Molecular Basis for the Cation Selectivity of Salmonella typhimurium Melibiose Permease

•Cation selectivity is an important attribute of cation-coupled symporters.•Novel insights into the molecular basis for Na+ and Li+ selectivity in MelB.•Li+ binding only requires a subset of residues in the Na+ binding site.•The charge balance of MelBSt cation-binding site plays an important role in the protein stability.•Our findings can be extended to other human cation-coupled transporters.