pH-dependent regulation in SLC38A9.

Cells rely on precise metabolic control to adapt to environmental cues. The mechanistic target of rapamycin complex 1 (mTORC1) senses nutrient availability, with amino acids serving as key signals. Lysosomes, which act as nutrient recycling centers, maintain amino acid homeostasis by breaking down macromolecules and releasing amino acids for cellular use. SLC38A9, a lysosomal amino acid transporter, functions as both a transporter and a sensor in the mTORC1 pathway. Here, we investigated whether SLC38A9 activity is regulated by pH. We show that arginine uptake by SLC38A9 is pH-dependent, and that the histidine residue His544 serves as the pH sensor. Mutating His544 abolishes the pH dependence of arginine uptake without impairing overall transport activity, indicating that His544 is not directly involved in substrate binding. Instead, protonation or deprotonation of His544 appears to influence transport through SLC38A9. To explore this mechanism, we compared two structures of SLC38A9 that we determined, one at high pH and one at low pH, and proposed a working model for pH-induced activation. These findings highlight the role of local ionic changes in modulating lysosomal transporters and underscore the intricate regulatory mechanisms that govern SLC38A9 function and, ultimately, mTORC1 signaling.