Metabotropic glutamate receptors play major roles in the activation of excitatory synapses in the central nerve system. We determined the crystal structure of the entire extracellular region of the group II receptor and that of the ligand-binding region of the group III receptor. A comparison among groups I, II, and III provides the structural basis that could account for the discrimination of groupspecific agonists. Furthermore, the structure of group II includes the cysteine-rich domain, which is tightly linked to the ligand-binding domain by a disulfide bridge, suggesting a potential role in transmitting a ligand-induced conformational change into the downstream transmembrane region. The structure also reveals the lateral interaction between the two cysteine-rich domains, which could stimulate clustering of the dimeric receptors on the cell surface. We propose a general activation mechanism of the dimeric receptor coupled with both ligand-binding and interprotomer rearrangements.crystallization ͉ cysteine-rich region ͉ ligand binding ͉ G protein-coupled receptor

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Superantigens (SAGs) are a class of immunostimulatory proteins of bacterial or viral origin that activate T cells by binding to the V beta domain of the T cell antigen receptor (TCR). The three-dimensional structure of the complex between a TCR beta chain (mouse V beta8.2) and the SAG staphylococcal enterotoxin B (SEB) at 2.4 A resolution reveals why SEB recognizes only certain V beta families, as well as why only certain SAGs bind mouse V beta8.2. Models of the TCR-SEB-peptide/MHC class II complex indicate that V alpha interacts with the MHC beta chain in the TCR-SAG-MHC complex. The extent of the interaction is variable and is largely determined by the geometry of V alpha/V beta domain association. This variability can account for the preferential expression of certain V alpha regions among T cells reactive with SEB.

Lectin-like, oxidized low-density lipoprotein (LDL) receptor 1, LOX-1, is the major receptor for oxidized LDL (OxLDL) in endothelial cells. We have determined the crystal structure of the ligand binding domain of LOX-1, with a short stalk region connecting the domain to the membrane-spanning region, as a homodimer linked by an interchain disulfide bond. In vivo assays with LOX-1 mutants revealed that the "basic spine," consisting of linearly aligned arginine residues spanning over the dimer surface, is responsible for ligand binding. Single amino acid substitution in the dimer interface caused a severe reduction in LOX-1 binding activity, suggesting that the correct dimer arrangement is crucial for binding to OxLDL. Based on the LDL model structure, possible binding modes of LOX-1 to OxLDL are proposed.