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) Belongs to the amino acid/polyamine transporter 2 family. SdaC/TdcC subfamily.