The glycosylation and structure of human serum IgA1, Fab, and Fc regions and the role of N-glycosylation on Fcα receptor interactions

The human serum immunoglobulins IgG and IgA1 are produced in bone marrow and both interact with specific cellular receptors that mediate biological events. In contrast to IgA1, the glycosylation of IgG has been well characterized, and its interaction with various Fe receptors (Fc gamma Rs) has been well studied. In this paper, we have analyzed the glycosylation of IgA1 and IgA1 Fab and Fc as well as three recombinant IgA1 molecules, including two N-glycosylation mutants. Amino acid sequencing data of the IgA1 Fc O-glycosylated hinge region indicated that O-glycans are located at Thr(228), Ser(230), and Ser(232), while O-glycan sites at Thr(228), Thr(236) are partially occupied. Over 90% of the N-glycans in IgA1 were sialylated, in contrast to IgG, where <10% contain sialic acid. This paper contains the first report of Fab glycosylation in IgA1, and (in contrast to IgG: Fab, which contains only N-linked glycans) both N- and O-linked oligosaccharides were identified. Analysis of the N-glycans attached to recombinant IgA1 indicated that the C alpha 2 N-glycosylation site contained mostly biantennary glycans, while the tailpiece site, absent in IgG, contained mostly triantennary structures. Further analysis of these data suggested that processing at one Fc N-glycosylation site affects the other. Neutrophil Fc alpha R binding studies, using recombinant IgA1, indicated that neither the tailpiece region nor the N-glycans in the C alpha 2 domain contribute to IgA1-neutrophil Fc alpha R binding. This contrasts with IgG, where removal of the Fc N-glycans reduces binding to the Fc gamma R. The primary sequence and disulfide bond pattern of IgA1, together with the crystal structures of IgG1 Fc and mouse IgA Fab and the glycan sequencing data, were used to generate a molecular model of IgA1. As a consequence of both the primary sequence and S-S bond pattern, the N-glycans in IgA1 Fc are not confined within the inter-alpha-chain space. The accessibility of the C alpha 2 N-glycans provides an explanation for the increased sialylation and galactosylation of IgA1 Fc over that of IgG Fc N-glycans, which are confined in the space between the two C gamma 2 domains. This also suggests why in contrast to IgG Fc, the IgA1 N-glycans are not undergalactosylated in rheumatoid arthritis.