Sigma 1 protein of mammalian reoviruses extends from the surfaces of viral particles

Electron microscopy revealed structures consisting of long fibers topped with knobs extending from the surfaces of virions of mammalian reoviruses. The morphology of these structures was reminiscent of the fiber protein of adenovirus. Fibers were also seen extending from the reovirus top component and intermediate subviral particles but not from cores, suggesting that the fibers consist of either the μ1C or σ1 outer capsid protein. Amino acid sequence analysis predicts that the reovirus cell attachment protein σ1 contains an extended fiber domain. When σ1 protein was released from viral particles with mild heat and subsequently obtained in isolation, it was found to have a morphology identical to that of the fiber structures seen extending from the viral particles. The identification of an extended form of σ1 has important implications for its function in cell attachment. Other evidence suggest that σ1 protein may occur in virions in both an extended and an unextended state