The Stoichiometry of P2X2/6 Receptor Heteromers Depends on Relative Subunit Expression Levels

Fast synaptic transmission involves the operation of ionotropic receptors, which are often composed of at least two types of subunit. We have developed a method, based on atomic force microscopy imaging to determine the stoichiometry and subunit arrangement within ionotropic receptors. We showed recently that the P2X2 receptor for ATP is expressed as a trimer but that the P2X6 subunit is unable to oligomerize. In this study we addressed the subunit stoichiometry of heteromers containing both P2X2 and P2X6 subunits. We transfected tsA 201 cells with both P2X2 and P2X6 subunits, bearing different epitope tags. We manipulated the transfection conditions so that either P2X2 or P2X6 was the predominant subunit expressed. By atomic force microscopy imaging of isolated receptors decorated with antiepitope antibodies, we demonstrate that when expression of the P2X2 subunit predominates, the receptors contain primarily 2 × P2X2 subunits and 1 × P2X6 subunit. In contrast, when the P2X6 subunit predominates, the subunit stoichiometry of the receptors is reversed. Our results show that the composition of P2X receptor heteromers is plastic and dependent on the relative subunit expression levels. We suggest that this property of receptor assembly might introduce an additional layer of subtlety into P2X receptor signaling.