LIGAND-GATED ion channels are multi-subunit complexes where each subunit-type is encoded by several related genes. Heterologous expression of any one of the neuronal nicotinic acetylcholine receptors (nAChR) alpha-type subunits, either alone or with any beta-type subunit, typically yields functional nAChR channels(1-5). A striking exception is the nAChR alpha 5 subunit : although apparently complexed with beta 2 and beta 4 nAChR subunits in neurons(6-8), and expressed in a subset of neurons within the central and peripheral nervous systems(9,10), heterologous expression of alpha 5, either alone or with any beta-type subunit has failed to yield functional channels(1.11). We demonstrate here that alpha 5 does participate in nAChRs expressed in heterologous systems and in primary neurons, and further that alpha 5 contributes to the lining of functionally unique nAChR channels, but only if coexpressed with both another alpha- and beta-type subunit. Furthermore, channels containing the alpha 5 subunit are potently activated and desensitized by nanomolar concentrations of nicotine.