Pro-opiomelanocortin Synthesized by Corticotrophs Bears Asparagine-linked Oligosaccharides Terminating with S04-4GalNAcf11,4GlcNAcf11,2Mana*

We have determined that 280% 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 S04-4GalNAc/31,4GlcNAc~1, 2Mana (S4GGnM). 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 S04-4-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 GalNAcpl,4GlcNAc/31, 2Man-4-sulfotransferase and the glycoprotein hormone-specific GalNAc-transferase are expressed in the corticotroph-derived AtT-PO cell line. A tripeptide recognition sequence, Pro-Val-Lys, similar to the ProLeu-Arg sequence required for recognition of glycoprotein hormone aand &subunits by the glycoprotein hormone-specific GalNAc-transferase, is present 8 residues amino-terminal to the glycosylated Asn of 15kDa 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.