Split Membrane Lipids and Polypeptides

It is currently accepted that the biological membrane is composed of amphiphilie lipids arranged in bilayers with functionally important proteins embedded in, or spanning, the hydrophobic interior of the bilayer (Singer, 1977). Support for this view comes from a variety of chemical and physical methods. Enzymatic and chemical modification reactions have established the basic property of membrane asymmetry for lipids (Verkleij et al., 1973) and proteins (Steck, 1978) and the results of many physical studies are most easily interpreted in the context of structural and functional asymmetry. Such studies often rely on the modification of biomembranes by probes; relatively few methods exist, however, to study the transbilayer concentration of native molecules directly. Moreover, the accessibility of reagent or probe molecules to membrane molecules or domains, or their distribution within or across the plane of the membrane, remain in some instances uncertain. In such instances it is desirable to have the means to determine their location directly by an independent method such as electron microscopy.