XENOPUS MUSCLE ACETYLCHOLINE-RECEPTOR ALPHA-SUBUNITS BIND LIGANDS WITH DIFFERENT AFFINITIES

The acetylcholine receptor (AChR) from vertebrate skeletal muscle is a pentameric complex composed of two identical alpha (the ligand-bindin g subunit) and one each of beta, gamma, and delta subunits. All muscle -like alpha subunits appear to be encoded by a single gene except for Xenopus where two genes have been identified (Xalpha1a and Xalpha1b). By establishing stable cell lines expressing each Xenopus alpha subuni t plus Torpedo betagammadelta (XaT and XbT), we were able to investiga te immunological and ligand-binding properties of these two Xenopus su bunits. We determined that each subunit was capable of proper assembly with betagammadelta subunits, each hybrid AChR cell line responded to bath-applied acetylcholine as determined by Na-22+ influx, but the tw o hybrid AChRs had distinct ligand-binding properties. The most striki ng difference between the two hybrids was observed with alpha-bungarot oxin (BuTx) binding. The K(D) for BuTx binding to XbT hybrids was virt ually identical to that of Torpedo and most other muscle AChRs, approx imately 10(-10)-10(-11) M. The K(D) for BuTx binding to XaT hybrids, h owever, was approximately 1000-fold weaker (6 x 10(-8) M). This weak b inding was due primarily to a very rapid BuTx off-rate (4.4 x 10(-2) s -1 for XaT AChRs compared with approximately 1.6 x 10(-5) s-1 for all- Torpedo and XbT AChRs).