Rab GTPase-regulated phagosome-lysosome fusion is bypassed by micromolar calcium.

Several ATP- and cytosol-dependent fusion processes between membranes of the endocytic and exocytic pathways have been biochemically reconstituted. We here present a phagosome-lysosome-fusion reaction driven by micromolar Ca2+ in the absence of ATP and cytosol. Investigating classical and Ca2+-driven fusion (CaFu) side-by-side in vitro, using the same membrane preparations, we show that CaFu is faster than standard fusion (StaFu), leads to larger fusion products, and is not blocked by established inhibitors of StaFu. Approximately 120 µM Ca2+ supports maximal membrane attachment and 15 µM Ca2+ maximal membrane fusion, assigning Ca2+ both a membrane binding and a fusion-promoting activity. StaFu and CaFu are inhibited by a mutant form α-SNAP that does not support SNARE (SNAP receptor) activation, and both are inhibited by a mixture of the cytosolic domains of three cognate Q-SNARE proteins, demonstrating a role of SNAREs in Ca2+-driven membrane merger. CaFu is independent of the calcium-regulated proteins synaptotagmin-7, calmodulin, and annexins A2 and A7. We propose that CaFu corresponds to the last step of phagosome-lysosome fusion, when a raised Ca2+ concentration from the compartment lumen activates SNAREs for fusion.