THE DIPEPTIDE 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 fo rce-dependent carrier protein. The gene (dtpT) encoding the di-tripept ide 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 dem onstrated by uptake studies of alanyl-[C-14]glutamate and other peptid es in E. coli cells. The di-tripeptide transport protein catalyzes pro ton motive force-driven peptide uptake and dipeptide exchange activity . The nucleotide sequence of dtpT was determined and the translated se quence corresponds with a protein of 463 amino acid residues. Hydropat hy profiling indicates that the protein could form 12 membrane-spannin g segments with the amino and carboxyl termini at the outer surface of the membrane. A secondary structure model is presented which is subst antiated by analysis of DtpT-PhoA fusion constructs. Amino acid sequen ce comparisons showed no significant homology with other bacterial pep tide transport systems nor with any other known protein. Flanking regi ons 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.