Structure of SARS-CoV-2 MTase nsp14 with the inhibitor STM957 reveals inhibition mechanism that is shared with a poxviral MTase VP39.

Nsp14 is an RNA methyltransferase (MTase) encoded by all coronaviruses. In fact, many viral families, including DNA viruses, encode MTases that catalyze the methylation of the RNA precap structure, resulting in fully capped viral RNA. This capping is crucial for efficient viral RNA translation, stability, and immune evasion. Our previous research identified nsp14 inhibitors based on the chemical scaffold of its methyl donor - the S-adenosyl methionine (SAM) - featuring a modified adenine base and a substituted arylsulfonamide. However, the binding mode of these inhibitors was based only on docking experiments. To uncover atomic details of nsp14 inhibition we solved the crystal structure of nsp14 bound to STM957. The structure revealed the atomic details of nsp14 inhibition such that the 7-deaza-adenine moiety of STM957 forms specific interactions with Tyr368, Ala353, and Phe367, while the arylsulfonamide moiety engages with Asn388 and Phe506. The large aromatic substituent at the 7-deaza position displaces a network of water molecules near the adenine base. Surprisingly, this was recently observed in the case of an unrelated monkeypox MTase VP39, where the 7-deaza modified SAH analogs also displaced water molecules from the vicinity of the active site.

Novel analogues of a nonnucleoside SARS-CoV-2 RdRp inhibitor as potential antivirotics.

The RNA-dependent RNA polymerase (RdRp) represents a prominent target in the discovery and development of new antivirotics against RNA viruses, inhibiting the replication process. One of the most targeted RNA viruses of the last years is, without doubt, SARS-CoV-2, the cause of the recent COVID-19 pandemic. HeE1-2Tyr, a known inhibitor of flaviviral RdRp, has been discovered to also have antiviral potency against this coronavirus. In this study, we report three distinct modifications of HeE1-2Tyr: conversion of the core from a benzothiazole to a benzoxazole moiety and two different scaffold simplifications, respectively. We provide a novel synthetic approach and, in addition, evaluate the final molecules in an in vitro polymerase assay for biological activity.

Structural and functional insights in flavivirus NS5 proteins gained by the structure of Ntaya virus polymerase and methyltransferase.

Flaviviruses are single-stranded positive-sense RNA (+RNA) viruses that are responsible for several (re)emerging diseases such as yellow, dengue, or West Nile fevers. The Zika epidemic highlighted their dangerousness when a relatively benign virus known since the 1950s turned into a deadly pathogen. The central protein for their replication is NS5 (non-structural protein 5), which is composed of the N-terminal methyltransferase (MTase) domain and the C-terminal RNA-dependent RNA-polymerase (RdRp) domain. It is responsible for both RNA replication and installation of the 5' RNA cap. We structurally and biochemically analyzed the Ntaya virus MTase and RdRp domains and we compared their properties to other flaviviral NS5s. The enzymatic centers are well conserved across Flaviviridae, suggesting that the development of drugs targeting all flaviviruses is feasible. However, the enzymatic activities of the isolated proteins were significantly different for the MTase domains.

New spiro tria(thia)zolidine-acridines as topoisomerase inhibitors, DNA binders and cytostatic compounds.

Three new diphenylsubstituted spirotriazolidine- and thiazolidinone-acridines were prepared and their interaction with calf thymus DNA investigated with UV-vis, fluorescence, circular dichroism spectroscopy and viscometry. The binding constants K were estimated to range from 0.34 to 0.93 × 10(4) M(-1). UV-vis, fluorescence and circular dichroism measurements indicated that the compounds act as effective DNA-interacting agents. Electrophoretic separation proved that ligands inhibited topoisomerase I and II. The biological activity of compounds 3, 5 &6 at several different concentrations (10, 20 and 50 µM) was evaluated both 48 h and 72 h following their addition to HL-60 cancer cells. The results were analysed using various different techniques (MMP detection, changes in metabolic activity/viability and analysis of cell cycle distribution). Acridine was also used as the positive control in these assays. The results from MMP analysis demonstrate the strong effect of 3-diphenylamino-2-(acridin-9-yl)imino-1,3-thiazolidin-4-one (5) on mitochondrial physiology. Cell viability analysis showed that acridine derivatives 3 and 6 were less effective than derivative 5 and the acridine control.