Sialic acid residues are required in cellular receptors for many different mammalian viruses. Sialic acid could have a direct role, being an integral part of the virus binding site on the receptor. Alternatively, negatively charged sialic acid could have an indirect role, being responsible for holding the receptor in the required configuration for virus recognition, for instance, by interacting with positively charged amino acid residues found in the polypeptide chain of receptors. We have investigated the role of sialic acid in virus attachment by studying the interaction of the small RNA virus encephalomyocarditis (EMC) with glycophorin A, its receptor on human erythrocytes. In several experiments, influenza virus A was used for control purposes. Blocking positive charges on glycophorin either in lysine residues by acetylation or in arginine residues with butanedione did not affect its interaction with EMC virus. In contrast, blocking negatively charged carboxyl groups in sialic acid residues by amidation destroyed the ability of glycophorin to inhibit EMC virus attachment suggesting an important role for this part of sialic acid in EMC virus attachment. Removal of the polyhydroxy side chain in sialic acid residues of glycophorin by mild oxidation with periodate followed by reduction with borohydride had little effect on its interaction with EMC virus. Further, sialic acid species with either an acetyl or glycolyl group attached to the amino group on position 5 interacted equally well with EMC virus.(ABSTRACT TRUNCATED AT 250 WORDS)