Precision Mapping of an In Vivo N-Glycoproteome Reveals Rigid Topological and Sequence Constraints

N-linked glycosylation is a biologically important protein modification, but only a small fraction of modification sites have been mapped. We developed a filter aided sample preparation (FASP)-based method in which glycopeptides are enriched by binding to lectins on the top of a filter and mapped 6367 N-glycosylation sites on 2352 proteins in four mouse tissues and blood plasma using high-accuracy mass spectrometry. We found 74% of known mouse N-glycosites and discovered an additional 5753 sites on a diverse range of proteins. Sites almost always have the N-!P-[SjT]-!P (where ! P is not proline) and rarely the N-X-C motif or nonconsensus sequences. Combining the FASP approach with analysis of subcellular glycosite localization reveals that the sites always orient toward the extracellular space or toward the lumen of ER, Golgi, lysosome, or peroxisome. The N-glycoproteome contains a plethora of modification sites on factors important in development, organ-specific functions, and disease.