Location and structural significance of the oligosaccharides in human Ig-A1 and IgA2 immunoglobulins.

The location, number, and kinds of oligosaccharides in human IgA1 and IgA2 immunoglobulins have been determined by amino acid sequence analysis of the alpha heavy chains. Both A2m allotypes of the alpha2 chain of IgA2 have two GlcN oligosaccharides that are absent in the alpha1 chain, but they lack GalN. The A2m(2) allotype has a fifth GlcN oligosaccharide. The alpha chains of IgA proteins also have subclass-specific and allotype-specific differences in amino acid sequence. Although other classes of human immunoglobulins differ in the number and kind of oligosaccharides, the sites are often homologous and are related to the immunoglobulin domain structure. Evolutionary preservation of the tripeptide acceptor sequence for GlcN probably indicates both a structural and biological role for carbohydrate.