Poly-N-acetyllactosamine Extension in N-Glycans and Core 2- and Core 4-branched O-Glycans Is Differentially Controlled by i-Extension Enzyme and Different Members of the b1,4-Galactosyltransferase Gene Family*

Poly-N-acetyllactosamines are attached to N-glycans, O-glycans, and glycolipids and serve as underlying glycans that provide functional oligosaccharides such as sialyl Lewis. Poly-N-acetyllactosaminyl repeats are synthesized by the alternate addition of b1,3-linked GlcNAc and b1,4-linked Gal by i-extension enzyme (iGnT) and a member of the b1,4-galactosyltransferase (b4GalT) gene family. In the present study, we first found that poly-N-acetyllactosamines in N-glycans are most efficiently synthesized by b4Gal-TI and iGnT. We also found that iGnT acts less efficiently on acceptors containing increasing numbers of N-acetyllactosamine repeats, in contrast to b4Gal-TI, which exhibits no significant change. In O-glycan biosynthesis, N-acetyllactosamine extension of core 4 branches was found to be synthesized most efficiently by iGnT and b4Gal-TI, in contrast to core 2 branch synthesis, which requires iGnT and b4Gal-TIV. Poly-N-acetyllactosamine extension of core 4 branches is, however, less efficient than that of N-glycans or core 2 branches. Such inefficiency is apparently due to competition between a donor substrate and acceptor in both galactosylation and N-acetylglucosaminylation, since a core 4-branched acceptor contains both Gal and GlcNAc terminals. These results, taken together, indicate that poly-N-acetyllactosamine synthesis in N-glycans and core 2and core 4-branched Oglycans is achieved by iGnT and distinct members of the b4Gal-T gene family. The results also exemplify intricate interactions between acceptors and specific glycosyltransferases, which play important roles in how poly-Nacetyllactosamines are synthesized in different acceptor molecules.