X-ray crystallography and mass spectroscopy reveal that the N-lobe of human transferrin expressed in Pichia pastoris is folded correctly but is glycosylated on serine-32

The ferric form of the N-lobe of human serum transferrin (Fe(III)-hTF/2N) has been expressed at high levels in Pichia pastoris. The Fe(III)-hTF/2N was crystallized in the space group P4(1)2(1)2, and X-ray crystallography was used to solve the structure of the recombinant protein at 2.5 Angstrom resolution. This represents only the second P. pastoris-derived protein structure determined to date, and allows the comparison of the structures of recombinant Fe(III)-hTF/2N expressed in P. pastoris and mammalian cells with serum-derived transferrin. The polypeptide folding pattern is essentially identical in all of the three proteins. Mass spectroscopic analyses of P. pastoris- hTF/2N and proteolytically derived fragments revealed glycosylation of Ser-32 with a single hexose. This represents the first localization of an O-linked glycan in a P. pastoris-derived protein. Because of its distance from the iron-binding site, glycosylation of Ser-32 should not affect the iron-binding properties of hTF/2N expressed in P. pastoris, making this an excellent expression system for the production of hTF/2N.