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Crystal structure of the photoprotein obelin solved at 1.1 angstrom Zhi-Jie Liu*1; ES Vysotski2; J Rose1; J Lee1; BC Wang1 (1) University of Georgia, Athens, GA 30602, USA; (2) Institute of Biophysics, RAS (SB), 660036 Krasnoyarsk, Russia      *(zjliu@arches.uga.edu) The crystal structure of the photoprotein obelin from Obelia longissima has been determined using 3 angstrom sulfur single-wavelength anomalous scattering (SAS) data collected using 1.74 angstrom X-rays. Crystals of obelin were grown in space group P62 with one molecule in the asymmetric unit. Protein phases were estimated from the anomalous signals of 8 sulfur atoms using the ISAS program and produced an electron density map of excellent quality that enabled the tracing of the complete polypeptide chain. The obelin structure contains 8 helices bundled in helix-turn-helix motifs characteristic of calcium-binding proteins. Contrary to the prediction of a peroxide, the ligand coelenterazine has only one oxygen atom bound at the C2-position. The same one-oxygen substitution at C2 has been confirmed in a new C2 crystal form which diffracted to 1.1 angstrom. This protein structure represents only the second de novo structure determined using the SAS signal of the sulfur substructure in the crystal. The method used here is theoretically different from that used for crambin in 1981 (4.72 kDa), and represents a significant advancement in protein crystal structure determination. Supported by ONR and RAS. [Talk: liu.zhijie.56131]

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