Production of heterologous glycoproteins by a glycosylation-defective alg3och1 mutant of Schizosaccharomyces pombe.

The early stages of N-linked glycosylation are highly conserved between fungal and mammalian cells. Such N-linked oligosaccharides are synthesized through the ordered assembly of a dolichyl pyrophosphate (Dol-PP)-linked Glc(3)Man(9)GlcNAc(2) structure by the sequential actions of several glycosyltransferases located in the endoplasmic reticulum (ER). Of the glycosyltransferase genes, Saccharomyces cerevisiae ALG3 has been identified to encode the Dol-P-Man:Man(5)GlcNAc(2)-PP-Dol α1,3-mannosyltransferase, and an alg3 mutant has been shown to accumulate an Endo H-resistant M5B (Manα1,2-Manα1,2-Manα1,3(Manα1,6-)-Manβ1,4-GlcNAcβ1,4-GlcNAc) structure. Although Schizosaccharomyces pombe contains a homolog of the ALG3 gene (SPAC7D4.06c), the role of this gene in oligosaccharide biosynthesis is not at all clear. In this study, we deleted the alg3(+) gene in the och1Δ mutant and analyzed the detailed oligosaccharide structures in alg3Δoch1Δ double mutant. The oligosaccharides were prepared from cell-surface glycoproteins by hydrazinolysis and fluorescent labeling with 2-aminopyridine. The labeled oligosaccharides were analyzed by high performance liquid chromatography, in combination with sequential glycosidase digestion and methylation analysis. These analyses revealed that the N-linked oligosaccharides of S. pombe alg3Δoch1Δ cells mainly consisted of two or three α-galactose-capped M5B structures. Finally, western blot analysis of recombinant human transferrin suggested that heterologously expressed glycoproteins in alg3Δoch1Δ cells have Endo H-resistant N-linked oligosaccharide structures similar to those of alg3Δoch1Δ cell-surface glycoproteins.