Structure-function studies of Ptr2p, the Saccharomyces cerevisiae di-/tripeptide transporter

Transport of di- and tripeptides in the yeast Saccharomyces cerevisiae is accomplished through the action of the membrane bound transporter, Ptr2p. Comparison of the deduced protein sequences from this transporter and its homologues in other organisms yielded a consensus sequence containing a highly conserved motif usually located in the fifth transmembrane domain of the proteins. This signature sequence was named the FING motif for the four amino acids that are faithfully maintained throughout the family of proteins. The conserved nature of this motif suggested that it played an important role in the peptide transport phenotype. Mutagenesis of the motif in Ptr2p has supported this idea. Alanine-scanning mutagenesis was done on each of the four completely conserved residues, F247, 1251, N252, and G254. Other mutations were also made. Growth on peptides was assayed, as well as sensitivity to toxic dipeptides and the ability to transport radiolabeled dileucine. Transport in each mutant was greatly reduced as measured by all three assay types; however, differences among the mutants could still be seen. In both the growth assay and the toxic peptide assay, the N252A mutant was more active than the others tested. Also, the F247A mutant, while not as active as N252A, was more active than the others. Taken together, these data support the hypothesis that the FING motif is vital to the function of the protein Ptr2p as a peptide transporter.