Ubiquitination plays an essential role in maintaining cellular homeostasis by regulating a multitude of essential processes. The ability to identify ubiquitinated proteins by MS currently relies on a strategy in which ubiquitinated peptides are identified by a 114.1 Da diglycine (GG) tag on lysine residues, which is derived from the C-terminus of ubiquitin, following trypsin digestion. In the following study, we report a more comprehensive approach for mapping ubiquitination sites by trypsin digestion and MS/MS analysis. We demonstrate that ubiquitination sites can be identified by signature peptides containing a GG-tag (114.1 Da) and an LRGG-tag (383.2 Da) on internal lysine residues as well as a GG-tag found on the C-terminus of ubiquitinated peptides. Application of this MS-based approach enabled the identification of 96 ubiquitination sites from proteins purified from human MCF-7 breast cancer cells, representing a 2.4-fold increase in the number of ubiquitination sites that could be identified over standard methods. Our improved MS-based strategy will aid future studies which aim to identify and/or characterize ubiquitinated proteins in human cells.

We describe the development of a glycoproteomic reactor that combines multiple biochemical and chemical protein processing into a single device for the study of N-glycosylated proteins. The glycoproteins are first enriched by concanavalin A affinity chromatography and then transferred onto and efficiently processed in the glycoproteomic reactor. This glycoproteomic reactor combines protein concentration and purification, disulfide bond reduction, peptide-N-glycosidase-mediated 18 O-labeling and deglycosylation, alkylation, tryptic digestion and pH based fractionation in a device that has an interstitial volume (reaction volume) of ∼1 µL. We demonstrated the potential of the glycoproteomic reactor using human plasma. Under stringent criteria, 82 unique glycopeptides representing 41 unique glycoproteins were identified from as little as 5 µL of human plasma. Our glycoproteomic reactor reduces the sample processing time to less than 1.5 h, reduces the reagent consumption while providing over 1000-fold concentration of the sample, provides efficient removal of high concentration of glycan buffer, and, finally, allows both glycopeptides and nonglycosylated tryptic peptides to be analyzed by the mass spectrometer which provides much greater protein coverage and more reliable identifications.

Mass spectrometry (MS) coupled to affinity purification is a powerful approach for identifying protein-protein interactions and for mapping post-translational modifications. Prior to MS analysis, affinity-purified proteins are typically separated by gel electrophoresis, visualized with a protein stain, excised, and subjected to in-gel digestion. An inherent limitation of this series of steps is the loss of protein sample that occurs during gel processing. Although methods employing in-solution digestion have been reported, they generally suffer from poor reaction kinetics. In the present study, we demonstrate an application of a microfluidic processing device, termed the Proteomic Reactor, for enzymatic digestion of affinity-purified proteins for liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. Use of the Proteomic Reactor enabled the identification of numerous ubiquitinated proteins in a human cell line expressing reduced amounts of the ubiquitin-dependent chaperone, valosin-containing protein (VCP). The Proteomic Reactor is a novel technology that facilitates the analysis of affinity-purified proteins and has the potential to aid future biological studies.