Cloning, expression, purification, crystallization and preliminary crystallographic analysis of selenomethionine‐labelled KaiC‐like protein PH0186 from Pyrococcus horikoshii OT3

KaiC is the central protein in the circadian‐clock system of cyanobacteria. A selenomethionine‐labelled KaiC‐homologous protein from Pyrococcus horikoshii OT3 (PH0186; 28 kDa) was crystallized by the sitting‐drop vapour‐diffusion method using ethanol as a precipitant. The crystals diffracted X‐rays to beyond 2.0 Å resolution using a synchrotron‐radiation source. The space group of the crystals was determined to be C 2, with unit‐cell parameters a = 173.7, b  = 51.8, c = 97.5 Å, β = 122.8°. The crystal contains three molecules in the asymmetric unit ( V M = 2.2 Å 3  Da −1 ) and has a solvent content of 43.5%. Sixfold noncrystallographic symmetry was identified from self‐rotation calculations, assuming the presence of a hexamer in the crystal.