Structural and Functional Features of the Reovirus σ1 Tail

Abstract

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. While it is established that different conformational states of attachment proteins of enveloped viruses mediate receptor binding and membrane fusion, less is understood about how such proteins mediate attachment and entry of nonenveloped viruses. The filamentous reovirus attachment protein σ1 binds cellular receptors; contains regions of predicted flexibility, including one at the fiber midpoint; and undergoes a conformational change during cell entry. Neither the nature of the structural change nor its contribution to viral infection is understood. We determined crystal structures of large σ1 fragments for two different reovirus serotypes. We observed an unexpectedly tight transition between two domains spanning the fiber midpoint, which allows for little flexibility. Studies of reoviruses with engineered changes near the σ1 midpoint suggest that the stabilization of this region is critical for function. Together with a previously determined structure, we now have a complete model of the full-length, elongated reovirus σ1 attachment protein.

Reovirus Neurotropism and Virulence Are Dictated by Sequences in the Head Domain of the Viral Attachment Protein

“…Alternatively, the viral RNA may contain an incompatibility between T1 and T3 sequences (29). Recently available structures of the T1 and T3 1 tail domains (12) should allow improved design of future 1-chimeric proteins.…”

“…1A), 1 extends from the virion surface and engages cellular receptors. While the T1 and T3 1 proteins are structurally similar (12,13), they share less than 30% amino acid identity (14) and are hypothesized to mediate tropism differences in the CNS by binding distinct receptors on the surfaces of ependyma and neurons.…”

Protein Mismatches Caused by Reassortment Influence Functions of the Reovirus Capsid

“…These data indicate that reassortment could influence how adjacent capsid proteins fit with each other and consequently influence the function of the capsid. The 1 protein folds into a homotrimeric structure with a distinct head, body, and tail domain (71,72). The N-terminal end of the protein is predicted to form an ␣-helical coiled coil (71,72).…”

“…The 1 protein folds into a homotrimeric structure with a distinct head, body, and tail domain (71,72). The N-terminal end of the protein is predicted to form an ␣-helical coiled coil (71,72). This region transitions into a body domain that is formed from ␤-spiral repeats (73).…”

Virus interactions with bacteria: Partners in the infectious dance

“…Reovirus virions and cell entry intermediates (infectious subvirion particles [ISVPs]) are stabilized by LPS and PG, suggesting the virus binds LPS and PG through the viral attachment fiber σ1 [9]. In contrast to poliovirus VP1, which intimately interacts with other capsid proteins [19], reovirus σ1 is a fibrous protein that protrudes up to 40 nm from the virion surface [20]. At least in the context of poliovirus VP1 and reovirus σ1, there is not a shared structure or fold that could be used to predict bacterial envelope component binding.…”