In meso Approaches to Membrane Protein Crystallization - Molecules: Nucleation, Aggregation and Crystallization - page 262

H. Michel, J. Deisenhofer and R. Huber received the Nobel Prize in
Chemistry in 1988 (Deisenftofer et al ., 1984).
In spite of this success, obtaining membrane protein crystals was still
a very difficult task. An illustration of this statement is the story of bacte-
riorhodopsin crystallization. Bacteriorhodopsin belongs to the family of
seven alpha-helical retinal proteins and plays an important role in the
bioenergetics of Halobacterim salinarum . Its chromophore retinal absorbs
a photon, isomerizes and triggers a sequence of further molecular events
leading to proton translocation against the electrochemical gradient of
membranes. Further, the energy of the gradient is used by another mem-
brane enzyme ATP synthase to catalyze the ADP-ATP reaction.
This protein is quite easy to solubilize and purify in large amounts,
and it was used as a major model system to study the molecular mecha-
nisms of proton transport (Landau, Rosenbusch, 1996). However, all
attempts to obtain crystals of the protein diffracting to high resolution
failed during a period of more than 20 years.
The success with bacteriorhodopsin (BR) crystallization came
unexpectedly. In 1996, Rosenbusch and Landau obtained high quality
BR crystals using a principally new approach: the protein was crystal-
lized in the lipidic cubic phase (Landau, Rosenbusch, 1996). The major
differences between the two approaches are illustrated in Fig. 2. A prin-
cipal feature of this novel method is that after solubilization the protein
Fig. 2. A schematic representation of an in meso approach to crystallization of mem-
brane protein: (1) solubilization and purification of proteins, (2) reconstitution of the
protein into a lipid bilayer, (3) initiation of crystal growth by direct addition of the precip-
itant to the lipid/protein phase, and (4) formation of crystals, usually of type I.