pH ‐mediated activation of the lysosomal arginine sensor SLC38A9

Cells rely on metabolic control; the mechanistic target of rapamycin complex 1 (mTORC1) senses nutrient availability, particularly amino acids. Lysosomes maintain amino acid homeostasis through recycling. SLC38A9, a lysosomal amino acid transporter, functions as a critical sensor in the mTORC1 pathway. Here, we investigate how pH regulates SLC38A9 activity. We show that arginine uptake is pH-dependent, with His544 residue serving as the pH sensor. Mutating His544 abolishes pH dependence without impairing overall transport, indicating His544 influences transport through protonation/deprotonation, instead of involving in the substrate binding. We propose a working model for pH-induced activation, through comparing two determined SLC38A9 structures at different pH. These findings reveal how local ionic shifts regulate lysosomal transporters and fine-tune SLC38A9 function to control mTORC1 signaling.