Fusion of bilayer membranes with leaves of different composition and of varying intermembrane separation
Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)Abstract
We calculate the barriers to fusion of planar bilayers which contain two
different amphiphiles, a lamellae former and a hexagonal former, using
self-consistent field theory. We first calculate these barriers as the
amount of hexagonal former is increased equally in both leaves to levels
appropriate to the plasma membrane of human red blood cells. We follow
these barriers as the composition of hexagonal formers is then increased
in the cis layer and decreased in the trans layer, again to an extent
comparable to the biological system. We find that, while the fusion
pathway exhibits two barriers, the magnitudes of these barriers are
comparable to one another, and small, on the order of 13 kT. As a
consequence, one expects that once the bilayers are brought sufficiently
close to one another to initiate the process, fusion should occur rapidly.
To determine just what is sufficiently close, we calculate the barriers to
fusion as a function of the distance between membranes. The bilayers are
under tension. We find that the largest barrier to fusion is that to
create the metastable stalk and is the smallest when the bilayers are at a
distance about 20 percent greater than the thickness of a single leaf, a
distance which would correspond to between two and three nanometers for
typical bilayers. The rate of increase of the free energy with distance
between bilayers is calculated for the metastable stalk itself and for the
barrier to the creation of this stalk. We find that they are sufficiently
large that external agents such as fusion peptides must be available in
order to bring bilayers sufficiently close in order that they fuse.