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In situ structure of transcriptional enzyme complex and asymmetric inner capsid protein of aquareovirus at primed state
Structural highlights
Publication Abstract from PubMedReoviruses carry out genomic RNA transcription within intact viruses to synthesize plus-sense RNA strands, which are capped prior to their release as mRNA. The in situ structures of the transcriptional enzyme complex (TEC) containing the RNA-dependent RNA polymerase (RdRp) and NTPase are known for the single-layered reovirus, cytoplasmic polyhedrosis virus (CPV), but not for multi-layered reoviruses, such as aquareoviruses (ARV), which possess a primed stage that CPV lacks. Consequently, how RNA genome and TEC respond to priming in reoviruses is unknown. Here, we have determined the near-atomic resolution asymmetric structure of ARV at the primed state by cryo electron microscopy (cryoEM), revealing the in situ structures of 11 TECs inside each capsid, and their interactions with the 11 surrounding dsRNA genome segments and with the 120 enclosing capsid shell protein (CSP) VP3 subunits. RdRp VP2 and NTPase VP4 associate with each other and with capsid vertices; both bind RNA in multiple locations, including a novel C-terminal domain of VP4. Structural comparison between the primed and quiescent states shows translocation of the dsRNA end from the NTPase to the RdRp during priming. The RNA template channel is open in both states, suggesting that channel-blocking is not a regulating mechanism between these states in ARV. Instead, NTPase's C-terminal domain appears to regulate RNA translocation between quiescent and primed states. Taken together, dsRNA viruses appear to have adapted divergent mechanisms to regulate genome transcription while retaining a similar mechanism to co-assemble their genome segments, TEC, and capsid proteins into infectious virions.IMPORTANCE Viruses in the family Reoviridae are characterized by their ability to endogenously synthesize nascent RNA within the virus. However, the mechanisms for assembling their RNA genome with transcriptional enzymes into a multi-layered virion and for priming such a virion for transcription are poorly understood. By cryoEM and novel asymmetric reconstruction, we have determined the atomic structure of the transcription complex inside aquareoviruses (ARV) that are primed for infection. The transcription complex is anchored by the N-terminal segments of enclosing capsid proteins and contains an NTPase and a polymerase, the former of which has a newly discovered domain that translocates 5' end of plus-sense RNA in segmented dsRNA genome from NTPase to polymerase when the virus changes from the inactive (quiescent) to the primed state. Conformation changes in capsid proteins and transcriptional complexes suggest a mechanism for relaying information from the outside to the inside of the virus during priming. In situ structures of polymerase complex and RNA genome show how aquareovirus transcription machineries respond to uncoating.,Ding K, Nguyen L, Zhou ZH J Virol. 2018 Aug 1. pii: JVI.00774-18. doi: 10.1128/JVI.00774-18. PMID:30068643[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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