A glycosyl hydrolase activity of mammalian reovirus sigma 1 protein can contribute to viral infection through a mucus layer

The mammalian reovirus ol protein is responsible for viral attachment to ho st cells and hemagglutination properties of the virus. In the present study , sequence similarity between al and chicken-type lysozymes prompted us to investigate additional functions of the ol protein. Expression in Pichia pa storis yeast cells showed that al can actually cleave lysozyme substrates, including complex sugars found in bacterial cell walls. Replacement by site -directed mutagenesis of acidic amino acid residues in ol by their respecti ve isosteric, uncharged, amino acid residues has allowed us to identify Glu 36 and Asp54 as the catalytic pair involved in ol-mediated glycosidase acti vity. The enzyme appears inactive in virions but its activity is unmasked u pon generation of infectious subviral particles (ISVPs) by partial proteoly tic removal of the outer capsid proteins. Purified ol protein and ISVPs can also hydrolyze mucins, heavily glycosylated glycoproteins that are a major component of the mucus layer overlaying the intestinal epithelium. Further more, 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 al mucinolytic activity in the intestine, consecutive to proteolytic cleavage of virions to ISVPs, thus likely contributes to the known increase in infectivity of reovirus I SVPs compared to complete virions. This work presents the first evidence th at some mammalian viruses have evolved mechanisms to facilitate their penet ration through the protective barrier of the mucus layer in the intestinal tract. (C) 1999 Academic Press.