Lipid-lipid interactions in bilayer membranes: Married couples and casual liaisons

There is considerable interest in lateral domain structure in biological membranes not least because a variety of physiological processes are believed to require assembly and mutual organization of particular membrane components for their execution. Domain structure is known to be created by differences in physicochemical properties between membrane lipids such as phase transition temperature, intermolecular hydrogen bonding and ionic functional groups. Domains are also created by specific interactions between different membrane lipids to form stoichiometric complexes. Such complexes often form ordered structures referred to as membrane rafts. The present challenge is to define the balance of line tension between lateral membrane domains in individual leaflets of the bilayer and coupling forces operating at the midplane of the bilayer responsible for maintaining lipids in opposing domains on either side of the structure in register. A review of the current evidence relating to these questions is presented.