Proinsulin cDNAs from the leopard frog, Rana pipiens: evolution of proinsulin processing

We have isolated a proinsulin cDNA from the Amphibian Rana pipiens. The pre dicted R. pipiens insulin A- and B-chain amino acid sequences differ from t hat deduced from the closely related Rana catesbeiana at one residue (Asp f or Pro at B2). The R, pipiens and Xenopus laevis proinsulin precursor seque nces are of identical length, with the amino acid sequences of the mature A - and B-chains being well conserved. The proinsulin C-peptide amino acid se quence is less well conserved between R. pipiens and X. laevis and also dif fers in length. The R. pipiens C-peptide is shorter than the homologous X. laevis sequence due to a two amino acid residue truncation. The truncation of the R. pipiens C-peptide compensates for a two amino acid residue extens ion observed at the N-terminal of;he A-chains of insulins from Ranid frogs. A change in the site of proinsulin processing can explain both the C-pepti de and A-chain length differences. The evolution of the new proinsulin proc essing site required two amino acid substitutions. (C) 2000 Elsevier Scienc e Inc. All rights reserved.