SNARE concentrations (Starai et al., 2007, 2008). Nonetheless, catalyze promises new insight into the control of biological the concept of fusion facilitators arranging a greater complex of membrane fusion. trans-SNARE pairs remains enticing. Imagining cooperative SNARE function during neurotransmitter release, an organiz- ing architectural factor was invoked that functioned to arrange multiple trans-SNARE pairs into a ring-like fusion machine (Rizo et al., 2006). Prm1p could regulate cell fusion in an analogous manner during yeast mating by interacting with and orienting core fusase molecules (Fig. 2 A, top). In the absence of Prm1p, a decreased ability to assemble active fusion ma- chines results in apposed but unfused membranes. Incorrectly assembled fusion machines may destabilize membranes, but not in a productive stalk-promoting manner, resulting in cell lysis (Fig. 2 A, bottom). Alternatively, instead of regulating protein fusases, integ- rity-promoting accessory factors could control lipid diffusion to control dangerous fusion intermediates such as the hypothetical membrane holes described above (Fig. 2 B). HA-mediated cell– cell fusion has been arrested in a state of hemifusion without lipid mixing; clustered HA trimers are believed to cause this re- striction (Chernomordik et al., 1998). Modeled on these obser- vations, Prm1 may act by preserving the lipidic environment set up by the core fusase or by stopping expansion of membrane holes (Shangguan et al., 1996; Jin et al., 2004). Consistent with this corral-like structural role, Prm1 forms covalent homodimers, but it is not known if these dimers further oligomerize (unpub- lished data). Conclusion Convergence of molecular simulations and experimental data suggests that lysis is not simply an irrelevant experimental arti- fact of membrane fusion assays. Accordingly, we must revisit the classical model of membrane fusion. The experimentally verified stalk structure is not in question, but different re- arrangements that risk loss of compartmental identity could occur before fusion pore formation. Specific factors might be involved containing or avoiding these risks, and identifying such factors would be an extremely valuable advance in our un- derstanding of how the activity of fusases is controlled to fuse membranes with high fidelity. Finding proteins that can sup- press vacuole lysis without lowering SNARE activity could help establish such late stage regulation. Moving from the other direction, the identification of proteins that interact with Prm1p may yield a fusase responsible for cell fusion. If the predictions outlined here are correct, removing this fusase should eliminate both fusion and lysis outcomes of prm1 × prm1 mating pairs. When available, comparing the mechanism by which the cell– cell fusase merges membranes to the mechanisms described for viral and intracellular fusion will describe the breadth of strat- egies for joining membranes of different character and in differ- ent contexts. Finally, our understanding of the diversity of fusion machines will greatly benefit with the characterization of the reovirus FAST proteins, a new class of fusion proteins that me- diate cell fusion (Salsman et al., 2005; Top et al., 2005). Given their small size (14 kD) and simple domain structure, answering questions about the arrangement and stoichiometry of the FAST proteins at the cell surface and the lipid rearrangements they We thank Michael Schick, Randy Schekman, and David Toczyski for critical reading and comments on the manuscript. P. Walter is an HHMI investigator. Submitted: 29 May 2008 Accepted: 10 September 2008 References Aguilar, P.S., A. Engel, and P. Walter. 2007. 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