Crystallization and preliminary crystallographic characterization of the extracellular Ig-like domain of human natural killer cell activating receptor NKp44

The cytolytic activity of human natural killer (NK) cells is regulated by fine balancing between activating and inhibitory signals related to distinct families of inhibitory and triggering receptor molecules. The recent identification of NKp44, a triggering receptor selectively expressed by activated NK cells and playing an important role in the natural cytotoxicity of these cells, opens the way to the characterization of the mechanisms involved in NKp44-mediated NK-cell activation on a structural basis. For this purpose, the NKp44 extracellular Ig-like domain was cloned, overexpressed, refolded and crystallized. Diffraction intensities were collected to a resolution limit of 2.2 A on a synchrotron source. NKp44 extracellular Ig-like domain crystals belong to the hexagonal space group P6(2)22 (or P6(4)22), with unit-cell parameters a = b = 60.4, c = 197.2 A, gamma = 120 degrees. Evaluation of the crystal packing parameter suggests the presence of one molecule (13.8 kDa) per asymmetric unit, with a solvent volume of 67%.