Current density-voltage characteristics are presented for a molecular structure of the form metal/organic-multilayer/metal for which the rectifierlike forward bias current density dependence is unequivocally associated with zwitterionic molecules. By placing passive organic barriers between the metal layers and the active molecules we prove that Schottky barrier effects are not important. This is the clearest evidence so far for molecularly controlled rectification, the basis for molecular electronics.

The MgI(LB mono1ayer)lPt structures of Z-p-( 1 -hexadecyl-4-quinolinium)-a-cyano-4-sty~/Idicyanomethanide (C16H33-Q3CNQ) show asymmetric current-voltage characteristics; the behaviour is attributed to the organic monolayer although whether it is due to the presence of the permanent dipole moment or molecular rectification is unclear.Aviram and Ratnerl postulated that oriented films of the donor part, TTF (tetrathiafulvalene), and electron affinity of bicyclo[2.2.2]octane bridged donor-acceptor molecule, TTF-the acceptor part, TCNQ (7,7,8,8-tetracyano-p-quinoo-TCNQ, would show rectifying characteristics because the dimethane), is 4.0 eV (cf. 9.6 eV when the roles are energy barrier to TTF+-o-TCNQ-is low relative to TTF--0-reversed).2 The molecule was never synthesised but suitable TCNQ+. The difference between the ionisation energy of the urethane bridged D-a-A materials (e.g. A , 5-bromo-TCNQ;

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Serotonin 5-HT 4(a) receptor, a G-protein-coupled receptor (GPCR), was produced as a functional isolated protein using Escherichia coli as an expression system. The isolated receptor was characterized at the molecular level by circular dichroism (CD) and steady-state fluorescence. A specific change in the near-UV CD band associated with the GPCR disulfide bond connecting the third transmembrane domain to the second extracellular loop (e2) was observed upon agonist binding to the purified receptor. This is a direct experimental evidence for a change in the conformation of the e2 loop upon receptor activation. Different variations were obtained depending whether the ligand was an agonist (partial or full) or an inverse agonist. In contrast, antagonist binding did not induce any variation. These observations provide a first direct evidence for the fact that free (or antagonist-occupied), active (partial-or full agonist-occupied) and silent (inverse agonist-occupied) states of the receptor involve different arrangements of the e2 loop. Finally, ligand-induced changes in the fluorescence emission profile of the purified receptor confirmed that the partial agonist stabilized a single, welldefined, conformational state and not a mixture of different states. This result is of particular interest in a pharmacological perspective since it directly demonstrates that the efficacy of a drug is likely due to the stabilization of a ligand-specific state rather than selection of a mixture of different conformational states of the receptor.G-protein-coupled receptors are versatile biological sensors. They are responsible for the majority of cellular responses to hormones and neurotransmitters, as well as sight, smell, and taste senses (1, 2). Signal transduction is specifically associated with GPCR 1 activation. Although significant progress has been made within the last few years in dissecting GPCR-mediated signal transduction pathways, understanding of the mechanisms underlying receptor activation is still hampered by the lack of information at the molecular level (3,4). This is largely due to the fact that very few expression systems have proven satisfactory for producing these receptors in a functional state and with sufficient yields for biophysical studies to be carried out (5-7). Most of the systems that have been developed to elucidate the mechanism of GPCR activation therefore essentially rely on the use of purified rhodopsin and ␤ 2 -adrenergic receptor (3,4,8). Interestingly, most of the results obtained so far report on the conformational events occurring at the level of the cytoplasmic side of the receptors. In contrast, few reports give indications on the possible conformational rearrangements certainly occurring in the extracellular part of the receptor, in particular in the extracellular loops.Several models have been developed to conceptualize the mechanisms of activation (9, 10). The two-state model and the extended ternary model assumes that the receptor exists in an equilibrium between a resting state (R) and...

G-protein-coupled receptors (GPCRs) are key players in cell communication. Although long considered as monomeric, it now appears that these heptahelical proteins can form homo-or heterodimers. Here, we analyzed the conformational changes in each subunit of a receptor dimer resulting from agonist binding to either one or both subunits by measuring the fluorescent properties of a leukotriene B 4 receptor dimer with a single 5-hydroxytryptophan-labeled protomer. We show that a receptor dimer with only a single agonist-occupied subunit can trigger G-protein activation. We also show that the two subunits of the receptor dimer in the G-protein-coupled state differ in their conformation, even when both are liganded by the agonist. No such asymmetric conformational changes are observed in the absence of G-protein, indicating that the interaction of the G-protein with the receptor dimer brings specific constraints that prevent a symmetric functioning of this dimer. These data open new options for the differential signaling properties of GPCR dimers.

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G protein-coupled receptors (GPCRs) are key players in signal recognition and cell communication and are among the most important targets for drug development. Direct structural information on the conformation of GPCR ligands bound to their receptors is scarce. Using a leukotriene receptor, BLT2, expressed under a perdeuterated form in Escherichia coli , purified in milligram amounts, and folded to its native state using amphipols, we have solved, by (1)H NMR, the structure of receptor-bound leukotriene B4 (LTB4). Upon binding, LTB4 adopts a highly constrained seahorse conformation, at variance with the free state, where it explores a wide range of conformations. This structure provides an experimentally determined template of a pro-inflammatory compound for further pharmacological studies. The novel approach used for its determination could prove powerful to investigate ligand binding to GPCRs and membrane proteins in general.

Two isolated recombinant fragments from human integrin, and induces a specific conformational adaptation of the fibronectin ligand. A two-site model for RGD binding to both ␣ and ␤ integrin components is inferred from our data using low molecular weight RGD mimetics. Integrins (IN)1 are a family of structurally and functionally related adhesion receptors that participate in cell-cell and cellextracellular matrix (EM) interactions (1). All integrins are heterodimers of non-covalently associated ␣ and ␤ subunits. At the functional level, the interactions between integrins and their EM protein ligands involve the following: (i) the extracellular integrin heteromeric "head" that encompasses the Nterminal halves from both ␣ and ␤ subunits and hosts the ligand-recognition pocket with a variety of binding sites on each subunit (2); (ii) short specific amino acid sequences (adhesion motifs) from the EM ligands. The prototype for these adhesion motifs is the Arg-Gly-Asp (RGD) sequence that is present in fibronectin, fibrinogen, vitronectin, and other EMadhesive proteins (3). The exact location of these binding loci in the integrin subunits, as well as their respective role on ligand binding energy and specificity, still remains an open question. The N-terminal half of the integrin ␣ subunits is characterized by the presence of seven N-terminal repeats of about 60 amino acids each (4, 5). Some of the ␣ subunits include an insertion domain, or I-domain, about 200 residues in length, between repeats II and III (2). The homologies between repeats I and VII essentially include the FG and GAP consensus sequences, so that these repeats are also referred as FG-GAP repeats (6). Three to four of these repeats (i.e. repeat IV or V to repeat VII) display sequences that resemble the EF-hand consensus sequence found in various divalent cation-binding proteins (7). However, the integrin EF-hand type sequences are systematically devoid of an acidic residue at their relative position 12, a highly invariant Glu residue in the typical EF-hands, that is replaced by a non-polar residue in integrins (8). Isolated recombinant integrin fragments encompassing repeats III to VII of ␣ IIb (9) and IV to VII of ␣ 5 (10) have been shown to mimic the ligand-binding features (divalent cation and RGD dependence) that are observed with native integrin receptors, indicating that the divalent cation-binding domains in the ␣ subunits are part of the ligand-recognition pocket of integrins. Repeats III and IV in the ␣ subunits have also been shown to be involved in cell spreading and in assembly of focal contacts at the cell surface (11).The N-terminal regions of the integrin ␤ subunits are characterized by a conserved domain that displays sequence homology with the I-domain found in several integrin ␣ subunits (12). This I-type domain in the integrin ␤ subunits includes the following: (i) a functional cation-binding site that displays strong similarities at the level of its metal-coordinating residues with the MIDAS site of the ␣ subunit I-domains (13, 14); ...