MHC Class I Recognition by NK Receptors in the Ly49 Family Is Strongly Influenced by the β2-Microglobulin Subunit

Abstract

NK cell recognition of targets is strongly affected by MHC class I specific receptors. The recently published structure of the inhibitory receptor Ly49A in complex with H-2Dd revealed two distinct sites of interaction in the crystal. One of these involves the α1, α2, α3, and β2-microglobulin (β2m) domains of the MHC class I complex. The data from the structure, together with discrepancies in earlier studies using MHC class I tetramers, prompted us to study the role of the β2m subunit in MHC class I-Ly49 interactions. Here we provide, to our knowledge, the first direct evidence that residues in the β2m subunit affect binding of MHC class I molecules to Ly49 receptors. A change from murine β2m to human β2m in three different MHC class I molecules, H-2Db, H-2Kb, and H-2Dd, resulted in a loss of binding to the receptors Ly49A and Ly49C. Analysis of the amino acids involved in the binding of Ly49A to H-2Dd in the published crystal structure, and differing between the mouse and the human β2m, suggests the cluster formed by residues Lys3, Thr4, Thr28, and Gln29, as a potentially important domain for the Ly49A-H-2Dd interaction. Another possibility is that the change of β2m indirectly affects the conformation of distal parts of the MHC class I molecule, including the α1 and α2 domains of the heavy chain.