In meso Approaches to Membrane Protein Crystallization - Molecules: Nucleation, Aggregation and Crystallization - page 270

spectroscopy.” Crystals of the reaction center from Rhodobacter
sphaeroides were obtained by a conventional hanging-drop experiment
and harvested directly without the addition of lipase or cryoprotectant,
and the structure was refined to 2.2 Å resolution. In contrast to the earlier
lipidic cubic phase reaction center structure (Katona et al ., 2003), the
mobile ubiquinone could be built and refined. In these experiments the
components similar to those for cubic phase crystallization (structural
resolution 2.35 Å) were used (Katona et al ., 2003): the MO/membrane protein/
detergent/buffer. The only additional component was a small amphiphilic
molecule, 1,2,3-heptanetriol or Jeffamine M600. The other work (Cherezov
et al ., 2006) is similar to the paper mentioned above. The light harvesting
II complex (LH2) was crystallized and a structural resolution of 2.45 Å
was achieved. Several additives were used in this study: KSCN, butanediol,
pentaerythritol propoxylate (PPO), t-butanol, Jeffamine, and 2-methyl-
2,4-pentanediol (MPD). A 2.0 Å structure was available for LH2. It was
obtained using vapor diffusion-grown crystals of the detergent-solubilized
complex (Papiz et al ., 2003).
Is the “sponge phase” approach better than crystallization in the cubic
phase? Unlike the former method, this approach has not led to a break-
through in the structural biology of membrane proteins. The method has
not resolved the structure of new membrane proteins nor has it achieved
considerable improvements in structural resolution. In addition, the use of
small additives to obtain the sponge phase can sometimes be harmful for
a membrane protein. We have conducted a test of this approach using BR
and the additives described in Cherezov et al . (2006). Some of the results
are presented in Table 2 (Moisseva et al ., unpublished).
These experiments show that, indeed, most of the additives are harm-
ful for bacteriorhodopsin and the approach does not allow one to obtain
BR crystals of the same quality as in the cubic phase approach. Nevertheless,
this does not mean that the sponge phase approach can be neglected; how-
ever, since for some proteins it will work well. Also, we cannot exclude the
possibility that there will be further improvements of the method. As in the
case of the previous approach, unfortunately, there is a lack of information
about the behavior of in meso systems in the course of crystallization.