Abstract
Many viruses utilize trimeric spikes to gain entry into host cells. However, without in situ structures of these trimeric spikes, a full understanding of this dynamic and essential process of viral infections is not possible. Here we present four in situ and one isolated cryoEM structures of the trimeric spike of the cytoplasmic polyhedrosis virus, a member of the non-enveloped Reoviridae family and a virus historically used as a model in the discoveries of RNA transcription and capping. These structures adopt two drastically different conformations, closed spike and opened spike, which respectively represent the penetration-inactive and penetration-active states. Each spike monomer has four domains: N-terminal, body, claw, and C-terminal. From closed to opened state, the RGD motif-containing C-terminal domain is freed to bind integrins, and the claw domain rotates to expose and project its membrane insertion loops into the cellular membrane. Comparison between turret vertices before and after detachment of the trimeric spike shows that the trimeric spike anchors its N-terminal domain in the iris of the pentameric RNA-capping turret. Sensing of cytosolic S-adenosylmethionine (SAM) and adenosine triphosphate (ATP) by the turret triggers a cascade of events: opening of the iris, detachment of the spike, and initiation of endogenous transcription.
Highlights
Many viruses utilize trimeric spikes to gain entry into host cells
To bridge that knowledge gap, we imaged cytoplasmic polyhedrosis virus (CPV) particles prepared in the absence of S-adenosylmethionine (SAM) and adenosine triphosphate (ATP), or presence of both SAM and ATP (SA-CPV) and employed a sub-particle reconstruction workflow[26] to obtain five near-atomic resolution structures: three different conformations of the trimeric spike, either on or detached from the virus; and two structures of the turret vertices, before and after the trimeric spike detaches
These two conformations explain how CPV attaches to the host cell surface and penetrates the endosomal membrane by sequentially exposing the functional components we discovered on the trimeric spike
Summary
Many viruses utilize trimeric spikes to gain entry into host cells. without in situ structures of these trimeric spikes, a full understanding of this dynamic and essential process of viral infections is not possible. We present four in situ and one isolated cryoEM structures of the trimeric spike of the cytoplasmic polyhedrosis virus, a member of the non-enveloped Reoviridae family and a virus historically used as a model in the discoveries of RNA transcription and capping. These structures adopt two drastically different conformations, closed spike and opened spike, which respectively represent the penetration-inactive and penetration-active states. High-resolution in situ structures in multiple states have remained unavailable for non-enveloped viral spike trimers, preventing comparison of these states and a complete understanding of the extremely dynamic process of cell entry. Even though several potential receptors, including sialic acid[31], glycosylphosphatidylinositolanchored alkaline phosphatase[29], and integrin[32], have been shown to participate in the CPV cell-entry process through the clathrin-mediated endocytosis[33], how CPV utilizes its structural proteins to facilitate the virion’s entry into the host cell remained unknown
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