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TMEM55B controls lipolysis via lysosomal pH and ER lysosome calcium signaling

Mukae, N.; Maki, H.; Shirane, M.

2025-12-23 cell biology
10.1101/2025.10.21.683636 bioRxiv
Show abstract

Endolysosomal maturation is accompanied by luminal acidification and increased Ca{superscript 2} concentration. Mature lysosomes function as major intracellular Ca{superscript 2} storage organelles, alongside the endoplasmic reticulum (ER) and mitochondria. Recent studies have highlighted a role for lysosomal Ca{superscript 2} in calcium-induced calcium release (CICR), where local lysosomal Ca{superscript 2} efflux triggers subsequent ER Ca{superscript 2} release at ER-lysosome membrane contact sites (MCSs). However, the molecular mechanisms underlying this process remain incompletely understood. In this study, we identified the lysosome-localized protein TMEM55B in a PDZD8-associated protein complex, a factor previously implicated in lipid transfer and membrane tethering at ER-late endosome/lysosome (LE/Ly) contacts. Loss of TMEM55B disrupted lysosomal acidification and impaired lipid droplet degradation. TMEM55B knockdown also reduced lysosomal Ca{superscript 2} release and uptake, accompanied by attenuated CICR at ER-LE/Ly MCSs. Together, these findings suggest that TMEM55B contributes to the regulation of lysosomal pH and Ca{superscript 2} dynamics and may influence Ca{superscript 2} signaling between lysosomes and the ER.

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