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Ca2+/calmodulin signals the completion of docking and triggers a late step of vacuole fusion

Nature volume 396, pages 575–580 (1998)Cite this article

Abstract

The basic reaction mechanisms for membrane fusion in the trafficking of intracellular membranes and in exocytosis are probably identical5. But in contrast to regulated exocytosis, intracellular fusion reactions are referred to as ‘constitutive’ as no final Ca2+-dependent triggering step has been observed. Although transport from the endoplasmic reticulum to the Golgi apparatus in the cell depends on Ca2+ (ref. 6), as does endosome fusion7 and assembly of the nuclear envelope8, it is unclear whether Ca2+ triggers these events. Membrane fusion involves several subreactions: priming, tethering and docking. Proteins that are needed for fusion include p115, SNAPs, NSF, SNAREs and small GTPases, which operate in these early reactions1,2,3 but the machinery that catalyses the final mixing of biological membranes is still unknown. Here we show that Ca2+ is released from the vacuolar lumen following completion of the docking step. We have identified calmodulin as the putative Ca2+ sensor and as the first component required in the post-docking phase of vacuole fusion. Calmodulin binds tightly to vacuoles upon Ca2+ release. Unlike synaptotagmin or syncollin in exocytosis4, calmodulin does not act as a fusion clamp but actively promotes bilayer mixing. Hence, activation of SNAREs is not sufficient to drive bilayer mixing between physiological membranes. We propose that Ca2+ control of the latest phase of membrane fusion may be a conserved feature, relevant not only for exocytosis, but also for intracellular, ‘constitutive’ fusion reactions. However, the origin of the Ca2+ signal, its receptor and its mode of processing differ.

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Figure 1: Effect of calmodulin antagonists on vacuole fusion.
Figure 5: Luminal Ca2+ is required for vacuole fusion.
Figure 2: Calmodulin is required for a late stage of vacuole fusion in vitro and invivo.
Figure 3: Calmodulin binds to vacuoles.
Figure 4: Calmodulin with a reduced affinity for Ca2+ inhibits fusion.
Figure 6: The efflux of Ca2+ from the lumen is needed in a late phase of fusion.

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Acknowledgements

We thank T. Davis for many strains and stimulating discussions; B. Wickner for the start-up kit of reagents; D. Botstein for the calmodulin ts mutants; and P. Overath and U. Henning for their support and the opportunity to use their facilities. We thank O. Müller, P. Overath and B. Wickner for critically reading the manuscript and A. Glatz and C. Baradoy for assistance. This work was supported by the Boehringer Ingelheim Foundation and the Deutsche Forschungsgemeinschaft.

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  1. Friedrich-Miescher-Laboratorium der Max-Planck-Gesellschaft, Spemannstrasse, 72076 37-39, Tübingen, Germany

    Christopher Peters & Andreas Mayer

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Correspondence to Andreas Mayer.

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Peters, C., Mayer, A. Ca2+/calmodulin signals the completion of docking and triggers a late step of vacuole fusion. Nature 396, 575–580 (1998). https://doi.org/10.1038/25133

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