In meso Approaches to Membrane Protein Crystallization - Molecules: Nucleation, Aggregation and Crystallization - page 265

β 2 -adrenergic G protein-coupled receptor (Cherezov et al ., 2007), is an
important breakthrough in the field. Indeed, GPCRs comprise the largest
integral membrane protein family in the human genome, with over one
thousand members. These receptors participate in the transduction of
signals across cellular membranes in response to a huge variety of extra-
cellular stimuli, including light, proteins, peptides, small molecules,
hormones, protons and ions. Activated GPCRs trigger a cascade of intra-
cellular responses, primarily through interactions with their cognate
heterotrimeric G proteins. In addition, GPCRs are associated with a mul-
titude of diseases, which makes members of this family very important
pharmacological targets.
These three examples show the power of the new approach. The
method deserves more detailed consideration. First of all, most crys-
tallizations were done using the monooleoyl (MO)/BR buffer system.
A phase diagram of a simple MO/water system is shown in Fig. 3. The
phase diagram is quite complex and comprises a wide range of phases:
from two lamellar ( Lc and L ) to two cubic bicontinuous phases of

different symmetries Ia3d and Pn3m (Briggs et al ., 1996). In this case
the membrane divides the solvent into two interpenetrating continuous
regions. Luzatti was the first to discover the lipid cubic phase and
recognize that the midsurfaces of bilayers are close to cubic minimal
surfaces, which have a zero mean curvature everywhere (Mariani
et al ., 1988). It is interesting to note that bicontinuous cubic phases
were observed in the cells (Landh, 1995), and they are used in the
food industry (Fontell, 1990) as well as for drug delivery (Ericsson
et al ., 1991).
Crystallization in the lipid cubic phase is very simple in practice.
An example — crystallization of BR — can be described by the following
procedure (Gordeliy et al ., 2003):

α

(i) Fill a 200 µ l PCR tube with monooleoyl powder (~4 mg).

(ii) C and spin the lysolipid down for 10 minutes at

13 000 g at room temperature.

Melt the MO at 40 °