Extraction of Membrane Proteins

The landmark paper on biomembrane structure in 1972 by Singer and Nicolson (1) is still widely accepted as an excellent working hypothesis (2), but a more rigid micro domain structure is now believed to exist in native membranes (3–5). The original fluid mosaic model of biomembrane structure basically defined two classes of membrane proteins that are associated, to varying degrees, with the phospholipid bilayer (1). In addition to peripheral and integral membrane proteins, a third class of membrane proteins is represented by lipid-anchored proteins (6). In addition to these protein-membrane interactions, certain components of the membrane cytoskeleton and the extracellular matrix are also directly or indirectly associated via binding-proteins or receptor molecules with membranes (7). Treatment of biological membranes with salt solutions or change in pH usually dissociates peripheral proteins because these extrinsic membrane proteins interact with the membrane surface mostly via electrostatic and hydrogen bonds. Integral membrane proteins that possess hydrophobic surfaces are more strongly associated with the bilayer and these intrinsic proteins extend across or are partially inserted into the lipid bilayer. Extraction of integral membrane proteins is commonly accomplished by solubilizing the protein-containing membrane fraction using a variety of detergents (8) (see Fig. 1).