Mechanistic insights into the inhibition of dengue virus NS5 methyltransferase by herbacetin
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Herbacetin (HC) is a naturally occurring flavonoid compound with a dual antiviral mechanism. It inhibits the polyamine pathway and targets the methyltransferase (MTase) enzyme of the dengue (DENV3) and chikungunya virus. However, understanding the detailed inhibition mechanism of DENV MTase by HC remains unclear. The crystal structure of DENV3 nonstructural protein 5 (NS5) MTase in complex with HC and S-adenosyl-L-homocysteine presented in this study gives structural insights into the inhibition mechnism. Structural analysis revealed that HC binds to the cap-0 RNA site near the GTP binding site in the DENV3 NS5 MTase and fluorescence polarization assay demonstrated HC inhibits GTP binding with an inhibition constant K i value of ∼ 0.43 µM. Further, the in vitro cell-based assays demonstrated that HC effectively inhibits replication of all the serotypes of DENV. This is the first study that identified an inhibitor that targets the conserved RNA-binding region of NS5 MTase, suggesting its potential as a highly effective scaffold for broad-spectrum antiviral agents against orthoflaviviruses.
Importance
Dengue virus (DENV) is a vector-borne disease affecting global health, and no antiviral treatment is available for Dengue fever. DENV NS5 methyltransferase (DENV NS5 MTase) is a viral RNA capping enzyme essential for the virus replication. This study demonstrated that herbacetin (HC), a natural flavonoid, blocked the binding of GTP to NS5 MTase and inhibited the enzyme. In addition to DENV3, HC effectively inhibited the replication of DENV1,2, and 4 serotypes. The crystal structure of the DENV3 NS5 MTase in complex with HC revealed that HC binds at the cap-0 RNA binding site, which is close to the GTP binding site and thereby perturbs the spatial arrangement of GTP binding site residues. HC, as the first identified inhibitor that targets the conserved RNA-binding site of DENV3 NS5 MTase, presents a potential scaffold for the development of new antiviral drugs.
