Crystallization of the PX domain of cytokine-independent survival kinase (CISK): improvement of crystal quality for X-ray diffraction with sodium malonate

Phox homology (PX) domains play critical roles in the intracellular localization of a variety of cell-signaling proteins through interactions with specific phosphoinositides. For cytokine-independent survival kinase (CISK), the PX domain also plays a role in the regulation of CISK activity in response to the activation of phosphatidylinositol-3 (PI-3) kinase. The PX domain of mouse CISK has been purified and crystallized, as well as its complex with a phosphoinositide ligand. The native PX domain was crystallized in space group I4 and the crystals diffracted to a maximum resolution of 1.6 Å. Selenomethionine-derivatized PX domain was also prepared and crystallized for MAD phasing. In this study, the use of sodium malonate is the key to both successful crystallization and cryoprotection of the PX domain of CISK.