Evidence that the transport-related proteins BAT and 4F2hc are not specific for amino acids: induction of Na+-dependent uridine and pyruvate transport activity by recombinant BAT and 4F2hc expressed in Xenopus oocytes

Abstract

Members of the BAT and 4F2hc gene family have one or, in the case of BAT, up to four transmembane domains and induce amino acid transport systems bo,+ (BAT) and y+L (4F2hc) when expressed in Xenopus oocytes. System bo,+ is a Na+-independent process with a broad tolerance for cationic and zwitterionic amino acids, whereas y+L exhibits Na+-independent transport of cationic amino acids (e.g., lysine) and Na+-dependent transport of zwitterionic amino acids (e.g., leucine). Mutations in the human BAT gene are associated with type I cystinuria, a genetic disease affecting the ability of intestinal and renal brush border membranes to transport cationic amino acids and cystine. An unresolved question is whether BAT and 4F2hc themselves have catalytic (i.e., transporting) activity or whether they operate as activators of other, as yet unidentified, transporter proteins. In this report, we have investigated the transport of representatives of four different classes of organic substrates in Xenopus oocytes following injection with rat BAT or 4F2hc RNA transcripts: leucine (a control amino acid substrate), uridine (a nucleoside), pyruvate (a monocarboxylate), and choline (an amine). Both recombinant proteins induced small, statistically significant Na+-dependent fluxes of uridine and pyruvate but had no effect on choline uptake. In contrast, control oocytes injected with transcripts for conventional nucleoside and cationic amino acid transporters (rat CNT1 and murine CAT1, respectively) showed no induction of transport of either leucine or pyruvate (CNT1) or uridine or pyruvate (CAT1). These findings support the idea that BAT and 4F2hc are transport activators and minimize the possibility that they have intrinsic transport capability. The transport-regulating functions of these proteins may extend to permeants other than amino acids.Key words: amino acid transport, uridine, pyruvate, BAT, 4F2hc, Xenopus oocytes.