A role for allosteric disulfides in regulating the function of human killer cell receptor KIR2DL4

Abstract

Abstract KIR2DL4 is an unusual member of the killer cell Ig-like receptor (KIR) family expressed in NK cells. It resides mostly in endosomes and activates NK cells in response to soluble agonists, such as soluble HLA-G and a monoclonal antibody. KIR2DL4 signals from endosomes for a unique proinflammatory and proangiogenic response, which has relevance to placental development due to the restricted expression of its ligand, HLA-G, by fetal trophoblast cells during early pregnancy. To understand the nature and regulation of KIR2DL4–HLA-G interactions, we used an unbiased random mutagenesis approach and screened mutants of KIR2DL4 in a cellular assay for their ability to load soluble HLA-G into KIR2DL4+ endosomes. With this approach, amino acid residues in KIR2DL4 that are important for endosomal localization and HLA-G recognition were identified. Specifically, we have identified a putative allosteric cysteine in the extracellular domain of this receptor, which may be involved in a disulfide switching mechanism to control receptor internalization and ligand binding. Protein disulfide isomerase (PDI) acting at the cell surface may be responsible for this switch as inhibition of PDI interfered with internalization of KIR2DL4 and uptake of HLA-G into endosomes. The configuration of the relevant disulfide bond is -RHstaple, which is a hallmark of many allosteric disulfides. Thus, KIR2DL4 localization and function is regulated by allosteric disulfide bonds at the cell surface. Studies are underway to explore the dynamic regulation of receptor structure by this allosteric disulfide switch to control ligand binding, localization and signaling by this receptor.