X-ray diffraction data is the main source of 3-dimensional structure information at atomic resolution for proteins. While a great deal of information is potentially available from this technique, its application requires the ability to prepare crystals of size and order suitable for X-ray analysis. This has proved especially difficult for membrane proteins. Complex II (succinate: ubiquinol oxidoreductase) is a membrane protein complex, which funnels electrons from succinate into the mitochondrial respiratory chain. The respiratory chain is responsible for biological oxidation and for conservation of the energy released in the form of a proton electrochemical potential gradient across the mitochondrial inner membrane. Energy from this gradient is then used to synthesize ATP or to do work by transporting substances across the membrane. A number of mitochondrial myopathies and CNS disorders have been shown to be due to defects in the mitochondrial electron transport chain and in some cases in complex II. In the course of our work on the mitochondrial bc1 complex we have prepared and solubilized mitochondria from a number of vertebrate sources. We have learned to prepare complex II as a by-product of the cytochrome bc1 preparation. In the case of the chicken enzyme, we have been able to crystallize the enzyme. The crystals can be flash-frozen for cryogenic data collection with diffraction up to 3.5 Angstrom units. The data can be indexed on an orthorhombic lattice with unit cell edges 69.036 x 83.735 x 290.214 Angstrom units, and systematic absences suggest the spacegroup P212121. It seems reasonable to expect that with improvement in size and mosaicity the crystals will diffract beyond 3.0 Angstrom units, which would be sufficient to build an atomic model of the protein once the data is phased. It is proposed to (a) Improve the quality of the crystals with respect to size and mosaicity, by using recrystallization and seeding techniques and more carefully controlling the crystallization conditions. (B) Collect diffraction data and phase it, either by molecular replacement using the coordinates from fumarate reductase of E. coli or W. succinogenes, or (in case neither of those structures is available) by isomorphous derivative and multiwavelength anomalous dispersion techniques using heavy atom derivatives and the intrinsic irons of the complex. (C) Build an atomic model of the protein into the electron density map and refine it against the diffraction data, for submission to the protein data bank.