O-glycosylation mimics N-glycosylation in the 16-kDa fragment of bovine pro-opiomelanocortin. The major O-glycan attached to Thr-45 carries SO4-4GalNAc beta 1-4GlcNAc beta 1-, which is the archetypal non-reducing epitope in the N-glycans of pituitary glycohormones.

The NH2-terminal domain of pro-opiomelanocortin, designated as the 16-kDa fragment, is highly conserved throughout the vertebrate family and is likely therefore to have an important functional role. Bovine 16-kDa fragment is a 77- residue glycopeptide, which has been found to be glycosylated at threonine 45 and asparagine 65. Available evidence suggests that glycoforms lacking glycans at the O-linked site are processed in the intermediate pituitary at -Arg49-Lys50- to give the residue 1-49 amino-terminal peptide and a carboxyl-terminal glycopeptide referred to as Lys1 gamma 3-melanotropin. Glycoforms carrying O-glycans remain unprocessed in the intermediate pituitary. Thus O-glycosylation is likely to play an important role in controlling the fate of the NH2-terminal portion of pro-opiomelanocortin, thereby affecting the biological events that are influenced by peptides and glycopeptides derived from this domain. In a recent study (Siciliano, R. A., Morris, H. R., McDowell, R. A., Azadi, P., Rogers, M. E., Bennett, H. P. J., and Dell, A. (1993) Glycobiology 3, 225-239), we sequenced the N-glycans attached to Asn-65 of bovine 16-kDa fragment and demonstrated that the acidic components contain, in addition to neutral antennae, a single SO4-4GalNAc beta 1-4GlcNAc beta 1- antenna, which is characteristic of the pituitary glycohormone N-glycans (Baenziger, J. U., and Green, E. D. (1988) Biochim. Biophys. Acta 947, 287-306). We now report the structural characterization of the O-linked oligosaccharides found in bovine 16-kDa fragment. The major component, which constitutes about 80% of the O-glycan population, is a novel sulfated tetrasaccharide, which carries the same sulfated epitope as the N-glycans. This is the first time that the SO4-4GalNAc beta 1-4GlcNAc beta 1- moiety has been observed in O-glycans, and it raises the interesting possibility that the beta-N-acetylgalactosaminyltransferase responsible for the addition of N-acetylgalactosamine to the pituitary glycohormones (Smith, P. L., and Baenziger, J. U. (1988) Science, 242, 930-933) might be capable of glycosylating both N- and O-linked acceptors.