Coronaviruses (CoV) redirect and rearrange host cell membranes as part of the viral genome replication and transcription machinery; they induce the formation of double-membrane vesicles in infected cells. CoV non-structural protein 6 (Nsp6), a transmembrane-containing protein, together with Nsp3 and Nsp4, have the ability to induce double-membrane vesicles that are similar to those observed in severe acute respiratory syndrome (SARS) coronavirus-infected cells. By itself, Nsp6 can generate autophagosomes from the endoplasmic reticulum. Autophagosomes are normally generated as a cellular response to starvation to carry cellular organelles and long-lived proteins to lysosomes for degradation. Degradation through autophagy may provide an innate defense against virus infection, or conversely, autophagosomes can promote infection by facilitating the assembly of replicase proteins. In addition to initiating autophagosome formation, Nsp6 also limits autophagosome expansion regardless of how they were induced, i.e. whether they were induced directly by Nsp6, or indirectly by starvation or chemical inhibition of MTOR signaling. This may favor coronavirus infection by compromising the ability of autophagosomes to deliver viral components to lysosomes for degradation.
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