Lipid Bilayer Fusion - Lipid Mechanism

Lipid Mechanism

Further information: Interbilayer forces in membrane fusion

There are four fundamental steps in the fusion process, although each of these steps actually represents a complex sequence of events. First, the involved membranes must aggregate, approaching each other to within several nanometers. Second, the two bilayers must come into very close contact (within a few angstroms). To achieve this close contact, the two surfaces must become at least partially dehydrated, as the bound surface water normally present causes bilayers to strongly repel at this distance. Third, a destabilization must develop at one point between the two bilayers, inducing a highly localized rearrangement of the two bilayers. Finally, as this point defect grows, the components of the two bilayers mix and diffuse away from the site of contact. Depending on whether hemifusion or full fusion occurs, the internal contents of the membranes may mix at this point as well.

The exact mechanisms behind this complex sequence of events are still a matter of debate. To simplify the system and allow more definitive study, many experiments have been performed in vitro with synthetic lipid vesicles. These studies have shown that divalent cations play a critical role in the fusion process by binding to negatively charged lipids such as phosphatidylserine, phosphatidylglycerol and cardiolipin. One role on these ions in the fusion process is to shield the negative charge on the surface of the bilayer, diminishing electrostatic repulsion and allowing the membranes to approach each other. This is clearly not the only role, however, since there is an extensively documented difference in the ability of Mg2+ versus Ca2+ to induce fusion. Although Mg2+ will induce extensive aggregation it will not induce fusion, while Ca2+ induces both. It has been proposed that this discrepancy is due to a difference in extent of dehydration. Under this theory, calcium ions bind more strongly to charged lipids, but less strongly to water. The resulting displacement of calcium for water destabilizes the lipid-water interface and promotes intimate interbilayer contact. A recently proposed alternative hypothesis is that the binding of calcium induces a destabilizing lateral tension. Whatever the mechanism of calcium-induced fusion, the initial interaction is clearly electrostatic, since zwitterionic lipids are not susceptible to this effect.

The role of lipid headgroup in the fusion process extends beyond charge density and can affect dehydration and defect nucleation independent of the effect of ions. The presence of the uncharged headgroup phosphatidylethanolamine (PE) increases fusion when incorporated into a phosphatidylcholine bilayer. This phenomenon has been explained by some as a dehydration effect similar to the influence of calcium. The PE headgroup binds water less tightly than PC and therefore may allow close apposition more easily. An alternate explanation is that the physical rather than chemical nature of PE may help induce fusion. According to the stalk hypothesis of fusion, a highly curved bridge must form between the two bilayers for fusion to occur. Since PE has a small headgroup and readily forms inverted micelle phases it should, according to the stalk model, promote the formation of these stalks. Further evidence cited in favor of this theory is the fact that certain lipid mixtures have been shown to only support fusion when raised above the transition temperature of these inverted phases. This topic also remains controversial, and even if there is a curved structure present in the fusion process, there is debate in the literature over whether it is a cubic, hexagonal or more exotic extended phase.