Pancreatic pre-proglucagons are encoded by two separate mRNAs

Abstract

Poly(A) RNA prepared from anglerfish islets programs the synthesis in a wheat germ cell-free system of two proteins of Mr = 14,500 and Mr = 12,500 that are both immunoprecipitated by an antiserum prepared to pancreatic glucagon of Mr = 3,500 (29 amino acids). We provide evidence by cDNA hybridizations and nucleotide sequence analyses that these two proteins are precursors of glucagon and that they are encoded by separate genes. Two cloned recombinant cDNAs prepared from the islet mRNAs individually hybrid-selected mRNAs that directed the cell-free synthesis of each of the two glucagon-related proteins. Nucleotide sequence analysis of the cDNA corresponding to the Mr = 14,500 protein revealed a coding sequence for a peptide of 29 amino acids flanked by Lys-Arg sequences typical of those found at the sites of post-translational cleavages of hormone precursors. Twenty of the 29 amino acids in the sequence are identical with those found in the sequence of mammalian pancreatic glucagon. The second of the two cDNAs completely hybrid-arrested the translation of the mRNA encoding the smaller glucagon precursor of Mr = 12,500 but had no effect on the translation of the mRNA encoding the Mr = 14,500 precursor. The cDNA corresponding to this Mr = 14,500 precursor hybridized to the DNA of bacterial colonies containing the cDNA for the Mr = 12,500 precursor. Thus, two separate but partially homologous mRNAs encode the two pre-proglucagons.