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THEORY AND SIMULATION OF BILAYER MEMBRANE FUSION:
FREE ENERGY AND STRUCTURE OF INTERMEDIATES
K. Katsov 1) , M. Schick 1) , M. Mueller 2)
1) Department of Physics, University of Washington, U.S.A.
2) Institut für Physik, Johannes-Gutenberg-Universität, Mainz, Germany
email: schick@mahler.phys.washington.edu
ABSTRACT
We combine Monte Carlo simulations and SCF theory to study the microscopic mechanism of bilayer
membrane fusion. Bilayers are composed of single chain amphiphiles in an excess homopolymer solvent.
The fusion mechanism we observe in simulations is very different from a widely accepted "stalk" hypothesis.
Instead of the radial stalk expansion into a hemifusion diaphragm with subsequent fusion pore formation, we
observe very strong bilayer destabilization after the first local connections (stalks) are formed. This
destabilization leads to a local rupture of the membranes in the vicinity of the stalk with a following sealing off
of the formed microscopic holes by propagation of the initial connection (stalk).
The observed mechanism is supported by our SCFT calculations of the structure and free energy of
different intermediates that could be involved in the fusion process. The structures that we are able to study
within the SCFT include bilayer edge, 3- and 4-junctions, point and linear stalks and some transition states.
We calculate both relative stability of the intermediates and barriers encountered along the alternative fusion
reaction paths. We also address the problem of mixed bilayers and find relatively strong deviations from the
ideal mixing.
The obtained results allow us to make very definite predictions that can be checked experimentally. This
includes, e.g. the effect of tension and amphiphile architecture on the fusion rate, and the fact that the
membrane fusion is a leaky process.