Regulation of poly-N-acetyllactosamine biosynthesis in O-glycans

Poly-N-acetyllactosamine is a unique carbohydrate composed of N-acetyllactosamine (LacNAc) repeats and provides the backbone structure for additional modifications such as sialyl Lewis(x). It is attached to N-glycans, O-glycans, and glycolipids and synthesized by the alternate addition of beta1,3-linked N-acetylglucosamine (GlcNAc) and beta1,3-linked galactose (Gal) by i-beta1,3-N-acetylglucosaminyltransferase (iGnT) and a member of the beta1,4-galactosyltransferase (beta 4Gal-T) gene family. Poly-N-acetyllactosamines in mucin-type O-glycans can be formed in core 2- and core 4-branched oligosaccharides, which are synthesized by core 2 beta1,6-N-acetylglucosaminyltransferase (C2GnT) and core 4 beta1,6-N-acetylglucosaminyltransferase (C4GnT), respectively. beta 4Gal-TIV was found to be most efficient in the addition of a single Gal residue to core 2-branched oligosaccharides among the members of the beta 4Gal-T gene family and to synthesize poly-N-acetyllactosamine in core 2-branched O-glycans together with iGnT. On the other hand, beta 4Gal-TI was shown to be most efficient for poly-N-acetyllactosamine synthesis in N-glycans. In contrast to beta 4Gal-TI, the efficiency of beta 4Gal-TIV decreases dramatically as the accepters contain more LacNAc repeats, consistent with the fact that core 2-branched O-glycans contain shorter poly-N-acetyllactosamines than N-glycans in many cells. Poly-N-acetyllactosamines in core 4-branched O-glycans were found to be synthesized most efficiently by iGnT and beta 4Gal-TI although the synthesis in core 4 branches is less efficient than in core 2 branches because of inefficient addition of GlcNAc to core 4 branches by iGnT. Thus, poly-N-acetyllactosamine extension in core 2- and core 4-branched O-glycans is differentially controlled by iGnT and different members of the beta 4Gal-T gene family.