Membrane Structure Studies by Means of Small-Angle Neutron Scattering (SANS)


The basic model for membrane structure - a lipid bilayer with imbedded proteins - was formulated 35 yearsago, however the detailed structure is still under active investigation using a variety of physical, chemical andcomputational techniques. Every biologically active cell is encapsulated by a plasma membrane with most cells alsoequipped with an extensive intracellular membrane system. The plasma membrane is an important boundary between thecytoplasm of the cell and the external environment, and selectively isolates the cell from that environment. Passivediffusion and/or active transport mechanisms are provided for water, ions, substrates etc. which are vital for cellmetabolism and viability. Membranes also facilitate excretion of substances either as useful cellular products or as waste.Despite their complexity and diverse function, plasma membranes from quite different cells have surprisingly similarcompositions. A typical membrane structure consists of a phospholipid bilayer with a number of proteins scatteredthroughout, along with carbohydrates (glycoproteins), glycolipids and sterols. The plasma membranes of mosteukaryotic cells contain approximately equal weights of lipid and protein, which corresponds to about 100 lipidmolecules per protein molecule. Clearly, lipids are a major constituent and the study of their structure and function inisolation provides valuable insight into the more complex intact multicomponent membrane. The membrane boundprotein is the other major constituent and is a very active area of research for a number of reasons including the fact thatover 60% of modern drugs act on their receptor sites. The interaction between the protein and the supporting lipidbilayer is clearly of major importance. Neutron scattering is a powerful technique for exploring the structure ofmembranes, either as reconstituted membranes formed from well characterised lipids, or as intact membranes isolatedfrom selected biological systems. A brief summary of membrane structure will be followed by an outline of the neutronscattering techniques used to understand membrane structure and dynamics. The emphasis will be on the small angleneutron scattering technique since there is a very powerful instrument at Serpong, however brief mention of othertechniques will be included to demonstrate how a multidisciplinary approach is usually requiredKeywords: neutron scattering, membrane structure, biology, SANS. R. B. Knottl,21Bragg Institute, ANSTO, Private Mail Bag, Menai NSW 2234, Australia2CSIRO Minerals, Box 312, Clayton South VIC 3169, Australia