Reovirus core proteins λ1 and σ2 promote stability of disassembly intermediates and influence early replication events

Abstract

ABSTRACTThe capsids of mammalian reovirus contain two concentric protein shells, the core and the outer capsid. The outer capsid is comprised of µ1-σ3 heterohexamers which surround the core. The core is comprised of λ1 decamers held in place by σ2. After entry into the endosome, σ3 is proteolytically degraded and µ1 is cleaved and exposed to form ISVPs. ISVPs undergo further conformational changes to form ISVP*s, resulting in the release of µ1 peptides which facilitate the penetration of the endosomal membrane to release transcriptionally active core particles into the cytoplasm. Previous work has identified regions or specific residues within reovirus outer capsid that impact the efficiency of cell entry. We examined the functions of the core proteins λ1 and σ2. We generated a reovirus T3D reassortant that carries strain T1L derived σ2 and λ1 proteins (T3D/T1L L3S2). This virus displays a lower ISVP stability and therefore converts to ISVP*s more readily. To identify the basis for lability of T3D/T1L L3S2, we screened for hyper-stable mutants of T3D/T1L L3S2 and identified three point mutations in µ1 that stabilize ISVPs. Two of these mutations are located in the C-terminal ϕ region of µ1, which has not previously been implicated in controlling ISVP stability. Independent from compromised ISVP stability, we also found that T3D/T1L L3S2 launches replication more efficiently and produces higher yields in infected cells. In addition to identifying a new role for the core proteins in disassembly events, these data highlight that core proteins may influence multiple stages of infection.IMPORTANCEProtein shells of viruses (capsids) have evolved to undergo specific changes to ensure the timely delivery of genetic material to host cells. The 2-layer capsid of reovirus provides a model system to study the interactions between capsid proteins and the changes they undergo during entry. We tested a virus in which the core proteins were derived from a different strain than the outer capsid. We found that this mismatched virus was less stable and completed conformational changes required for entry prematurely. Capsid stability was restored by introduction of specific changes to the outer capsid, indicating that an optimal fit between inner and outer shells maintains capsid function. Separate from this property, mismatch between these protein layers also impacted the capacity of virus to initiate infection and produce progeny. This study reveals new insights into the roles of capsid proteins and their multiple functions during viral replication.