The functional binding site for the C-type lectin-like natural killer cellreceptor Ly49A spans three domains of its major histocompatibility complexclass I ligand

Natural killer (NK) cells express receptors that recognize major histocompa tibility complex (MHC) class I molecules and regulate cytotoxicity of targe t cells. In this study, we demonstrate that Ly49A, a prototypical C-type le ctin-like receptor expressed on mouse NK cells, requires species-specific d eterminants on beta2-microglobulin (beta 2m) to recognize its mouse MHC cla ss 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 a lpha3 domains of H-2D(d) that are critical for the recognition of H-2D(d) o n target cells by Ly49A. In the crystal structure of the Ly49A/H-2D(d) comp lex, 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 ind icate that the functional effect of Ly49A as an MHC class I-specific NK cel l receptor is mediated by binding to a concave region formed by three struc tural domains of H-2D(d), which partially overlaps the CD8 binding site.