An autophagy-independent role for LC3 in equine arteritis virus replication.

Equine arteritis virus (EAV) is an enveloped, positive-strand RNA virus. Genome replication of EAV has been associated with modified intracellular membranes that are shaped into double-membrane vesicles (DMVs). We showed by immuno-electron microscopy that the DMVs induced in EAV-infected cells contain double-strand (ds)RNA molecules, presumed RNA replication intermediates, and are decorated with the autophagy marker protein microtubule-associated protein 1 light chain 3 (LC3). Replication of EAV, however, was not affected in autophagy-deficient cells lacking autophagy-related protein 7 (ATG7). Nevertheless, colocalization of DMVs and LC3 was still observed in these knockout cells, which only contain the nonlipidated form of LC3. Although autophagy is not required, depletion of LC3 markedly reduced the replication of EAV. EAV replication could be fully restored in these cells by expression of a nonlipidated form of LC3. These findings demonstrate an autophagy-independent role for LC3 in EAV replication. Together with the observation that EAV-induced DMVs are also positive for ER degradation-enhancing α-mannosidase-like 1 (EDEM1), our data suggested that this virus, similarly to the distantly-related mouse hepatitis coronavirus, hijacks the ER-derived membranes of EDEMosomes to ensure its efficient replication.