Membranes: reading between the lines

Abstract

Membranes are remarkable entities; only ~40Å thick, they maintain the territorial integrity of a cell or organelle while regulating the flow of matter, information and energy between the interior and the external environment. The lipids and proteins that dominate the composition of membranes exhibit a characteristic architecture in which the lipids adopt a bilayer arrangement penetrated by integral membrane proteins [1]. In the common shorthand representation for this organization, the membrane bilayer is represented by two parallel lines with ellipsoidal membrane proteins embedded in a nonpolar continuum. While this depiction is obviously understood as a cartoon version of reality, it does convey a misleading impression of order in a biological membrane that belies the inherent compositional, structural and dynamic complexities of this system. Even for the simplest case of a bilayer composed of a single lipid type, a sharp interface does not exist between the lipid bilayer and surrounding aqueous solution, but rather there are substantial displacements of headgroups from the average plane, and the aliphatic chains in the bilayer interior are disordered owing to low torsional angle barriers and the steric consequences of cis-double bounds [[2], [3] and [4]]. The associated packing defects even allow polar compounds to penetrate into the membrane interior; as one consequence, water has an appreciable bilayer permeability [5]. The picture that emerges from such studies is that the biologically relevant fluid phase of membranes is highly dynamic, and the time-averaged structure can only be defined probabilistically.