Differential glycosylation of N-POMC 1–77 regulates the production of γ 3-MSH by purified pro-opiomelanocortin converting enzyme A possible mechanism for tissue-specific processing

Abstract

The amino terminus of bovine pro-opiomelanocortin (N-POMC 1–77) is partially processed in the intermediate lobe of the pituitary to N-POMC 1–49 and lys-γ 3 -melanotropin. Two pools of N-POMC 1–77 were isolated which were differentially glycosylated at threonine 45, while N-POMC 1–49 isolated from bovine intermediate lobe extracts existed in a non-glycosylated form. This suggested that differential O-linked glycosylation of N-POMC 1–77 may regulate cleavage at the Arg 49-Lys 50 processing site. We tested this hypothesis by incubating N-POMC 1–77 glycoforms with purified pro-opiomelanocortin converting enzyme. Only non- O-glycosylated N-POMC 1–77 and O-glycosylated N-POMC 1–77 with truncated oligosaccharide sidechains were sensitive to cleavage and generated predominantly lys-γ 3 -melanotropin, identified by high-performance liquid chromatography. These data provide the first functional evidence to support a role for differential O-linked glycosylation in the regulation of the processing of the N-terminus of bovine POMC.