Nucleotide sequence of a cDNA clone encoding human preproinsulin

Abstract

Insulin consists of two polypeptide chains, A (21 amino acids) and B (30 amino acids), linked by disulphide bonds. Both chains are derived from one precursor, proinsulin, which includes a connecting peptide (C) between the A and B chains, and which is excised before the secretion of insulin from the pancreatic B cells¹. The observation that the C-peptide varies between species, in contrast to the highly conserved A and B sequences^1–3, is consistent with the theory that it serves a purely structural function in insulin synthesis. During in vitro translation of insulin mRNA, a larger peptide containing about 25 additional residues at the N-terminal end (preproinsulin) is the primary product^4–7. The prepeptide is cleaved to leave proinsulin during transport into the endoplasmic reticulum and is thought to direct this process specifically⁸. Using automated amino acid sequence analysis, the partial amino acid sequences of the prepeptide regions of bovine, rat, sea raven and anglerfish preproinsulin have been established^4,5,7. The nucleotide sequences of the cloned cDNA and gene coding for rat insulin I have confirmed the amino acid sequence of rat proinsulin I, and have also predicted the sequence of the prepeptide^9–11. Like the prepeptides of other secreted proteins, this prepeptide has a prominent hydrophobic region¹². We report here the cloning of a cDNA prepared from human insulin mRNA and an analysis of the nucleotide sequence of the cloned molecule including the region coding for the prepeptide and portions of the 5′- and the 3′-untranslated regions of the molecule. We also compare the structure of the human molecule with the previously reported rat mRNA^9–11.