The functional binding site for the C-type lectin-like natural killer cell receptor Ly49A spans three domains of its major histocompatibility complex class I ligand

Natural killer (NK) cells express receptors that recognize major histocompatibility complex (MHC) class I molecules and regulate cytotoxicity of target cells. In this study, we demonstrate that Ly49A, a prototypical C-type lectin-like receptor expressed on mouse NK cells, requires species-specific determinants on beta2-microglobulin (beta 2m) to recognize its mouse MHC class I ligand, H-2D(d). The involvement of beta 2m in the interaction between Ly49A and H-2D(d) is also demonstrated by the functional effects of a beta 2m-specific antibody. We also define three residues in alpha1/alpha2 and alpha3 domains of H-2D(d) that are critical for the recognition of H-2D(d) on target cells by Ly49A. In the crystal structure of the Ly49A/H-2D(d) complex, these residues are involved in hydrogen bonding to Ly49A in one of the two potential Ly49A binding sites on H-2D(d). These data unambiguously indicate that the functional effect of Ly49A as an MHC class I-specific NK cell receptor is mediated by binding to a concave region formed by three structural domains of H-2D(d), which partially overlaps the CD8 binding site.