CLONING AND CHARACTERIZATION OF THE OSNRAMP FAMILY FROM ORYZA-SATIVA,A NEW FAMILY OF MEMBRANE-PROTEINS POSSIBLY IMPLICATED IN THE TRANSPORT OF METAL-IONS

The mammalian Nramp1 protein is an integral membrane protein expressed exclusively in macrophages, where it plays a critical role in the abi lity of these cells to destroy ingested microbes. The bactericidal mec hanism of action of Nramp1 remains unknown. We report the identificati on and characterization of cDNA clones corresponding to three homologu es of the mammalian Nramp1 gene from the genome of Oryza sativa, OsNra mp1, OsNramp2, and OsNramp3. These three genes encode a novel group of highly similar hydrophobic polypeptides sharing between 64% and 75% s equence similarity, that show similar hydropathy profiles, and predict ed secondary structure, including the same number, position, and seque nce characteristics (including conserved charges) of transmembrane dom ains. Together, these define a highly conserved membrane associated hy drophobic core. The three plant proteins show a remarkable degree of s equence similarity with their mammalian counterpart (60% to 70% simila rity, including primary and secondary structure elements previously de scribed 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 expr essed in both tissues. The recent discovery that the yeast Nramp homol ogue SMF1 functions as a manganese transporter raises the exciting pos sibility that OsNramp encodes a family of metal ion transporters in pl ants.