The σ3 protein, whose crystal structure is known ( Olland et al., 2001), is thought to serve as a protective cap for μ1, and cleavage of σ3 by endosomal proteases during viral infection results in the loss of σ3 and generation of infectious subvirion particles. A second layer of proteins (μ1, σ1 and σ3) forms the reovirus outer capsid, with μ1 and σ3 comprising the bulk of this capsid and σ1 protruding from the 12 vertices of the icosahedron. Five of these (λ1, λ2, λ3, μ2 and σ2) form the ‘core’ or inner capsid particle, the crystal structure of which has been determined ( Reinisch et al., 2000). The virions measure ∼850 Å in diameter and are composed of eight structural proteins. Reoviruses form non-enveloped, icosahedral particles ( Dryden et al., 1993) that contain a segmented double-stranded (ds) RNA genome. The architecture of the trimer interface and the observed flexibility indicate that σ1 is a metastable structure poised to undergo conformational changes upon viral attachment and cell entry. The fibrous tail is mainly responsible for σ1 trimer formation, and it contains a highly flexible region that allows for significant movement between the base of the tail and the head.
A prominent loop in the σ1 head contains a cluster of residues that are conserved among reovirus serotypes and are likely to form a binding site for junction adhesion molecule, an integral tight junction protein that serves as a reovirus receptor.
Numerous structural and functional similarities between reovirus σ1 and the adenovirus fiber suggest an evolutionary link in the receptor-binding strategies of these two viruses. The crystal structure of a receptor-binding fragment of σ1 reveals an elongated trimer with two domains: a compact head with a new β-barrel fold and a fibrous tail containing a triple β-spiral. Reovirus attaches to cellular receptors with the σ1 protein, a fiber-like molecule protruding from the 12 vertices of the icosahedral virion.
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