Involved in the import of threonine and serine into the cell, with the concomitant import of a proton (symport system). H(+)(in) + L-threonine(in) = H(+)(out) + L-threonine(out) H(+)(in) + L-serine(in) = H(+)(out) + L-serine(out) Inhibited by the proton ionophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) (PubMed:2115866, PubMed:9498571). Partially inhibited by the respiratory chain inhibitor KCN (PubMed:2115866). Activity is Na(+) independent (PubMed:2115866, PubMed:9498571). Induced in amino acid-rich medium under anaerobic conditions. Belongs to the amino acid/polyamine transporter 2 family. SdaC/TdcC subfamily.