THE GLYCOSYLATION OF THE COMPLEMENT REGULATORY PROTEIN, HUMAN ERYTHROCYTE CD59

Human erythrocyte CD59 contains N- and O-glycans and a glycosylphospha tidylinositol (GPI) anchor, all of which have been analyzed in this st udy. The anchor consists principally of the minimum core glycan sequen ce Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN-linked to a phosphatidy linositol moiety with the structure sn-1-O-alkyl(C18:0 and rol-3-phosp ho-1-(2-O-palmitoyl(C16:0))myo-inositol This structure is essentially identical to that of human erythrocyte cholinesterase (Deeg, M. A., Hu mphrey, D. R., Yang, S. H., Ferguson, T. R., Reinhold, V. N., and Rose nberry, T. L. (1992) J. Biol. Chem. 267, 18573-18580). This first comp arison of GPI anchors from different proteins expressed in the same ti ssue suggests that human reticulocytes produce only one type of anchor structure. The N- and O-glycans were sequenced using a novel approach involving digestion of the total glycan pool with multiple enzyme arr ays. The N-glycan pool contained families of bi-antennary complex-type structures with and without lactosamine extensions and outer arm fuco se residues. The predominant O-glycans were Neu-NAc alpha 2-3Gal beta 1-3GalNAc and Gal beta 1-3[NeuNAc alpha 2-3]Gal-NAc. Inspection of a m olecular model of CD59, based on the NMR solution structure of the ext racellular domain and the structural data from this study, suggested s everal roles for the glycans, including spacing and orienting CD59 on the cell surface and protecting the molecule from proteases. This work completes the initial structural analysis of CD59, providing the most complete view of any cell surface glycoprotein studied to date.