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 A resolution using a synchrotron-radiation source. The space group of the crystals was determined to be C2, with unit-cell parameters a = 173.7, b = 51.8, c = 97.5 A, beta = 122.8 degrees. The crystal contains three molecules in the asymmetric unit (V(M) = 2.2 A(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.