Early O-glycosidic glycosylation of proglucagon in pancreatic islets: an unusual type of prohormonal modification.

Proglucagon from rat islets is identified as a glycoprotein by its binding to soybean lectin and by the biosynthetic incorporation of [14C]galactosamine. Glycosylation can be demonstrated for both forms of proglucagon, i.e. the primary translation product which is detectable as early as 30 s after incubation of isolated islets with radioactive amino acids (proglucagon a), and its conversion product of slightly higher electrophoretic mobility which is formed after 5-10 min of incubation (proglucagon b). This glycosylation is determined to be of the O-glycosidic type by the following criteria: rat proglucagon has previously been shown to lack an acceptor sequence for N-glycosidic linkage of sugars, the sugar bond in rat proglucagon is labile under mild alkaline conditions, glycosylated serine is demonstrated in proteolytic lysates of both the early and the late form of this prohormone. O-glycosidic linkage of sugars has not been reported for other prohormones. Its early formation and the apparent absence of N-glycosidically bound sugars in proglucagon give evidence for an unusual type of protein glycosylation.