Originally cloned from rat brain, the P2X₇ receptor is homologous to other P2X receptors but possesses a unique C-terminal tail that is required for the lytic actions of ATP that result in membrane disruption and both apoptotic and necrotic cell death (Surprenant et al. 1996). During kidney development, P2X₇ is expressed in differentiating collecting duct epithelia, and in the cpk/cpk mouse model of autosomal recessive polycystic kidney disease, collecting ducts express P2X₇ during fulminant cystogenesis (Hillman et al. 2002). Presently, it is not clear where in collecting duct cells P2X₇ is expressed, or by what mechanism(s) polarized membrane targeting occurs. Recent work suggests that the C-terminus is an important determinant of membrane trafficking of P2X receptors in neuronal cells (Chaumont et al. 2004). In this study we have tested whether the carboxy terminal domain is also important in polarized targeting of P2X₇ in renal epithelial cells. MDCK monolayers grown on permeable filter inserts were transiently transfected with cDNA encoding wild type (WT) or mutant P2X₇ receptors in which the C-terminus (amino acids 362-595) was truncated either at residues 418 (G418) or at 570 (P570). All constructs were N terminally-tagged with the Glu-Glu epitope. At 72 h post transfection, the monolayers were fixed and the subcellular localization of each construct was determined by indirect immunofluorescence and confocal microscopy, using a commercially available antibody against the Glu-Glu epitope and an anti-rabbit Alexafluor⁴⁸⁸ secondary antibody. The localization of each construct was compared with that of endogenous domain-specific markers for (a) the basolateral membrane (β-catenin), detected by indirect immunofluorescence using a commercially available primary antibody and a Cy5-conjugated secondary antibody and (b) the apical membrane detected with rhodamine-conjugated peanut agglutinin (PNA). Transfections were replicated between 3 and 5 times for each construct. All appropriate antibody controls were negative. WT P2X₇ colocalized with β-catenin in the basolateral membrane and no fluorescence could be detected in the apical membrane. In contrast, both the G418 and P570 P2X₇ constructs colocalized both with β-catenin in the basolateral membrane, but also with PNA in the apical membrane. From these experiments, we conclude that rat P2X₇ is targeted to the basolateral membrane of renal tubular cells and that the C-terminus is important in determining appropriate membrane targeting. From the targeting pattern of the G418 and P570 mutants we propose that amino acids 570 – 595 may contain a motif (yet to be identified) that is necessary for discrete basolateral sorting of P2X₇.