Crystal Structure of the Avian Reovirus Inner Capsid Protein σA

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Abstract
Avian reovirus, an important avian pathogen, expresses eight structural and four nonstructural proteins. The structural sigma A protein is a major component of the inner capsid, clamping together lambda A building blocks. sigma A has also been implicated in the resistance of avian reovirus to the antiviral action of interferon by strongly binding double-stranded RNA in the host cell cytoplasm and thus inhibiting activation of the double-stranded RNA-dependent protein kinase. We have solved the structure of bacterially expressed sigma A by molecular replacement and refined it using data to 2.3-angstrom resolution. Twelve sigma A molecules are present in the P1 unit cell, arranged as two short double helical hexamers. A positively charged patch is apparent on the surface of sigma A on the inside of this helix and mutation of either of two key arginine residues (Arg155 and Arg273) within this patch abolishes double-stranded RNA binding. The structural data, together with gel shift assay, electron microscopy, and sedimentation velocity centrifugation results, provide evidence for cooperative binding of sigma A to double-stranded RNA. The minimal length of double-stranded RNA required for sigma A binding was observed to be 14 to 18 bp.