JCB: MINI-REVIEW Membrane lysis during biological membrane fusion: collateral damage by misregulated fusion machines Alex Engel and Peter Walter Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158 Y G O L O I B L L E C F O L A N R U O J E H T In the canonical model of membrane fusion, the integrity of the fusing membranes is never compromised, preserv- ing the identity of fusing compartments. However, recent molecular simulations provided evidence for a pathway to fusion in which holes in the membrane evolve into a fusion pore. Additionally, two biological membrane fusion models—yeast cell mating and in vitro vacuole fusion—have shown that modifying the composition or altering the relative expression levels of membrane fusion complexes can result in membrane lysis. The convergence of these ﬁndings showing membrane integrity loss during biological membrane fusion suggests new mechanistic models for membrane fusion and the role of membrane fusion complexes. Introduction Membrane fusion is a ubiquitous process in biology that allows for delivery, mixing, and sorting of soluble and membrane inte- grated macromolecules across membrane barriers. Despite enormous diversity of fusion reactions, the job description compo- nents catalyzing membrane fusion remain simple: tether, desta- bilize, and fuse membranes without allowing contents leakage across the bilayer (Jahn et al., 2003; Sollner, 2004; Wickner and Schekman, 2008). In the prevailing model of membrane fusion, the catalyst that drives the coalescence of juxtaposed bilayers, termed a fusase, initiates the formation of a hemifusion stalk, a nonbilayer intermediate that joins the apposed leaflets of the fusing membranes (Fig. 1, stage 2) (Chernomordik and Kozlov, 2008). Axial expansion of the stalk leads to a single bilayer con- sisting of the other two leaflets—termed a hemifusion diaphragm— that separates the two compartments (stage 3). Rupture of the hemifusion diaphragm results in a fusion pore (stage 4). At no point in this process are the contents of the two fusing mem- brane exposed to the environment between the membranes; thus, compartmental identity is preserved. This characteristic of the fusion process is considered vital to biological membrane Correspondence to Peter Walter: pwalter@biochem.ucsf.edu Abbreviation used in this paper: HA, hemagglutinin. The Rockefeller University Press $30.00 J. Cell Biol. Vol. 183 No. 2 181–186 www.jcb.org/cgi/doi/10.1083/jcb.200805182 fusion because leakiness in the fusion pathway could have di- sastrous consequences for the cell. Depending on their longev- ity and degree of occlusion, uncontained membrane holes would allow the dissipation of ion gradients, the escape of potentially harmful hydrolases from intracellular compartments, and cell lysis if plasma membranes were compromised during cell–cell or cell–virus fusion events. Thus, it comes as a surprise that re- cent work has shown that vacuole fusion and yeast mating are prone to lysis when the balance of fusion players is altered (Jin et al., 2004; Aguilar et al., 2007; Starai et al., 2007), and some reports suggest that viral fusases may also cause membrane holes (Shangguan et al., 1996; Blumenthal and Morris, 1999; Frolov et al., 2003). Here we review those perturbations that cause fusases to make holes instead of nonleaky fusion pores and discuss how fusase organization and hypothetical fidelity factors could promote formation of fusion pores over membrane lysis. Lysis during biological membrane fusion SNARE-driven vacuole lysis. Analogous to lyso- somes, yeast vacuoles are an acidified compartment specialized for protein and membrane degradation. These large (0.5–1 μm in diameter) organelles undergo fusion and fission and are main- tained at 1–5 vacuoles per cell (Wang et al., 2002). The SNARE- dependent fusion of yeast vacuoles has been extensively studied in vitro. Before fusion, Rab-dependent docking results in expansive membrane contact, termed boundary membrane, between neighboring vacuoles. The ring-shaped vertex micro- domain at the edges of this boundary domain accumulates many fusion-relevant proteins, including the Rab GTPase Ypt7p, the HOPS Rab effector complex, and the vacuolar SNAREs (Wang et al., 2002). Fusion initiates around the vertex ring, resulting in fused vacuoles with the boundary membranes released into the lumenal space. Wickner and colleagues created a strain of yeast with GFP in the vacuole lumen (Starai et al., 2007). By monitoring the re- lease of lumenal GFP in the in vitro vacuole fusion assay, they were able to assess vacuole lysis during the fusion reaction. With the physiological ratio of Rab, effector complex, and SNAREs, they observed a low background of vacuole lysis (which was likely a result of handling the purified vacuoles). Surprisingly, © 2008 Engel and Walter This article is distributed under the terms of an Attribution– Noncommercial–Share Alike–No Mirror Sites license for the ﬁrst six months after the publica- tion date (see http://www.jcb.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/). JCB 181