Structural basis for phosphorylation-induced inactivation of the rice ammonium transporter AMT1;1

Ammonium uptake and transport in plants occur through ammonium transporters (AMTs), which are regulated by phosphorylation to prevent excessive ammonium uptake and subsequent toxicity. The precise mechanism of this phosphorylation-induced inactivation, however, has not been fully understood. In this report, we present the cryo-EM structures of the rice (Oryza sativa) ammonium transporter OsAMT1;1 in both its wild-type and phosphomimetic variant T452D.

OsAMT1;1 organizes into a stable homotrimer, with conserved motifs confirming a shared transport mechanism within the AMT/MEP/Rh superfamily. Through structural analysis, we discovered that the T452D phosphomimetic variant enhances the stability of an intracellular salt-bridge network, thereby maintaining the channel in a closed state.

In this closed state, the ICL5 loop positions the hydrophobic plug residue Leu360 near the cytoplasmic exit, effectively blocking the channel exit and preventing ammonium transport. Our findings shed light on the phosphorylation-induced inactivation mechanism in the plant AMT1 subfamily, offering a structural framework for the post-translational regulation of nutrient uptake.