[Show abstract][Hide abstract] ABSTRACT: 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-rhodamine-maleimide and showed significant changes
in the emission of the fluorescence probe on application of the agonists ATP and benzoyl-benzoyl-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.
Proceedings of the National Academy of Sciences 07/2012; 109(28):11396-11401. DOI:10.1073/pnas.1118759109 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.