Proton-dependent permease that transports di- and tripeptides. Shows significantly higher specificity towards dipeptides than tripeptides. Has a preference for dipeptides with a C-terminal Lys residue. Can bind Ala-Lys, Lys-Ala, Ala-Ala. Can also transport alanine and trialanine. Optimum pH is around 6.5. Monomer. Belongs to the major facilitator superfamily. Proton-dependent oligopeptide transporter (POT/PTR) (TC 2.A.17) family.