STRATEGY TO OBTAIN HIGH RESOLUTION STRUCTURE OF MEMBRANE PROTEINS BY X-RAY CRYSTALLOGRAPHY

The membrane proteins, which have key roles in various biological processes such as photosynthesis, respiration, ion pump, and so on, are estimated for 25–30% of all the species of proteins. The membrane proteins whose tertiary structures are known are, however, less than one % of those of water-soluble proteins. It is because both purification and crystallization of membrane proteins are more difficult than other proteins. The procedures of purification and crystallization were summarized for the membrane proteins whose structures were determined by X-ray method. Each membrane protein has suitable detergents for respective purification and crystallization. There are two types of crystals of membrane proteins diffracting at high resolution. In the type II crystal hydrophilic surfaces of molecules contribute mainly to inter-molecular interactions, while in the type I crystals transmembrane surfaces with hydrophobic nature contact with each other. The quality of membrane protein crystals can be improved by adjusting solvent contents as that of water-soluble proteins.