Catalyzes the export of L-threonine and L-serine from the cell to the extracellular environment (PubMed:11514515). Export is dependent on the proton motive force (PubMed:11514515). H(+)(out) + L-threonine(in) = H(+)(in) + L-threonine(out) Transport is inhibited by the proton ionophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). Mutant shows increased threonine peptide sensitivity (PubMed:11514515). Mutant still exports L-threonine, but at a reduced rate (PubMed:11514515). Belongs to the ThrE exporter (TC 2.A.79) family.