Lysosomal TRPML1 is H ⁺ -permeable but not H ⁺ -selective. Whole lysosome recordings from TRPML1-transfected HEK293T cells. Pipette solution contained NMDG but no K⁺ (pH 4.6), and bath solutions (pH 7.2) contained no K⁺ (NMDG-containing) or 150 mM K⁺ as indicated. A voltage ramp protocol (−120 to +120 mV in 1 s, Ψ_h = 0 mV, before LJP correction) was used. (A) H⁺ currents were recorded from each lysosome with a bath containing 150 mM NMDG followed by the addition of PI(3,5)P₂ (1 μM). Left: Representative traces. Right: Summary of current amplitudes at −100 and +100 mV. Each circle represents the current from a lysosome patch: n = 5. (B and C) Each lysosome was recorded with baths containing 150 mM NMDG or 150 mM K⁺ with and without PI(3,5)P₂ (1 μM), in the sequence as indicated by circled numbers in B. (B) Representative I-Ψ relationships extracted from ramps under each condition. (C) Summary of inward (at −100 mV) and outward (at +100 mV) currents recorded in the four bath solutions outlined in B. Each circle represents one lysosome: n = 5. Error bars are SEM.