Disulfide bonding among μ1 trimers in mammalian reovirus outer capsid: A late and reversible step in virion morphogenesis

We examined how a particular type of intermolecular disulfide (ds) bond is formed in the capsid of a cytoplasmically replicating nonenveloped animal virus despite the normally reducing environment inside cells. The μ1 protein, a major component of the mammalian reovirus outer capsid, has been implicated in penetration of the cellular membrane barrier during cell entry. A recent crystal structure determination supports past evidence that the basal oligomer of μ1 is a trimer and that 200 of these trimers surround the core in the fenestrated T=13 outer capsid of virions. We found in this study that the predominant forms of μ1 seen in gels after the nonreducing disruption of virions are ds-linked dimers. Cys679, near the carboxyl terminus of μ1, was shown to form this ds bond with the Cys679 residue from another μ1 subunit. The crystal structure in combination with a cryomicroscopy-derived electron density map of virions indicates that the two subunits that contribute a Cys679 residue to each ds bond must be from adjacent μ1 trimers in the outer capsid, explaining the trimer-dimer paradox. Successful in vitro assembly of the outer capsid by a nonbonding mutant of μ1 (Cys679 substituted by serine) confirmed the role of Cys679 and suggested that the ds bonds are not required for assembly. A correlation between μ1-associated ds bond formation and cell death in experiments in which virions were purified from cells at different times postinfection indicated that the ds bonds form late in infection, after virions are exposed to more oxidizing conditions than those in healthy cells. The infectivity measurements of the virions with differing levels of ds-bonded μ1 showed that these bonds are not required for infection in culture. The ds bonds in purified virions were susceptible to reduction and reformation in situ, consistent with their initial formation late in morphogenesis and suggesting that they may undergo reduction during the entry of reovirus particles into new cells.