Pro-opiomelanocortin synthesized by corticotrophs bears asparagine-linked oligosaccharides terminating with SO₄-4GalNAcβ1,4GlcNAcβ1,2Manα

We have determined that ≥80% of the Asn-linked oligosaccharides on the glycosylated form of mouse adrenocorticotropin (15-kDa adrenocorticotropin (ACTH)) bear one or more branches terminating with the sequence SO₄-4GalNAcβ1,4GIcNAcβ1, 2Manα (84GGnM). Proopiomelanocortin (POMC), the precursor of ACTH, is the first example of a glycoprotein that is not a member of the glycoprotein hormone family to bear such sulfated structures. Like lutropin and thyrotropin, 15-kDa ACTH bears dibranched oligosaccharides terminating with SO₄-4-GalNAc; however, at least half of the oligosaccharides on 15-kDa ACTH terminating with SO₄-4-GalNAc consist of more highly branched structures that have not previously been described. Both the GalNAcβ1,4GlcNAcβ1, 2Man-4-sulfotransferase and the glycoprotein hormone-specific GalNAc-transferase are expressed in the corticotroph-derived AtT-20 cell line. A tripeptide recognition sequence, Pro-Val-Lys, similar to the ProLeu-Arg sequence required for recognition of glycoprotein hormone α- and β-subunits by the glycoprotein hormone-specific GalNAc-transferase, is present 8 residues amino-terminal to the glycosylated Asn of 15-kDa ACTH. Thus, POMC has the features expected for specific addition of the S4GGnM sequence to its oligosaccharides. The recent discovery of a receptor in hepatic endothelial cells that recognizes oligosaccharides terminating with S4GGnM suggests these sulfated oligosaccharides will regulate the circulatory half-life of glycosylated POMC cleavage products.