Protein glycosylation is a prevalent and important super-class of post-translational modifications (PTMs) involved in regulation of a wide array of intra- and inter-cellular functions. Glycosylation accounts for the greatest proteome diversity over any other PTM, as approximately half of all expressed proteins undergo this modification; characterizing the complex structures and functions of the glycoproteome is integral to advancing our understanding of cell biology and disease. That said, strikingly little is known about exactly which glycans modify which proteins and where, indicating a technology gap exists for this key PTM. Today's technology relies upon mass spectrometry (MS), a high-throughput and sensitive technique with the ability to localize modifications to a single amino acid, lending it great power in PTM analysis - global studies of protein phosphorylation, for example, routinely harbor tens of thousands of sites in a single experiment. Methods of detecting O-glycosylation, however, lag behind other PTMs due to several inherent analytical challenges, including: poor ionization as cations, broad structural heterogeneity, no known consensus sequence motif, and a lack of methods for concurrent peptide and glycan characterization. We conclude that the best way to improve O-glycopeptide analysis is to fundamentally change our analytical methods to better pair with the chemical characteristic of the analytes. Specifically, we will develop a negative ion mode MS method that will embrace the acidity of the glycopeptide to achieve preferential, rather than discriminatory, ionization. This has been heretofore challenging, due to a lack of dissociation methods compatible with peptide anions. Over the last several years, however, we have developed the negative ion analog of ETD - termed negative ETD (NETD). Here, we propose the first implementation of NETD for global glycoproteomic analyses. In NETD, glycopeptide anions are oxidized by radical reagent cations, promoting electron rearrangements that cleave the C-Ca backbone bond to produce a- and x-type product ions, leaving the glycan modification intact for site-specific localization To provide concurrent glycan structure elucidation we shall couple NETD with photo-activation (simultaneously, i.e., activated-ion NETD, AI-NETD) to enable intact glycopeptide (glycan and peptide) identification in single tandem MS analysis.
We are developing a transformative approach for the analysis of O-glycosylation. This posttranslational modification is exceedingly difficult to characterize wih current technologies, despite its important roles in cancer biology, immune response, and cell development. The proposed technology will enable the first high-throughput platform for O-glycosylation analysis, furthering our understanding of how its regulation affects a wide array of health and disease states.
