Crystallization of Two Membrane Proteins: Bacteriorhodopsin and Photosynthetic Reaction Centres

The only method to determine the spatial structure of proteins at a resolution sufficient to trace the peptide chain and to position the amino acids is X-ray crystallography. Crystallography depends on the availability of large, well-ordered crystals. Membrane proteins could not be crystallized until recently. The main reason for the lack of success in the crystallization of membrane proteins lies in the amphiphilic nature of the membrane proteins surface: Those surface domains exposed to the aqueous phases on both sides of the membrane are hydrophilic, like the surface of globular proteins, whereas the surface domains in contact with the alkane chains of the lipids must be highly hydrophobic. As a consequence, typical membrane proteins can only be solubilized with the help of detergents: Detergent micelles replace the lipids of the membrane and shield the hydrophobic surface domains of the membrane proteins against water. Mild detergents have to be used to solubilize membrane proteins in a functionally active state.