Structure and expression of the nicotinic acetylcholine receptor beta subunit of Xenopus laevis
A cDNA encoding the beta subunit of the Xenopus muscle nicotinic acetylcholine receptor (AChR) was cloned from an embryonic Xenopus cDNA library. The predicted mature polypeptide has 469 amino acids and four membrane spanning regions corresponding to the M1-M4 regions identified in other AChR subunit clones. The polypeptide bears greater homology to beta subunits of Torpedo and mouse than to alpha, gamma or delta subunits of Xenopus. The earliest beta subunit transcripts were detected by RNase protection assays at the neural plate stage of development (stage 14) and the level of transcripts, as a fraction of total RNA, continued to increase through the age of hatching (stages 34-36). Co-injection of Xenopus alpha, beta, gamma and delta cRNAs into Xenopus oocytes led to expression of functional AChRs. Micromolar concentrations of ACh activated depolarizing AChR currents which reversed at -5 mV and were blocked by alpha bungarotoxin. Injection of alpha, gamma and delta subunits alone did not yield detectable ACh responses. With the cloning of the Xenopus beta subunit, structure/function relations of AChRs can now be studied using receptors composed entirely of Xenopus subunits.
© 1994 Harwood Academic. (Received 12 July 1993; in final form 10 October 1993) The authors wish to thank Paul Brehm and Nancy Murray for their valuable contributions throughout the course of this study, and Irene Gallion and Reese Bolinger for expert technical assistance. WLT would like to acknowledge support and encouragement from Rob Steele. This work was supported by a McKnight Foundation Scholar's Award to SEF, by postdoctoral fellowships from MDA and NIH to WLT, by the University of Alaska Biomedical Program, and by NIH grants NS24078, NS18205, NS22518.