One and Two Dimensional Arrays of Membrane Proteins Stabilized by Amphipol

All living cells are sounded by a lipid bilayer which forms the cell membrane and separates the aqueous intraand extra cellular compartments. The lipid bilayer itself creates a five nanometer thick environment which is hydrophobic at its center and hydrophilic where it meets the bounding environments (Fig1). As throughout the rest of the cell, the membrane contains a repertoire of proteins which confer on it active functions such as transport, metabolism or molecular recognition. However, unlike the water soluble proteins found in the cytoplasm, the membrane proteins are highly hydrophobic where they are buried in the bilayer but hydrophilic where they face the compartments on either side. Because of this the study of purified membrane proteins is complicated by the need to recreate the mixed environment of the membrane. In some cases membranes containing only one type of protein are found in nature and one of these, the purple membrane from photosynthetic bacteria, was used to obtain the first molecular structure of a membrane protein using electron microscopy by Henderson and Unwin [1]. In most cases the membranes contain low levels of the required protein in a mixture with others and thus the protein needs to be extracted from the original membrane and purified before it can be studied in a reductionist way. To extract the protein from the membrane workers generally use detergents which dissolve the membrane and from a protective micelle around the hydrophobic regions of the membrane protein (Fig.1). Whilst the basis of many successful studies, detergents have negative aspects and, in particular, they need to be present at a concentration above that needed to from micelles. Under these conditions a complex mixture exists of protein containing micelles, empty micelles and free detergent. Furthermore the micelle is dynamic and does not recreate the surface pressure of the bilayer and can be destabilizing.