Characterization of the PT Clade of Oligopeptide Transporters in Rice

Oligopeptide transporters (OPTs) are a group of membrane-localized proteins that have a broad range of substrate transport capabilities and that are thought to contribute to many biological processes. Nine OPTs belonging to the peptide transport (PT) clade were identified in the rice (Oryza sativa L.) genome, based on their predicted amino acid sequences containing the two highly conserved motifs (NPG and KIPPR) that define the PT-OPT subgroup. Protein identity analysis comparing the rice OPTs of the PT and yellow stripe (YS) clades revealed low levels of similarity between members of these two OPT subgroups. Sequence analysis of the predicted proteins of the PT clade, named OsOPT1 to OsOPT9, revealed all to be integral membrane proteins. Functional analysis of five of the OsOPTs, using a yeast complementation assay, demonstrated that OsOPT1, OsOPT3, OsOPT4, OsOPT5, and OsOPT7 could transport ferrous and/or ferric iron chelated to nicotianamine, a nonpeptidyl metal-chelate. Semi quantitative reverse-transcriptase polymerase chain reaction verified expression of the OsOPTs in roots, stems, leaves, hulls, pedicels, and embryos, but transcripts for each OsOPT were not found in every tissue. The identification of multiple PT-clade OPT genes in rice with different expression patterns indicates that they may have different biological roles in the plant.