Knockout of mouse β1,4-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 β1,4-galactosyltransferase (β4Gal-T)-1 to galactosylation in vivo, N-glycans of hepatic membrane glycoproteins and plasma glycoproteins from β4Gal-T1 wild-type (β4Gal-T1+/+) and β4Gal-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 β1,4-linked to GlcNAc residues in the β4Gal-T1+/+ mouse glycans but β1,3-linked in the knockout mouse glycans, thus resulting in the shift of the backbone structure from type 2 chain (Galβ1 → 4GlcNAc) to type 1 chain (Galβ1 → 3GlcNAc). Detailed analysis of plasma glycoproteins revealed that the expression of sialyl linkage in N-glycans was shifted from the Siaα2 → 6Gal to the Siaα2 → 3Gal, and oversialylated type 1 chains were, remarkably, found in the knockout mouse glycans. Thus β4Gal-T1 deficiency was primarily compensated for by β1,3-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 β4Gal-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.