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)6 polymers. In most species, mu- 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 alpha2,6 sialyltransferase [alpha2,6ST(N)] or by a hybridoma expressing this enzyme (B1.8 cells). The absence 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 two cell lines, these results indicate that a competition reaction occurs in vivo between alpha2,6ST(N) and alpha1,3Gal-T. In the alpha2,6ST(N) deficient transfectants, mu-chains lacking the C-subterminal Cys575 residue, which are secreted mainly in the form of mu2L2 monomers, are more efficiently capped by alpha1, 3-galactosyl residues, confirming that polymerization significantly reduces the accessibility of mu-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.
Smilovich
D
D
Department of Experimental Pathology, University of Bologna, Bologna, Italy and DIBIT-San Raffaele Scientific Institute, Milano, Italy.
Malagolini
N
N
Fagioli
C
C
de Lalla
C
C
Sitia
R
R
Serafini-Cessi
F
F
D000818:Animals; D001704:Biopolymers; D003165:Complement System Proteins / Q000378:metabolism; D004187:Disaccharides / Q000378:metabolism*; D000939:Epitopes / Q000378:metabolism*; D016695:Glycosyltransferases / Q000378:metabolism; D007075:Immunoglobulin M / Q000378:metabolism; D051379:Mice; D009101:Multiple Myeloma / Q000378:metabolism* / Q000473:pathology; D011134:Polysaccharides / Q000378:metabolism; D014407:Tumor Cells, Cultured
D001704:Biopolymers; D004187:Disaccharides; D000939:Epitopes; D007075:Immunoglobulin M; D011134:Polysaccharides; C055075:galactosyl-(1-3)galactose; D003165:Complement System Proteins; D016695:Glycosyltransferases