We next considered whether SSA 1 could function only as a selective K + uniporter, or whether other biologically relevant cations may also be transported. To investigate this, we adapted our K + /Cl -co-transport assay to enable us to assay a potential Na + /Cl -symport process in the presence of anionophore 4 (Section S2.7). POPC vesicles were prepared containing 489 mM NaCl, buffered to pH 7.2 with 5 mM sodium phosphate salts. The vesicles were suspended in 167 mM K2SO4, buffered to pH 7.2. In short, we reversed the cation gradients from our previously developed assay. The results are shown in Figure and Table . We found that SSA 1 retained some activity in the Na + /Cl -co-transport assay with a reduction in the observed activity, which is consistent with the Hofmeister classification and the greater hydrophilicity of Na + vs K + , making it a more challenging ion to transport.