Domains of P2X receptors involved in desensitization

ATP-gated ion channels (P2X receptors) are abundantly expressed in both neuronal and nonneuronal tissues, where they can serve as postsynaptic receptors. The response to ATP shows marked desensitization in some tissues but not others. Currents induced by ATP in Xenopus oocytes expressing cloned P2X 1 (or P2X 3 ) receptors had strong desensitization, whereas currents in cells expressing P2X 2 receptors desensitized relatively little (90% vs. 14% decline of current in a 10-s application). In chimeric receptors, substitution into the P2X 1 receptor of either one of two 34-residue segments from the P2X 2 receptor removed the desensitization; these segments included the first or the second hydrophobic domain. In contrast, desensitization was introduced into the P2X 2 receptor only by providing both these segments of the P2X 1 (or P2X 3 ) receptor. This suggests that desensitization requires interaction between the two hydrophobic domains of the receptor, and supports the view that these are membrane-spanning segments.