The di- and tripeptide transport protein of Lactococcus lactis. A new type of bacterial peptide transporter

Lactococcus lactis takes up di- and tripeptides via a proton motive force-dependent carrier protein. The gene (dtpT) encoding the di-tripeptide transport protein of L. lactis was cloned by complementation of a dipeptide transport-deficient and proline auxotrophic Escherichia coli strain. Functional expression of the dipeptide transport gene was demonstrated by uptake studies of alanyl-[14C]glutamate and other peptides in E. coli cells. The di-tripeptide transport protein catalyzes proton motive force-driven peptide uptake and dipeptide exchange activity. The nucleotide sequence of dtpT was determined and the translated sequence corresponds with a protein of 463 amino acid residues. Hydropathy profiling indicates that the protein could form 12 membrane-spanning segments with the amino and carboxyl termini at the outer surface of the membrane. A secondary structure model is presented which is substantiated by analysis of DtpT-PhoA fusion constructs. Amino acid sequence comparisons showed no significant homology with other bacterial peptide transport systems nor with any other known protein. Flanking regions of the di-tripeptide transport gene were used to delete dtpT from the chromosome of L. lactis. Genetic and biochemical characterization of this mutant indicates that DtpT is the only transport protein in L. lactis for hydrophilic di- and tripeptides.