A REPORTER MUTATION APPROACH SHOWS INCORPORATION OF THE ORPHAN SUBUNIT BETA-3 INTO A FUNCTIONAL NICOTINIC RECEPTOR

We have investigated whether the neuronal nicotinic subunit beta 3 can participate in the assembly of functional recombinant receptors, Alth ough beta 3 is expressed in several areas of the central nervous syste m, it does not form functional receptors when expressed heterologously together with an alpha or another beta nicotinic subunit, We inserted into the human beta 3 subunit a reporter mutation (V273T), which, if incorporated into a functional receptor, would be expected to increase its agonist sensitivity and maximum response to partial agonists, Exp ressing the mutant beta 3(V273T) in Xenopus oocytes together with both the alpha 3 and the beta 4 subunits resulted in the predicted changes in the properties of the resulting nicotinic receptor when compared w ith those of alpha 3 beta 4 receptors. This indicated that some of the receptors incorporated the mutant beta 3 subunit, as part of a ''trip let'' alpha 3 beta 4 beta 3 receptor. The proportion of triplet recept ors was dependent on the ratios of the alpha 3:beta 4:beta 3 cRNA inje cted. We conclude that, like the related alpha 5 subunit, the beta 3 s ubunit can form functional receptors only if expressed together with b oth alpha and beta subunits.