IgM are glycoproteins secreted by plasma cells as (μ2L2)5+J or (μ2L2)6 polymers. In most species, μ- and J-chains bear five and one N-glycans, respectively. Here we compare the terminal glycosylation patterns of 4-hydroxy-3-nitrophenylacetyl (NP)-specific IgM secreted by transfectants of the J558L mouse myeloma deficient in the α2,6 sialyltransferase [α2,6ST(N)] or by a hybridoma expressing this enzyme (B1.8 cells). The absence of α2,6-sialylation results in an increased addition of α1,3-galactosyl residues to μ- and J-chain N-glycans. Since α1,3-galactosyltransferase (α1,3Gal-T) is similarly expressed in the two cell lines, these results indicate that a competition reaction occurs in vivo between α2,6ST(N) and α1,3Gal-T. In the α2,6ST(N) deficient transfectants, μ-chains lacking the C-subterminal Cys575 residue, which are secreted mainly in the form of μ2L2 monomers, are more efficiently capped by α1,3-galactosyl residues, confirming that polymerization significantly reduces the accessibility of μ-chain glycans to the Golgi processing enzymes involved in the biogenesis of antennary sugars. Functional assays indicate that IgM sialylation affects antigen-binding and complement-dependent hemolysis of haptenated red blood cells.
- α1,3 galactosyltransferase
- α2,6 sialyltransferase-deficiency
- IgM glycosylation
- IgM polymerization
ASJC Scopus subject areas