THE STRUCTURE OF AQUAREOVIRUS SHOWS HOW THE DIFFERENT GEOMETRIES OF THE 2 LAYERS OF THE CAPSID ARE RECONCILED TO PROVIDE SYMMETRICAL INTERACTIONS AND STABILIZATION

Background: Aquareoviruses are important pathogens of aquatic animals and have severe consequences in aquaculture, These viruses belong to t he family Reoviridae. A structural feature common to members of the Re oviridae is a multilayered capsid, formed by several concentric icosah edral shells with different protein compositions. How these proteins, which often are present in unequal stoichiometries, interact between i cosahedral layers to stabilize the capsid is not well understood, Resu lts: We have determined the three-dimensional structure of aquareoviru s to 23 Angstrom resolution using electron cryomicroscopy and computer image analysis. The protein capsid is composed of two structurally di stinct icosahedral layers: an outer layer similar to 100 Angstrom thic k, with incomplete T=13 left-handed symmetry, surrounds an inner layer 600 Angstrom in diameter that has T=1 symmetry and is perforated by c hannels near the fivefold awes. There are 120 subunits, arranged in di mers, in the inner layer, each of which interacts with two of the 600 subunits in the outer layer. A separate set of closely interacting pro teins forms the fivefold awes of the virus structure, forming continuo us density throughout both layers of the capsid. Comparison of full an d empty (lacking RNA) virus structures reveals an RNA shell that lies directly beneath the inner layer. Conclusions: Our aquareovirus struct ure displays marked similarity to the mammalian reovirus intermediate subviral particles, suggesting a close evolutionary relationship. Howe ver, the noticeable distinction is that aquareovirus lacks the hemaggl utinin spike observed in reovirus. The T=1 inner layer organization ob served in the aquareovirus appears to be common to other members of th e Reoviridae. Such organization may be of fundamental significance in the endogenous transcription of the genome in these viruses.