucleotide-binding proteins

Histidine Triad Nucleotide-Binding Protein 1 Improves Critical Limb Ischemia by Regulating Mitochondrial Homeostasis

Critical limb ischemia (CLI) is a common complication of diabetes mellitus that typically occurs in the later stages of the disease. Vascularization is indeed an important physiological process involving the formation of new blood vessels from existing ones. It occurs in response to various normal and pathophysiological conditions, and one of its critical roles is to compensate for inadequate oxygen supply, which is often seen in situations like chronic limb ischemia (CLI). Histidine triad nucleotide-binding protein 1 (Hint1) is a member of the Hint family that has been shown to attenuate cardiac hypertrophy, but its role in vascularization still needs to be clarified. In this study, we investigated the role of Hint1 in CLI. We found that Hint1 is significantly reduced in the muscle tissue of STZ-induced diabetic mice and high-glucose (HG)-treated endothelial cells (ECs). Hint1 deletion impaired blood flow recovery and vascularization, whereas Hint1 overexpression promoted these processes. In addition, our in vitro study showed that Hint1 deficiency aggravated mitochondrial dysfunction in ECs, as evidenced by impaired mitochondrial respiration, decreased mitochondrial membrane potential, and increased reactive oxygen species. Our findings suggest that Hint1 deficiency impairs blood perfusion by damaging mitochondrial function and that Hint1 may represent a potential therapeutic target for treating CLI.

Interactive effects of Pseudomonas entomophila strain 23S and Clavibacter michiganensis subsp. michiganensis on proteome and anti-Cmm compound production

Pseudomonas entomophila strain 23S is an effective biocontrol bacterium for tomato bacterial canker caused by Clavibacter michiganensis subsp. michiganensis (Cmm); it produces an inhibitory compound affecting the growth of Cmm. In this study, the interactions between pure cultures of P. entomophila 23S and Cmm were investigated. First, the population dynamics of each bacterium during the interaction was determined using the selective media. Second, the amount of anti-Cmm compound produced by P. entomophila 23S in the presence of Cmm was quantified using HPLC. Lastly, a label-free shotgun proteomics study of P. entomophila 23S, Cmm, and a co-culture was conducted to understand the effects of the interaction of each bacterium at the proteomic level. Compared with the pure culture grown, the total number of proteins decreased in the interaction for both bacteria. P. entomophila 23S secreted stress-related proteins, such as chaperonins, peptidases, ABC-transporters and elongation factors. The bacterium also produced more proteins related with purine, pyrimidine, carbon and nitrogen metabolisms in the presence of Cmm. The population enumeration study revealed that the Cmm population declined dramatically during the interaction, while the population of P. entomophila 23S maintained. The quantification of anti-Cmm compound indicated that P. entomophila 23S produced significantly higher amount of anti-Cmm compound when it was cultured with Cmm. Overall, the study suggested that P. entomophila 23S, although is cidal to Cmm, was also negatively affected by the presence of Cmm, while trying to adapt to the stress condition, and that such an environment favored increased production of the anti-Cmm compound by P. entomophila 23S. SIGNIFICANCE: Pseudomonas entomophila strain 23S is an effective biocontrol bacterium for tomato bacterial canker caused by Clavibacter michiganensis subsp. michiganensis (Cmm); it produces an inhibitory compound affecting the growth of Cmm. In this study, secreted proteome of pure cultures of P. entomophila 23S and Cmm, and also of a co-culture was first time identified. Furthermore, the study found that P. entomophila strain 23S produced significantly higher amount of anti-Cmm compound when the bacterium was grown together with Cmm. Co-culture enhancing anti-Cmm compound production by P. entomophila 23S is useful information, particularly from a commercial point of view of biocontrol application, and for scale-up of anti-Cmm compound production.

Mycobacteriophage Rita: a cluster F1 phage discovered in North Easton, Massachusetts

Mycobacteriophage Rita infects Mycobacterium smegmatis mc2155 and was isolated from a soil sample collected in North Easton, Massachusetts. Assigned to cluster F1 based on sequence similarity to other phages in the same cluster, Rita has a 58,771 bp genome and encodes 104 genes. Rita is 98% similar to phage Bipolar.

Native Glucagon Amyloids Catalyze Key Metabolic Reactions

Glucagon is a prominent peptide hormone, playing central roles in the regulation of glucose blood-level and lipid metabolism. Formation of glucagon amyloid fibrils has been previously reported, although no biological functions of such fibrils are known. Here, we demonstrate that glucagon amyloid fibrils catalyze biologically important reactions, including esterolysis, lipid hydrolysis, and dephosphorylation. In particular, we found that glucagon fibrils catalyze dephosphorylation of adenosine triphosphate (ATP), a core metabolic reaction in cell biology. Comparative analysis of several glucagon variants allowed mapping the catalytic activity to an enzymatic pocket-like triad formed at the glucagon fibril surface, comprising the histidyl-serine domain at the N-terminus of the peptide. This study may point to previously unknown physiological roles and pathological consequences of glucagon fibrillation and supports the hypothesis that catalytic activities of native amyloid fibrils play functional roles in human physiology and disease.

Insights into agar and secondary metabolite pathways from the genome of the red alga Gracilaria domingensis (Rhodophyta, Gracilariales)

Gracilariales is a clade of florideophycean red macroalgae known for being the main source of agar. We present a de novo genome assembly and annotation of Gracilaria domingensis, an agarophyte alga with flattened thallus widely distributed along Central and South American Atlantic intertidal zones. In addition to structural analysis, an organizational comparison was done with other Rhodophyta genomes. The nuclear genome has 78 Mbp, with 11,437 predicted coding genes, 4,075 of which did not have hits in sequence databases. We also predicted 1,567 noncoding RNAs, distributed in 14 classes. The plastid and mitochondrion genome structures were also obtained. Genes related to agar synthesis were identified. Genes for type II galactose sulfurylases could not be found. Genes related to ascorbate synthesis were found. These results suggest an intricate connection of cell wall polysaccharide synthesis and the redox systems through the use of L-galactose in Rhodophyta. The genome of G. domingensis should be valuable to phycological and aquacultural research, as it is the first tropical and Western Atlantic red macroalgal genome to be sequenced.

Protective effect of HINT2 on mitochondrial function via repressing MCU complex activation attenuates cardiac microvascular ischemia-reperfusion injury

Current evidence indicates that coronary microcirculation is a key target for protecting against cardiac ischemia–reperfusion (I/R) injury. Mitochondrial calcium uniporter (MCU) complex activation and mitochondrial calcium ([Ca²⁺]_m) overload are underlying mechanisms involved in cardiovascular disease. Histidine triad nucleotide-binding 2 (HINT2) has been reported to modulate [Ca²⁺]_m via the MCU complex, and our previous work demonstrated that HINT2 improved cardiomyocyte survival and preserved heart function in mice with cardiac ischemia. This study aimed to explore the benefits of HINT2 on cardiac microcirculation in I/R injury with a focus on mitochondria, the MCU complex, and [Ca²⁺]_m overload in endothelial cells. The present work demonstrated that HINT2 overexpression significantly reduced the no-reflow area and improved microvascular perfusion in I/R-injured mouse hearts, potentially by promoting endothelial nitric oxide synthase (eNOS) expression and phosphorylation. Microvascular barrier function was compromised by reperfusion injury, but was repaired by HINT2 overexpression via inhibiting VE-Cadherin phosphorylation at Tyr⁷³¹ and enhancing the VE-Cadherin/β-Catenin interaction. In addition, HINT2 overexpression inhibited the inflammatory response by suppressing vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). Mitochondrial fission occurred in cardiac microvascular endothelial cells (CMECs) subjected to oxygen–glucose deprivation/reoxygenation (OGD/R) injury and resulted in mitochondrial dysfunction and mitochondrion-dependent apoptosis, the effects of which were largely relieved by HINT2 overexpression. Additional experiments confirmed that [Ca²⁺]_m overload was an initiating factor for mitochondrial fission and that HINT2 suppressed [Ca²⁺]_m overload via modulation of the MCU complex through directly interacting with MCU in CMECs. Regaining [Ca²⁺]_m overload by spermine, an MCU agonist, abolished all the protective effects of HINT2 on OGD/R-injured CMECs and I/R-injured cardiac microcirculation. In conclusion, the present report demonstrated that HINT2 overexpression inhibited MCU complex-mitochondrial calcium overload-mitochondrial fission and apoptosis pathway, and thereby attenuated cardiac microvascular ischemia–reperfusion injury.

Comparative Proteomic Study Shows the Expression of Hint-1 in Pituitary Adenomas

Pituitary adenomas (PAs) can be unpredictable and aggressive tumors. No reliable markers of their biological behavior have been found. Here, a proteomic analysis was applied to identify proteins in the expression profile between invasive and non-invasive PAs to search for possible biomarkers. A histopathological and immunohistochemical (adenohypophyseal hormones, Ki-67, p53, CD34, VEGF, Flk1 antibodies) analysis was done; a proteomic map was evaluated in 64 out of 128 tumors. There were 107 (84%) invasive and 21 (16%) non-invasive PAs; 80.5% belonged to III and IV grades of the Hardy–Vezina classification. Invasive PAs (n = 56) showed 105 ± 43 spots; 86 ± 32 spots in non-invasive PAs (n = 8) were observed. The 13 most prominent spots were selected and 11 proteins related to neoplastic process in different types of tumors were identified. Hint1 (Histidine triad nucleotide-binding protein 1) high expression in invasive PA was found (11.8 ± 1.4, p = 0.005), especially at high index (>10; p = 0.0002). High Hint1 expression was found in invasive VEGF positive PA (13.8 ± 2.3, p = 0.005) and in Flk1 positive PA (14.04 ± 2.28, p = 0.006). Hint1 is related to human tumorigenesis by its interaction with signaling pathways and transcription factors. It could be related to invasive behavior in PAs. This is the first report on Hint expression in PAs. More analysis is needed to find out the possible role of Hint in these tumors.

Deacetylation by SIRT1 promotes the tumor-suppressive activity of HINT1 by enhancing its binding capacity for β-catenin or MITF in colon cancer and melanoma cells

Histidine triad nucleotide-binding protein 1 (HINT1), which belongs to the evolutionarily conserved HIT superfamily, has been shown to possess a tumor-suppressive function by binding to and inhibiting several oncogenic transcription factors, such as β-catenin and microphthalmia transcription factor (MITF), in various types of cancer cells. However, the regulatory mechanism that mediates the binding capacity of HINT1 for partner transcription factors remains elusive. Here, we report that HINT1 is acetylated by CBP at K21 and K30 and deacetylated by SIRT1. Deacetylation of HINT1 by SIRT1 increases the capacity of HINT1 to bind to β-catenin or MITF. As a result, the tumor-suppressive function of HINT1 is increased. In support of this, the deacetylation mimetic HINT1 mutant HINT1 2KR was found to significantly reduce cellular proliferation in colon cancer and melanoma cells and tumorigenesis in xenograft assays. Thus, this study reveals an acetylation-dependent regulatory mechanism that governs the tumor-suppressive function of HINT1.

A crucial modification to the tumor-suppressing gene HINT1 helps slow the spread of colon cancer and melanoma according to researchers in South Korea. HINT1 is known to bind to and inhibit several tumor-promoting transcription factors, but it is unclear how this process is regulated. Hyun-Seok Kim at Ewha Womans University in Seoul and co-workers focused on SIRT1, an enzyme that deacetylates, i.e., removes acetyl groups from, various important proteins. They found that the deacetylation of HINT1 by SIRT1 promotes the capacity of HINT1 to bind to transcription factors, thereby enhancing its tumor-suppressing function. Mutant colon cancer and melanoma cell lines with completely deacetylated HINT1 showed significantly reduced growth. The researchers suggest that acetylation and other reversible modifications of HINT1, such as phosphorylation, could be useful in clinical treatments.

Histidine triad nucleotide-binding proteins HINT1 and HINT2 share similar substrate specificities and little affinity for the signaling dinucleotide Ap4A

Human histidine triad nucleotide-binding protein 2 (hHINT2) is an important player in human mitochondrial bioenergetics, but little is known about its catalytic capabilities or its nucleotide phosphoramidate prodrug (proTide)-activating activity akin to the cytosolic isozyme hHINT1. Here, a similar substrate specificity profile (k_cat /K_m ) for model phosphoramidate substrates was found for hHINT2 but with higher k_cat and K_m values when compared with hHINT1. A broader pH range for maximum catalytic activity was determined for hHINT2 (pK₁  = 6.76 ± 0.16, pK₂  = 8.41 ± 0.07). In addition, the known hHINT1-microphthalmia-inducing transcription factor-regulating molecule Ap₄ A was found to have no detectable binding to HINT1 nor HINT2 by isothermal titration calorimetry. These results demonstrate that despite differences in their sequence and localization, HINT1 and HINT2 have similar nucleotide substrate specificities, which should be considered in future proTide design and in studies of their natural function.

Metal binding selectivity of an N-terminally free multihistidine peptide HAVAHHH-NH2

For NH₂-HAVAHHH-NH₂ peptide the coordination to internal histidines is preferred for copper(ii), while it is the opposite for nickel(ii).

The histidine triad nucleotide-binding protein 2 (HINT-2) positively regulates hepatocellular energy metabolism

The histidine triad nucleotide-binding protein 2 (HINT-2) is a mitochondrial adenosine phosphoramidase expressed in hepatocytes. The phenotype of Hint2 knockout ( Hint2^-/-) mice includes progressive hepatic steatosis and lysine hyperacetylation of mitochondrial proteins, which are features of respiratory chain malfunctions. We postulated that the absence of HINT-2 induces a defect in mitochondria bioenergetics. Isolated Hint2^-/- hepatocytes produced less ATP and generated a lower mitochondrial membrane potential than did Hint2^+/+ hepatocytes. In extracellular flux analyses with glucose, the basal, ATP-linked, and maximum oxygen consumption rates (OCRs) were decreased in Hint2^-/- hepatocytes and in HepG2 cells lacking HINT-2. Conversely, in HINT-2 overexpressing SNU-449 and HepG2 cells, the basal, ATP-linked, and maximum OCRs were increased. Similarly, with palmitate, basal and maximum OCRs were decreased in Hint2^-/- hepatocytes, but they were increased in HINT-2 overexpressing HepG2 cells. When assayed with radiolabeled substrate, palmitate oxidation was reduced by 25% in Hint2^-/- mitochondria. In respirometry assays, complex I- and II-driven, coupled and uncoupled respirations and complex IV KCN-sensitive respiration were reduced in Hint2^-/- mitochondria. Furthermore, HINT-2 associated with cardiolipin and glucose-regulated protein 75 kDa. Our study shows decreased electron transfer and oxidative phosphorylation capacity in the absence of HINT-2. The bioenergetics deficit accumulated over time in hepatocytes lacking HINT-2 likely leads to the secondary outcome of steatosis.-Rajasekaran, R., Felser, A., Nuoffer, J.-M., Dufour, J.-F., St-Pierre, M. V. The histidine triad nucleotide-binding protein 2 (HINT-2) positively regulates hepatocellular energy metabolism.

Loss of fragile histidine triad (Fhit) protein expression alters the translation of cancer-associated mRNAs

Objectives

In > 50% of cancers tumor development involves the early loss of Fhit (fragile histidine triad) protein expression, yet the mechanistic pathway(s) by which Fhit mediates its tumor suppressor functions are not fully understood. Earlier attempts to identify a Fhit-deficient gene expression profile relied on total cellular RNA and microarray analysis. The data here used RNA sequencing (RNA-Seq) of Fhit-negative and Fhit-positive cells as proof of principle for the impact of Fhit on specific mRNAs, and to lay the foundation for a study using ribosome profiling to identify mRNAs whose translation is affected by FHIT loss.

Data description

RNA-Seq was performed on RNA from lines of Fhit-expressing and Fhit-deficient lung cancer cells. This identified changes in the levels of mRNAs for a number of cell survival and cell cycle progression genes. Polysome profile analysis performed on cytoplasmic extracts from Fhit-negative and Fhit-positive cells showed changes in the sedimentation of select mRNAs consistent with changes in translation efficiency. The impact of differential Fhit expression on the turnover of selected cancer-linked mRNAs was determined by RT-qPCR of cytoplasmic RNA isolated at intervals after treating cells with a transcription inhibitor.

FHIT promoter DNA methylation and expression analysis in childhood acute lymphoblastic leukemia

Fragile histidine triad (FHIT) is a tumor suppressor gene, which is involved in several malignancies. Epigenetic alterations in FHIT have been hypothesized to contribute to tumorigenesis. The present study aimed to examine DNA promoter methylation and gene expression levels of FHIT in childhood acute lymphoblastic leukemia (ALL), in a sample of Iranian patients. The promoter methylation status of FHIT was analyzed in 100 patients diagnosed with ALL and 120 healthy control patients. mRNA expression levels were assessed in 30 new cases of ALL compared with 32 healthy controls. Hypermethylation of the FHIT promoter was significantly more frequent in patients with ALL than in healthy controls (OR=3.83, 95% CI=1.51-9.75, P=0.007). Furthermore, FHIT mRNA expression levels were significantly reduced in childhood ALL patients compared with healthy controls (P=0.032). The results of the present study revealed that dysregulation of the FHIT gene may contribute to the pathogenesis of childhood ALL. Future studies investigating a larger sample population with greater ethnic diversity would be beneficial, to confirm the results from the present study.

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.

Errors in Crystal structure of HINT from Helicobacter pylori

Inaccuracies in the article, Crystal structure of HINT from Helicobacter pylori by Tarique et al. [(2016) Acta Cryst. F72, 42-48] are presented, and a brief history of HINT nomenclature is discussed.

Crystal structure of HINT from Helicobacter pylori

Proteins belonging to the histidine triad (HIT) superfamily bind nucleotides and use the histidine triad motif to carry out dinucleotidyl hydrolase, nucleotidyltransferase and phosphoramidite hydrolase activities. Five different branches of this superfamily are known to exist. Defects in these proteins in humans are linked to many diseases such as ataxia, diseases of RNA metabolism and cell-cycle regulation, and various types of cancer. The histidine triad nucleotide protein (HINT) is nearly identical to proteins that have been classified as protein kinase C-interacting proteins (PKCIs), which also have the ability to bind and inhibit protein kinase C. The structure of HINT, which exists as a homodimer, is highly conserved from humans to bacteria and shares homology with the product of fragile histidine triad protein (FHit), a tumour suppressor gene of this superfamily. Here, the structure of HINT from Helicobacter pylori (HpHINT) in complex with AMP is reported at a resolution of 3 Å. The final model has R and Rfree values of 26 and 28%, respectively, with good electron density. Structural comparison with previously reported homologues and phylogenetic analysis shows H. pylori HINT to be the smallest among them, and suggests that it branched out separately during the course of evolution. Overall, this structure has contributed to a better understanding of this protein across the animal kingdom.

A knockdown with smoke model reveals FHIT as a repressor of Heme oxygenase 1

Fragile histidine triad (FHIT) gene deletions are among the earliest and most frequent events in carcinogenesis, particularly in carcinogen-exposed tissues. Though FHIT has been established as an authentic tumor suppressor, the mechanism underlying tumor suppression remains opaque. Most experiments designed to clarify FHIT function have analyzed the consequence of re-expressing FHIT in FHIT-negative cells. However, carcinogenesis occurs in cells that transition from FHIT-positive to FHIT-negative. To better understand cancer development, we induced FHIT loss in human bronchial epithelial cells with RNA interference. Because FHIT is a demonstrated target of carcinogens in cigarette smoke, we combined FHIT silencing with cigarette smoke extract (CSE) exposure and measured gene expression consequences by RNA microarray. The data indicate that FHIT loss enhances the expression of a set of oxidative stress response genes after exposure to CSE, including the cytoprotective enzyme heme oxygenase 1 (HMOX1) at the RNA and protein levels. Data are consistent with a mechanism in which Fhit protein is required for accumulation of the transcriptional repressor of HMOX1, Bach1 protein. We posit that by allowing superinduction of oxidative stress response genes, loss of FHIT creates a survival advantage that promotes carcinogenesis.

Purification and functional characterization of diadenosine 5',5‴-P(1),P(4)-tetraphosphate phosphorylases from Mycobacterium smegmatis and Mycobacterium avium

We recently demonstrated that the Rv2613c protein from Mycobacterium tuberculosis H37Rv is a novel diadenosine 5',5‴-P(1),P(4)-tetraphosphate (Ap4A) phosphorylase (MtAPA) that forms a tetramer. Mycobacterium avium and Mycobacterium smegmatis express proteins named MAV_3489 and MSMEG_2932, respectively, that are homologous to MtAPA. Here we showed that the MAV_3489 and MSMEG_2932 proteins possess Ap4A phosphorylase activity and enzymatic properties similar to those of MtAPA. Furthermore, gel-filtration column chromatography revealed that MAV_3489 and MSMEG_2932 assembled into homotetramers in solution, indicating that they may also form unique Ap4A-binding sites composed of tetramers.

Differential proteomic analysis of the anti-depressive effects of oleamide in a rat chronic mild stress model of depression

Depression is a complex psychiatric disorder, and its etiology and pathophysiology are not completely understood. Depression involves changes in many biogenic amine, neuropeptide, and oxidative systems, as well as alterations in neuroendocrine function and immune-inflammatory pathways. Oleamide is a fatty amide which exhibits pharmacological effects leading to hypnosis, sedation, and anti-anxiety effects. In the present study, the chronic mild stress (CMS) model was used to investigate the antidepressant-like activity of oleamide. Rats were exposed to 10weeks of CMS or control conditions and were then subsequently treated with 2weeks of daily oleamide (5mg/kg, i.p.), fluoxetine (10mg/kg, i.p.), or vehicle. Protein extracts from the hippocampus were then collected, and hippocampal maps were generated by way of two-dimensional gel electrophoresis (2-DE). Altered proteins induced by CMS and oleamide were identified through mass spectrometry and database searches. Compared to the control group, the CMS rats exhibited significantly less body weight gain and decreased sucrose consumption. Treatment with oleamide caused a reversal of the CMS-induced deficit in sucrose consumption. In the proteomic analysis, 12 protein spots were selected and identified. CMS increased the levels of adenylate kinase isoenzyme 1 (AK1), nucleoside diphosphate kinase B (NDKB), histidine triad nucleotide-binding protein 1 (HINT1), acyl-protein thioesterase 2 (APT-2), and glutathione S-transferase A4 (GSTA4). Compared to the CMS samples, seven spots changed significantly following treatment with oleamide, including GSTA4, glutathione S-transferase A6 (GSTA6), GTP-binding nuclear protein Ran (Ran-GTP), ATP synthase subunit d, transgelin-3, small ubiquitin-related modifier 2 (SUMO2), and eukaryotic translation initiation factor 5A-1 (eIF5A1). Of these seven proteins, the level of eIF5A1 was up-regulated, whereas the remaining proteins were down-regulated. In conclusion, oleamide has antidepressant-like properties in the CMS rat model. The identification of proteins altered by CMS and oleamide treatment provides support for targeting these proteins in the development of novel therapies for depression.

A 24-48 h fed Amblyomma americanum tick saliva immuno-proteome

BACKGROUND

Multiple tick saliva proteins, the majority of which are unknown, confer tick resistance in repeatedly infested animals. The objective of this study was to identify the 24-48 h fed Amblyomma americanum tick saliva immuno-proteome. The 24-48 h tick-feeding phase is critical to tick parasitism as it precedes important events in tick biology, blood meal feeding and disease agent transmission. Fed male, 24 and 96 h fed female phage display cDNA expression libraries were biopanned using rabbit antibodies to 24 and 48 h fed A. americanum female tick saliva proteins. Biopanned immuno-cDNA libraries were subjected to next generation sequencing, de novo assembly, and bioinformatic analysis.

RESULTS

More than 800 transcripts that code for 24-48 h fed A. americanum immuno-proteins are described. Of the 895 immuno-proteins, 52% (464/895) were provisionally identified based on matches in GenBank. Of these, ~19% (86/464) show high level of identity to other tick hypothetical proteins, and the rest include putative proteases (serine, cysteine, leukotriene A-4 hydrolase, carboxypeptidases, and metalloproteases), protease inhibitors (serine and cysteine protease inhibitors, tick carboxypeptidase inhibitor), and transporters and/or ligand binding proteins (histamine binding/lipocalin, fatty acid binding, calreticulin, hemelipoprotein, IgG binding protein, ferritin, insulin-like growth factor binding proteins, and evasin). Others include enzymes (glutathione transferase, cytochrome oxidase, protein disulfide isomerase), ribosomal proteins, and those of miscellaneous functions (histamine release factor, selenoproteins, tetraspanin, defensin, heat shock proteins).

CONCLUSIONS

Data here demonstrate that A. americanum secretes a complex cocktail of immunogenic tick saliva proteins during the first 24-48 h of feeding. Of significance, previously validated immunogenic tick saliva proteins including AV422 protein, calreticulin, histamine release factor, histamine binding/lipocalins, selenoproteins, and paramyosin were identified in this screen, supporting the specificity of the approach in this study. While descriptive, this study opens opportunities for in-depth tick feeding physiology studies.

Decapping Scavenger (DcpS) enzyme: advances in its structure, activity and roles in the cap-dependent mRNA metabolism

Decapping Scavenger (DcpS) enzyme rids eukaryotic cells of short mRNA fragments containing the 5' mRNA cap structure, which appear in the 3'→5' mRNA decay pathway, following deadenylation and exosome-mediated turnover. The unique structural properties of the cap, which consists of 7-methylguanosine attached to the first transcribed nucleoside by a triphosphate chain (m(7)GpppN), guarantee its resistance to non-specific exonucleases. DcpS enzymes are dimers belonging to the Histidine Triad (HIT) superfamily of pyrophosphatases. The specific hydrolysis of m(7)GpppN by DcpS yields m(7)GMP and NDP. By precluding inhibition of other cap-binding proteins by short m(7)GpppN-containing mRNA fragments, DcpS plays an important role in the cap-dependent mRNA metabolism. Over the past decade, lots of new structural, biochemical and biophysical data on DcpS has accumulated. We attempt to integrate these results, referring to DcpS enzymes from different species. Such a synergistic characteristic of the DcpS structure and activity might be useful for better understanding of the DcpS catalytic mechanism, its regulatory role in gene expression, as well as for designing DcpS inhibitors of potential therapeutic application, e.g. in spinal muscular atrophy.

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.

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.

Synthesis and evaluation of potential inhibitors of human and Escherichia coli histidine triad nucleotide binding proteins

Based on recent substrate specificity studies, a series of ribonucleotide based esters and carbamates were synthesized and screened as inhibitors of the phosphoramidases and acyl-AMP hydrolases, Escherichia coli Histidine Triad Nucleotide Binding Protein (ecHinT) and human Histidine Triad Nucleotide Binding Protein 1 (hHint1). Using our established phosphoramidase assay, K(i) values were determined. All compounds exhibited non-competitive inhibition profiles. The carbamate based inhibitors were shown to successfully suppress the Hint1-associated phenotype in E. coli, suggesting that they are permeable intracellular inhibitors of ecHinT.

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.

Hits, Fhits and Nits: beyond enzymatic function

We have briefly summarized what is known about these proteins, but in closing wish to feature the outstanding questions. Hint1 was discovered mistakenly as an inhibitor of Protein Kinase C and designated Pkci, a designation that still confuses the literature. The other Hint family members were discovered by homology to Hint1. Aprataxin was discovered as a result of the hunt for a gene responsible for AOA1. Fhit was discovered through cloning of a familial chromosome translocation breakpoint on chromosome 3 that interrupts the large FHIT gene within an intron, in the FRA3B chromosome region (Ohta et al., 1996), now known to be the region of the human genome most susceptible to DNA damage due to replication stress (Durkin et al., 2008). The NitFhit fusion genewas discovered during searches for Fhit homologs in flies and worms because the fly/worm Nit polypeptide is fused to the 5'-end of the Fhit gene; the mammalian Nit gene family was discovered because of the NitFhit fusion gene, in searches for homologs to the Nit polypeptide of the NitFhit gene. Each of the Hit family member proteins is reported to have enzymatic activities toward putative substrates involving nucleosides or dinucleosides. Most surprisingly, each of the Hit family proteins discussed has been implicated in important DNA damage response pathways and/or tumor suppression pathways. And for each of them it has been difficult to assign definite substrates, to know if the substrates and catalytic products have biological functions, to know if that function is related to the DNA damage response and suppressor functions, and to precisely define the pathways through which tumor suppression occurs. When the fly Nit sequence was found at the 5'-end of the fly Fhit gene, this gene was hailed as a Rosetta stone gene/protein that would help in discovery of the function of Fhit, because the Nit protein should be in the same signal pathway (Pace et al., 2000). However, the mammalian Nit family proteins have turned out to be at least as mysterious as the Fhit proteins, with the Nit1 substrate still unknown and the surprising finding that Nit proteins also appear to behave as tumor suppressor proteins. Whether the predicted enzymatic functions of these proteins are relevant to the observed biological functions, remain among the outstanding unanswered puzzles and raise the question: have these mammalian proteins evolved beyond the putative original enzymatic purpose, such that the catalytic function is now vestigial and subservient to signal pathways that use the protein-substrate complexes in pathways that signal apoptosis or DNA damage response? Or can these proteins be fulfilling catalytic functions independently but in parallel with signal pathway functions, as perhaps observed for Aprataxin? Or is the catalytic function indeed part of the observed biological functions, such as apoptosis and tumor suppression? Perhaps the recent, post-genomic focus on metabolomics and genome-wide investigations of signal pathway networks will lead to answers to some of these outstanding questions.

Probing the impact of the echinT C-terminal domain on structure and catalysis

Histidine triad nucleotide binding protein (Hint) is considered as the ancestor of the histidine triad protein superfamily and is highly conserved from bacteria to humans. Prokaryote genomes, including a wide array of both Gram-negative bacteria and Gram-positive bacteria, typically encode one Hint gene. The cellular function of Hint and the rationale for its evolutionary conservation in bacteria have remained a mystery. Despite its ubiquity and high sequence similarity to eukaryote Hint1 [Escherichia coli Hint (echinT) is 48% identical with human Hint1], prokaryote Hint has been reported in only a few studies. Here we report the first conformational information on the full-length N-terminal and C-terminal residues of Hint from the E. coli complex with GMP. Structural analysis of the echinT-GMP complex reveals that it crystallizes in the monoclinic space group P2(1) with four homodimers in the asymmetric unit. Analysis of electron density for both the N-terminal residues and the C-terminal residues of the echinT-GMP complex indicates that the loops in some monomers can adopt more than one conformation. The observation of conformational flexibility in terminal loop regions could explain the presence of multiple homodimers in the asymmetric unit of this structure. To explore the impact of the echinT C-terminus on protein structure and catalysis, we conducted a series of catalytic radiolabeling and kinetic experiments on the C-terminal deletion mutants of echinT. In this study, we show that sequential deletion of the C-terminus likely has no effect on homodimerization and a modest effect on the secondary structure of echinT. However, we observed a significant impact on the folding structure, as reflected by a significant lowering of the T(m) value. Kinetic analysis reveals that the C-terminal deletion mutants are within an order of magnitude less efficient in catalysis compared to wild type, while the overall kinetic mechanism that proceeds through a fast step, followed by a rate-limiting hydrolysis step, was conserved. Nevertheless, the ability of the C-terminal deletion mutants to hydrolyze lysyl-AMP generated by LysU was greatly impaired. Taken together, our results highlight the emerging role of the C-terminus in governing the catalytic function of Hints.

Immunohistochemical FHIT expression still exists in early lesions of basal cell carcinoma

In this study, we evaluated the expression of Fragile Histidine Triad (FHIT) in basal cell carcinoma (BCC). The FHIT locus was found to be altered in numerous types of cancer [6,7,18,20,22,25,26]. However, we found only one study dealing with FHIT expression in BCC [11]. In our study, we used immunohistochemical methods for the evaluation of FHIT expression in tissue samples of 42 BCC cases. The control group was formed by intradermal melanocytic nevi (IMN). Ki-67 labeling index was used to compare cellular proliferation of BCC with internal and external controls. The study group was further separated into two subgroups, according to the intensity of FHIT staining. The Ki-67 indexes of these subgroups were also compared with each other. As a primary result, there was no significant decrease in FHIT expression in early lesions of BCC. As a second finding, there was no correlation between the intensity of FHIT staining and Ki-67 labeling index. As a third finding, there was no difference in Ki-67 labeling index between early lesions of BCC and non-neoplastic epidermis. The results were unexpected, since FHIT expression has been reported to be lost in an above mentioned study [11]. We concluded that FHIT expression remains to be positive, at least in early lesions of BCC.

Pathology and biology associated with the fragile FHIT gene and gene product

More than 12 years and >800 scientific publications after the discovery of the first gene at a chromosome fragile site, the FHIT gene at FRA3B, there are still questions to pursue concerning the selective advantage conferred to cells by loss of expression of FHIT, the most frequent target of allele deletion in precancerous lesions and cancers. These questions are considered in light of recent investigations of genetic and epigenetic alterations to the locus and in a retrospective consideration of biological roles of the Fhit protein discovered through functional studies.

An interaction-motif-based scoring function for protein-ligand docking

Background

A good scoring function is essential for molecular docking computations. In conventional scoring functions, energy terms modeling pairwise interactions are cumulatively summed, and the best docking solution is selected. Here, we propose to transform protein-ligand interactions into three-dimensional geometric networks, from which recurring network substructures, or network motifs, are selected and used to provide probability-ranked interaction templates with which to score docking solutions.

Results

A novel scoring function for protein-ligand docking, MotifScore, was developed. It is non-energy-based, and docking is, instead, scored by counting the occurrences of motifs of protein-ligand interaction networks constructed using structures of protein-ligand complexes. MotifScore has been tested on a benchmark set established by others to assess its ability to identify near-native complex conformations among a set of decoys. In this benchmark test, 84% of the highest-scored docking conformations had root-mean-square deviations (rmsds) below 2.0 Å from the native conformation, which is comparable with the best of several energy-based docking scoring functions. Many of the top motifs, which comprise a multitude of chemical groups that interact simultaneously and make a highly significant contribution to MotifScore, capture recurrent interacting patterns beyond pairwise interactions.

Conclusions

While providing quite good docking scores, MotifScore is quite different from conventional energy-based functions. MotifScore thus represents a new, network-based approach for exploring problems associated with molecular docking.

CK2 phosphorylation-dependent interaction between aprataxin and MDC1 in the DNA damage response

Aprataxin, defective in the neurodegenerative disorder ataxia oculomotor apraxia type 1, resolves abortive DNA ligation intermediates during DNA repair. Here, we demonstrate that aprataxin localizes at sites of DNA damage induced by high LET radiation and binds to mediator of DNA-damage checkpoint protein 1 (MDC1/NFBD1) through a phosphorylation-dependent interaction. This interaction is mediated via the aprataxin FHA domain and multiple casein kinase 2 di-phosphorylated S-D-T-D motifs in MDC1. X-ray structural and mutagenic analysis of aprataxin FHA domain, combined with modelling of the pSDpTD peptide interaction suggest an unusual FHA binding mechanism mediated by a cluster of basic residues at and around the canonical pT-docking site. Mutation of aprataxin FHA Arg29 prevented its interaction with MDC1 and recruitment to sites of DNA damage. These results indicate that aprataxin is involved not only in single strand break repair but also in the processing of a subset of double strand breaks presumably through its interaction with MDC1.

Fragile histidine triad protein: structure, function, and its association with tumorogenesis

BACKGROUND

The human fragile histidine triad (FHIT) gene is a putative tumor suppressor gene, which is located at chromosome region 3p14.2. It was suggested that the loss of heterozygosity (LOH), homozygous deletions, and abnormal expression of the FHIT gene were involved in several types of human malignancies.

MATERIALS AND METHODS

To determine the role of FHIT in various cancers, we have performed structural and functional analysis of FHIT in detail.

RESULTS AND DISCUSSION

The protein FHIT catalyzes the Mg(2+) dependent hydrolysis of P1-5 cent-O-adenosine-P3-5 cent-O-adenosine triphosphate, Ap3A, to AMP, and ADP. The reaction is thought to follow a two-step mechanism. Histidine triad proteins, named for a motif related to the sequence H-cent-H-cent-H-cent-cent- (cent, a hydrophobic amino acid), belong to superfamily of nucleotide hydrolases and transferases. This enzyme acts on the R-phosphate of ribonucleotides, and contain a approximately 30-kDa domain that is typically a homodimer of approximately 15 kDa polypeptides with catalytic site.

CONCLUSION

Here we have gathered information is known about biological activities of FHIT, the structural and biochemical bases for their functions. Our approach may provide a comparative framework for further investigation of FHIT.

Anti-depressant and anxiolytic like behaviors in PKCI/HINT1 knockout mice associated with elevated plasma corticosterone level

BACKGROUND

Protein kinase C interacting protein (PKCI/HINT1) is a small protein belonging to the histidine triad (HIT) family proteins. Its brain immunoreactivity is located in neurons and neuronal processes. PKCI/HINT1 gene knockout (KO) mice display hyper-locomotion in response to D-amphetamine which is considered a positive symptom of schizophrenia in animal models. Postmortem studies identified PKCI/HINT1 as a candidate molecule for schizophrenia and bipolar disorder. We investigated the hypothesis that the PKCI/HINT1 gene may play an important role in regulating mood function in the CNS. We submitted PKCI/HINT1 KO mice and their wild type (WT) littermates to behavioral tests used to study anti-depressant, anxiety like behaviors, and goal-oriented behavior. Additionally, as many mood disorders coincide with modifications of hypothalamic-pituitary-adrenal (HPA) axis function, we assessed the HPA activity through measurement of plasma corticosterone levels.

RESULTS

Compared to the WT controls, KO mice exhibited less immobility in the forced swim (FST) and the tail suspension (TST) tests. Activity in the TST tended to be attenuated by acute treatment with valproate at 300 mg/kg in KO mice. The PKCI/HINT1 KO mice presented less thigmotaxis in the Morris water maze and spent progressively more time in the lit compartment in the light/dark test. In a place navigation task, KO mice exhibited enhanced acquisition and retention. Furthermore, the afternoon basal plasma corticosterone level in PKCI/HINT1 KO mice was significantly higher than in the WT.

CONCLUSION

PKCI/HINT1 KO mice displayed a phenotype of behavioral and endocrine features which indicate changes of mood function, including anxiolytic-like and anti-depressant like behaviors, in conjunction with an elevated corticosterone level in plasma. These results suggest that the PKCI/HINT 1 gene could be important for the mood regulation function in the CNS.

Purification and molecular characterization of a novel diadenosine 5',5'''-P(1),P(4)-tetraphosphate phosphorylase from Mycobacterium tuberculosis H37Rv

In this study, Rv2613c, a protein that is encoded by the open reading frame Rv2613c in Mycobacterium tuberculosis H37Rv, was expressed, purified, and characterized for the first time. The amino acid sequence of Rv2613c contained a histidine triad (HIT) motif consisting of H-phi-H-phi-H-phi-phi, where phi is a hydrophobic amino acid. This motif has been reported to be the characteristic feature of several diadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap4A) hydrolases that catalyze Ap4A to adenosine 5'-triphosphate (ATP) and adenosine monophosphate (AMP) or 2 adenosine 5'-diphosphate (ADP). However, enzymatic activity analyses for Rv2613c revealed that Ap4A was converted to ATP and ADP, but not AMP, indicating that Rv2613c has Ap4A phosphorylase activity rather than Ap4A hydrolase activity. The Ap4A phosphorylase activity has been reported for proteins containing a characteristic H-X-H-X-Q-phi-phi motif. However, no such motif was found in Rv2613c. In addition, the amino acid sequence of Rv2613c was significantly shorter compared to other proteins with Ap4A phosphorylase activity, indicating that the primary structure of Rv2613c differs from those of previously reported Ap4A phosphorylases. Kinetic analysis revealed that the K(m) values for Ap4A and phosphate were 0.10 and 0.94mM, respectively. Some enzymatic properties of Rv2613c, such as optimum pH and temperature, and bivalent metal ion requirement, were similar to those of previously reported yeast Ap4A phosphorylases. Unlike yeast Ap4A phosphorylases, Rv2613c did not catalyze the reverse phosphorolysis reaction. Taken together, it is suggested that Rv2613c is a unique protein, which has Ap4A phosphorylase activity with an HIT motif.

Identification of the HIT-45 protein from Trypanosoma brucei as an FHIT protein/dinucleoside triphosphatase: substrate specificity studies on the recombinant and endogenous proteins

A new member of the FHIT protein family, designated HIT-45, has been identified in the African trypanosome Trypanosoma brucei. Recombinant HIT-45 proteins were purified from trypanosomal and bacterial protein expression systems and analyzed for substrate specificity using various dinucleoside polyphosphates, including those that contain the 5'-mRNA cap, i.e., m(7)GMP. This enzyme exhibited typical dinucleoside triphosphatase activity (EC 3.6.1.29), having its highest specificity for diadenosine triphosphate (ApppA). However, the trypanosome enzyme contains a unique amino-terminal extension, and hydrolysis of cap dinucleotides with monomethylated guanosine or dimethylated guanosine always yielded m(7)GMP (or m(2,7)GMP) as one of the reaction products. Interestingly, m(7)Gpppm(3)(N6, N6, 2'O)A was preferred among the methylated substrates. This hypermethylated dinucleotide is unique to trypanosomes and may be an intermediate in the decay of cap 4, i.e., m(7)Gpppm(3)(N6, N6, 2'O)Apm(2'O)Apm(2'O)Cpm(2)(N3, 2'O)U, that occurs in these organisms.

Crystallization and preliminary X-ray crystallographic analysis of SMU.412c protein from the caries pathogen Streptococcus mutans

The smu.412c gene encodes a putative histidine triad-like protein (SMU.412c) with 139 residues that is involved in cell-cycle regulation in Streptococcus mutans. The gene was cloned into the expression vector pET28a and subsequently expressed in Escherichia coli strain BL21 (DE3) to give a substantially soluble form of SMU.412c with a His(6) tag at its N-terminus. The recombinant protein was purified to homogeneity in a two-step procedure involving Ni(2+)-chelating and size-exclusion chromatography. Crystals suitable for X-ray diffraction were obtained using the sitting-drop vapour-diffusion method and diffracted to 1.8 A resolution on beamline BL6A at Photon Factory, Tsukuba, Japan. The crystal belonged to space group P4(1)2(1)2, with unit-cell parameters a = b = 53.5, c = 141.1 A.

Hint2 is expressed in the mitochondria of H295R cells and is involved in steroidogenesis

Hint2 belongs to the superfamily of histidine triad hydrolase enzymes. Recently, it has been shown to influence the mitochondria-dependent apoptosis occurring in hepatocytes, but its mechanism of action is still obscure. Here, we demonstrate that Hint2 is expressed in the mitochondria of H295R cells and in normal adrenals, and that this protein is involved in steroidogenesis. The presence of Hint2 in H295R cells was revealed by RT-PCR and by immunoblot analysis of subcellular fractions. The protein appeared associated with mitochondrial membranes, probably facing the interior of the organelle. Hint2 overexpression in H295R cells had no effect on pregnenolone secretion elicited by angiotensin II or K+, whereas protein silencing with specific small interfering RNA resulted in a marked reduction of the steroidogenic response. The duration of the mitochondrial calcium signal induced by angiotensin II was also reduced upon Hint2 down-regulation with small interfering RNA, but not affected after its overexpression, suggesting that under basal conditions, Hint2 is optimally expressed, and not rate limiting in steroidogenesis. Moreover, Hint2 also appeared involved in Ca2+-independent pathways leading to steroid formation. Indeed, pregnenolone formation in response to either forskolin or a hydroxyl analog of cholesterol was markedly reduced after Hint2 silencing. Calcium-dependent and calcium-independent actions of Hint2 on steroidogenesis could be related to its ability to maintain a favorable mitochondrial potential. In conclusion, these data suggest that, in H295R cells, Hint2 is required for an optimal steroidogenic response, possibly because of a particular signalling function exerted within the mitochondria and that still remains to determine at the molecular level.

Approaching the minimal metal ion binding peptide for structural and functional metalloenzyme mimicking

The peptides Ac-His-Pro-His-Pro-His-NH(2) (L1) and Ac-Lys-His-Pro-His-Pro-His-Gln-NH(2) (L2) have been prepared and the equilibria of their proton, copper(II) and zinc(II) complexes in aqueous solution have been studied by the combination of pH-potentiometric titrations, UV/visible and circular dichroism (CD) spectroscopy. The latter methods also provided information on solution structure of the complexes formed under different conditions. Both ligands formed complexes with three imidazole nitrogens around the metal ion at pH ~7. In the L1 containing system precipitation of either copper(II) or zinc(II) complexes occurred upon slight increase of the pH. The re-titration of the filtered and acidified precipitates revealed that the insoluble materials were neutral complexes rather than metal-hydroxides. Indeed, by attaching amino acids with polar side-chains to the His-Pro-His-Pro-His template in L2 we could prevent any precipitation, and the soluble complexes around pH ~7 exerted three imidazole nitrogens and a (deprotonated) water molecule around the metal ions. To our knowledge L2 provides the first example of a short peptide preventing both the amide nitrogen coordination in copper(II) and the formation of copper(II) and zinc(II) hydroxides. The zinc(II) and copper(II) complexes at pH ~7 having similar structure to the natural hydrolytic and redox enzymes, respectively, showed considerable activity in hydrolytic cleavage assays with a model substrate (2-hydroxypropyl-4-nitrophenyl phosphate), as well as with native plasmid DNA, and in a superoxide dismutase-like reaction.

Mechanistic and kinetic analysis of the DcpS scavenger decapping enzyme

Decapping is an important process in the control of eukaryotic mRNA degradation. The scavenger decapping enzyme DcpS functions to clear the cell of cap structure following decay of the RNA body by catalyzing the hydrolysis of m(7)GpppN to m(7)Gp and ppN. Structural analysis has revealed that DcpS is a dimeric protein with a domain-swapped amino terminus. The protein dimer contains two cap binding/hydrolysis sites and displays a symmetric structure with both binding sites in the open conformation in the ligand-free state and an asymmetric conformation with one site open and one site closed in the ligand-bound state. The structural data are suggestive of a dynamic decapping mechanism where each monomer could alternate between an open and closed state. Using transient state kinetic studies, we show that both the rate-limiting step and rate of decapping are regulated by cap substrate. A regulatory mechanism is established by the intrinsic domain-swapped structure of the DcpS dimer such that the decapping reaction is very efficient at low cap substrate concentrations yet regulated with excess cap substrate. These data provide biochemical evidence to verify experimentally a dynamic and mutually exclusive cap hydrolysis activity of the two cap binding sites of DcpS and provide key insights into its regulation.