Yu, F.; Huang, X.; Wang, S.; An, N.; Zhang, P.; Huang, J.; Zhang, X.

Ammonium (NH4+) uptake and transport in plants are mediated by ammonium transporters (AMTs), which are tightly regulated by phosphorylation to prevent excess ammonium influx and toxic accumulation. However, the mechanism of phosphorylation-mediated inactivation is unique and remains unclear. Here, we report cryo-EM structures of the rice (Oryza sativa) ammonium transporter OsAMT1;1 in its wild-type and phosphomimetic variant T452D states. OsAMT1;1 forms a stable homotrimer, with conserved motifs affirming a shared transport mechanism within the AMT/MEP/Rh superfamily. Our structural analysis reveals that T452D phosphomimetic stabilizes an intracellular salt-bridge network and maintains the channel in a closed state, in which the ICL5 loop positions the hydrophobic plug residue Leu360 near the cytoplasmic exit to occlude the channel exit and abolish complementation capacity. Our findings elucidate phosphorylation-induced inactivation mechanism for the plant AMT1 subfamily, providing a structural paradigm for the post-translational regulation of nutrient uptake.