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Manganese triggers phosphorylation-mediated endocytosis of the Arabidopsis metal transporter NRAMP1.

The NATURAL RESISTANCE ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1) transporter guarantees survival of the plant to Mn deficiency by mediating Mn entry into root cells. Unlike other high-affinity metal transporters, NRAMP1 is only slightly regulated at the transcriptional level. We show here that adequate Mn content in tissues is safeguarded through a tight control of the amount of NRAMP1 present at the surface of root cells. Depending on Mn availability, an NRAMP1-GFP fusion protein cycles dynamically between the plasma membrane (PM) and endosomal compartments. This involves a clathrin-mediated endocytosis pathway since disrupting the latter in auxilin-overexpressor lines prevents NRAMP1 internalization. Mutation of the phosphorylated serine residues 20, 22 and 24 in the cytosol-exposed N-terminus of NRAMP1 alters its membrane distribution. Indeed, a phosphodead mutation stabilizes NRAMP1 at the PM regardless of the Mn regime and dramatically reduces plant tolerance to Mn toxicity. Conversely a phosphomimetic mutant is constitutively internalized into endosomes. Together these data establish that phosphorylation of NRAMP1 is the trigger for its Mn-induced endocytosis and represents the main level of regulation of this transporter. Furthermore, the extent of Mn toxicity observed when interrupting NRAMP1 membrane cycling undermines the dogma in which Mn is only marginally toxic to plants.