Preparation and X-ray crystallographic analysis of recombinant obelin crystals diffracting to beyond 1.1 Å

Crystals of recombinant obelin, the Ca(2+)-regulated photoprotein from the marine hydroid Obelia longissima, have been grown from a solution containing PEG 8000 and potassium phosphate. Hexaminecobalt trichloride was used as an additive to increase the chance of crystallization. The crystals grow in a light yellow cubic form (0.5 x 0.5 x 0.45 mm) which diffracts to beyond 1.1 A resolution. The crystals belong to the space group C2, with unit-cell parameters a = 83.43, b = 54.92, c = 52.99 A, beta = 112.00 degrees. The asymmetric unit contains one molecule. Crystals exposed to calcium ion before and after X-ray irradiation emit light, confirming that the crystals consist of an active photoprotein.