Engineering of an artificial glycosylation pathway blocked in core oligosaccharide assembly in the yeast Pichia pastoris: production of complex humanized glycoproteins with terminal galactose.

A significant percentage of eukaryotic proteins contain posttranslational modifications, including glycosylation, which are required for biological function. However, the understanding of the structure-function relationships of N-glycans has lagged significantly due to the microheterogeneity of glycosylation in mammalian produced proteins. Recently we reported on the cellular engineering of yeast to replicate human N-glycosylation for the production of glycoproteins. Here we report the engineering of an artificial glycosylation pathway in Pichia pastoris blocked in dolichol oligosaccharide assembly. The PpALG3 gene encoding Dol-P-Man:Man(5)GlcNAc(2)-PP-Dol mannosyltransferase was deleted in a strain that was previously engineered to produce hybrid GlcNAcMan(5)GlcNAc(2) human N-glycans. Employing this approach, combined with the use of combinatorial genetic libraries, we engineered P. pastoris strains that synthesize complex GlcNAc(2)Man(3)GlcNAc(2) N-glycans with striking homogeneity. Furthermore, through expression of a Golgi-localized fusion protein comprising UDP-glucose 4-epimerase and beta-1,4-galactosyl transferase activities we demonstrate that this structure is a substrate for highly efficient in vivo galactose addition. Taken together, these data demonstrate that the artificial in vivo glycoengineering of yeast represents a major advance in the production of glycoproteins and will emerge as a practical tool to systematically elucidate the structure-function relationship of N-glycans.