Enzymatic Tagging of Glycoproteins on the Cell Surface for Their Global and Site-Specific Analysis with Mass Spectrometry.

The cell surface is normally covered with sugars that are bound to lipids or proteins. Surface glycoproteins play critically important roles in many cellular events, including cell-cell communications, cell-matrix interactions, and response to environmental cues. Aberrant protein glycosylation on the cell surface is often a hallmark of human diseases such as cancer and infectious diseases. Global analysis of surface glycoproteins will result in a better understanding of glycoprotein functions and the molecular mechanisms of diseases and the discovery of surface glycoproteins as biomarkers and drug targets. Here, an enzyme is exploited to tag surface glycoproteins, generating a chemical handle for their selective enrichment prior to mass spectrometric (MS) analysis. The enzymatic reaction is very efficient, and the reaction conditions are mild, which are well-suited for surface glycoprotein tagging. For biologically triplicate experiments, on average 953 N-glycosylation sites on 393 surface glycoproteins per experiment were identified in MCF7 cells. Integrating chemical and enzymatic reactions with MS-based proteomics, the current method is highly effective to globally and site-specifically analyze glycoproteins only located on the cell surface. Considering the importance of surface glycoproteins, this method is expected to have extensive applications to advance glycoscience.