DIFFERENTIAL RECOGNITION OF GLYCOPROTEIN ACCEPTORS BY TERMINAL GLYCOSYLTRANSFERASES

The factors regulating modifications of terminal beta-Gal residues in lactosaminyl (Gal beta 1-->4GlcNAc beta-->R) units in intact glycoprot eins are not well understood, To examine these factors, rat liver alph a 2,3 sialyltransferase (alpha 2,3ST) and alpha 2,6 sialyltransferase (alpha 2,6ST) and the murine alpha 1,3 galactosyltransferase (alpha 1, 3GT) were incubated with a variety of well-defined desialylated glycop roteins and with glycoproteins in extracts of the Lec2 mutant CHO cell s, Lec2 cells constitutively synthesize nonsialylated glycoproteins wi th terminal lactosaminyl sequences, The results demonstrate that each enzyme displays preferences for glycoprotein accepters and in the type s of N-glycans recognized, The alpha 2,3ST, in contrast to the alpha 2 ,6ST and alpha 1,3GT, prefers more branched N-glycans compared to dian tennary N-glycans, However, only the alpha 1,3GT is able to efficientl y modify polylactosamines (3Gal beta 1-->4GlcNAc beta 1-->)(n) in N-gl ycans, Glycopeptides mere also prepared by proteolysis of Lec2 glycopr oteins and tested as accepters compared to intact Lec2 glycoproteins, The alpha 2,6ST and alpha 1,3GT utilized intact glycoproteins and glyc opeptides with a 2-fold preference for the former over the latter, In contrast, the alpha 2,3ST showed a 20-fold preference for intact glyco proteins over glycopeptides. These results demonstrate that each of th ese terminal glycosyltransferases differentially recognizes glycans an d glycoprotein accepters, and that the alpha 2,3ST requires peptide fe atures for efficient utilization of branched N-glycan accepters.