Complementary LC-MS/MS-Based N-Glycan, N-Glycopeptide, and Intact N-Glycoprotein Profiling Reveals Unconventional Asn71-Glycosylation of Human Neutrophil Cathepsin G.

Neutrophil cathepsin G (nCG) is a central serine protease in the human innate immune system, but the importance of its N-glycosylation remains largely undescribed. To facilitate such investigations, we here use complementary LC-MS/MS-based N-glycan, N-glycopeptide, and intact glycoprotein profiling to accurately establish the micro- and macro-heterogeneity of nCG from healthy individuals. The fully occupied Asn71 carried unconventional N-glycosylation consisting of truncated chitobiose core (GlcNAcß: 55.2%; Fucα1,6GlcNAcß: 22.7%), paucimannosidic N-glycans (Manß1,4GlcNAcß1,4GlcNAcß: 10.6%; Manß1,4GlcNAcß1,4(Fucα1,6)GlcNAcß: 7.9%; Manα1,6Manß1,4GlcNAcß1,4GlcNAcß: 3.7%, trace level of Manα1,6Manß1,4GlcNAcß1,4(Fucα1,6)GlcNAcß), and trace levels of monoantennary α2,6- and α2,3-sialylated complex N-glycans. High-resolution/mass accuracy LC-MS profiling of intact nCG confirmed the Asn71-glycoprofile and identified two C-terminal truncation variants at Arg243 (57.8%) and Ser244 (42.2%), both displaying oxidation of solvent-accessible Met152. Asn71 appeared proximal (~19 Å) to the active site of nCG, but due to the truncated nature of Asn71-glycans (~5-17 Å) we questioned their direct modulation of the proteolytic activity of the protein. This work highlights the continued requirement of using complementary technologies to accurately profile even relatively simple glycoproteins and illustrates important challenges associated with the analysis of unconventional protein N-glycosylation. Importantly, this study now facilitates investigation of the functional role of nCG Asn71-glycosylation.

Was the serine protease cathepsin G discovered by S. G. Hedin in 1903 in bovine spleen?

In the beginning of the 20th century, enzymes with proteolytic activity were classified as peptidases, Erepsin, and proteases. Among these, pepsin, trypsin, and autolytic enzymes were of the protease class. Spleen-derived proteases were poorly characterized until Sven Gustaf Hedin performed several digestion experiments with bovine spleen. He incubated minced bovine spleen under acidic or neutral conditions and characterized two active proteases; the results were published in 1903. The first protease was named α-protease and was active under neutral conditions. The second was named ß-protease and was active under acidic conditions. We replicated Hedin's experiments according to his methods and found, by using activity-based probes to visualize proteases, that the historical α-protease is the present-day serine protease cathepsin G (CatG), which is known to be important in several immune processes, including antigen processing, chemotaxis, and activation of surface receptors. The ß-protease, however, comprised different proteases including CatX, B, S, and D. We suggest that Hedin described CatG activity in bovine spleen over 100 years ago.