Preparation and preliminary study of crystals of the recombinant calcium-regulated photoprotein obelin from the bioluminescent hydroid Obelia longissima

Crystals of recombinant obelin, the Ca2+-regulated photoprotein from the ma rine hydroid Obelia longissima, have been grown from sodium citrate solutio ns. Crystals grow as hexagonal light-yellow rods (0.1 x 0.1 x 1.0 mm) which diffract to beyond 1.8 Angstrom with synchrotron radiation of 1.0 Angstrom wavelength. The crystals have a primitive hexagonal lattice with unit-cell parameters a = 81.55, c = 86.95 Angstrom. The asymmetric unit contains two molecules. This represents the successful preparation of single crystals o f a photoprotein obelin which have promising diffraction properties.