Energy for membrane fusion

Energy for membrane fusion

(click on each corresponding publication to see it)

Membrane fusion is the cell’s delivery process, enabling its many compartments to receive cargo and machinery for cell growth and inter-cellular communication. The overall activation energy of the process must be large enough to prevent frequent and non-specific spontaneous fusion events, yet must be low enough to allow it to be overcome upon demand by specific fusion proteins such as SNAREs. Remarkably, to the best of our knowledge, the activation energy for spontaneous bilayer fusion has never been measured. Multiple models have been developed and refined to estimate the overall activation energy and its component parts, and they span a very broad range from 20 kBT to 150 kBT depending on the assumptions. In this study, using a bulk lipid mixing assay at various temperatures, we report that the activation energy of complete membrane fusion is at the lowest range of these theoretical values. Typical lipid vesicles were found to slowly and spontaneously fully fuse with activation energies of ~30 kBT. Our data demonstrate that the merging of membranes is not nearly as energy-consuming as anticipated by many models, and is ideally positioned to minimize spontaneous fusion while enabling rapid, SNARE-dependent fusion upon demand. This was published in PNAS, 2017.

Estimate of the activation energy of POPC and DOPC vesicles fusion. (A) Fusion assays are performed at different temperatures (27°C – 47°C). The averages of six independent experiments are represented. The initial time (t=0) is the time where the temperature was stabilized. The speed of fusion increases with temperature. Error bars are standard errors on the mean. (B) Initial spontaneous fusion speeds are represented vs. the temperature (average of nine independent experiments for POPC and four for DOPC, error bars being standard deviations), and fitted by exponentials. Speeds of fusion were determined thanks to the initial slope of the curve representing the percentage of fused vesicles per minute. The exponential fits allow the determination of the activation energies for both reactions, here fusion. Independent fits were also performed for the different experiments, hence allowing the estimation of the error on the measurement (standard error on the mean): 26.4 ± 1 kBT for POPC, and 34.3 ± 0.8 kBT for DOPC.

 

In collaboration with James Rothman (Yale and UCL)