Structure-Based Engineering of Lithium-Transport Capacity in an Archaeal Sodium–Calcium Exchanger

Members of the Ca<sup>2+</sup>/cation exchanger superfamily (Ca<sup>2+</sup>/CA) share structural similarities (including highly conserved ion-coordinating residues) while exhibiting differential selectivity for Ca<sup>2+</sup>, Na<sup>+</sup>, H<sup>+</sup>, K<sup>+</sup>, and Li<sup>+</sup>. The archaeal Na<sup>+</sup>/Ca<sup>2+</sup> exchanger (NCX_Mj) and its mammalian orthologs are highly selective for Na<sup>+</sup>, whereas the mitochondrial ortholog (NCLX) can transport either Li<sup>+</sup> or Na<sup>+</sup> in exchange with Ca<sup>2+</sup>. Here, structure-based replacement of ion-coordinating residues in NCX_Mj resulted in a capacity for transporting either Na<sup>+</sup> or Li<sup>+</sup>, similar to the case for NCLX. This engineered protein may serve as a model for elucidating the mechanisms underlying ion selectivity and ion-coupled alternating access in NCX and similar proteins.