Biology Reference
In-Depth Information
Why do crystals grow in the lipidic cubic phase? There are several
possible reasons for the efficiency of the method, which have been dis-
cussed in the literature. However, the first question is whether the system
is really in this phase. In fact, the system for crystallization is different
from the pure MO/water system. Indeed, it contains salts in the buffer and,
most importantly, a certain amount of the detergent. A priori it was not
evident that the phase diagram shown in Fig. 3 somehow describes the
lipidic/membrane protein/detergent/salt/water system as well. Hence, the
detailed kinetics of a proteolipidic cubic phase was studied by neutron
scattering in the course of crystallization of the membrane protein bacte-
riorhodopsin. It was shown that the initiation of the crystallization process
by salt addition leads to a dramatic decrease in the lattice constant, but no
phase transition takes place. The cubic phase of Pn3m symmetry is
observed during the entire crystallization process. No other phases are
present in a macroscopic amount (Efremov et al ., 2005). However, this
does not exclude the presence of a small amount of phases other than the
cubic phase (Efremov et al ., 2005). There is evidence of the presence of a
lamellar phase around the growing protein crystals but there is no definite
proof of this (Cherezov et al ., 2007). What can one speculate about the
mechanism of membrane protein crystal growth? There has been a theo-
retical attempt to understand the mechanism of membrane protein crystal
growth in the lipidic cubic phase (Grabe et al ., 2003). Theoretical calcu-
lations showed that the elastic energy of the deformation of the curved
bilayer due to embedded proteins may be the driving force for crystal-
lization. Unfortunately, this theory is incomplete, and it is not yet known
whether the model of the crystallization system used for the
calculation is completely correct. Further experimental and theoretical
studies are required to elucidate the mechanism of crystallization.
Crystallization in the sponge phase
The sponge ( L 3 ) phase can be imagined as a disordered cubic phase. The
ordered cubic phase structure is perturbed by thermally excited shape fluc-
tuations of the membranes. The sponge phase, which is still bicontinuous,
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