Folding and crystallization conditions of menbrane protein supercomplexes

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This research project has provided the following conclusion about the folding mechanism and the crystallization method of membrane protein. Three dimensional structure of the membrane protein supercomplexe spanned across the biological membrane in the cell is stabilized by hydrophilic loops exposed to aquaous medium as well as by alpha-helices placed under the ydrophobic environment in the membrane which provide hydrogen bonds and electrostatic interactions between these helices. Thus, solubilization with a detergent which covers the hydrophobic surface seems the best method for isolation of this type of protein complex without denaturation. The specific interaction between the protein melocules which stabilizes the crystalline state is likely to be the one between the hydrophilic surfaces, suggesting that larger homologous membrane protein is better for crystallization than the smaller one. This conclusion is suppoted by the fact that only the biggest cytochrome c oxidase and cytochrome bc1 complex, isolated from beef heart mitochondria, among many homologous proteins have been crystallized. Structure of the detergent molecules which attach to the hydrophobic surfaces of membrane proteins could control the specific interactions between the hydrophilic surface of the protein moleules to determine the crystallization conditions. The control mechanism is not so simple as has been proposed by Michel et al.