Abstract
The homodimeric, activating natural killer cell receptor NKG2D interacts with multiple monomeric ligands polyspecifically, yet without central conformational flexibility. Crystal structures of multiple NKG2D-ligand interactions have identified the NKG2D tyrosine pair Tyr 152 and Tyr 199 as forming multiple specific but diverse interactions with MICA and related proteins. Here we systematically altered each tyrosine to tryptophan, phenylalanine, isoleucine, leucine, valine, serine, and alanine to measure the effect of mutation on affinity and thermodynamics for binding a range of similar ligands: MICA, the higher-affinity ligand MICB, and MICdesign, a high-affinity version of MICA that shares all NKG2D contact residues with MICA. Affinity and residue size were related: tryptophan could often substitute for tyrosine without loss of affinity; loss of the tyrosine hydroxyl through mutation to phenylalanine was tolerated more at position 152 than 199; and the smallest residues coincide with lowest affinities in general. NKG2D mutant van't Hoff binding thermodynamics generally show that substitution of other residues for tyrosine causes a moderate positive or flat van't Hoff slope consistent with moderate loss of binding enthalpy. One set of NKG2D mutations caused MICA to adopt a positive van't Hoff slope corresponding to absorption of heat, and another set caused MICB to adopt a negative slope of greater heat release than wild-type. MICdesign shared one example of the first set with MICA and one of the second set with MICB. When the NKG2D mutation affinities were arranged according to change in nonpolar surface area and compared to results from specific antibody-antigen and protein-peptide interactions, it was found that hydrophobic surface loss in NKG2D reduced binding affinity less than reported in the other contexts. The hydrophobic effect at the center of the NKG2D binding appears more similar to that at the periphery of an antibody-antigen binding site than at its center. Therefore the polyspecific NKG2D binding site is more tolerant of structural alteration in general than either an antibody-antigen or protein-peptide binding site, and this tolerance may adapt NKG2D to a broad range of protein surfaces with micromolar affinity.