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

Abstract

The location, number, and kinds of oligosaccharides in human IgAl and IgA2 immunoglobulins have been determined by amino acid sequence analysis of the a heavy chains. Both A2m allotypes of the a2 chain of IgA2 have two GlcN oligosaccharides that are absent in the al chain, but they lack GalN. The A2m(2) allotype has a fifth GlcN oligosaccharide. The a 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.Although the five classes of human immunoglobulins differ greatly in their content of carbohydrate and in its distribution along the polypeptide chain, the carbohydrate often is in homologous positions and usually seems to lie in between the compact domains of the heavy chain or on the surface of a domain. With rare exceptions governed by the presence of a signal acceptor tripeptide sequence, the carbohydrate is present only on the heavy chain and is confined to the constant (C) region of the heavy chain. We earlier reported the location and nature of the five oligosaccharides of the At chain of human IgM (1, 2). In contrast, the oy chain of human IgG has a single oligosaccharide (3), and the E chain of human IgE is reported to have six (4). In all these cases the oligosaccharide contains glucosamine (GlcN), which is attached to asparagine by an N-glycosidic linkage. The IgAl subclass is unusual among glycoproteins in having two types of linkage to the polypeptide chain, the N-glycosidic linkage of GlcN to asparagine and an O-glycosidic linkage of galactosamine (GalN) to serine (5, 6). Recently we reported (7) the complete amino acid sequence of a human IgAl immunoglobulin (designated Bur), including the location of five GalN carbohydrates in the hinge region, two GlcN oligosaccharides in the Fc region, and an adventitious GlcN oligosaccharide in the variable (V) region of this al heavy chain. We have now determined the amino acid sequence of the C region of two a2 heavy chains representing different allotypes of the IgA2 subclass, namely, the A2m (2) MATERIALS AND METHODSHuman IgA was prepared from the serum of patients with multiple myeloma who produced large amounts of monoclonal IgA. The procedure involved ammonium sulfate precipitation, ion exchange chromatography, and gel filtration techniques (9, 10). The purity of the IgA was tested by immunodiffusion and immunoelectrophoresis with monospecific antisera and by gel electrophoresis and ultracentrifugation (9, 10). The purified IgA proteins were reduced at room temperature under N2 with 0.01 M dithiothreitol for 1 hr followed by alkylation with iodoacetic acid (10% molar excess) or ethylene imine. The heavy and light chains were separated on a Sephadex G-100 column. The...

A comparison of anti-HER2 IgA and IgG1 in vivo efficacy is facilitated by high N-glycan sialylation of the IgA

“…16 The main differences between IgA1 and IgA2 are located in the heavy chain hinge region where the IgA1 isotype is 13-amino acid residues longer than IgA2, and where only this IgA isotype is decorated with up to 5 O-glycans. 17 In addition, IgA1 has 2 N-glycosylation sites, whereas the IgA2m1 and IgA2m2 isotypes comprise 4 or 5 Nglycosylation sites, respectively, containing N-glycans with higher complexity than what is usually found for IgG antibodies. 18 The IgA2m1 allotype is remarkable among human IgA for lacking a covalent disulfide bond between heavy and light chains.…”

Cleavage of the Human Immunoglobulin A1 (IgA1) Hinge Region by IgA1 Proteases Requires Structures in the Fc region of IgA

“…IgA1 has a 22-amino-acid hinge region containing up to five O-linked glycans at serine and threonine residues. In contrast, IgA2 has a 9-amino-acid hinge and lacks the O-linked glycans (8,16,37). While IgA1 contains two N-linked glycans in the Fc region, those at Asn263 and Asn459, IgA2m (1) contains four and IgA2m (2) and the novel IgA2 allotype IgA2(n) contain five N-linked glycans.…”

Initiation of O-Glycan Synthesis in IgA1 Hinge Region Is Determined by a Single Enzyme, UDP-N-Acetyl-α-d-galactosamine:PolypeptideN-Acetylgalactosaminyltransferase 2

“…To explain why IgA1, but neither IgA2 nor IgG is deposited, it was proposed that O-glycan structures in the hinge region of IgA1 are profoundly involved in the deposition. IgA1 is characterized by a long extended polyproline structure and distinctive O-glycan side chains in its hinge region (27,28). The other immunoglobulins, except for IgA1 and IgD, do not have O-glycans in their hinge regions.…”