Prospects for X-ray crystal structure analysis of selenoproteins with SPring-8 synchrotron radiation

The impact of synchrotron radiation as a new X-ray source with its polychromatic nature and associated high intensity and fine collimation has brought important advances in the field of macromolecular crystallography. It has extended structure determinations of proteins to higher resolution, allowed use of smaller crystals with larger unit cells. In particular, selenoprotein is a suitable material for X-ray crystal structure analysis with synchrotron radiation, since its polychromatic nature and anomalous diffraction from the selenium atom(s) in the protein allow the multiple-wavelength anomalous-diffraction (MAD) method to be used for phase determination. RIKEN beam line I (BL45XU), installed in the SPring-8 synchrotron radiation facility, has been designed and developed to optimize MAD data collection based on a 'trichromatic concept'. This concept facilitates simultaneous data collection by use of a 'trichromator', of three intensity data-sets at three different wavelengths from a single protein crystal, and thus results in the minimization of systematic errors in the measurement of anomalous diffraction by the MAD method. The X-ray crystallographic analysis of selenoprotein with SPring-8 synchrotron radiation, therefore, results in very fast data collection and high resolution structural analysis using a single protein crystal, which should lead to elucidation of the structure-function relationship of selenoprotein.