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Low Energy Cost for Optimal Speed and Control of Membrane Fusion

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Low Energy Cost for Optimal Speed and Control of Membrane Fusion Low energy cost for optimal speed and control of membrane fusion Claire François-Martina,b,c, James E. Rothmand,e,1, and Frederic Pinceta,b,c,d,e,1 aLaboratoire de Physique Statistique, Ecole Normale Supérieure, Paris Sciences et Lettres Research University, 75005 Paris, France; bLaboratoire de Physique Statistique, Université Paris Diderot Sorbonne Paris Cité, 75005 Paris, France; cLaboratoire de Physique Statistique, Sorbonne Universités, Université Pierre et Marie Curie, Univ Paris 06, CNRS, 75005 Paris, France; dDepartment of Cell Biology, School of Medicine, Yale University, New Haven, CT 06520; and eNanobiology Institute, School of Medicine, Yale University, New Haven, CT 06520 Contributed by James E. Rothman, December 28, 2016 (sent for review December 15, 2016; reviewed by Gary Bryant and Ivan Lopez-Montero) Membrane fusion is the cell’s delivery process, enabling its many Whereas key aspects of the transition states remain unclear, compartments to receive cargo and machinery for cell growth and lipid bilayer fusion likely involves intermediates (19) and is there- intercellular communication. The overall activation energy of the fore kinetically complex. With this in mind, we take a more global process must be large enough to prevent frequent and nonspecific view, in which kinetic complexity and molecular rearrangements are spontaneous fusion events, yet must be low enough to allow it to averaged to enable a simple experimental approach to measure be overcome upon demand by specific fusion proteins [such as activation energy by population analysis. Working in bulk with small soluble N-ethylmaleimide–sensitive factor attachment protein re- phospholipid vesicles that undergo random Brownian motion pro- ceptors (SNAREs)]. Remarkably, to the best of our knowledge, the vides an ideally controlled minimal system to monitor fusion on a activation energy for spontaneous bilayer fusion has never been large scale. Vesicles collide and, on very rare occasions, these col- measured. Multiple models have been developed and refined to lisions are sufficiently energy yielding to trigger fusion. When fusion estimate the overall activation energy and its component parts, is intentionally triggered by proteins like SNAREs or physico- and they span a very broad range from 20 kBT to 150 kBT, depend- chemical factors such as osmotic pressure, conditions are invariably ing on the assumptions. In this study, using a bulk lipid-mixing chosen to minimize the rate of this spontaneous fusion so the fusion assay at various temperatures, we report that the activation en- signal resulting from these rare spontaneous events is negligible ergy of complete membrane fusion is at the lowest range of these (3, 20). In principle, at high collision frequency, these fusion BIOPHYSICS AND theoretical values. Typical lipid vesicles were found to slowly and events can become numerous enough to be observable. Hence, COMPUTATIONAL BIOLOGY spontaneously fully fuse with activation energies of ∼30 kBT. Our to observe and quantify spontaneous fusion, we chose to use a data demonstrate that the merging of membranes is not nearly as classical bulk fusion assay with increased collision rate by working energy consuming as anticipated by many models and is ideally at high vesicle concentration with refined analysis. positioned to minimize spontaneous fusion while enabling rapid, Fusion among ∼60-nm diameter phosphatidylcholine (PC) SNARE-dependent fusion upon demand. vesicles at 37 °C was monitored using a well-established lipid- mixing assay (21) (SI Methods). Briefly, two sets of vesicles were membrane fusion | activation energy | liposome | energy landscape | mixed together. One set of vesicles contained two types of fluo- lipid-mixing assay rescent lipids tagged with either 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) or lissamine rhodamine B (Rh). These fluorescent lipids iving organisms and cells are composed of different com- were present at concentrations at which NBD is largely quenched Lpartments delimited by a membrane. These compartments by Rh through Förster resonance energy transfer (FRET). The have their own function and integrity but nevertheless need to other set of vesicles was not fluorescent. When a fusion event communicate with one another. A common pathway by which occurs between a fluorescent vesicle and a nonfluorescent vesicle, exchanges can occur between them is membrane fusion, a crucial process leading to the opening of a fusion pore connecting two Significance compartments and allowing their respective contents to mix or react (1, 2). The global effective activation energy of the process Membrane fusion is a key process for cell growth and in- must be large enough to avoid frequent spontaneous membrane tercellular communication. There are many models for fusion fusion events. Nevertheless, it must remain sufficiently low so with widely divergent activation energies. Surprisingly, no that proteins like soluble N-ethylmaleimide–sensitive factor at- comprehensive quantification of fusion was ever experimen- tachment protein receptors (SNAREs) (3–5) are able to over- tally performed. Probably this is because of the difficulty of come it and induce fusion. If multiple models (6–16) have been observing and quantifying rare spontaneous fusion events and developed and refined to estimate this activation energy, there is equally the difficulty of establishing that such events are bona still a lack of experimental data to provide its actual value and fide fusion events. Here, we find that the activation energy is validate these models. Activation energies have been reported lower by far than in most predictions. The biological impor- between intermediate states of the fusion process (17) or with tance of this low energy value is that it explains how cells can nonphospholipid surfactants (18). They were obtained with maintain traffic among distinct compartments without mixing nonspontaneous fusion triggered by an external source such as them up, preventing spontaneous fusion but allowing specific osmotic pressure or mechanical shear. delivery of cargo as soon as fusion-inducing proteins are Here, by using a minimal membrane model system, we show in place. that the activation energy of complete and spontaneous mem- Author contributions: C.F.-M., J.E.R., and F.P. designed research; C.F.-M. performed re- brane fusion is in the lowest range of the predicted values. Lipid search; C.F.-M., J.E.R., and F.P. analyzed data; and C.F.-M., J.E.R., and F.P. wrote the paper. sn vesicles composed of 1-palmitoyl-2-oleoyl- -glycero-3-phosphocho- Reviewers: G.B., RMIT University; and I.L.-M., Universidad Complutense de Madrid. sn line (POPC) or 1,2-dioleoyl- -glycero-3-phosphocholine (DOPC) The authors declare no conflict of interest. were found to slowly and spontaneously fully fuse with respective 1 ± k T ± k T To whom correspondence may be addressed. Email: [email protected] or james. activation energies of 26.4 1 B and 34.3 0.8 B . Our data [email protected]. demonstrate that the merging of membranes is not as energy This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. consuming as anticipated in the early models. 1073/pnas.1621309114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1621309114 PNAS Early Edition | 1of4 Downloaded by guest on September 25, 2021 the mixing of their lipids leads to dilution of the dyes in the suggesting that detectable fusion-like events were occurring in the resulting combined membrane. This dilution is associated with a vesicle solution on experimental timescales (∼1h). decrease of FRET and can be experimentally observed as an in- Before quantifying the energies involved, the remaining diffi- crease in NBD fluorescence that, when monitored in bulk, directly culty was to determine what these events actually correspond to: yields the number of fusion events per second, i.e., fusion speed Are they full fusion, intermediate fusion states, or merely lipid (3). To optimize the collision rate, vesicles were incubated at exchange without fusion? To discriminate among these possi- unusually high concentrations (18 mM PC). The results presented bilities, we monitored lipid mixing of the inner leaflets only. The in Fig. 1A show that lipid mixing was indeed readily observed, fluorescent-lipid–containing vesicles were first preincubated with dithionite (SI Methods), which quenches NBD’s fluorescence as it chemically reduces the NBD groups on the external leaflet only because dithionite does not cross lipid bilayers (22). Be- cause the resulting vesicles had only fluorescent NBD in their inner leaflets (Fig. S1), any FRET changes resulting from in- cubation with the unlabeled vesicles could only be due to a di- lution of their inner leaflet lipids, i.e., full fusion events, and not hemifusion (mixing of outer leaflets without inner leaflet fusion) or exchange of lipids between vesicles via their outer leaflets. In- ner leaflet mixing was indeed observed at essentially the same rate as total lipid mixing (Fig. 1A), implying that the vast majority of the FRET signal was due to full fusion. To independently confirm this conclusion, we observed the samples by cryo-electron mi- croscopy following incubation (60 min at 37 °C). Stable, extended hemifusion structures are readily observable by this method (23). However, we observed no such hemifusion diaphragms among the 4,215 vesicles. Because FRET experiments show that ∼2% of the vesicles have fused at the end of the experiment at 37 °C (or 4% have hemifused), ∼150
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