Background: Aquareoviruses are important pathogens of aquatic animals and have severe consequences in aquaculture. These viruses belong to the family Reoviridae. A structural feature common to members of the Reoviridae is a multilayered capsid, formed by several concentric icosahedral shells with different protein compositions. How these proteins, which often are present in unequal stoichiometries, interact between icosahedral layers to stabilize the capsid is not well understood.
Results: We have determined the three-dimensional structure of aquareovirus to 23 A resolution using electron cryomicroscopy and computer image analysis. The protein capsid is composed of two structurally distinct icosahedral layers: an outer layer approximately 100 A thick, with incomplete T=13 left-handed symmetry, surrounds an inner layer 600 A in diameter that has T=1 symmetry and is perforated by channels near the fivefold axes. There are 120 subunits, arranged in dimers, in the inner layer, each of which interacts with two of the 600 subunits in the outer layer. A separate set of closely interacting proteins forms the fivefold axes of the virus structure, forming continuous density throughout both layers of the capsid. Comparison of full and empty (lacking RNA) virus structures reveals an RNA shell that lies directly beneath the inner layer.
Conclusions: Our aquareovirus structure displays marked similarity to the mammalian reovirus intermediate subviral particles, suggesting a close evolutionary relationship. However, the noticeable distinction is that aquareovirus lacks the hemagglutinin spike observed in reovirus. The T=1 inner layer organization observed in the aquareovirus appears to be common to other members of the Reoviridae. Such organization may be of fundamental significance in the endogenous transcription of the genome in these viruses.