Screening for N ‐glycosylated proteins by liquid chromatography mass spectrometry

Abstract

In the last few years mass spectrometry has become the method of choice for characterization of post‐translationally modified proteins. Whereas most protein chemical modifications are binary in the sense that only one change can be associated with a given residue, many different oligosaccharides can be attached to a glycosylation site residue. The detailed characterization of glycoproteins in complex biological samples is extremely challenging. However, information on N ‐glycosylation can be gained at an intermediary level. Here we demonstrate a procedure for mapping N ‐glycosylation sites in complex mixtures by reducing sample complexity and enriching glycoprotein content. Glycosylated proteins are selected by an initial lectin chromatography step and digested with endoproteinase Lys‐C. Glycosylated peptides are then selected from the digest mixture by a second lectin chromatography step. The glycan components are removed with N ‐glycosidase F and the peptides digested with trypsin before analysis by on‐line reversed‐phase liquid chromatography mass spectrometry. Using two different lectins, concanavalin A and wheat germ agglutinin, this procedure was applied to human serum and a total of 86 N ‐glycosylation sites in 77 proteins were identified.