MUTATIONS IN TYPE-3 REOVIRUS THAT DETERMINE BINDING TO SIALIC-ACID ARE CONTAINED IN THE FIBROUS TAIL DOMAIN OF VIRAL ATTACHMENT PROTEIN SIGMA-1

The reovirus attachment protein, sigma 1, determines numerous aspects of reovirus-induced disease, including viral virulence, pathways of sp read, and tropism for certain types of cells in the central nervous sy stem. The sigma 1 protein projects from the virion surface and consist s of two distinct morphologic domains, a virion-distal globular domain known as the head and an elongated fibrous domain, termed the tail, w hich is anchored into the virion capsid. To better understand structur e-function relationships of sigma 1 protein, we conducted experiments to identify sequences in al important for viral binding to sialic acid , a component of the receptor for type 3 reovirus. Three serotype 3 re ovirus strains incapable of binding sialylated receptors were adapted to growth in murine erythroleukemia (MEL) cells, in which sialic acid is essential for reovirus infectivity. MEL-adapted (MA) mutant viruses isolated by serial passage in MEL cells acquired the capacity to bind sialic acid containing receptors and demonstrated a dependence on sia lic acid for infection of MEL cells. Analysis of reassortant viruses i solated from crosses of an MA mutant virus and a reovirus strain that does not bind sialic acid indicated that the sigma 1 protein is solely responsible for efficient growth of MA mutant viruses in MEL cells. T he deduced al amino acid sequences of the MA mutant viruses revealed t hat each strain contains a substitution within a short region of seque nce in the al tail predicted to form beta-sheet. These studies identif y specific sequences that determine the capacity of reovirus to bind s ialylated receptors and suggest a location for a sialic acid-binding d omain. Furthermore, the results support a model in which type 3 sigma 1 protein contains discrete receptor binding domains, one in the head and another in the tail that binds sialic acid.