Cloning and characterization of the OsNramp family from Oryza sativa, a new family of membrane proteins possibly implicated in the transport of metal ions

Abstract

The mammalian Nramp1 protein is an integral membrane protein expressed exclusively in macrophages, where it plays a critical role in the ability of these cells to destroy ingested microbes. The bactericidal mechanism of action of Nramp1 remains unknown. We report the identification and characterization of cDNA clones corresponding to three homologues of the mammalian Nramp1 gene from the genome of Oryza sativa, OsNramp1, OsNramp2, and OsNramp3. These three genes encode a novel group of highly similar hydrophobic polypeptides sharing between 64% and 75% sequence similarity, that show similar hydropathy profiles, and predicted secondary structure, including the same number, position, and sequence characteristics (including conserved charges) of transmembrane domains. Together, these define a highly conserved membrane associated hydrophobic core. The three plant proteins show a remarkable degree of sequence similarity with their mammalian counterpart (60% to 70% similarity), including primary and secondary structure elements previously described in ion transporters and channels. Expression studies in normal plant tissues indicate that while OsNramp1 is expressed primarily in roots, and OsNramp2 is primarily expressed in leaves, OsNramp3 is expressed in both tissues. The recent discovery that the yeast Nramp homologue SMF1 functions as a manganese transporter raises the exciting possibility that OsNramp encodes a family of metal ion transporters in plants.