Crystal structure of the human natural killer (NK) cell activating receptor NKp46 reveals structural relationship to other leukocyte receptor complex immunoreceptors.

Natural cytotoxicity receptors (NCR) mediate lysis of a variety of tumor and virus-infected cells by natural killer (NK) cells. Upon engagement, NCR trigger the cytolytic activity and cytokine release of NK cells through association with ITAM-containing signaling molecules. To further understand the function of these receptors in activation of natural cytotoxicity, we determined the crystal structure of the extracellular ligand binding domain of human NKp46, one of three known NCR, at 2.2-A resolution. The overall fold and disposition of the two C2-set immunoglobulin domains are similar to the D1D2 domains of inhibitory killer cell Ig-like receptor (KIR) and Ig-like transcript (ILT) receptors. As the cellular ligands of NKp46 have not yet been defined, the known ligand binding sites of KIR and ILT were compared with the corresponding structural regions of NKp46 in an effort to identify potential areas suitable for molecular recognition. A potential binding site for influenza hemagglutinin is located near the interdomain hinge, a region that mediates ligand binding in KIR. The structural similarity of NKp46 to inhibitory KIR receptors may have implications for how NK cells balance activating and inhibitory signals.