Structure and mechanisms of ion-exchange

Na+/H+ exchangers catalyse an ion-exchange mechanism 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 details of the NHA2 ion-exchange mechanism have been lacking.

In this application we show that we have recently been able to obtain the cryo-EM structure of mammalian NHA2 at 3.0 Å in an outward-facing conformation.

The ion-binding site, oligomerization, and energetics of mammalian NHA2 is very different from any other Na+/H+ antiporter we have seen to date, consistent with it being a mammalian “outlier” by bioinformatic analysis.

Indeed, in contrast to other mammalian Na+/H+ exchangers belonging to the SLC9A family, NHA2 shares closer structural similarity to the bacterial Na+/H+ antiporter NapA, which our lab has used as a model system to establish general mechanisms of Na+/H+ exchange.

Here we plan to use a combination of cryo EM, nanodiscs, Solid State Membrane-electrophysiology, transport assays, fluorescence spectroscopy and lipid-interaction studies to establish a detailed Na+/H+ exchange mechanism for NHA2 and its upregulation by changes in cell volume.

We also highlight our progress towards the first structure of a K+/H+antiporter to establish the molecular basis for Na+ versus K+-selectivity.