1. Natural Killer Cell Inhibitory Receptors CD94/NKG2, with its cytoplasmic immuno-tyrosine inhibitory motif (ITIM), defines a family of MHC class I molecule mediated negative/positive signaling receptors on the surface of natural killer (NK) cells. Both chains are members of the type II transmembrane C-lectin family of receptors. Recently, the ligand of this receptor family has been identified to be HLA-E, which presents the signal peptides of other class I MHC molecules. We have expressed and reconstituted several forms of CD94 and NKG2A extracellular ligand binding domains. The crystallization of CD94 yielded crystals diffracting to 2.6 angstrom resolution. The structure of CD94 revealed a novel C-type lectin fold with the second canonical C-type lectin alpha helix being distorted into a looped conformation to form a CD94/CD94 dimer interface. Another family of inhibitory receptors, termed Killer Immunoglobulin-like Receptors (KIR), has been identified recently on the surface of human NK cells to mediate self versus non-self recognitions. To understand the function of these receptors, we have expressed and reconstituted a truncated form of KIR2DL2, using a bacteria expression system and in vitro refolding. Crystallization of 2DL2 has led to the solution of its molecular structure at 2.9 angstrom resolution. The structure reveals a fold similar to that of the human growth hormone receptor. The hinge angle between the two immunoglobulin domains, D1 and D2, however, is significantly smaller than that observed in the human growth hormone receptor, and is stabilized by a set of hydrophobic residues at the location of the hinge. Structural comparison with a homologous molecule, KIR2DL1, shows that the hinge angle of 2DL2 is also different from that of KIR2DL1, implying that the hinge is not rigid. An orderly receptor aggregate of 2DL2 is observed in the orthorhombic crystal form where the D1 domain of one receptor molecule dimerizes onto the D2 domain of an adjacent receptor molecule. We propose this D1/D2 cross- dimerization as a model for killer cell ligand induced receptor aggregation and activation. 2. Transcription Regulator C/EBPThe CCAAT- enhancer binding protein (C/EBP) is a transcription factor which regulates cell proliferation and differentiation, particularly in cell growth arrest and terminal differentiation. It belongs to a family of leucine zipper DNA-binding proteins with a basic region responsible for DNA sequence recognition. We have crystallized the DNA binding domain of C/EBP in complex with its cognate DNA. - Natural killer cell receptors, CD94/NKG2, KIR, crystal structure, C/EBP
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