Proteins related to St. John's Wort p27SJ, a suppressor of HIV-1 expression, are ubiquitous in plants

DING Proteins Extend to the Extremophilic World

The DING proteins are ubiquitous in the three domains of life, from mesophiles to thermo- and hyperthermophiles. They belong to a family of more than sixty members and have a characteristic N-terminus, DINGGG, which is considered a "signature" of these proteins. Structurally, they share a highly conserved phosphate binding site, and a three dimensional organization resembling the "Venus Flytrap", both reminding the ones of PstS proteins. They have unusually high sequence conservation, even between distantly related species. Nevertheless despite that the genomes of most of these species have been sequenced, the DING gene has not been reported for all the relative characterized DING proteins. Identity of known DING proteins has been confirmed immunologically and, in some cases, by N-terminal sequence analysis. Only a few of the DING proteins have been purified and biochemically characterized. DING proteins are heterogeneous for their wide range of biological activities and some show different activities not always correlated with each other. Most of them have been originally identified for different biological properties, or rather for binding to phosphate and also to other ligands. Their involvement in pathologies is described. This review is an update of the most recent findings on old and new DING proteins.

Comparison of the DING protein from the archaeon Sulfolobus solfataricus with human phosphate-binding protein and Pseudomonas fluorescence DING counterparts

DING proteins represent a new group of 40 kDa-related members, ubiquitous in living organisms. The family also include the DING protein from Sulfolobus solfataricus, functionally related to poly(ADP-ribose) polymerases. Here, the archaeal protein has been compared with the human Phosphate-Binding Protein and the Pseudomonas fluorescence DING enzyme, by enzyme assays and immune cross-reactivity. Surprisingly, as the Sulfolobus enzyme, the Human and Pseudomonas proteins display poly(ADP-ribose) polymerase activity, whereas a phosphatase activity was only present in Sulfolobus and human protein, despite the conserved phosphate-binding site residues in Pseudomonas DING. All proteins were positive to anti-DING antibodies and gave a comparable pattern of anti-poly(ADP-ribose) polymerase immunoreactivity with two bands, at around 40 kDa and roughly at the double of this molecular mass. The latter signal was present in all Sulfolobus enzyme preparations and proved not due to either a contaminant or a precursor protein, but likely being a dimeric form of the 40 kDa polypeptide. The common immunological and partly enzymatic behavior linking human, Pseudomonas and Sulfolobus DING proteins, makes the archaeal protein an important model system to investigate DING protein function and evolution within the cell.

Biochemical characterization and immunolocalization studies of a Capsicum chinense Jacq. protein fraction containing DING proteins and anti-microbial activity

The DING protein family consists of proteins of great biological importance due to their ability to inhibit carcinogenic cell growth. A DING peptide with Mr ∼7.57 kDa and pI ∼5.06 was detected in G10P1.7.57, a protein fraction from Capsicum chinense Jacq. seeds. Amino acid sequencing of the peptide produced three smaller peptides showing identity to the DING protein family. G10P1.7.57 displayed a phosphatase activity capable of dephosphorylating different phosphorylated substrates and inhibited the growth of Saccharomyces cerevisiae cells. Western immunoblotting with a custom-made polyclonal antibody raised against a sequence (ITYMSPDYAAPTLAGLDDATK), derived from the ∼7.57 kDa polypeptide, immunodetected an ∼ 39 kDa polypeptide in G10P1.7.57. Purification by electroelution followed by amino acid sequencing of the ∼39 kDa polypeptide yielded seven new peptide sequences and an additional one identical to that of the initially identified peptide. Western immunoblotting of soluble proteins from C. chinense seeds and leaves revealed the presence of the ∼39 kDa polypeptide at all developmental stages, with increased accumulation when the organs reached maturity. Immunolocalization using Dabsyl chloride- or Alexa fluor 488-conjugated antibodies revealed a specific fluorescent signal in the cell cytoplasm at all developmental stages, giving support to the idea that the ∼39 kDa polypeptide is a soluble DING protein. Thus, we have identified and characterized a protein fraction with a DING protein from C. chinense.

Biochemical, kinetic, and in silico characterization of DING protein purified from probiotic lactic acid bacteria Pediococcus acidilactici NCDC 252

DING proteins are intriguing proteins characterized by conserved N-terminal sequence. In spite of unusually high sequence conservation even between distantly related species, DING proteins exhibit outstanding functional diversity. An extracellular caseinolytic alkaline enzyme was purified to homogeneity from a probiotic lactic acid bacteria Pediococcus acidilactici NCDC 252 using a simple procedure involving ammonium sulphate precipitation and gel filtration chromatography. This was purified 45.72-fold with a yield and specific activity of 43.5 % and 250 U/mg, respectively. The calculated molecular weight was 38.7 and 38.9 kDa by MALDI and SDS-PAGE, respectively, and pI was 7.77. The enzyme exhibited optimal activity at pH 8.0 and 40 °C. It was considerably stable up to pH 12. For casein, the enzyme had K m of 20 μM with V max of 26 U/ml. The enzyme was resistant to organic solvents but sensitive to DTNB and EDTA that confirmed it as thiol protein with involvement of metal ions in catalysis. Its tryptic peptide fragments showed 95 % similarity with eukaryotic DING, i.e., human phosphate binding protein (HPBP). Homology-based structure evaluation using HBPB as template revealed both to be structurally conserved and also possessing conserved phosphate binding motifs.

DING proteins: numerous functions, elusive genes, a potential for health

DING proteins, named after their conserved N-terminus, form an overlooked protein family whose members were generally discovered through serendipity. It is characterized by an unusually high sequence conservation, even between distantly related species, and by an outstanding diversity of activities and ligands. They all share a demonstrated capacity to bind phosphate with high affinity or at least a predicted phosphate-binding site. However, DING protein genes are conspicuously absent from databases. The many novel family members identified in recent years have confirmed that DING proteins are ubiquitous not only in animals and plants but probably also in prokaryotes. At the functional level, there is increasing evidence that they participate in many health-related processes such as cancers as well as bacterial (Pseudomonas) and viral (HIV) infections, by mechanisms that are now beginning to be understood. They thus represent potent targets for the development of novel therapeutic approaches, especially against HIV. The few genomic sequences that are now available are starting to give some clues on why DING protein genes and mRNAs are well conserved and difficult to clone. This could open a new era of research, of both fundamental and applied importance.

Growth inhibition of malignant glioblastoma by DING protein

Malignant gliomas are a highly aggressive type of brain tumor with extremely poor prognosis. These tumors are highly invasive and are often surgically incurable and resistant to chemotherapeutics and radiotherapy. Thus, novel therapies that target pathways involved in growth and survival of the tumor cells are required for the treatment of this class of brain tumors. Previous studies revealed that epidermal growth factor receptor and extracellular-signal-regulated kinases (ERKs), which are involved in the induction of cell proliferation, are activated in the most aggressive type of glioma, i.e. glioblastoma multiforme (GBM). In fact, GBMs with increased levels of ERK activity exhibit a more aggressive phenotype than the others with moderate ERK activity, pointing to the importance of ERK and its kinase activity in the development and progression of these tumors. In this study, we have evaluated the effect of p38SJ, a novel member of the DING family of proteins, derived from Hypericum perforatum calluses, on the growth of malignant glioma cell lines, T98G and U-87MG by focusing on cell cycle and signaling pathways controlled by phosphorylation of various regulatory proteins including ERK. p38SJ, which exhibits profound phosphatase activity, shows the capacity to affect the phosphorylation status of several important kinases modulating signaling pathways, and cell growth and proliferation. Our results demonstrate that p38SJ reduces glioma cell viability and arrests cell cycle progression at G0/G1. The observed growth inhibitory effect of p38SJ is likely mediated by the downregulation of several cell cycle gatekeeper proteins, including cyclin E, Cdc2, and E2F-1. These results suggest that p38SJ may serve as a potential candidate for development of a therapeutic agent for the direct treatment of malignant gliomas and/or as a potential radiosensitizer.

Suppression of HIV-1 transcriptional elongation by a DING phosphatase

HIV-1 gene transcription is controlled by the cooperation of viral and host factors which bind to specific DNA sequences within the viral promoter spanning the long terminal repeat (LTR). Previously we showed that the St. John's Wort DING phosphatase, p27SJ, suppresses HIV-1 gene transcription by binding to the viral protein Tat and preventing its nuclear import. Here, we describe the inhibitory effect of p27SJ on the phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (RNAPII). This inhibition leads to the suppression of the association of RNAPII with the LTR. Inhibition of binding of RNAPII to LTR by p27SJ resulted in the suppression of LTR transcription elongation and a decrease in LTR transcriptional activity. Another form of the St. John's Wort DING phosphatase, p38SJ, also suppressed binding of RNAPII to the LTR, reduced transcription elongation and was even more powerful than p27SJ in inhibiting the transcriptional activity of the LTR. Our data suggest a possible mechanism by which the p27SJ/p38SJ DING phosphatase can regulate HIV-1 LTR expression by inhibiting phosphorylation of the CTD of RNAPII and suppressing LTR transcription elongation.

Eukaryotic DING proteins are endogenous: an immunohistological study in mouse tissues

Background

DING proteins encompass an intriguing protein family first characterized by their conserved N-terminal sequences. Some of these proteins seem to have key roles in various human diseases, e.g., rheumatoid arthritis, atherosclerosis, HIV suppression. Although this protein family seems to be ubiquitous in eukaryotes, their genes are consistently lacking from genomic databases. Such a lack has considerably hampered functional studies and has fostered therefore the hypothesis that DING proteins isolated from eukaryotes were in fact prokaryotic contaminants.

Principal Findings

In the framework of our study, we have performed a comprehensive immunological detection of DING proteins in mice. We demonstrate that DING proteins are present in all tissues tested as isoforms of various molecular weights (MWs). Their intracellular localization is tissue-dependant, being exclusively nuclear in neurons, but cytoplasmic and nuclear in other tissues. We also provide evidence that germ-free mouse plasma contains as much DING protein as wild-type.

Significance

Hence, data herein provide a valuable basis for future investigations aimed at eukaryotic DING proteins, revealing that these proteins seem ubiquitous in mouse tissue. Our results strongly suggest that mouse DING proteins are endogenous. Moreover, the determination in this study of the precise cellular localization of DING proteins constitute a precious evidence to understand their molecular involvements in their related human diseases.

p38SJ, a novel DINGG protein protects neuronal cells from alcohol induced injury and death

Ethanol induces neuronal cell injury and death by dysregulating several signaling events that are controlled, in part, by activation of MAPK/ERK1/2 and/or inactivation of its corresponding phosphatase, PP1. Recently, we have purified a novel protein of 38 kDa in size, p38SJ, from a callus culture of Hypericum perforatum, which belongs to an emerging DINGG family of proteins with phosphate binding activity. Here, we show that treatment of neuronal cells with p38SJ protects cells against injury induced by exposure to ethanol. Furthermore, pre-treatment of neuronal cells with p38SJ diminishes the level of the pro-apoptotic protein Bax and some events associated with apoptosis such as caspase 3 cleavage. In addition, by inducing stress, alcohol can elevate production of reactive oxygen species (ROS) that leads to a decrease in the activity of superoxide dismutase (SOD). Our results showed that p38SJ restores the activity of SOD in the ethanol treated neuronal cells. These observations provide a novel biological tool for developing new approaches for preventing neuronal cell death induced by ethanol and possibly treatment of neurological disorders associated with alcohol abuse.

Evidence for phosphatase activity of p27SJ and its impact on the cell cycle

p27SJ, a novel protein isolated from St John's wort (Hypericum perforatum), belongs to an emerging family of DING proteins that are related to a prokaryotic phosphate-binding protein superfamily. Here we demonstrate that p27SJ exhibits phosphatase activity and that its expression in cells decreases the level of phosphorylated Erk1/2, a key protein of several signaling pathways. Treatment of p27SJ-expressing cells with phosphatase inhibitors including okadaic acid, maintained Erk1/2 in its phosphorylated form, suggesting that dephosphorylation of Erk1/2 is mediated by p27SJ. Further, expression of p27SJ affects Erk1/2 downstream regulatory targets such as STAT3 and CREB. Moreover, the level of expression of cyclin A that associates with active ERK1/2 and is regulated by CREB, was modestly reduced in p27SJ-expressing cells. Accordingly, results from in vitro kinase assays revealed a noticeable decrease in the activity of cyclin A in cells expressing p27SJ. Cell cycle analysis demonstrated dysregulation at S and G2/M phases in cells expressing p27SJ, supporting the notion that a decline in cyclin A activity by p27SJ has a biological impact on cell growth. These observations provide evidence that p27SJ alters the state of Erk1/2 phosphorylation, and impacts several biological events associated with cell growth and function.

For whom the bell tolls? DING proteins in health and disease

DING proteins, identified mainly by their eponymous N-terminal sequences, are ubiquitous in living organisms. Amongst bacteria, they are common in pseudomonads, and have been characterised with respect to genetics and structure. They form part of a wider family of phosphate-binding proteins, with emerging roles in phosphate acquisition and pathogenicity. Many DING proteins have been isolated in eukaryotes, in which they have been associated with very diverse biological activities, often in the context of possible signalling roles. Disease states in which DING proteins have been implicated include rheumatoid arthritis, lithiasis, atherosclerosis, some tumours and tumour-associated cachexia, and bacterial and viral adherence. Complete genetic and structural characterisation of eukaryotic DING genes and proteins is still lacking, though the phosphate-binding site seems to be conserved. Whether as bacterial proteins related to bacterial pathogenicity, or as eukaryotic components of biochemical signalling systems, DING proteins require further study.

The DING family of proteins: ubiquitous in eukaryotes, but where are the genes?

PstS and DING proteins are members of a superfamily of secreted, high-affinity phosphate-binding proteins. Whereas microbial PstS have a well-defined role in phosphate ABC transporters, the physiological function of DING proteins, named after their DINGGG N termini, still needs to be determined. PstS and DING proteins co-exist in some Pseudomonas strains, to which they confer a highly adhesive and virulent phenotype. More than 30 DING proteins have now been purified, mostly from eukaryotes. They are often associated with infections or with dysregulation of cell proliferation. Consequently, eukaryotic DING proteins could also be involved in cell-cell communication or adherence. The ubiquitous presence in eukaryotes of proteins structurally and functionally related to bacterial virulence factors is intriguing, as is the absence of eukaryotic genes encoding DING proteins in databases. DING proteins in eukaryotes could originate from unidentified commensal or symbiotic bacteria and could contribute to essential functions. Alternatively, DING proteins could be encoded by eukaryotic genes sharing special features that prevent their cloning. Both hypotheses are discussed.

The ADP-ribosylating thermozyme from Sulfolobus solfataricus is a DING protein

The partial amino acid sequence of the sulfolobal thermoprotein biochemically characterized as poly(ADP-ribose)polymerase-like enzyme overlaps those of DING proteins. This group of proteins, widely occurring in animals, plants and eubacteria, shows a characteristic and highly conserved N-terminus, DINGGGATL. The sequence of the N-terminal region and of the analyzed tryptic peptides of the sulfolobal thermozyme shows a high similarity with most of the DING proteins from databases. This is the first example of a DING protein from a sulfolobal source.