Reoviruses are double-stranded RNA viruses that lack a membrane envelope. Their capsid is organised in two concentric icosahedral layers: an inner core and an outer capsid layer. The sigma1 protein is found in the outer capsid, and the sigma2 protein is found in the core. There are four other kinds of protein (besides sigma2) in the core, termed lambda 1-3, mu2. Interactions between sigma2 and lambda 1 and lambda 3 are thought
to initiate core formation, followed by mu2 and lambda2 .
Sigma1 is a trimeric protein, and is positioned at the 12 vertices of the icosahedral outer capsid layer. Its N-terminal fibrous tail, arranged as a triple coiled coil,
anchors it in the virion, and a C-terminal globular head interacts with the
cellular receptor . These two parts form by separate trimerization events.
The N-terminal fibrous tail forms on the polysome, without the involvement
of ATP or chaperones. The post- translational assembly of the C-terminal
globular head involves the chaperone activity of Hsp90, which is associated
with phosphorylation of Hsp90 during the process . Sigma1 protein acts
as a cell attachment protein, and determines viral virulence, pathways of
spread, and tropism. Junctional adhesion molecule has been identified as a
receptor for sigma1 . In type 3 reoviruses, a small region, predicted to
form a beta sheet, in the N-terminal tail was found to bind target cell surface
sialic acid (i.e. sialic acid acts as a co-receptor) and promote apoptosis .
The sigma1 protein also binds to the lambda2 core protein .