Role of aromatic transmembrane residues of the organic anion transporter, rOAT3, in substrate recognition

Organic anion transporters (OATs, SLC21) are important in the excretion of endogenous and exogenous compounds in the kidney. The rat organic anion transporter, rOAT3, mediates the transport of organic anions such as p-aminohippurate (PAH) and estrone sulfate as well as the basic compound, cimetidine. In the present study, we examined the role of conserved transmembrane aromatic amino acid residues of rOAT3 in substrate recognition and transport. Alanine scanning followed by amino acid replacements was used to construct mutants of rOAT3. The uptake of model compounds was studied in Xenopus laevis oocytes expressing the mutant transporters. We observed that four mutants in transmembrane domain 7 (TMD 7), W334A, F335A, Y341A, and Y342Q, and one mutant in transmembrane domain 8 (TMD 8), F362S, exhibited a less than 2-fold enhanced uptake of PAH and cimetidine in comparison to wild-type rOAT3, which exhibited a 16-fold enhanced uptake of PAH and an 8-fold enhanced uptake of cimetidine. Estrone sulfate uptake in oocytes expressing any one of these five mutants remained at least 8-fold enhanced. The data suggest that the five residues, W334, F335, Y341, Y342, and F362, contribute differently to the transport of the small hydrophilic organic substrates PAH and cimetidine in comparison to the large hydrophobic organic substrate estrone sulfate. The effects of side chains of these five residues on transporter functions were also evaluated by constructing conservative mutations. We observed that the residues contribute to PAH and cimetidine transport in different ways: the -OH group of Y342, the indole ring of W334, and the aromatic rings of F335, Y341, and F362 are important for PAH and cimetidine transport by rOAT3. These data suggest that there is an aromatic pocket composed mainly of residues in TMD 7 in the translocation pathway of rOAT3, which is important for the transport of PAH and cimetidine. Aromatic residues in this pocket may interact directly with substrates of rOAT3 through hydrogen bonds and pi-pi interactions.