Identification of Critical Amino Acid Residues Required for the Polar Localization of a Rice Manganese Transporter

Rice has developed an efficient system for manganese (Mn) uptake, mediated by two distinct transporters, OsNramp5 and OsMTP9. These transporters exhibit polar localization at the root exodermis and endodermis; however, the mechanisms underlying their polar localization and their role in Mn uptake remain unclear. Here, we identified key amino acid residues critical for the polar localization of OsNramp5 at the distal side. Through analysis of chimeric proteins between OsNramp5 and its non-polar homologues, we found that the C-terminal cytosolic region of OsNramp5 is essential for its polar localization. Site-directed mutagenesis further revealed that aspartate 500 and four valine residues at positions 494, 495, 498 and 506 are crucial for polarity. Substitution of these valine residues with isoleucine, leucine, phenylalanine, or threonine partially or fully maintained polar localization, whereas substitution with alanine, serine, or asparagine resulted in loss of polarity. These findings suggest that β-branching and high hydrophobicity of amino acid side chains are likely required for OsNramp5 polarity. Furthermore, we found that adaptor protein 2-dependent clathrin-mediated endocytosis is not involved in the polar localization of OsNramp5. Finally, we provided experimental evidence showing the significant role of OsNramp5 polarity in efficient Mn uptake in rice; plants expressing non-polarly localized OsNramp5 exhibited reduced Mn uptake compared to those with polarly localized OsNramp5. In addition, we found that cadmium accumulation in shoots could be reduced by manipulating OsNramp5 polarity in combination with its overexpression, without a growth penalty.