Preliminary Cryocrystallographic Study of the Mitochondrial Cytochromebc1Complex: Improved Crystallization and Flash-cooling of a Large Membrane Protein

Ubiquinol-cytochrome c reductase is a crucial integral membrane protein in the mitochondrial respiratory cycle. Eleven subunits containing three cytochrome heme groups and a 2Fe-2S Rieske center make up this 240 kDa enzyme complex. Previously, many different crystal forms of the bc 1 complex have displayed diffraction to as far as 4.5 A. However, rapid degradation of the protein in the X-ray beam at room temperature has obstructed the collection of a full data set from a single crystal. As slight heterogeneities between crystals severely hampered merging of data from different crystals, we sought a m ethod to stabilize the protein crystal in the X-ray beam in order to collect a full data set from one crystal sample. To this end, water soluble protein crystals are frequently flash-cooled to cryogenic temperatures; however, there is no report of cryocrystallography for membrane proteins. In this communication, we report on a successful experiment in which flash-cooled bc 1 membrane protein crystals have given rise to sustained diffraction over a 60 hour data collection period at a synchrotron source. Furthermore, we present an improved purification and crystallization protocol yielding crystals readily diffracting out to 3.3 A. These results should greatly aid in the future realization of the molecular structure of the bc 1 complex as well as other membrane proteins.