Uncovering the "secret" lives of vacuolar fusion pores in living cells.

Non-expanding fusion pores have been predicted to constitute rate-limiting metastable intermediates in a broad range of fusion processes ranging from exocytosis to cell-cell fusion. However, their existence and nature, in particular in intracellular non-exocytic compartments, remained largely intangible. In this issue, D'Agostino etal demonstrate that non-expanding pores are likely long-lived steady-state intermediates in yeast vacuoles allowing the cells to adjust their volume to rapidly changing physiology. These pores are stabilized against closure by SNAREs and the attached vacuolar vesicle tethering complex HOPS. Their expansion is controlled by increasing membrane tension.