Zinc bound to the killer cell-inhibitory receptor modulates the negative signal in human NK cells

The lysis of target cells by human NK cells is inhibited by several kinds of receptors with varying specificities for the MHC class I molecules of target cells. The requirements for complete inhibition of NK cytotoxicity appear to be complex and not well defined. The HLA-C-specific members of the killer cell-inhibitory receptor (KIR) family, carrying two Ig domains (KIR2D), are unusual among Ig superfamily members in their ability to bind zinc. A role for the zinc-binding site in KIR-mediated inhibition was demonstrated in this study using a functional reconstitution system in NK cells. Replacement of six histidines by alanine residues in putative zinc binding sites of a KIR2D ablated zinc binding and markedly impaired its inhibitory function, but left intact its ability to bind HLA-C and to transduce a positive signal through an immunoreceptor tyrosine-based activation motif grafted onto its cytoplasmic tail. Thus, zinc modulates specifically the negative signal transmitted by this KIR molecule. Mutation of an exposed amino-terminal zinc-binding motif alone was sufficient to impair the inhibitory function of KIR. The data suggest that complete inhibition of HLA-C-specific NK cells requires a zinc-dependent protein-protein interaction via the amino-terminal end of KIR2D.