SPONTANEOUS AND AGONIST-INDUCED OPENINGS OF AN ACETYLCHOLINE-RECEPTOR CHANNEL COMPOSED OF BOVINE MUSCLE ALPHA-SUBUNIT, BETA-SUBUNIT AND DELTA-SUBUNIT

During the development of mammalian muscle the γ-subunit of the nicotinic acetylcholine receptor (AChR) is replaced by the e{open}-subunit to produce well-defined alterations in the conductance and gating of the channel. To gain a better unterstanding of the functional role of the γ and e{open}-subunits, we have studied the properties of an AChR channel lacking these subunits. The AChR expressed in Xenopus oocytes injected with the bovine α-, β and δ-subunit-specific mRNAs (referred to as αβδ-AChR) is unusual in that its channel opens spontaneously at a high frequency in the absence of agonist. From a comparison of the αβδ-AChR with complete receptors containing either the γ or e{open}-subunit, we conclude that the γ and e{open}-subunits influence most channel properties, including agonist binding, and are especially important for stabilizing the closed state of the unliganded receptor channel. The αβδ-AChR can form when a complete set of four subunit-specific mRNAs is injected. The ease with which it is assembled raises the possibility that the αβδ- AChR contributes to some of the variations in receptor properties that occur during development. © 1990 Springer-Verlag.