PT - JOURNAL ARTICLE AU - Selvan Bavan AU - Louise Farmer AU - Shire K Singh AU - Volko A Straub AU - Felix D Guerrero AU - Steven J Ennion TI - The penultimate arginine of the carboxy terminus determines slow desensitization in a P2X receptor from the cattle tick <em>Boophilus microplus</em> AID - 10.1124/mol.110.070037 DP - 2011 Jan 01 TA - Molecular Pharmacology PG - mol.110.070037 4099 - http://molpharm.aspetjournals.org/content/early/2011/01/06/mol.110.070037.short 4100 - http://molpharm.aspetjournals.org/content/early/2011/01/06/mol.110.070037.full AB - P2X ion channels have been functionally characterized from a range of eukaryotes. Whilst 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 channel from the cattle tick Boophilus microplus (BmP2X) displaying extremely slow current kinetics, little desensitization during ATP application and marked run-down in current amplitude between sequential responses. ATP (EC50 67.1 μM) evoked concentration dependent currents at BmP2X which 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 HdP2X receptor from Hypsibius dujardini. Exchange of N- or C-termini between these fast and slow desensitizing receptors altered the rate of current desensitization towards that of the donor channel. Truncation of the BmP2X C-terminus identified the penultimate residue (R413) 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 run-down 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 run-down and desensitization are governed by distinct mechanisms.