A Molecular Mechanism of Lipid Membrane Fusion

Abstract

Membrane fusion is an essential molecular event involved in many cellular processes, such as exocytosis, endocytosis, intracellular vesicle trafficking, fertilization, and viral infection to target cells. In spite of extensive studies of membrane fusion, however, the basic molecular mechanisms in biological systems are not well understood. Probably, it is due to the complex nature of biological membranes and the variety of possible molecular pathways for membrane fusion. We have studied the membrane fusion process, particularly ion-induced membrane fusion. Biological membrane fusion seems to occur with either ion-induced or non-ion-induced membrane process, particularly the later case is for virus membrane fusion system. Dr. Chernomordik and his co-workers have studied on non-ion-induced lipid membrane fusion and developed the so-called “Stalk-intermediate model” before total membrane fusion. That fusion model has been well received by many membrane fusion investigators, particularly in the virus fusion field. Stalk formation between two lipid membranes may occur due to undulation of lipid molecules or local binding of the lipid bilayers, which results in the formation of a local region of outer monolayer fusion. The stalk hypothesis can be described by macroscopic models treating bilayers and monolayers as homogeneous elastic surfaces. We have also studied non-ion-induced bilayer membrane fusion. Our membrane fusion theory is based on the interaction energies between the two membranes due to alternation of the membrane surface properties, e.g., hydrophilicity and hydrohobicity of interacting membranes, and then lipid membrane close approach and due to membrane curvature. Although the membrane interaction processes are different between the two models, these membrane fusion properties are the same as those of our ion-induced lipid membrane fusion.