STRUCTURE AND EXPRESSION OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR BETA-SUBUNIT OF XENOPUS-LAEVIS

A cDNA encoding the beta subunit of the Xenopus muscle nicotinic acety lcholine receptor (AChR) was cloned from an embryonic Xenopus cDNA lib rary. The predicted mature polypeptide has 469 amino acids and four me mbrane spanning regions corresponding to the M1-M4 regions identified in other AChR subunit clones. The polypeptide bears greater homology t o beta subunits of Torpedo and mouse than to alpha, gamma or delta sub units of Xenopus. The earliest beta subunit transcripts were detected by RNase protection assays at the neural plate stage of development (s tage 14) and the level of transcripts, as a fraction of total RNA, con tinued to increase through the age of hatching (stages 34-36). Co-inje ction of Xenopus alpha, beta, gamma and delta cRNAs into Xenopus oocyt es 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 delt a subunits alone did not yield detectable ACh responses. With the clon ing of the Xenopus beta subunit, structure/function relations of AChRs can now be studied using receptors composed entirely of Xenopus subun its.