ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

Voltage-gated channels open paths for ion permeation upon changes in membrane potential, but how voltage changes are coupled to gating is not entirely understood. Two modules can be recognized in voltage-gated potassium channels, one responsible for voltage sensing (transmembrane segments S1 to S4), the other for permeation (S5 and S6). It is generally assumed that the conversion of a conformational change in the voltage sensor into channel gating occurs through the intracellular S4–S5 linker that provides physical continuity between the two regions. Using the pathophysiologically relevant KCNH family, we show that truncated proteins interrupted at, or lacking the S4–S5 linker produce voltage-gated channels in a heterologous model that recapitulate both the voltage-sensing and permeation properties of the complete protein. These observations indicate that voltage sensing by the S4 segment is transduced to the channel gate in the absence of physical continuity between the modules.

P2X receptors (P2XRs) are ligand-gated ion channels activated by extracellular ATP. Although the crystal structure of the zebrafish P2X4R has been solved, the exact mode of ATP binding and the conformational changes governing channel opening and desensitization remain unknown. Here, we used voltage clamp fluorometry to investigate movements in the cysteine-rich head domain of the rat P2X1R (A118-I125) that projects over the proposed ATP binding site. On substitution with cysteine residues, six of these residues (N120-I125) were specifically labeled by tetramethyl-rhodaminemaleimide and showed significant changes in the emission of the fluorescence probe on application of the agonists ATP and benzoylbenzoyl-ATP. Mutants N120C and G123C showed fast fluorescence decreases with similar kinetics as the current increases. In contrast, mutants P121C and I125C showed slow fluorescence increases that seemed to correlate with the current decline during desensitization. Mutant E122C showed a slow fluorescence increase and fast decrease with ATP and benzoyl-benzoyl-ATP, respectively. Application of the competitive antagonist 2′,3′-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) resulted in large fluorescence changes with the N120C, E122C, and G123C mutants and minor or no changes with the other mutants. Likewise, TNP-ATP-induced changes in control mutants distant from the proposed ATP binding site were comparably small or absent. Combined with molecular modeling studies, our data confirm the proposed ATP binding site and provide evidence that ATP orients in its binding site with the ribose moiety facing the solution. We also conclude that P2XR activation and desensitization involve movements of the cysteine-rich head domain. P 2X receptors (P2XRs) represent a family of nonselective cation channels gated by extracellular ATP. They are widely distributed in mammalian tissues and have been shown to be involved in diverse physiological functions (1). The seven known subunits all contain two transmembrane domains linked by a large extracellular loop. Functional receptors are homo-or heteromeric trimers (2, 3).Based on mutagenesis studies, it has been suggested that conserved positively charged and aromatic residues are crucial for ATP binding, presumably by interacting with the negatively charged phosphate chain of ATP (4-6) and its adenine ring (6), respectively. We have previously shown that replacement of two of these residues, K68 and F291, by cysteine residues allows disulfide cross-linking between neighboring P2X1 subunits and that this reaction is prevented in the presence of ATP. Based on these data, we concluded that the ATP binding sites are located at the subunit interfaces (7,8). This conclusion is in good agreement with the positions of the relevant amino acids in the crystal structure of the unliganded P2X4R from zebrafish (2). This zP2X4 structure revealed an ion channel architecture that resembles a dolphin, with the transmembrane helices and the extracellular region forming the fluke and the upper body, respectively. Att...

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