Mass Tags Quantify Glycosylation

Researchers using a new mass-tagging strategy have found a way to quantify the stoichiometry of O-GlcNAc glycosylation in proteins (Nat. Chem. Biol., DOI: 10.1038/nchembio.412). In this type of posttranslational modification, which plays a role in many biological processes, the sugar N-acetyl-D-glucosamine is added to serine or threonine residues. Because the modification is labile and occurs at low levels, it has been difficult to quantify. Linda C. Hsieh-Wilson of California Institute of Technology and coworkers at Caltech and UCLA developed a gel-based approach using polyethylene glycol mass tags that they attach to proteins through a chemoenzymatic labeling method. When the proteins are separated by gel electrophoresis and visualized via fluorescently labeled antibodies, shifts in the protein bands reveal whether a protein has been singly, doubly, or more heavily glycosylated. For example, the researchers found that cyclic AMP-response element-binding protein exists as a mixture of mono- and diglycosylated forms in rat neurons. In addition, they determined the interplay of glycosylation and phosphorylation in the transcription repressor MeCP2. The team discovered that glycosylation is induced to a greater extent in phosphorylated MeCP2.