RT Journal Article SR Electronic T1 The Penultimate Arginine of the Carboxyl Terminus Determines Slow Desensitization in a P2X Receptor from the Cattle Tick Boophilus microplus JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 776 OP 785 DO 10.1124/mol.110.070037 VO 79 IS 4 A1 Selvan Bavan A1 Louise Farmer A1 Shire K. Singh A1 Volko A. Straub A1 Felix D. Guerrero A1 Steven J. Ennion YR 2011 UL http://molpharm.aspetjournals.org/content/79/4/776.abstract AB P2X ion channels have been functionally characterized from a range of eukaryotes. Although these receptors can be broadly classified into fast and slow desensitizing, the molecular mechanisms underlying current desensitization are not fully understood. Here, we describe the characterization of a P2X receptor from the cattle tick Boophilus microplus (BmP2X) displaying extremely slow current kinetics, little desensitization during ATP application, and marked rundown in current amplitude between sequential responses. ATP (EC50, 67.1 μM) evoked concentration-dependent currents at BmP2X that were antagonized by suramin (IC50, 4.8 μM) and potentiated by the antiparasitic drug amitraz. Ivermectin did not potentiate BmP2X currents, but the mutation M362L conferred ivermectin sensitivity. To investigate the mechanisms underlying slow desensitization we generated intracellular domain chimeras between BmP2X and the rapidly desensitizing P2X receptor from Hypsibius dujardini. Exchange of N or C termini between these fast- and slow-desensitizing receptors altered the rate of current desensitization toward that of the donor channel. Truncation of the BmP2X C terminus identified the penultimate residue (Arg413) as important for slow desensitization. Removal of positive charge at this position in the mutant R413A resulted in significantly faster desensitization, which was further accentuated by the negatively charged substitution R413D. R413A and R413D, however, still displayed current rundown to sequential ATP application. Mutation to a positive charge (R413K) reconstituted the wild-type phenotype. This study identifies a new determinant of P2X desensitization where positive charge at the end of the C terminal regulates current flow and further demonstrates that rundown and desensitization are governed by distinct mechanisms.