Cell-free synthesis and processing of multiple precursors to glucagon

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Abstract
Glucagon, a polypeptide hormone of 29 amino acids, is synthesized in the islets of Langerhans and immunoreactive forms of the molecule were found in several tissues. Like many other polypeptide hormones, glucagon is synthesized via a larger precursor molecule, proglucagon; estimates of its size vary considerably and the biosynthetic relationship between some of the putative precursors and authentic secreted glucagon is unclear. The primary translation product of anglerfish glucagon mRNA was investigated to relate its size to that of previously described glucagon precursors. Evidence is provided for 3 distinct immunoreactive preproglucagon molecules, 2 of which have an apparent MW of .apprx. 14,000 (14K), the 3rd with a MW of .apprx. 16,000 (16K). When dog microsomal membranes were present during translation, the nascent 14K preproglucagon polypeptides were processed to proglucagon with a higher apparent MW of 15,000. The nascent 16K preproglucagon was co-translationally processed to a slightly smaller polypeptide. Apparently the 14K and 16K preproglucagons undergo different types of posttranslational modification.