On the theory of membrane fusion. The adhesion-condensation mechanism.

The primary act of the adhesion-condensation mechanism underlying membrane fusion is considered. This act involves the formation of a close dehydrated contact between membranes and the subsequent crystallization of molecules of the external monolayers in the contact region. Crystallization associated with a decrease in the area per molecule gives rise to elastic stresses which cause a disruption of the external monolayer of the membrane in the contact region. This disruption results in the formation of a trilaminar structure (a monolayer fusion occurs). It has been shown that for the formation of a trilaminar structure between liposomes with a radius of 20 nm the contact area must be at least 22% of that external monolayer. Moreover, the membrane has to overcome an energy barriers; according to estimates for 20 nm liposomes, the maximum value of the barrier is approximately 20 kT. The height of the disruption barrier decreases with increasing area of the contact region. Estimates have been obtained for the minimum area of a "hole" in the contracting monolayers, which arises from their disruption for 20 nm liposomes, this area is approximately 2 X 10(2) nm2. The developed theory explains the data obtained by the Papahadjopoulos group in their experiments on the fusion of phosphatidylserine liposomes (Portis et al. 1979; Wilshut et al. 1980; Düzgünes et al. 1981); in addition, it enables the description of the mechanism underlying the disruption of a liposome as a result of expansion of its membrane. The process of disruption was studied by Kwok and Evans (1981).