Production of sialylated O-linked glycans in Pichia pastoris

The methylotrophic yeast, Pichia pastoris, is an important organism used for the production of therapeutic proteins. Previously, we have reported the glycoengineering of this organism to produce human-like N-linked glycans but up to now no one has addressed engineering the O-linked glycosylation pathway. Typically, O-linked glycans produced by wild-type P. pastoris are linear chains of four to five alpha-linked mannose residues, which may be capped with beta- or phospho-mannose. Previous genetic engineering of the N-linked glycosylation pathway of P. pastoris has eliminated both of these two latter modifications, resulting in O-linked glycans which are linear alpha-linked mannose structures. Here, we describe a method for the co-expression of an alpha 1,2-mannosidase, which reduces these glycans to primarily a single O-linked mannose residue. In doing so, we have reduced the potential of these glycans to interact with carbohydrate-binding proteins, such as dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin. Furthermore, the introduction of the enzyme protein-O-linked-mannose beta-1,2-N-acetylglucosaminyltransferase 1, resulted in the capping of the single O-linked mannose residues with N-acetylglucosamine. Subsequently, this glycoform was extended into human-like sialylated glycans, similar in structure to alpha-dystroglycan-type glycoforms. As such, this represents the first example of sialylated O-linked glycans being produced in yeast and extends the utility of the P. pastoris production platform beyond N-linked glycosylated biotherapeutics to include molecules possessing O-linked glycans.