Knockout of mouse beta 1,4-galactosyltransferase-1 gene results in a dramatic shift of outer chain moieties of N-glycans from type 2 to type 1 chains in hepatic membrane and plasma glycoproteins.
To understand the contribution of beta 1,4-galactosyltransferase (beta 4Gal-T)-1 to galactosylation in vivo, N-glycans of hepatic membrane glycoproteins and plasma glycoproteins from beta 4Gal-T1 wild-type (beta 4Gal-T1(+/+)) and beta 4Gal-T1 knockout mice were compared. Unexpectedly, glycoproteins from the knockout mice were found to express considerable amounts of sialylated, galactosylated N-glycans. A striking contrast was that galactose residues were largely beta 1,4-linked to GlcNAc residues in the beta 4Gal-T1(+/+) mouse glycans but beta 1,3-linked in the knockout mouse glycans, thus resulting in the shift of the backbone structure from type 2 chain (Gal beta 1-->4GlcNAc) to type 1 chain (Gal beta 1-->3GlcNAc). Detailed analysis of plasma glycoproteins revealed that the expression of sialyl linkage in N-glycans was shifted from the Sia alpha 2-->6Gal to the Sia alpha 2-->3Gal, and oversialylated type 1 chains were, remarkably, found in the knockout mouse glycans. Thus beta 4Gal-T1 deficiency was primarily compensated for by beta1,3-galactosyltransferases, which resulted in different sialyl linkages being formed on the outer chains and altered backbone structures, depending on the acceptor specificities of sialyltransferases. These results suggest that beta 4Gal-T1 in mouse liver plays a central role in the synthesis of type 2 chain and is also involved in the regulation of sialylation of N-glycans. The knockout mice may prove useful in investigation of the mechanism which regulates the tissue-dependent terminal glycosylation.
Kotani
N
N
Division of Biochemistry, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
Asano
M
M
Iwakura
Y
Y
Takasaki
S
S
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D005700:Galactosyltransferases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D011134:Polysaccharides / Q000378:metabolism*
D008562:Membrane Glycoproteins; D011134:Polysaccharides; D005700:Galactosyltransferases; C119701:beta-1,4-galactosyltransferase I; D005690:Galactose