Roles of accessory subunits in alpha4beta2(*) nicotinic receptors.

Accessory subunits in heteromeric nicotinic receptors (AChRs) do not take part in forming ACh binding sites. alpha5 and beta3 subunits can function only as accessory subunits. We show that both alpha5 and beta3 efficiently assemble in human alpha4beta2(*) AChRs expressed in permanently transfected human embryonic kidney (HEK) cell lines. Only (alpha4beta2)(2)alpha5, not (alpha4beta2)(2)beta3 AChRs, have been detected in brain. The alpha4beta2alpha5 line expressed 40% more AChRs than the parent alpha4beta2 line and was equally sensitive to up-regulation by nicotine. The alpha4beta2beta3 line expressed 25-fold more AChRs than the parental line and could not be further up-regulated by nicotine. Relative sensitivity to activation by ACh depends on the accessory subunit, beta2 conferring the greatest sensitivity, alpha5 less, and beta3 and alpha4 much less. Accessory subunits form binding sites for positive allosteric modulators, as illustrated by the observation that alpha5 conferred high sensitivity to galanthamine. In the presence of alpha5 or beta3, stable, partially degraded, dead end intermediates accumulated within the cells. These may have the form alpha5alpha4beta2alpha5. The efficiency with which alpha5 and beta3 assemble with alpha4 and beta2 and the necessity of avoiding formation of potentially toxic intermediates may explain why alpha5 and beta3 seem to be transcribed at low levels in brain. Autosomal dominant nocturnal frontal lobe epilepsy can be caused by the alpha4 mutation S247F. This mutant did not produce functional AChRs unless cells were cotransfected with alpha5, beta3, or alpha6 to replace alpha4 as accessory subunit.