Characterization of membrane transport processes: Lessons from the study of BSP, bilirubin, and fatty acid uptake

Recent efforts to identify plasma membrane transporters mediating the selective uptake of specific classes of organic anions have employed expression cloning techniques. The transporters identified by these procedures have, almost without exception, differed from putative transporters identified earlier by classical methods. In this review, the classical and molecular approaches to the identification of membrane transporters are examined and compared, with particular attention paid to the results obtained by each with respect to sulfobromophthalein, bilirubin, and fatty acid transport. The classical approach requires the initial identification and purification of a candidate transport protein, proof of its function as a transporter by antibody inhibition or liposome reconstitution studies, and, following the cloning of its cDNA, genetic expression in transfected mammalian cells or in Xenopus laevis oocytes. Expression cloning affords more rapid identification of proteins which (by definition) influence uptake, avoids several potential artifacts (e.g., of conventional cDNA library screening techniques), and yields rapid access to sequence information and derived structural characteristics. However, it is ultimately necessary to express and purify the recombinant protein product, produce antibodies against synthetic peptides and/or the purified recombinant protein, use the antibodies to identify the subcellular location of the cloned protein, demonstrate that the protein binds the ligand of interest, and document that the protein mediates a facilitated process with the characteristics of the one under study. Hence, the two approaches ultimately require similar efforts. It is argued that the different putative BSP/bilirubin and fatty acid transporters identified by the two approaches may mediate different parallel transport pathways.