Structure and lipid-mediated remodelling mechanism of the Na+/H+ exchanger NHA2

Abstract

Na+/H+ exchangers catalyse an ion-exchange activity that is carried out in most, if not all cells. SLC9B2, also known as NHA2, correlates with the long-sought after sodium/lithium (Na+/Li+) exchanger linked to the pathogenesis of diabetes mellitus and essential hypertension in humans. Despite its functional importance, structural information and the molecular basis of its ion-exchange mechanism have been lacking. Here, we report the cryo EM structures of bison NHA2 in detergent and in nanodiscs at 3.0 and 3.5 Å resolution, respectively. NHA2 shares closest structural similarity to the bacterial electrogenic Na+/H+ antiporter NapA, rather than other mammalian SLC9A members. Nevertheless, SSM-based electrophysiology results with NHA2 show the catalysis of electroneutral rather than electrogenic ion exchange, and the ion-binding site is quite distinctive, with a tryptophan-arginine- glutamate triad separated from the well-established ion-binding aspartates. These triad residues fine-tune ion binding specificity, as demonstrated by a salt-bridge swap mutant that converts NHA2 into a Li+-specific transporter. Strikingly, an additional N-terminal helix in NHA2 establishes a unique homodimer with a large ∼ 25 Å intracellular gap between protomers. In the presence of phosphatidylinositol lipids, the N-terminal helix rearranges and closes this gap. We confirm that dimerization of NHA2 is required for activity in vivo, and propose that the N- terminal helix has evolved as a lipid-mediated remodelling switch for regulation of transport activity.