Molecular Mechanisms of Reovirus Capsid Assembly

Abstract

Protein folding encompasses the biological process by which a polypeptide achieves its functional three-dimensional conformation. Although some proteins can spontaneously reach their native shape, others cannot fold without the assistance of molecular chaperones. Viruses have evolved within the context of the host protein folding machinery, and many viruses co-opt cellular chaperones to replicate. In my dissertation research, I discovered that mammalian reoviruses employ the host TRiC chaperonin to fold the σ3 outer-capsid protein for assembly onto progeny virions. TRiC is a ubiquitous molecular chaperone present in all eukaryotic cells, which folds a subset of cytosolic proteins through an ATP-dependent mechanism. TRiC renders σ3 into a conformation capable of assembling onto its cognate binding partner in the reovirus outer capsid, μ1, forming stable and soluble μ1/σ3 complexes. This folding reaction is ATP-dependent and occurs rapidly after incubating σ3 with TRiC in the presence of μ1. Multiple regions within σ3 interact with TRiC, suggesting that independent binding events between TRiC subunits and σ3 influence the folding process. TRiC is incapable of solubilizing or refolding aggregated σ3, demonstrating that σ3 must fold through TRiC immediately following translation to avoid entering thermodynamically favorable misfolded conformations. The structure of the TRiC/σ3 complex reveals a defined σ3 orientation inside the TRiC folding chamber, which may be the conformation of σ3 capable of complexing with μ1 to form the reovirus outer capsid. These findings elucidate the maturation steps in reovirus outer capsid assembly and provide insight into mechanisms by which TRiC folds proteins into their native conformation.