A. Broer et al., Neutral amino acid transporter ASCT2 displays substrate-induced Na+ exchange and a substrate-gated anion conductance, BIOCHEM J, 346, 2000, pp. 705-710
The neutral amino acid transporter ASCT2 mediates electroneutral obligatory
antiport but at the same time requires Na+ for its function. To elucidate
the mechanism, ASCT2 was expressed in Xenopus laevis oocytes and transport
was analysed by flux studies and two-electrode voltage clamp recordings. Fl
ux studies with (NaCl)-Na-22 indicated that the uptake of one molecule of g
lutamine or alanine is accompanied by the uptake of four to seven Na+ ions.
Similarly to the transport of amino acids, the Na+ uptake was mediated by
an obligatory Na+ exchange mechanism that depended on the presence of amino
acids but was not stoichiometrically coupled to the amino acid transport.
Other cations could not replace Na+ in this transport mechanism. When NaCl
was replaced by NaSCN in the transport buffer, the superfusion of oocytes w
ith amino acid substrates resulted in large inward currents, indicating the
presence of a substrate-gated anion channel in the ASCT2 transporter. The
K-m for glutamine derived from these experiments is in good agreement with
the K-m, derived from flux studies; it varied between 40 and 90 mu M at hol
ding potentials of - 60 and - 20 mV respectively. The permeability of the s
ubstrate-gated anion conductance decreased in the order SCN- much greater t
han NO3- > I- > Cl- and also required the presence of Na+.