SNARE‐mediated membrane fusion arrests at pore expansion to regulate the volume of an organelle

• Published in: The EMBO journal Vol. 37; no. 19
• Main Authors: D'Agostino, Massimo, Risselada, Herre Jelger, Endter, Laura J, Comte‐Miserez, Véronique, Mayer, Andreas
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.10.2018; John Wiley and Sons Inc
• Subjects: Cell fusion; Corresponding states; Docking; endosomes; lysosomes; Membrane fusion; Membranes; Molecular dynamics; Organelles; Osmosis; Osmotic pressure; Pores; Porosity; Pressure gradients; Proteins; SNAP receptors; SNAREs; Tethering; Vacuoles; Yeast; Yeasts; endosomes; lysosomes; vacuoles; SNAREs; membrane fusion
• ISSN: 0261-4189; 1460-2075
• DOI: 10.15252/embj.201899193

Constitutive membrane fusion within eukaryotic cells is thought to be controlled at its initial steps, membrane tethering and SNARE complex assembly, and to rapidly proceed from there to full fusion. Although theory predicts that fusion pore expansion faces a major energy barrier and might hence be a rate‐limiting and regulated step, corresponding states with non‐expanding pores are difficult to assay and have remained elusive. Here, we show that vacuoles in living yeast are connected by a metastable, non‐expanding, nanoscopic fusion pore. This is their default state, from which full fusion is regulated. Molecular dynamics simulations suggest that SNAREs and the SM protein‐containing HOPS complex stabilize this pore against re‐closure. Expansion of the nanoscopic pore to full fusion can thus be triggered by osmotic pressure gradients, providing a simple mechanism to rapidly adapt organelle volume to increases in its content. Metastable, nanoscopic fusion pores are then not only a transient intermediate but can be a long‐lived, physiologically relevant and regulated state of SNARE‐dependent membrane fusion. Synopsis Membrane tethering and docking have been assumed to control fusion reactions. New data show that non‐expanding, nanoscopic pores connect yeast vacuoles in vivo and regulate fusion between two organelles according to the physiological status of the cell. Non‐expanding, nanoscopic fusion pores connect vacuoles in vivo. Fusion pores are long‐lived structures. Metastable fusion pores allow organelles to rapidly adapt to osmotic pressure changes. SNARE proteins and the HOPS complex stabilize pores against re‐closure. Metastable nanoscopic pores serve as a control point for the fusion reaction. Metastable nanoscopic fusion pores connect yeast vacuoles and serve as hubs for the regulation of fusion reactions.