Membrane lipids: where they are and how they behave

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Abstract
Lipids function as essential structural components of membranes, as signalling molecules, as chemical identifiers of specific membranes and as energy storage molecules. The synthesis of lipids is non-uniformly distributed among a few cellular membranes, which requires most organelles to depend on lipid transport processes to achieve their full complement of lipids. Organelles differ both quantitatively and qualitatively in their lipid content. In many organelles, the lipid composition of separate leaflets of the membrane bilayer is significantly different, which produces asymmetry across the bilayer — a situation that is maintained by ATP-dependent flippases. Lipid transport between and within organelles is poorly understood, but a growing number of genes that are involved in these processes have been unambiguously identified, and their mechanisms of action are under active investigation. There is clear evidence that several lipids are transported between organelles by non-vesicular mechanisms that involve zones of apposition between donor and acceptor compartments, and macromolecular assemblies that involve multiple lipids and proteins. Several dozen lipids participate in intra- and intercellular signalling processes. In most instances, the levels of signalling molecules are exceedingly low compared with the complement of structural lipids that is present in membranes. Lipids adopt defined phases depending on their molecular structure and the physical conditions. In lipid mixtures, two fluid phases can coexist with different physical properties: liquid-disordered and liquid-ordered. Liquid-ordered assemblies in biomembranes, known as lipid rafts, are small and transient but can coalesce and become stabilized during signalling and vesicle budding. How proteins contribute to phase separation and preferentially distribute into one of the two different phases (or at their interface) is presently unclear.

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