Cell-free synthesis and processing of multiple precursors to glucagon

Abstract

Glucagon, a polypeptide hormone of 29 amino acids, is synthesized in the islets of Langerhans and immunoreactive forms of the molecule have been found in several tissues¹. Like many other polypeptide hormones, glucagon is synthesized via a larger precursor molecule, proglucagon^2–7; however, estimates of its size vary considerably and the biosynthetic relationship between some of the putative precursors and authentic secreted glucagon is unclear. Consequently, it was of interest to investigate the primary translation product of glucagon mRNA to relate its size to that of previously described glucagon precursors^5,6. Here we provide evidence for three distinct immunoreactive preproglucagon molecules, two of which have an apparent molecular weight (MW) of ∼14,000 (14K), the third with a MW of ∼16,000 (16K). Furthermore, when microsomal membranes were present during translation, the nascent 14K preproglucagon polypeptides were processed to proglucagon with a higher apparent MW of 15,000. In contrast, the nascent 16K preproglucagon was co-translationally processed to a slightly smaller polypeptide. The data indicate that the 14K and 16K preproglucagons undergo different types of post-translational modification.