NKG2D is a potent activating receptor on natural killer cells, T cells, and macrophages. Mouse NKG2D interacts with two cell surface ligands related to class I MHC molecules: RAE1 and H60. We used soluble versions of NKG2D, RAE1, and H60 to characterize their interactions. RAE1 and H60 each bind NKG2D with nanomolar affinities, indicating tighter binding than most cell surface immune interactions, but NKG2D binds to H60 with ∼25-fold higher affinity than to RAE1. RAE1 and H60 compete directly for occupancy of NKG2D, and, thus, NKG2D can be occupied by only one ligand at a time. The NKG2D-H60 interaction is more temperature dependent and makes greater use of electrostatic interactions than the NKG2D-RAE1 interaction. The distinct thermodynamic profiles provide insights into the different molecular mechanisms of the binding interactions.

© 2001 Cell Press. Received 1 May 2001, Revised 21 June 2001, Available online 27 August 2001. C.A.O'C. is an MRC Fellow. B.E.W. is a Wellcome Trust Fellow. A.C. and L.L.L. are supported by NIH grant CA 89189. We are grateful to Roland Strong and members of the Bjorkman and Lanier labs for helpful discussions and critical reading of the manuscript and to Sandy Zurawski for the original Ig-fusion constructs.