Required for Na(+) uptake into the cell, especially at low external Na(+) concentrations or low Na(+)/K(+) ratios. May be part of a sodium cycle that permits re-entry of sodium into the cell. Mutant cannot grow at low sodium concentrations and exhibits an increased sensitivity towards potassium. Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family.