Pre-Proglucagon Messenger Ribonucleic Acid: Nucleotide and Encoded Amino Acid Sequences of the Rat Pancreatic Complementary Deoxyribonucleic Acid*

Abstract

Glucagon, a pancreatic peptide hormone of 29 amino acids that regulates carbohydrate, fat and protein metabolism, is one of a family of structurally similar regulatory peptides which include GH[growth hormone]-releasing hormone, vasoactive intestinal peptide, secretin and gastric inhibitory peptide. The synthesis of glucagon involves its specific proteolytic cleavage from preproglucagon, a large polyprotein precursor. To facilitate analyses of the cellular processing of pre-proglucagon and to begin studies of the regulation of glucagon gene expression in the rat, 2 cDNA derived from rat neonatal pancreas were cloned and sequenced. The cDNA represent close to the entire transcriptional sequence of the glucagon gene and encode a pre-proglucagon of 180 amino acids. The coding region of the pre-proglucagon contains, in addition to the sequence of glucagon, the sequences of 2 peptide domains that are related in their structures to glucagon. Glucagon and the 2 glucagon-like peptides are flanked in the precursor by pairs of basic amino acids characteristic of the sites that are cleaved during the posttranslational processing of prohormones. Northern blot analyses indicate the presence of a single mRNA of 1400 .+-. 100 nucleotides in the rat pancreas, and results of Southern blotting of rat liver genomic DNA are consistent with the existence of a single chromosomal gene. Comparisons of the nucleotide sequence of the rat pre-proglucagon cDNA with those of the 2 pre-proglucagons of the anglerfish pancreas encoded by 2 separate genes indicate that the pre-proglucagon genes probably evolved by intragenic duplications of a DNA segment corresponding to the coding sequences of glucagon and the glucagon-like peptides.