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First published online August 31, 2004
Journal of Experimental Biology 207, 3581-3590 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01204
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Phylogenetic conservation of disulfide-linked, dimeric acetylcholine receptor pentamers in southern ocean electric rays

M. L. Tierney1,*, K. E. Osborn1, P. J. Milburn2, M. H. B. Stowell3 and S. M. Howitt1

1 School of Biochemistry & Molecular Biology, Australian National University, Canberra, ACT 0200, Australia
2 John Curtin School of Medical Research, Australian National University, Canberra, ACT 0200, Australia
3 MCD Biology, University of Colorado, Boulder, CO 80309, USA

* Author for correspondence (e-mail: louise.tierney{at}anu.edu.au)

Accepted 15 July 2004

Intact acetylcholine receptors have been purified on a novel affinity resin from three electric fish endemic to Australian waters. Their binding properties and morphology are compared with those of their northern hemisphere homolog, Torpedo marmorata. All four exhibit apparent dissociation constants, Kd, in the nanomolar range for the snake neurotoxin {alpha}-bungarotoxin and have a distinctive rosette-like appearance when viewed in negative stain under the electron microscope. Furthermore, these rosettes are paired, indicating that acetylcholine receptors from southern ocean electric fish exist as dimers, in the same fashion as their northern hemisphere counterparts. The cDNAs of the receptor's four subunits were sequenced from Hypnos monopterigium and the northern hemisphere counterpart, Torpedo marmorata, while cDNAs from only two subunits, {alpha} and {delta}, were able to be sequenced from Narcine tasmaniensis. The penultimate amino acid in the {delta} subunit of each of the newly sequenced fish species is a cysteine residue. Its conservation suggests that the mechanism for the observed dimerization of acetylcholine receptors is disulfide bond formation between the {delta} subunit of adjacent receptors, analogous to acetylcholine receptor dimers observed in other electric fish. It appears that this mechanism for receptor clustering is unique to acetylcholine receptors packed and organized in the specialized organs of electric fish. Alignment of the deduced protein sequences with the equivalent sequences from Torpedo californica and humans reveals a high degree of homology.

Key words: acetylcholine receptor, ligand-gated ion channel, electric fish, dimer, clustering, Torpedo macneilli, Torpedo marmorata, Hypnos monopterigium, Narcine tasmaniensis


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© The Company of Biologists Ltd 2004