A reporter mutation approach shows incorporation of the "orphan" subunit β3 into a functional nicotinic receptor

We have investigated whether the neuronal nicotinic subunit beta 3 can participate in the assembly of functional recombinant receptors, Although beta 3 is expressed in several areas of the central nervous system, 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, Expressing 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 with those of alpha 3 beta 4 receptors. This indicated that some of the receptors incorporated the mutant beta 3 subunit, as part of a "triplet" alpha 3 beta 4 beta 3 receptor. The proportion of triplet receptors was dependent on the ratios of the alpha 3:beta 4:beta 3 cRNA injected. We conclude that, like the related alpha 5 subunit, the beta 3 subunit can form functional receptors only if expressed together with both alpha and beta subunits.