Amino acid residues involved in the substrate specificity of TauT/SLC6A6 for taurine and γ-aminobutyric acid.

Taurine transporter (TauT/SLC6A6) is an "honorary" γ-aminobutyric acid (GABA) transporter because of its low affinity for GABA. The sequence analysis of TauT implied the role of Gly57, Phe58, Leu306 and Glu406 in the substrate recognition of TauT, and amino acid-substitutions were performed. Immunocytochemistry supported no marked effect of mutations on the expression of TauT. TauT-expressing oocytes showed a reduction in [(3)H]taurine uptake by G57E, F58I, L306Q and E406C, and change in [(3)H]GABA uptake by G57E and E406C, suggesting their significant roles in the function of TauT. G57E lost the activity of [(3)H]taurine and [(3)H]GABA uptake, suggesting that Gly57 is involved in the determination of substrate pocket volume and in the interaction with substrates. E406C exhibited a decrease and an increase in the affinity for taurine and GABA, respectively, suggesting the involvement of Glu406 in the substrate specificity of TauT. The inhibition study supported the role of Glu406 in the substrate specificity since [(3)H]taurine and [(3)H]GABA uptake by E406C was less sensitive to taurine and β-alanine, and more sensitive to GABA and nipecotic acid than was the case with wild type of TauT. F58I had an increased affinity for GABA, suggesting the involvement of Phe58 in the substrate accessibility. The kinetic parameters showed the decreased and increased affinities of L306Q for taurine and GABA, respectively, supporting that substrate recognition of TauT is conformationally regulated by the branched-side chain of Leu306. In conclusion, the present results suggest that these residues play important roles in the transport function and substrate specificity of TauT.