MCOLN1/TRPML1-MCOLN3/TRPML3 heteromer and its coupling to Ca 2+ sensor SYT5 regulates autophagosome-lysosome fusion in a PtdIns4P-dependent manner

Macroautophagy/autophagy progresses through Ca2+-dependent multiple fusion events. Recently, we reported that the intracellular Ca2+ channel MCOLN3/TRPML3 provides Ca2+ for membrane fusion during autophagosome formation as a downstream effector of phosphatidylinositol-3-phosphate (PtdIns3P). However, the molecular mechanism of Ca2+ signaling in the late stage of autophagy remains unknown. Here, we show that the MCOLN1/TRPML1-MCOLN3/TRPML3 heteromer is the Ca2+ provider for autophagosome-lysosome fusion. MCOLN1-MCOLN3 functions downstream of PtdIns4P to release Ca2+ from autophagosomes, and the Ca2+ signal via PtdIns4P-MCOLN1-MCOLN3 is decoded by the Ca2+ sensor SYT5 (synaptotagmin 5). The binding of Ca2+ and PtdIns4P to SYT5 is critical for autophagosome-lysosome fusion by forming a fusion complex. Collectively, these results reveal that PtdIns4P-MCOLN1-MCOLN3-SYT5 constitutes the Ca2+ signaling complex in autophagosome-lysosome fusion and that MCOLN3 also regulates the late stage of autophagy through heteromerization with MCOLN1 in a phosphoinositide (PI)-dependent manner.