Localization of an organic anion transporter-GFP fusion construct (rROAT1-GFP) in intact proximal tubules

The organic anion transporter, rROAT1, is a dicarboxylate/ organic anion exchanger, a function associated with the basolateral membrane in rat proximal tubule. To directly establish the subcellular localization of rROAT1 in renal epithelia, we made a rROAT1-green fluorescent protein (GFP) fusion construct (rROAT1-GFP). Plasma membrane-associated fluorescence was observed in rROAT1-GFP-expressing Xenopus oocytes examined by confocal microscopy. Uptake of (3)H-labeled p-aminohippurate (PAH) increased 2.5-fold in rROAT1-GFP-expressing Xenopus oocytes, and this increase was abolished by 1 mM probenecid. Thus the construct was capable of specific organic anion transport. Cultured renal epithelial cell lines (MDCK and LLC-PK1) transfected with the vector pEGFP-C3 showed a diffuse, evenly distributed cytoplasmic signal. However, when transfected with pEGFPC3/rROAT1 (vector coding for rROAT1-GFP), both cell lines showed predominantly plasma membrane fluorescence. The expression and distribution of rROAT1-GFP in intact renal proximal tubules was also investigated. Isolated killifish (Fundulus heteroclitus) renal tubules transfected with pEGFPC3/rROAT1 showed marked basal and lateral membrane-associated fluorescence, but no detectable signal in the nucleus or the apical pole of tubule cells. Tubules transfected with pEGFP-C3 showed diffuse cytoplasmic fluorescence. Function of the rROAT1-GFP construct was demonstrated in transfected killifish tubules by fluorescein transport assay. These results demonstrate the basolateral subcellular localization of rROAT1 in polarized renal epithelia and validate a new technique for localizing cloned transporters within intact renal tubules.