ABSTRACT
Before considering the topology of membrane proteins it is necessary to define the two sides of the membrane bilayer. In eukaryotic cells the plasma membrane can be considered as having an extracellular surface and a cytosolic surface, while the intracellular organelles have a cytosolic surface and a lumenal or cisternal surface. Cytochemical demonstration of enzyme activity at the electron microscopic level – Using either an electron-dense enzyme product or antibody against know membrane protein labeled with an electron-dense particle such as gold, the orientation of membrane vesicles can be demonstrated and compared with same marker used in whole tissue. Membrane proteins may also be modified by covalent attachment of fatty acids myristate (C14:0) or palmitate (C16:0) or the isoprenoid chains farnesyl (C15) or geranyl-geranyl (C20). Membrane proteins are difficult to purify and because of their hydrophobic nature are not easily crystallized. Therefore most experimental studies of the orientation of the polypeptide chain with respect to the bilayer have used probes.
