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1 Janelia Publications

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    Clapham LabCryo-Electron MicroscopyScientific Computing Systems
    01/25/23 | The SARS-CoV-2 accessory protein Orf3a is not an ion channel, but does interact with trafficking proteins
    Alexandria N. Miller , Patrick R. Houlihan , Ella Matamala , Deny Cabezas-Bratesco , Gi Young Lee , Ben Cristofori-Armstrong , Tanya L. Dilan , Silvia Sanchez-Martinez , Doreen Matthies , Rui Yan , Zhiheng Yu , Dejian Ren , Sebastian E. Brauchi , David E. Clapham
    eLife. 2023 Jan 25:. doi: 10.7554/eLife.84477

    The severe acute respiratory syndrome associated coronavirus 2 (SARS-CoV-2) and SARS-CoV-1 accessory protein Orf3a colocalizes with markers of the plasma membrane, endocytic pathway, and Golgi apparatus. Some reports have led to annotation of both Orf3a proteins as a viroporin. Here we show that neither SARS-CoV-2 nor SARS-CoV-1 form functional ion conducting pores and that the conductances measured are common contaminants in overexpression and with high levels of protein in reconstitution studies. Cryo-EM structures of both SARS-CoV-2 and SARS-CoV-1 Orf3a display a narrow constriction and the presence of a basic aqueous vestibule, which would not favor cation permeation. We observe enrichment of the late endosomal marker Rab7 upon SARS-CoV-2 Orf3a overexpression, and co-immunoprecipitation with VPS39. Interestingly, SARS-CoV-1 Orf3a does not cause the same cellular phenotype as SARS-CoV-2 Orf3a and does not interact with VPS39. To explain this difference, we find that a divergent, unstructured loop of SARS-CoV-2 Orf3a facilitates its binding with VPS39, a HOPS complex tethering protein involved in late endosome and autophagosome fusion with lysosomes. We suggest that the added loop enhances SARS-CoV-2 Orf3a ability to co-opt host cellular trafficking mechanisms for viral exit or host immune evasion.

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