ROLE OF THE PUTATIVE TRANSMEMBRANE SEGMENT M3 IN GATING OF NEURONAL NICOTINIC RECEPTORS

The involvement of some structural domains in the gating of the neuron al nicotinic acetylcholine receptor (AChR) was studied by expressing f unctional alpha 7/alpha 3 chimeric subunits in Xenopus oocytes. Substi tution of the M3 transmembrane segment in the alpha 7 subunit modifies the kinetic properties of the chimeric AChRs as follows: (a) a 6-fold reduction in the maximal current evoked by nicotinic agonists, (b) a 10-fold decrease in the macroscopic desensitization rate, (c) an incre ase of almost 1 order of magnitude in the apparent affinity for acetyl choline and nicotine, and (d) a decrease in the affinity for alpha-bun garotoxin. Computer simulations showed that the first three effects co uld be accounted for by a simple kinetic model in which chimeric AChRs presented a smaller ratio of the gating rates, beta/alpha, and a slig htly slower desensitization rate. It is concluded that the M3 domain i nfluences the gating of neuronal AChRs.