A pathway leading to a cation-binding pocket determines the selectivity of the NhaP2 antiporter in Vibrio cholerae.

The Vc-NhaP2 antiporter from Vibrio cholerae exchanges H+ for K+ or Na+ but not for the smaller Li+ . The molecular basis of this unusual selectivity remains unknown. Phyre2 and Rosetta software were used to generate a structural model of the Vc-NhaP2. The obtained model suggested that a cluster of residues from different transmembrane segments (TMSs) forms a putative cation-binding pocket in the middle of the membrane: D133 and T132 from TMS V together with D162 and E157 of TMS VI. The model also suggested that L257, G258, and N259 from TMS IX together with T276, D273, Q280, and Y251 from TMS X as well as L289 and L342 from TMS XII form a transmembrane pathway for translocated ions with a built-in filter determining cation selectivity. Alanine-scanning mutagenesis of the identified residues verified the model by showing that structural modifications of the pathway resulted in altered cation selectivity and transport activity. In particular, L257A, G258A, Q280A, and Y251A variants gained Li+ /H+ antiport capacity that was absent in the nonmutated antiporter. T276A, D273A, and L289A variants exclusively exchanged K+ for H+ , while a L342A variant mediated Na+ /H+ exchange only, thus maintaining strict alkali cation selectivity.