On the Asymmetric Composition of Plasma Membranes

Abstract

By intuition, we always have known that there was asymmetry in the function and composition of plasma membranes. After all, we had to accommodate into that structure specific ion channels, pumps, transporters, receptors and many other functions which clearly demand highly differentiated molecular assemblies. Until recently, we have visualized the plasma membrane primarily as a smooth contiguum which, depending somewhat on cell type, stretched over a round, polygonal or spindle-shaped cell body. This original concept of the structure of the plasma membrane had been strongly influenced by the appearance of the erythrocyte under the microscope. Reinforcement of this image occurred when the first laboratory preparations of isolated plasma membranes, cell ghosts, synaptosomes, etc. were inspected at higher resolutions with the transmission electron microscope; the prevailing image was one of a fairly smooth cytoplasmic envelope. The concurrent availability of cell ghosts and of purified plasma membrane preparations instigated numerous studies on the molecular composition of the membrane. One of the first chemical analyses of the erythrocyte membrane led to the formulation of the familiar lipid bi-layer model and such bi-layers were eventually reconstituted between two aqueous compartments (GOERTER & GRENDEL, 1925; DANIELLI & DAVSON, 1935; MUELLER et al., 1962). Addition of peptides, proteins, antibiotics and ionophores changed the functional characteristics of those bi-layer membranes, but on visual inspection these systems revealed remarkably smooth surfaces.