Alpha 5, alpha 3, and non-alpha 3. Three clustered avian genes encoding neuronal nicotinic acetylcholine receptor-related subunits.

Abstract

In vertebrates, neuronal nicotinic acetylcholine receptors (nAChRs) assemble in an unknown stoichiometry from two homologous subunits, an alpha and a non-alpha. How large is the repertoire of these subunits and how many subtypes of functionally different nAChRs can they constitute? We found in the avian genome a cluster of three closely linked genes spanning 28 kilobase pairs and encoding three proteins, n alpha 3, alpha 3, and alpha 5, that have the features expected of neuronal nAChR subunits. Gene n alpha 3 lies 5' of alpha 3 (whose role in cholinoception has already been established) and is transcribed from the same DNA strand, whereas alpha 5 lies 3' of alpha 3 and is transcribed from the opposite DNA strand. The structure of the n alpha 3 and alpha 5 genes consists of six exons with precisely conserved splice sites and is identical to the structure of the previously characterized avian neuronal receptor subunit genes alpha 2, alpha 3, alpha 4, and n alpha 1. alpha 3, n alpha 3, and alpha 5 transcripts are rare in the central nervous system, but alpha 3 and n alpha 3 are readily detectable in embryonic superior cervical and ciliary ganglia. In order to assay function, the gene encoding n alpha 3 and the cDNAs encoding alpha 3, alpha 4, alpha 5, and n alpha 1 were subcloned into an expression vector, and the constructs were injected into Xenopus oocyte nuclei, either singly or in pairwise combinations of one alpha and one non-alpha. One to five days later, ACh sensitivity of the injected oocytes was examined in voltage clamp. The n alpha 3 gene and n alpha 1 cDNA elicited assembly of nAChRs when coinjected with alpha 3 or alpha 4 cDNA and the electrophysiological properties of the four pairwise combinations were significantly different. alpha 5, however, did not direct the assembly of functional nAChRs when injected alone or in combination with n alpha 1 or n alpha 3.