Lipids in biological membrane fusion

The results reviewed suggest that membrane fusion in diverse biological fusion reactions involves formation of some specific intermediates: stalks and pores. Energy of these intermediates and, consequently, the rate and extent of fusion depend on the propensity of the corresponding monolayers of membranes to bend in the required directions.Proteins and peptides can control the bending energy of membrane monolayers in a number of ways. Monolayer lipid composition may be altered by different phospholipases 50, 85, 90], flipases and translocases 4, 50]. Proteins and peptides can change monolayer spontaneous curvature or hydrophobic void energy by direct interaction with membrane lipids 20, 32, 111]. Proteins may also provide some barriers for lipid diffusion in the plane of the monolayer 83, 141]. If diffusion of lipids at some specific membrane sites (e.g., in the vicinity of fusion protein) is somehow hindered, the energy of the bent fusion intermediates would reflect the elastic properties of these particular sites rather than the spontaneous curvature of the whole monolayers. Proteins may deform membranes while bringing them locally into close contact. The alteration of the geometric (external) curvature will certainly change the elastic energy of the initial state and, thus affect the energetic barriers of the formation of the intermediates 143]. In addition, the area and the energy of the stalk can be reduced by preliminary bending of the contacting membranes 111]. The possible effects of proteins and polymers on local elastic properties and local shapes of the membranes have been recently analyzed 22, 39, 45, 63]. These studies may provide a good basis for future development of theoretical models of protein-mediated fusion.