The role of glycine residues in the function of human organic anion transporter 4

Human organic anion transporter 4 (hOAT4) belongs to a superfamily of organic ion transporters that play critical roles in the body disposition of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. In this study, we investigated the role of conserved glycine residues in hOAT4 function. We mutagenized each of the six glycine residues (at positions 11, 241, 383, 388, 400, and 466) to serine, and their functional properties were analyzed in COS-7 cells by measuring the uptake of [³H]estrone sulfate. Our results showed that mutants G11S, G383S, G388S, and G466S exhibited transport activities comparable with those of wild-type hOAT4. In contrast, mutants G241S and G400S almost completely lost transport function. We then further characterized Gly-241 and Gly-400 by mutagenizing these residues to amino acids with varying sizes of side chains, including alanine, valine, and leucine. We demonstrated that increasingly larger side chains at positions 241 and 400 increasingly impaired hOAT4 function. Cell-surface biotinylation using an impermeant biotinylating reagent showed that mutations of Gly-241 and Gly-400 interfered with the trafficking of the transporter onto cell surface. Immunofluorescence analysis of mutant-transfected cells confirmed these results. Substitutions of amino acids with large side chains at positions 241 and 400 resulted in decreased V _max and increased K_m. These results suggest that Gly-241 and Gly-400 are important both in targeting the transporter to the plasma membrane and in substrate binding. This is the first identification and characterization of critical amino acid residues in hOAT4 and may provide important insights into the structure-function relationships of the organic ion transporter family.