Involved in the import of serine and threonine into the cell, with the concomitant import of sodium (symport system). L-serine(in) + Na(+)(in) = L-serine(out) + Na(+)(out) L-threonine(in) + Na(+)(in) = L-threonine(out) + Na(+)(out) Activated by valinomycin and inhibited by monensin. Optimum pH is 7.0. Optimum temperature is 40 degrees Celsius. Repressed by tryptophan. Belongs to the dicarboxylate/amino acid:cation symporter (DAACS) (TC 2.A.23) family.