Revisiting mRNA capping in Mononegavirales : a promising, yet underexplored antiviral target

ABSTRACT mRNA capping is a co-transcriptional, nuclear process in eukaryotes that adds a cap onto the 5' end of RNA to confer stability, efficient translation, and nuclear export. Viruses have evolved diverse strategies by mimicking or subverting host pathways to install an mRNA cap for efficient gene expression and evasion of the host innate immune response. The conventional mRNA capping pathway, employed by many RNA and DNA viruses, is mediated by dedicated RNA 5′-triphosphatase, guanylyltransferase, and methyltransferase enzymes. In contrast, many RNA viruses utilize unconventional capping mechanisms that diverge from the canonical pathway, offering unique opportunities to develop virus family-specific antiviral therapeutics. Among negative-sense RNA viruses, members of the order Mononegavirales harbor a unique capping domain, polyribonucleotidyltransferase (PRNTase), embedded within the viral RdRp large (L) protein. Unlike the conventional guanylyltransferase-mediated mechanism, PRNTase catalyzes mRNA capping through the formation of a covalent L-pRNA intermediate rather than the canonical GMP-enzyme intermediate. The HR motif in PRNTase, involved in covalent linking to mRNA, is highly conserved in non-segmented negative-strand RNA viruses. This review focuses on structural and functional insights into Mononegavirales capping, highlighting the conserved motifs and domain architecture that make it a promising therapeutic target.