Electrogenic amino acid exchange via the rBAT transporter

Abstract

A cDNA clone was isolated from rabbit renal cortex using DNA-mediated expression cloning, which caused alanine-dependent outward currents when expressed in Xenopus oocytes. The cDNA encodes rBAT, a Na-independent amino acid transporter previously cloned elsewhere. Exposure of cDNA-injected oocytes to neutral amino acids led to voltage-dependent outward currents, but inward currents were seen upon exposure to basic amino acids. Assuming one charge/alanine, the outward current represented 38% of the rate of uptake of radiolabelled alanine, and was significantly reduced by prolonged preincubation of oocytes in 5 mM alanine. The currents were shown to be due to countertransport of basic amino acids for external amino acids using the cut-open oocyte system. This transport represents a major mode of action of this protein, and may help in defining a physiological role for rBAT in the apical membrane of renal and intestinal cells.

The rBAT Gene Is Responsible for L-Cystine Uptake via the bo,+-like Amino Acid Transport System in a “Renal Proximal Tubular” Cell Line (OK Cells)

“…3 and 6B). The b 0,ϩ -like transport activity expressed by rBAT cRNA in oocytes shows hetero-exchange between L-arginine and L-leucine (3)(4)(5). This has also been suggested for this transport activity in OK cells in the present study (Fig.…”

“…The exchange of amino acids via system b 0,ϩ -like in oocytes expressing rBAT (3)(4)(5), and also suggested for this transport activity in OK cells by the results presented here, offers an additional mechanism of accumulation for cystine and dibasic amino acids in the brush border membranes of the epithelial cells of the S3 segment of the nephron; exchange with intracellular neutral amino acids could drive the active reabsorption of cystine and dibasic amino acids. Indeed, rBAT-specific accumulation of cystine occurs in oocytes where reduction to cysteine is prevented by diamide.…”

“…Heteroexchange diffusion of these amino acids has been reported for this transport activity expressed by rabbit (3,4), rat (5), and human 1 rBAT cRNA in oocytes. rBAT protein is expressed in the brush border plasma membrane of both the proximal straight tubules of the nephron and the small intestine (6,7).…”

“…Recently, it has been reported that system b 0,ϩ -like expressed by rBAT in oocytes shows trans-stimulation, suggesting that this transport system is an amino acid exchanger (3)(4)(5). To provide further evidence for the presence of system b 0,ϩ -like transport activity in OK cells, we searched for transstimulation of L-arginine efflux by different amino acids.…”

Progressive C-terminal Deletions of the Renal Cystine Transporter, NBAT, Reveal a Novel Bimodal Pattern of Functional Expression

“…A more detailed investigation of the transport induced in oocytes by NBAT has shown that this transport is in fact an exchange transport in which uptake of a cationic amino acid is enhanced by the outward flux of neutral amino acids and uptake of a neutral amino acid is stimulated by counter-transport of a cationic substrate (41,42). It was, therefore, essential to determine whether the bimodal pattern of expression also applies to the exchange transport activity.…”