Abstract
Secondary active membrane transporters harness the energy of ion gradients to concentrate their substrates. Homologous transporters evolved to couple transport to different ions in response to changing environments and needs. The bases of such diversification and, thus, principles of ion coupling are unexplored. Here, using phylogenetics and ancestral protein reconstruction, we investigated sodium-coupled transport in prokaryotic glutamate transporters, a mechanism ubiquitous across life domains and critical to neurotransmitter recycling in humans by excitatory amino acid transporters from the solute carrier 1 family. By inferring ancestral prokaryotic transporter sequences during a change in the ion-coupling mechanism, we found an evolutionary transition from sodium-dependent to independent substrate binding and transport. Structural and functional experiments on ancestral transporters suggest that the transition involved allosteric mutations, rendering sodium binding dispensable without affecting the ion-binding sites. Allosteric tuning of transporters' energy landscapes might be a widespread route of their functional diversification.