The vesicles stock suspensions were diluted to POPC concentrations of 1 mM (ISE assay) and 0.1 mM using the required external solution. DMSO solutions of transporters were added to the vesicle suspensions and the membrane transport kinetics was studied by measuring the chloride concentrations in the external solutions using a chloride selective electrode (ISE assay) or by monitoring the ratiometric fluorescence response from intravesicular pH indicator HPTS (HPTS assay). See Supplemental Material for detailed assay conditions.