Structure-function correlation among some erythrocyte membrane sialoglycoproteins

To define the epitope of the Paul-Bunnell (PB) heterophile antibody of infectious mononucleosis, a sialoglycopeptide with PB activity was purified from the N-terminus of a bovine erythrocyte membrane sialoglycoprotein. The structure determined is shown below.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)A related series of PB active horse, sheep, goat and dog erythrocyte membrane sialoglycoproteins share a blocked N-terminus and the presence of O-linked sialo-oligosaccharides of this same size as the one shown above. The horse sialoglycoprotein, like the bovine glycopeptide has a penultimate threonine carrying a sialylated Gal-GalNAc as above. The role of the PCA residue was highlighted by showing that its removal from the horse sialoglycoprotein decreased that glycoconjugates PB activity.A second receptor property shared by this series of erythrocyte membrane sialoglycoproteins is their ability to interact with the human T lymphocyte. Horse, sheep and dog sialoglycoproteins were potent inhibitors of E-rosetting while their asialo-derivatives were inactive. In contrast the horse, sheep, goat, bovine and human sialoglycoproteins--as well as their asialo derivates--were all shown here to inhibit (although not completely) the binding of an anti-CD2 monoclonal antibody to the human T-lymphocyte. They also inhibited proliferation of T-cells stimulated by suboptimal concentrations of phytohaemagglutinin. The only structure featured shared by all these sialoglycoproteins is the rather common Gal-GalNAc suggesting that the determinant binding to the receptor is a conformational one