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When starting a membrane protein crystallization project, scientists are faced with unknowns such as: 1. is the purity of the starting material sufficient? and 2. which type of crystallization experiment is the most promising to conduct? The difficulty in purifying functional membrane protein samples for crystallization trials and the substantial costs that are associated with producing such samples require a pragmatic approach. Also, practical guidelines are desired to increase the efficiency of membrane protein crystallization trials. In order to address these questions, we have undertaken a systematic, comparative study investigating the effects of typical impurities on various membrane-protein crystallization regimes (detergent, vapor diffusion, micro batch, plug-based, crystallization in lipidic cubic phases). The results of this study has recently been published at: C. A. Kors, E. Wallace, D. R. Davies, L. Li, P. D. Laible and P. Nollert Effects of impurities on membrane-protein crystallization in different systems Acta Cryst. (2009). D65, 1062-1073
In short, it was found that the Lipidic Cubic Phase (LCP) based crystallization methodology is the most robust crystallization technique. Crystallization in detergent environments using vapor diffusion or microbatch approaches did not tolerate contamination levels in the forms of protein, lipid or other general membrane components that were tolerated by the LCP approach. 
Figure 1. The LCP crystallization methodology tolerates more than 50% impurities. Images are shown of crystallization experiments with photosynthetic reaction centre samples of increasing purity (as reflected by the A280/A800 ratio) using LCP, microfluidics or sitting-drop vapor-diffusion techniques. Overview of crystallization trials shown at uniform scale, for comparison of crystal size and habit.
The LCP-based crystallizations produced X-ray diffracting crystals of the photosynthetic reaction center (RC) of Rhodobacter sphaeroides from samples with significant levels of residual impurities. Indeed, crystals were grown from samples with protein contamination levels of up to 50%. Further experiments showed that the deliberate contamination of pure RC samples with lipid material and membrane fragments to pure samples had little effect on the number or on the quality of crystals grown in LCP-based crystallization screens. A kinetic exclusion model was devised to explain these results (Fig. 2). 
Figure 2: Why does the LCP methodology have a high tolerance for contaminating protein species (black and green)? Kinetic exclusion model: impurities are excluded from the crystal-growth process, essentially providing a microenvironment enriched in the crystallizing species (gray and yellow), thus favoring crystal growth. Contaminating protein species with large hydrophilic or hydrophobic moieties face an energy penalty for diffusion in curved membranes with small channels, resulting in less unproductive aggregation and hence less interference with the desired crystallization process. Similarly, lipidic cubic phases form substantial diffusion barriers for soluble proteins trapping soluble contaminating proteins within the small hydrophilic channels of the LCP matrix, where they are excluded from poisoning the crystal-growth surface. The local absence of contaminating species allows crystals to grow as they would in solution-based crystallization approaches (batch and vapor diffusion) using samples of higher purity. While this surprising finding was demonstrated for RC only, it hints toward advantages of the LCP methodology. If generally applicable, this tolerance for impurities would avoid the requirement for ultra high purity samples for initial crystallization screening trials. Once the membrane protein target crystallizability is determined, the found crystallization conditions that can be optimized in many ways, including further purified target protein.
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