Structural and Functional Features of the Reovirus σ1 Tail.

Mammalian orthoreovirus attachment to target cells is mediated by the outer capsid protein σ1, which projects from the virion surface. The σ1 protein is a homotrimer consisting of a filamentous tail, which is partly inserted into the virion; a body domain constructed from β-spiral repeats; and a globular head with receptor-binding properties. The σ1 tail is predicted to form an α-helical coiled coil. Although σ1 undergoes a conformational change during cell entry, the nature of this change and its contributions to viral replication are unknown. Electron micrographs of σ1 molecules released from virions identified three regions of flexibility, including one at the midpoint of the molecule, that may be involved in its structural rearrangement. To enable a detailed understanding of essential σ1 tail organization and properties, we determined high-resolution structures of the reovirus type 1 Lang (T1L) and type 3 Dearing (T3D) σ1 tail domains. Both molecules feature extended α-helical coiled coils, with T1L σ1 harboring central chloride ions. Each molecule displays a discontinuity (stutter) within the coiled coil and an unexpectedly seamless transition to the body domain. The transition region features conserved interdomain interactions and appears rigid rather than highly flexible. Functional analyses of reoviruses containing engineered σ1 mutations suggest that conserved residues predicted to stabilize the coiled-coil-to-body junction are essential for σ1 folding and encapsidation, whereas central chloride ion coordination and the stutter are dispensable for efficient replication. Together, these findings enable modeling of full-length reovirus σ1 and provide insight into the stabilization of a multidomain virus attachment protein.