Differential expression of Galalpha1,3Gal epitope in polymeric and monomeric IgM secreted by mouse myeloma cells deficient in alpha2, 6- sialyltransferase

IgM are glycoproteins secreted by plasma cells as (mu2L2)5+J or (mu2L2)6polymers. In most species, mu- and J-chains bear five and one N -glycans,respectively. Here we compare the terminal glycosylation patterns of4-hydroxy-3-nitrophenylacetyl (NP)-specific IgM secreted by transfectantsof the J558L mouse myeloma deficient in the alpha2,6 sialyltransferase[alpha2,6ST(N)] or by a hybridoma expressing this enzyme (B1.8 cells). Theabsence of alpha2,6-sialylation results in an increased addition of alpha1,3-galactosyl residues to mu- and J-chain N-glycans. Since alpha1,3-galactosyltransferase (alpha1,3Gal-T) is similarly expressed in the twocell lines, these results indicate that a competition reaction occurs invivo between alpha2,6ST(N) and alpha1,3Gal-T. In the alpha2,6ST(N)deficient transfectants, mu-chains lacking the C-subterminal Cys575residue, which are secreted mainly in the form of mu2L2 monomers, are moreefficiently capped by alpha1, 3- galactosyl residues, confirming thatpolymerization significantly reduces the accessibility of mu-chain glycansto the Golgi processing enzymes involved in the biogenesis of antennarysugars. Functional assays indicate that IgM sialylation affectsantigen-binding and complement-dependent hemolysis of haptenated red bloodcells.