title: Molecular basis of selective antagonism of the P2X1 receptor for ATP by NF449 and suramin: contribution of basic amino acids in the cysteine-rich loop.
abstract: OBJECTIVE
  The cysteine-rich head region, which is adjacent to the proposed ATP-binding pocket in the extracellular ligand-binding loop of P2X receptors for ATP, is absent in the antagonist-insensitive Dictyostelium receptors. In this study we have determined the contribution of the head region to the antagonist action of NF449 and suramin at the human P2X1 receptor.
  
  
  METHODS
  Chimeras and point mutations in the cysteine-rich head region were made between human P2X1 and P2X2 receptors. Mutant receptors were expressed in Xenopus oocytes and P2X receptor currents characterized using two-electrode voltage clamp.
  
  
  RESULTS
  The chimera replacing the region between the third and fourth conserved cysteine residues of the P2X1 receptor with the corresponding part of P2X2 reduced NF449 sensitivity a thousand fold from an IC(50) of ∼1 nM at the P2X1 receptor to that of the P2X2 receptor (IC(50) ∼1 µM). A similar decrease in sensitivity resulted from mutation of four positively charged P2X1 receptor residues in this region that are absent from the P2X2 receptor. These chimeras and mutations were also involved in determining sensitivity to the antagonist suramin. Reciprocal chimeras and mutations in the P2X2 receptor produced modest increases in antagonist sensitivity.
  
  
  CONCLUSIONS
  These data indicate that a cluster of positively charged residues at the base of the cysteine-rich head region can account for the highly selective antagonism of the P2X1 receptor by the suramin derivative NF449.
authors.affiliation: Department of Cell Physiology and Pharmacology, University of Leicester, Leicester, UK.
authors.forename: S
authors.initials: S
authors.lastname: El-Ajouz
authors.forename: D
authors.initials: D
authors.lastname: Ray
authors.forename: R C
authors.initials: RC
authors.lastname: Allsopp
authors.forename: R J
authors.initials: RJ
authors.lastname: Evans
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000595:Amino Acid Sequence; D024361:Amino Acids, Basic / Q000737:chemistry; D000818:Animals; D001557:Benzenesulfonates / Q000494:pharmacology*; D003545:Cysteine / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000187:drug effects / Q000502:physiology; D058920:Purinergic P2X Receptor Antagonists / Q000494:pharmacology*; D058474:Receptors, Purinergic P2X1 / Q000737:chemistry* / Q000235:genetics; D058476:Receptors, Purinergic P2X2 / Q000737:chemistry / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013498:Suramin / Q000494:pharmacology*; D014982:Xenopus laevis
chemical_list: C110258:4,4,',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis(benzene-1,3-disulfonate); D024361:Amino Acids, Basic; D001557:Benzenesulfonates; D058920:Purinergic P2X Receptor Antagonists; D058474:Receptors, Purinergic P2X1; D058476:Receptors, Purinergic P2X2; D013498:Suramin; D000255:Adenosine Triphosphate; D003545:Cysteine