The cell attachment proteins of type 1 and type 3 reovirus are differentially susceptible to trypsin and chymotrypsin

Purified native sigma 1 proteins from [35S]methionine-labeled reovirions [serotypes 1 (T1) and 3 (T3)] were subjected to limited trypsin and chymotrypsin digestion. It was found that T1 sigma 1 was resistant to both trypsin and chymotrypsin, whereas T3 sigma 1 (49K molecular weight) was cleaved by trypsin to yield a 24K and a 25K fragment, and by chymotrypsin to yield a 42K fragment. The 24K tryptic fragment, but not the 25K tryptic fragment, was shown to possess L-cell binding capacity, and represents the carboxy-terminal half of T3 sigma 1 since it contains the single cysteine residue (amino acid 351) as revealed by tryptic analysis of [35S]cysteine-labeled sigma 1. Neither tryptic fragment was able to bind to glycophorin, the reovirus receptor on human erythrocytes. Thus, the mechanism of reovirus host cell attachment is distinct from that of reovirus hemagglutination. The two tryptic fragments were recognized by different neutralizing monoclonal anti-sigma 1 antibodies, indicating that neutralizing and cell attachment sites are not necessarily equivalent. The 42K chymotryptic fragment of T3 sigma 1 was shown to be generated by a cleavage proximal to the carboxy-terminus. Like intact T3 sigma 1, the 42K protein retained its capacity to bind to both L cells and glycophorin, and was recognized by all the neutralizing monoclonal anti-sigma 1 antibodies tested. Thus, the host cell receptor binding site on T3 sigma 1 is located between the trypsin-sensitive and the chymotrypsin-sensitive sites.