Crystallization Methods of Membrane Proteins: Practical Aspects of Crystallizing Plant Light-Harvesting Complexes
The light reactions in photosynthesis involve a series of membrane protein complexes residing in photosynthetic membranes. The three-dimensional structures of these complexes are indispensable for a better understanding of the underlying molecular mechanism of light reactions. The key step in the structure determination of these complexes by X-ray crystallography is crystallization. Here we review the major aspects concerning membrane protein crystallization. Membrane protein crystals can be classifi ed into three basic types according to their different packing patterns. Membrane proteins are purifi ed and crystallized in detergent solutions. The basic properties of detergents and some considerations about detergents are discussed. A homogeneous membrane protein sample with detergent concentration properly controlled is a good start for crystallization. It is not diffi cult to fi nd initial crystallization conditions for membrane proteins, but further optimization for high-quality crystals suitable for structure determination is a challenging task. The optimization strategies include additive screen, detergent exchange and protein modifi cation, in addition to adjustments on regular factors like temperature, pH and precipitants. In the second part of this chapter, we present a practical case of photosynthetic membrane protein crystallization and optimization. The plant major light-harvesting complex of Photosystem II was crystallized in four different forms. One of them was highly ordered in all three dimensions and could diffract X-rays to 2.5 Å resolution. It was obtained after careful optimization of detergent, lipid and additives. This crystal form belongs to a novel type of membrane protein crystal, type III, which is built from icosahedral proteoliposome vesicles. The formation of rigid and homogenous proteoliposomes is a prerequisite for the growth of well-ordered type III crystals.
KeywordsMembrane Protein Crystal Packing Detergent Concentration Crystallization Method Membrane Protein Complex
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