Abstract
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.