Given a moderately diffracting crystal, does the choice of data reduction approach effect [sic] the S-SAD phasing result?
Swindell, James Tucker
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The inherent difficulty of solving the phase problem in macromolecular crystallography has been somewhat alleviated due to the advances in all areas of the diffraction experiment including protein purification, crystallization, X-ray sources, cryo-crystallography, detection technologies, and data reduction. A phasing approach, Sulfur-ISAS (Iterative Single Wavelength Anomalous Scattering), aimed at removing the generic necessity of either including selenium via protein engineering or derivatization using heavy atoms was hypothesized in 1985 by B.C. Wang and is, albeit gradually, increasing in popularity. We have recently determined the structure of AF1382, a small 95-residue protein encoded by Archaeoglobus fulgidus by S-SAD (Single Wavelength Anomalous Diffraction) phasing using two 360° data sets collected on a moderately (2.65Å) diffracting crystal. Producing an interpretable electron density map at the time of data collection required additional assistance from the SER-CAT support staff. The eventual phase solution was achieved by merging the two data sets in conjunction with expert processing, involving both the HKL2000-GUI and command line scaling via SCALEPACK. The downsides associated with removing the need for experienced crystallographers when dealing with data reduction becomes most evident when a data set does not yield an immediate structure solution, as in the AF1382 case. Due to the reliance on a single data reduction program, characteristic of many of this generations Structural Biologist, using point-an-click processing without a fundamental understanding programs operation causes data to often be discarded in lieu of mounting a second or third crystal in hopes of a better processing result. Discarding data for this reason illustrates pitfalls from an experimental point of view. First “difficult” proteins may produce only a few or even a single crystal and second, employing more than one data reduction program during phasing efforts could be advantageous to phasing results. Considering the difficulties involved with generating the AF1832 phases an obvious question presented itself. “Given a moderately diffracting crystal such as AF1382, does the choice of data reduction approach affect the S-SAD phasing results?” Herein we report a comparative analysis of data sets produced by five data reduction programs (HKL2000, d*TREK, XDS, MOSFLM, PROTEUM2) to the success rate of S-SAD phasing for data collected on a moderately diffracting crystal using 1.9Å SER-CAT (22ID) X-rays.