Hepatitis C virus (HCV) is the major etiological agent of non‐A, non‐B post‐transfusion hepatitis. Its genome, a (+)‐stranded RNA molecule of approximately 9.4 kb, encodes a large polyprotein that is processed by viral and cellular proteases into at least nine different viral polypeptides. As with other (+)‐strand RNA viruses, the replication of HCV is thought to proceed via the initial synthesis of a complementary (‐) RNA strand, which serves, in turn, as a template for the production of progeny (+)‐strand RNA molecules. An RNA‐dependent RNA polymerase has been postulated to be involved in both of these steps. Using the heterologous expression of viral proteins in insect cells, we present experimental evidence that an RNA‐dependent RNA polymerase is encoded by HCV and that this enzymatic activity is the function of the 65 kDa non‐structural protein 5B (NS5B). The characterization of the HCV RNA‐dependent RNA polymerase product revealed that dimer‐sized hairpin‐like RNA molecules are generated in vitro, indicating that NS5B‐mediated RNA polymerization proceeds by priming on the template via a ‘copy‐back’ mechanism. In addition, the purified HCV NS5B protein was shown to perform RNA‐ or DNA oligonucleotide primer‐dependent RNA synthesis on templates with a blocked 3′ end or on homopolymeric templates. These results represent a first important step towards a better understanding of the life cycle of the HCV.

Histone deacetylases (HDACs) are a family of enzymes involved in the regulation of gene expression, DNA repair, and stress response. These processes often are altered in tumors, and HDAC inhibitors have had pronounced antitumor activity with promising results in clinical trials. Here, we report the crystal structure of human HDAC8 in complex with a hydroxamic acid inhibitor. Such a structure of a eukaryotic zinc-dependent HDAC has not be described previously. Similar to bacterial HDAC-like protein, HDAC8 folds in a single ␣͞␤ domain. The inhibitor and the zinc-binding sites are similar in both proteins. However, significant differences are observed in the length and structure of the loops surrounding the active site, including the presence of two potassium ions in HDAC8 structure, one of which interacts with key catalytic residues. CD data suggest a direct role of potassium in the fold stabilization of HDAC8. Knockdown of HDAC8 by RNA interference inhibits growth of human lung, colon, and cervical cancer cell lines, highlighting the importance of this HDAC subtype for tumor cell proliferation. Our findings open the way for the design and development of selective inhibitors of HDAC8 as possible antitumor agents.T he epigenetic control of gene expression is operated through a series of posttranslational modifications of chromatin that influence the electrostatics of DNA-protein interactions and generate docking sites for a large number of chromatininteracting proteins (1, 2). The acetylation status of lysine residues found in the accessible N termini of core histones is one of the posttranslational chromatin modifications that impinge on gene expression. Acetylation and deacetylation of histones are controlled by the enzymatic activity of histone acetyltransferases and histone deacetylases (HDACs) (3, 4). Alterations of gene expression are a hallmark of cancer, and mounting evidence suggests that at least a part of these alterations is mediated by epigenetic mechanisms (5, 6). Importantly, the aberrant recruitment of HDACs has been mechanistically linked to malignancy in leukemias and lymphomas (7,8), and small-molecule HDAC inhibitors show antitumor activity in preclinical models and in clinical trials and have the promise to become effective, new antineoplastic therapeutics (9).At least 18 HDAC subtypes exist, and they are subdivided into three classes (10): class I (HDACs 1-3 and 8), homologous to the yeast Rpd3 deacetylase; class II (HDACs 4-7, 9, and 10), related to the yeast Hda1 deacetylase; and class III proteins (Sirtuins 1-7), which are yeast Sir2 homologs. HDAC11 has homology to both class I and II enzymes but cannot unambiguously be assigned to either class. Class I and II HDACs, as well as HDAC11, are all zinc-dependent hydrolases. The therapeutically relevant HDAC inhibitors are thought to be nonselective or poorly selective inhibitors of all or most of class I and II enzymes but do not inhibit class III HDACs (9). It is not clear whether the antitumor properties of HDAC inhibitors are due to their l...

SummaryTumor-infiltrating regulatory T lymphocytes (Treg) can suppress effector T cells specific for tumor antigens. Deeper molecular definitions of tumor-infiltrating-lymphocytes could thus offer therapeutic opportunities. Transcriptomes of T helper 1 (Th1), Th17, and Treg cells infiltrating colorectal or non-small-cell lung cancers were compared to transcriptomes of the same subsets from normal tissues and validated at the single-cell level. We found that tumor-infiltrating Treg cells were highly suppressive, upregulated several immune-checkpoints, and expressed on the cell surfaces specific signature molecules such as interleukin-1 receptor 2 (IL1R2), programmed death (PD)-1 Ligand1, PD-1 Ligand2, and CCR8 chemokine, which were not previously described on Treg cells. Remarkably, high expression in whole-tumor samples of Treg cell signature genes, such as LAYN, MAGEH1, or CCR8, correlated with poor prognosis. Our findings provide insights into the molecular identity and functions of human tumor-infiltrating Treg cells and define potential targets for tumor immunotherapy.

Previous findings have suggested that class IIa histone deacetylases (HDACs) (HDAC4, -5, -7, and -9) are inactive on acetylated substrates, thus differing from class I and IIb enzymes. Here, we present evidence supporting this view and demonstrate that class IIa HDACs are very inefficient enzymes on standard substrates. We identified HDAC inhibitors unable to bind recombinant human HDAC4 while showing inhibition in a typical HDAC4 enzymatic assay, suggesting that the observed activity rather reflects the involvement of endogenous copurified class I HDACs. Moreover, an HDAC4 catalytic domain purified from bacteria was 1,000-fold less active than class I HDACs on standard substrates. A catalytic Tyr is conserved in all HDACs except for vertebrate class IIa enzymes where it is replaced by His. Given the high structural conservation of HDAC active sites, we predicted the class IIa His-N2 to be too far away to functionally substitute the class I Tyr-OH in catalysis. Consistently, a Tyr-to-His mutation in class I HDACs severely reduced their activity. More importantly, a His-976-Tyr mutation in HDAC4 produced an enzyme with a catalytic efficiency 1,000-fold higher than WT, and this ''gain of function phenotype'' could be extended to HDAC5 and -7. We also identified trifluoroacetyl-lysine as a class IIa-specific substrate in vitro. Hence, vertebrate class IIa HDACs may have evolved to maintain low basal activities on acetyl-lysines and to efficiently process restricted sets of specific, still undiscovered natural substrates.catalytic domain ͉ enzymatic activity ͉ trifluoroacetyl-lysine ͉ gain of function

We report the crystal structure of the RNA-dependent RNA polymerase of hepatitis C virus, a major human pathogen, to 2.8-Å resolution. This enzyme is a key target for developing specific antiviral therapy. The structure of the catalytic domain contains 531 residues folded in the characteristic fingers, palm, and thumb subdomains. The fingers subdomain contains a region, the ''fingertips,'' that shares the same fold with reverse transcriptases. Superposition to the available structures of the latter shows that residues from the palm and fingertips are structurally equivalent.