A voltage-dependent K channel in the lysosome is required for refilling lysosomal Ca stores.

The resting membrane potential (Δψ) of the cell is negative on the cytosolic side and determined primarily by the plasma membrane's selective permeability to K We show that lysosomal Δψ is set by lysosomal membrane permeabilities to Na and H, but not K, and is positive on the cytosolic side. An increase in juxta-lysosomal Ca rapidly reversed lysosomal Δψ by activating a large voltage-dependent and K-selective conductance (LysoK). LysoK is encoded molecularly by SLO1 proteins known for forming plasma membrane BK channels. Opening of single LysoK channels is sufficient to cause the rapid, striking changes in lysosomal Δψ. Lysosomal Ca stores may be refilled from endoplasmic reticulum (ER) Ca via ER-lysosome membrane contact sites. We propose that LysoK serves as the perilysosomal Ca effector to prime lysosomes for the refilling process. Consistently, genetic ablation or pharmacological inhibition of LysoK, or abolition of its Ca sensitivity, blocks refilling and maintenance of lysosomal Ca stores, resulting in lysosomal cholesterol accumulation and a lysosome storage phenotype.