Abstract
Lysosomes degrade excess or damaged cellular components and recycle their building blocks through membrane transporters. They also act as nutrient-sensing signaling hubs to coordinate cell responses. The membrane protein PQ-loop repeat-containing protein 2 (PQLC2; "picklock two") is implicated in both functions, as it exports cationic amino acids from lysosomes and serves as a receptor and amino acid sensor to recruit the C9orf72/SMCR8/WDR41 complex to lysosomes upon nutrient starvation. Its transport activity is essential for drug treatment of the rare disease cystinosis. Here, we quantitatively studied PQLC2 transport activity using electrophysiological and biochemical methods. Charge/substrate ratio, intracellular pH, and reversal potential measurements showed that it operates in a uniporter mode. Thus, PQLC2 is uncoupled from the steep lysosomal proton gradient, unlike many lysosomal transporters, enabling bidirectional cationic amino acid transport across the organelle membrane. Surprisingly, the specific presence of arginine, but not other substrates (lysine, histidine), in the discharge ("trans") compartment impaired PQLC2 transport. Kinetic modeling of the uniport cycle recapitulated the paradoxical substrate-yet-inhibitor behavior of arginine, assuming that bound arginine facilitates closing of the transporter's cytosolic gate. Arginine binding may thus tune PQLC2 gating to control its conformation, suggesting a potential mechanism for nutrient signaling by PQLC2 to its interaction partners.