A glycosyl hydrolase activity of mammalian reovirus sigma1 protein can contribute to viral infection through a mucus layer

The mammalian reovirus sigma1 protein is responsible for viral attachment to host cells and hemagglutination properties of the virus. In the present study, sequence similarity between sigma1 and chicken-type lysozymes prompted us to investigate additional functions of the sigma1 protein. Expression in Pichia pastoris yeast cells showed that sigma1 can actually cleave lysozyme substrates, including complex sugars found in bacterial cell walls. Replacement by site-directed mutagenesis of acidic amino acid residues in sigma1 by their respective isosteric, uncharged, amino acid residues has allowed us to identify Glu36 and Asp54 as the catalytic pair involved in sigma1-mediated glycosidase activity. The enzyme appears inactive in virions but its activity is unmasked upon generation of infectious subviral particles (ISVPs) by partial proteolytic removal of the outer capsid proteins. Purified sigma1 protein and ISVPs can also hydrolyze mucins, heavily glycosylated glycoproteins that are a major component of the mucus layer overlaying the intestinal epithelium. Furthermore, reovirus infection of epithelial Madin Darby canine kidney cells was inhibited tenfold in cells expressing mucin at their apical surface, while this inhibition was overcome by ISVPs. Unmasking of sigma1 mucinolytic activity in the intestine, consecutive to proteolytic cleavage of virions to ISVPs, thus likely contributes to the known increase in infectivity of reovirus ISVPs compared to complete virions. This work presents the first evidence that some mammalian viruses have evolved mechanisms to facilitate their penetration through the protective barrier of the mucus layer in the intestinal tract.