Annexin A2 stabilizes the endoplasmic reticulum and actin cytoskeleton and influences the formation of reovirus factories.

Early in infection, mammalian orthoreoviruses (reoviruses) build neo-organelles called viral factories (VFs). These structures incorporate fragments of endoplasmic reticulum (ER) and serve as sites of viral genome replication and particle assembly. Two reovirus nonstructural proteins, σNS and µNS, remodel the ER to produce the vesicles and tubules that form the VF matrix. Using co-immunoprecipitation assays followed by mass spectrometry, we identified annexin A2 (ANXA2), which binds actin and cellular membranes, as a host factor that interacts with reovirus nonstructural proteins. In the absence of ANXA2, VF formation was accelerated in the early phases of reovirus infection, and yields of reovirus were increased. Moreover, organization of the ER network and structure of the actin cytoskeleton were altered in the absence of ANXA2, suggesting that ANXA2 binding to actin is required to maintain ER network morphology. These findings provide evidence that interactions of reovirus nonstructural proteins directly or indirectly with ANXA2 are required for ER remodeling and formation of the membranous fragments that serve as the matrix to assemble reovirus factories.IMPORTANCEReovirus uses ER fragments to build the membranous scaffold of viral factories (VFs). Host proteins that participate in the ER remodeling that precedes factory biogenesis are not known. We identified actin-binding protein ANXA2 as a cellular factor required for maintenance of ER morphology. The absence of ANXA2 destabilizes the actin cytoskeleton and consequently the ER, which accelerates VF biogenesis and enhances reovirus replication. Uncovering the cellular factors used by viruses to form VFs deepens an understanding of viral cell biology and highlights new targets for antiviral drug development.