O-Glycosylation of G-protein-coupled receptor, octopus rhodopsin. Direct analysis by FAB mass spectrometry

In addition to the N-glycan that is evidently conserved in G-protein-coupled receptors (GPCRs), O-glycans in the N-terminus of GPCRs have been suggested. Using a combination of enzymatic and manual Edman degradation in conjunction with G-protein coupled receptor mass spectrometry, the structure and sites of O-glycans in octopus rhodopsin are determined. Two N-acetylgalactosamine residues are O-linked to Thr4 and Thr5 in the N-terminus of octopus rhodopsin. Further, we found chicken iodopsin, but not bovine rhodopsin, contains N-acetylgalactosamine. This is the first direct evidence to determine the structure and sites of O-glycans in GPCRs.