Noncanonical P gene mRNA editing in Cedar virus generates a U protein that is required for efficient virion production

Abstract Highly pathogenic Hendra and Nipah viruses encode accessory P gene products (C, V and W) that antagonize innate immunity and contribute to pathogenicity. Cedar virus (CedV), an apathogenic bat-borne henipavirus, is presumed to lack P gene mRNA editing and therefore is unable to express V and W proteins. Here, we identify CedV peptides originating from a frameshifted P gene open reading frame and demonstrate a previously unrecognized, noncanonical editing site at a homopolymeric adenine tract that introduces single-nucleotide A or G insertion. This mRNA editing produces a protein that we refer to as U protein, whose C-terminal domain shares sequence and predicted structural features with those of the henipavirus V protein. Recombinant CedV mutants defective in mRNA editing were only recoverable by trans-complementation and showed markedly reduced release of infectious virus in cell culture and attenuated replication in mice lacking type I interferon receptor. Our data revise the CedV gene expression models and reveal a noncanonical editing mechanism that supports the production of a U protein critical for efficient infectious virus release. These results expand the fundamental concepts of paramyxovirus gene expression and reveal an unexpected requirement for P-gene editing in efficient infectious-virus production, with implications for the evaluation of potentially high-consequence paramyxoviruses. Author Summary Cedar virus is a close relative of the highly pathogenic Nipah and Hendra viruses but is considered non-pathogenic. Unlike these viruses, Cedar virus was thought to lack a mechanism called P-gene mRNA editing, which allows related viruses to produce additional proteins that support infection. Here, we show that Cedar virus does use mRNA editing, but at an unusual sequence and by inserting either an adenine or a guanine nucleotide. This editing event generates a previously unknown protein, termed U, that shares selected features with the V and W proteins of other henipaviruses. Most strikingly, viruses unable to produce U released far fewer infectious virus, showed abnormal membrane-associated structures, and replicated less efficiently in susceptible mice. These findings revise the current model of Cedar virus gene expression and reveal that mRNA editing can contribute directly to efficient virus production, not only to immune evasion. More broadly, our results highlight the need to search for unconventional editing sites when annotating and assessing newly discovered paramyxoviruses.