Contributions of the gamma and epsilon subunit family to nicotinic acetylcholine receptor function

Hybrid acetylcholine (ACh) receptors expressed in Xenopus oocytes were used to study the role of the gamma/epsilon subunit family in determining channel function. The receptors were composed of either a mouse gamma, Torpedo gamma, Xenopus gamma or mouse epsilon subunit in combination with mouse alpha, beta and delta subunits. The effects of different gamma and epsilon subunits on channel conductance and open time were paralleled by their structural relatedness. Of the four subunits studied, mouse gamma and mouse epsilon are the most divergent in structure and the receptors containing these subunits were also the most divergent in function. Torpedo and Xenopus gamma subunits have structural features intermediate to both mouse epsilon and gamma and correspondingly imparted intermediate functional properties. Of particular interest, ACh receptors formed in combination with a Torpedo gamma subunit had a significantly lower conductance than those containing a mouse epsilon subunit, despite the presence of identical amino acids in all charge positions previously shown to affect conductance.