Crystallization and preliminary X-ray analysis of human uridine-cytidine kinase 2

Uridine-cytidine kinase (UCK), which converts uridine and cytidine to their corresponding monophosphates, is a rate-limiting enzyme involved in the salvage pathway of pyrimidine-nucleotide biosynthesis. Two members of human UCK, named UCK1 and UCK2, were cloned recently and UCK2 was reported to play a crucial role in activating anti-tumour pro-drugs in human cancer cells. Human UCK2 was expressed, purified and crystallized alone or in complex with various ligands. Free UCK and UCK complexes were crystallized in six crystal forms. Form I (ligand-free) belongs to space group P2₁2₁2, with unit-cell parameters a = 83.1, b = 93.7, c = 157.1 Å. Forms IIa (with CTP), IIb (with UTP) and IIc (with cytidine) belong to space group F222, with unit-cell parameters a = 133.3, b = 247.3, c = 91.6 Å (IIa), a = 132.1, b = 247.0, c = 91.5 Å (IIb) and a = 136.7, b = 246.3, c = 90.4 Å (IIc), respectively. Form III (with ATPγS) belongs to space group C222₁, with unit-cell parameters a = 70.3, b = 149.9, c = 117.2 Å. Form IV (with cytidine and ATP) belongs to space group C2, with unit-cell parameters a = 89.0, b = 109.7, c = 64.8 Å, β = 95.3°. Diffraction data were collected from these crystals; form IV diffracted to 1.8 Å resolution.