Submitted to: The Plant Genome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2008
Publication Date: 11/1/2008
Citation: Vasconcelos, M.W., Li, G.W., Lubkowitz, M.A., Grusak, M.A. 2008. Characterization of the PT clade of oligopeptide transporters in rice. The Plant Genome. 1(2):77-88. Interpretive Summary: There are many small molecules that are moved from cell to cell within plants, with this movement often involving transport across biological membranes. Because these molecules include compounds of potential nutritional value, to the plant or to humans who eat these plants, we have been interested in understanding how these molecules are transported across cells. Using rice, we studied a group of nine related proteins called oligopeptide transporters, which are known in other plants to move small molecules across cell membranes. We studied the DNA structure of the genes that encode these proteins, and we investigated the ability of these proteins to transport compounds that bind iron. Based on the DNA sequences, we demonstrated which proteins were most closely related to each other, and their relationship to known oligopeptide transporters from other plants. We also showed that three of the rice transporters were able to move iron bound to specific small molecules. Furthermore, we identified where the genes for each of these proteins are turned on throughout the plant. Various oligopeptide transporter genes were found to be active in roots, stems, leaves, hulls, pedicels, and embryos. Our results will help us devise strategies for improving the nutritional value of plant foods, such as rice, for human or animal consumption.
Technical Abstract: Oligopeptide transporters (OPTs) are a group of membrane-localized proteins with a broad range of substrate transport capabilities, and which 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, and OsOPT4 could transport ferrous and/or ferric iron chelated to nicotianamine, a metal-chelating tripeptide. Semi quantitative RT-PCR 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.