The enlightening encounter between structure and function in the NhaA Na+–H+ antiporter

Na+–H+ antiporters are integral membrane proteins that exchange Na+ for H+ across the cytoplasmic membrane and many intracellular membranes. They are essential for Na+, pH, and volume homeostasis, which are processes crucial for cell viability. Accordingly, antiporters are important drug targets in humans and underlie salt resistance in plants. Many Na+–H+ antiporters are tightly regulated by pH. Escherichia coli NhaA, a prototype pH-regulated antiporter, exchanges 2H+ for 1Na+ (or Li+). The NhaA crystal structure has provided insight into the pH-regulated mechanism of antiporter action and revealed transmembrane segments, which are interrupted by extended mid-membrane chains that have since been found with variations in other ion-transport proteins. This novel structural fold creates a delicately balanced electrostatic environment in the middle of the membrane, which might be essential for ion binding and translocation.