The structure of Mordacia mordax insulin supports the monophyly of the Petromyzontiformes and an ancient divergence of Mordaciidae and Geotriidae

The phylogenetic relationships between the two southern hemisphere lamprey families (Geotriidae and Mordaciidae) and their northern hemisphere counter parts (Petromyzontidae) are unresolved. Insulin was isolated from an extrac t of islet-containing intestinal tissue from ammocoetes of the Australian l amprey, Mordacia mordax. Its primary structure was established as A-chain: GIVEQCCHRK(10)CSIYDMENYC(20)N and B-chain: SALMGT-GGTH(10) LCGSHLVEAL(20) Y VVCGQRGFF(30) YTP[SKG]. Although the residues in parentheses are only tenta tively assigned, mass spectrometry supports the proposed sequence and demon strates that Mordacia proinsulin, unlike proinsulin from Geotria australis, is fully processed to mature insulin. Insulins from M.mordax and G. austra lis and from the northern hemisphere lampreys Petromyzon marinus and Lampet ra fluviatilis share a pentapeptide extension to N-terminus of the B-chain (Ser-Ala-Leu-Xaa-Gly) that has never been found in the insulins of any othe r vertebrate class. This observation provides support for the claim that th e Petromyzontiformes constitute a monophyletic group. M. mordax insulin dif fers from that of G. australis by 18 amino acid residues but by only four r esidues from the common sequence of P. marinus and L. fluviatilis insulin. These data are consistent with the view that Geotriidae and Mordaciidae hav e been separated for a long period and suggest that G. australis insulin ha s undergone an accelerated rate of molecular evolution. (C) 2001 Elsevier S cience Inc. All rights reserved.