Status lab-led Areas Influenza Genome Regulation and Replication, Innate Immune Signaling and the Interferon Response
Nucleoprotein acts as an elongation factor, dispensable on templates up to about 76 nucleotides and supporting only diminished synthesis to about 125, which had been read as a way of prioritising viral protein synthesis before genome replication. It also shields viral RNA from host nucleases. Whether the amount of nucleoprotein governs host detection independently of its role in supporting replication had not been tested, because infection does not allow protein supply and genome supply to be separated.
Nilsson-Payant 2021 separated them by placing microRNA target sites downstream of the nucleoprotein open reading frame in recombinant influenza A and Sendai viruses, so nucleoprotein messenger RNA is degraded while the genomic RNA the polymerase copies is untouched, with matched nonfunctional-target viruses as controls. Silencing abolished detectable viral protein and full-length replication and yet strongly raised interferon-stimulated gene induction relative to the viral material present. Sequencing showed reduced coverage with enrichment at segment termini and increased noncanonical junction reads, and Northern blotting against the conserved 5 prime promoter showed mini-viral RNA accumulating from three hours and tracking with interferon beta induction. Titrating nucleoprotein against fixed polymerase in a reconstituted complex containing no NS1 reproduced the inverse relationship between full-length product and mini-viral RNA, and reporter cells placed the response through RIG-I and MAVS rather than MDA5. The same pairing of lost replication with IFIT1 induction held across seven negative-sense viruses in six families.
The claim this establishes is that a single viral protein sets a point where replication competence and immune invisibility cannot be traded independently, and that a perturbation lowering it buys reduced replication at the cost of a stronger response. A practical corollary is that the choice of drug target decides whether an antiviral also engages host defence, since nucleozin induced IFIT1 where the polymerase inhibitor baloxavir marboxil did not. The authors offer bystander priming as an extrapolation from two compounds in cell culture, and it should be read that way.
Boundaries are specific. The relative contribution of mini-viral RNA against longer defective genomes is left unresolved by the authors, and junction-read counting substantially undercounts defective genomes. Knockdown of the SARS-CoV-2 nucleocapsid transcript reduced replication without inducing interferon, a contrast the paper does not resolve. All of the work is in cell lines, with no animal arm, so what nucleoprotein scarcity does in tissue is untested. The related finding that the host factor ANP32A is required at both steps of genome replication and acts on the actively synthesising rather than the encapsidating polymerase comes from Nilsson-Payant 2022, a collaborative study led by the te Velthuis laboratory at Princeton with the first author based in this laboratory, and it is that laboratory's result rather than this program's.
Substantiated by - Reduced Nucleoprotein Availability Impairs Negative-Sense RNA Virus Replication and Promotes Host Recognition, separates protein supply from genome supply during infection and identifies the short products that RIG-I detects when nucleoprotein is scarce
Restricting nucleoprotein availability in negative-sense RNA virus infections both blocks full-length genome replication and increases production of aberrant replication products that are sensed through RIG-I and MAVS, so a strong interferon response accompanies rather than follows successful infection.