Analysis of conserved NCS2 motifs in the Escherichia coli xanthine permease XanQ.

The xanthine permease XanQ of Escherichia coli is a paradigm for transporters of the evolutionarily broad family nucleobase-cation symporter-2 (NCS2) that transport key metabolites or anti-metabolite analogs. Most functionally known members are xanthine/uric acid transporters related to XanQ and belong to a distinct phylogenetic cluster of the family. Here, we present a comprehensive mutagenesis of XanQ based on the identification and Cys-scanning analysis of conserved sequence motifs in this cluster. Results are interpreted in relation to homology modeling on the structurally known template of UraA and previous data on critical binding-site residues in transmembrane segments (TMs) 3, 8 and 10. The current analysis, of motifs distant to the binding site, revealed a set of functionally important residues in TMs 2, 5, 12 and 13, including seven irreplaceable ones, of which six are Gly residues in the gate domain (159, 369, 370, 383, 409) and in TM2 (Gly-71), and one is polar (Gln-75). Gln-75 (TM2) is probably crucial in a network of hydrogen-bonding interactions in the middle of the core domain involving another essential residue, Asp-304 (TM9). Although the two residues are irreplaceable individually, combinatorial replacement of Gln-75 with Asn and of Asp-304 with Glu rescues significant transport activity.