Characterization of the sialic acid binding activity of transmissible gastroenteritis coronavirus by analysis of haemagglutination-deficient mutants

Transmissible gastroenteritis coronavirus (TGEV) agglutinates erythrocytes of several species by virtue of sialic acid binding activity of the surface protein S, We have isolated and characterized five haemagglutination-defec tive (HAD) mutants. In contrast to the parental virus, the mutants were una ble to bind to porcine submandibulary mucin, a substrate rich in sialic aci d. Each of the mutants was found to contain a single point mutation in the S protein (Cys 155Phe, Met195Val, Arg196Ser, Asp208Asn or Leu209Pro), indic ating that these amino acids are affecting the sialic acid binding site. In four of the HAD mutants a nearby antigenic site is affected in addition to the sialic acid binding site, as indicated by reactivity with monoclonal a ntibodies. The parental virus was found to have an increased resistance to the detergent octylglucoside compared to the HAD mutants. This effect depen ded on cellular sialoglycoconjugates bound to the virion. If the binding of sialylated macromolecules was prevented by neuraminidase treatment, the pa rental virus was as sensitive to octylglucoside as were the HAD mutants. We discuss the possibility that the sialic acid binding activity helps TGEV t o resist detergent-like substances encountered during the gastrointestinal passage and thus facilitates the infection of the intestinal epithelium, An alternative function of the sialic acid binding activity - accessory bindi ng to intestinal tissues - is also discussed.