Development of an ELISA assay for screening inhibitors against divalent metal ion dependent alphavirus capping enzyme

Alphavirus non-structural protein, nsP1 has a distinct molecular mechanism of capping the viral RNAs than the conventional capping mechanism of host. Thus, alphavirus capping enzyme nsP1 is a potential drug target. nsP1 catalyzes the methylation of guanosine triphosphate (GTP) by transferring the methyl group from S-adenosylmethionine (SAM) to a GTP molecule at its N7 position with the help of nsP1 methyltransferase (MTase) followed by guanylylation (GT) reaction which involves the formation of m⁷GMP-nsP1 covalent complex by nsP1 guanylyltransferase (GTase). In subsequent reactions, m⁷GMP moiety is added to the 5ʹ end of the viral ppRNA by nsP1 GTase resulting in the formation of cap0 structure. In the present study, chikungunya virus (CHIKV) nsP1 MTase and GT reactions were confirmed by an indirect non-radioactive colorimetric assay and western blot assay using an antibody specific for the m⁷G cap, respectively. The purified recombinant CHIKV nsP1 has been used for the development of a rapid and sensitive non-radioactive enzyme linked immunosorbent assay (ELISA) to identify the inhibitors of CHIKV nsP1. The MTase reaction is followed by GT reaction and resulted in m⁷GMP-nsP1 covalent complex formation. The developed ELISA nsP1 assay measures this m⁷GMP-nsP1 complex by utilizing anti-m⁷G cap monoclonal antibody. The mutation of a conserved residue Asp63 to Ala revealed its role in nsP1 enzyme reaction. Inductively coupled plasma mass spectroscopy (ICP-MS) was used to determine the presence of magnesium ions (Mg²⁺) in the purified nsP1 protein. The divalent metal ion selectivity and investigation show preference for Mg²⁺ ion by CHIKV nsP1. Additionally, using the developed ELISA nsP1 assay, the inhibitory effects of sinefungin, aurintricarboxylic acid (ATA) and ribavirin were determined and the IC₅₀ values were estimated to be 2.69 μM, 5.72 μM and 1.18 mM, respectively.