G protein-coupled receptors (GPCRs) have been found as monomers but also as dimers or higher-order oligomers in cells. The relevance of the monomeric or dimeric receptor state for G protein activation is currently under debate for class A rhodopsin-like GPCRs. Clarification of this issue requires the availability of well defined receptor preparations as monomers or dimers and an assessment of their ligand-binding and G protein-coupling properties. We show by pharmacological and hydrodynamic experiments that purified neurotensin receptor NTS1, a class A GPCR, dimerizes in detergent solution in a concentration-dependent manner, with an apparent affinity in the low nanomolar range. At low receptor concentrations, NTS1 binds the agonist neurotensin with a Hill slope of Ϸ1; at higher receptor concentrations, neurotensin binding displays positive cooperativity with a Hill slope of Ϸ2. NTS1 monomers activate G␣q␤ 1␥2, whereas receptor dimers catalyze nucleotide exchange with lower affinity. Our results demonstrate that NTS1 dimerization per se is not a prerequisite for G protein activation.dimer ͉ G protein activation ͉ G protein-coupled receptor ͉ monomer D imerization of G protein-coupled receptors (GPCRs) has been the subject of intense interest. Class C GPCRs, such as metabotropic glutamate receptors and ␥-aminobutyric acid type B (GABA B ) receptors, clearly form homo-and heterodimeric structures, essential both for trafficking of receptors to the cell surface and for ligand-induced activation of receptors and G protein coupling. Detailed models for receptor and G protein activation have been proposed that account for the multidomain structure of class C GPCRs (for review, see ref. 1).In contrast, no conclusion has yet been reached as to the importance of dimerization for class A receptor function; the role of receptor monomers (2) or dimers (3, 4) in signal transduction is controversial. Models proposed to explain the mechanism of receptor-catalyzed G protein activation (5-7) assumed the interaction of G protein with a receptor monomer. More recently, class A GPCRs have been described as monomers (8) and dimers (see refs. 9 and 10) in living cells by resonance energy transfer methods. For rhodopsin, dimer particles and higher-order oligomers have been visualized in disk membranes by atomic force microscopy (11). Based on structural constraints, a model was suggested in which a receptor dimer provides an extensive contact area for the heterotrimeric G protein; the surface area of a GPCR monomer was deemed insufficiently large to anchor both the G␣ and G␤␥ subunits simultaneously (3, 4, 12). However, alternate models for the interaction of a monomeric rhodopsin with a G protein heterotrimer have also been proposed (2).The concentration of rhodopsin in rod outer segment disk membranes is high (Ϸ2.5 mM; for review, see ref. 13), and rhodopsin may therefore exist only as dimers, as seen by atomic force microscopy (11). Because a rod cell can respond to a single photon (14, 15), only one activated rhodopsin protomer, w...

The plant terpenoids encompass a diversity of structures and have many functional roles in nature, including protection against pest arthropods. Previous studies in this laboratory have identified naturally occurring sesquiterpenes contained in essential oils from two plants, amyris (Amyris balsamifera) and Siam-wood (Fokienia hodginsii), that are significantly repellent to a spectrum of arthropod pests. In efforts to further examine the biological activity of this class of compounds 12 of these plant-derived sesquiterpenes have been isolated, purified, and assayed for spatial and contact repellency against the yellow fever mosquito, Aedes aegypti . These data were used to develop quantitative structure-activity relationships that identified key properties of the sesquiterpene molecule, including electronic and structural parameters that were used to predict optimal repellent activity. There were notable similarities in the models developed for spatial repellency over five time points and for contact repellency. Vapor pressure was an important component of all repellency models. Initial levels of spatial repellency were also related to polarizability of the molecule and lowest unoccupied molecular orbital (LUMO) energy, whereas the equation for late spatial repellency was dependent on other electronic features, including Mulliken population and electrotopological state descriptors. The model identified for contact repellency was the best fit and most significant model in this analysis and showed a relationship with vapor pressure, Mulliken population, and total energy.

Monoterpenoids are naturally occurring compounds that are found in higher-order plants. These compounds are secondary metabolites that seem to play no major role in the metabolic functioning of the plants. One role of monoterpenoids in the plants is to defend against plant-directed pathogens, herbivores, or competing plant species. These compounds are good leads for synthesis or isolation of more effective insecticides. To accomplish these goals, we developed quantitative structure−activity relationships (QSARs) in order to predict insect toxicity of monoterpenoids and derivatives that have not yet been synthesized or experimentally tested. Correlations were found between toxicity and certain quantum and traditional chemical parameters. We found a linear relationship between LD50 values for house fly toxicity and Mulliken populations in aromatic monoterpenoids. Multiple linear regression of an E-State descriptor and a GETAWAY (GEometry, Topology and Atomic Weights AssemblY) descriptor also showed a relationship with house fly toxicity for a wide range of monoterpenoids. KeywordsQuantitative structure−activity relationship (QSAR), monoterpenoid, insecticide, Mulliken population, quantum descriptor, electrotopological state Disciplines Entomology CommentsReprinted with permission from Journal of Agricultural and Food Chemistry 50 (2002) Monoterpenoids are naturally occurring compounds that are found in higher-order plants. These compounds are secondary metabolites that seem to play no major role in the metabolic functioning of the plants. One role of monoterpenoids in the plants is to defend against plant-directed pathogens, herbivores, or competing plant species. These compounds are good leads for synthesis or isolation of more effective insecticides. To accomplish these goals, we developed quantitative structureactivity relationships (QSARs) in order to predict insect toxicity of monoterpenoids and derivatives that have not yet been synthesized or experimentally tested. Correlations were found between toxicity and certain quantum and traditional chemical parameters. We found a linear relationship between LD 50 values for house fly toxicity and Mulliken populations in aromatic monoterpenoids. Multiple linear regression of an E-State descriptor and a GETAWAY (GEometry, Topology and Atomic Weights AssemblY) descriptor also showed a relationship with house fly toxicity for a wide range of monoterpenoids.

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In the presence of arginine vasopressin (AVP), somatostatin increases [Ca 2ϩ ] i , leading to a transient increase in insulin release from clonal ␤ cells HIT-T15 via G i/o and phospholipase C (PLC) pathway (Cheng et al., 2002a). The present study was to elucidate the mechanisms underlying somatostatin-induced [Ca 2ϩ ] i increase in the presence of AVP. We found that the effect of somatostatin was mediated by ␤␥ subunits but not by the ␣ subunit of G i/o . Because somatostatin alone failed to increase [Ca 2ϩ ] i , we hypothesized that somatostatin increases phosphatidylinositol 4,5-bisphosphate (PIP 2 ) synthesis, providing extra substrate for preactivated PLC-␤ to generate inositol 1,4,5-trisphosphate (IP 3 ). Somatostatin alone did not increase IP 3 levels, but AVP ϩ somatostatin did. Somatostatin increased PIP 2 levels but decreased phosphatidylinositol 4-phosphate levels. We further hypothesized that PLD mediates somatostatin-induced changes in PIP 2 levels. Both the phospholipase D (PLD) inhibitors and antibody versus PLD1 antagonized AVPsomatostatin-induced increases in [Ca 2ϩ ] i . PLD inhibitor also antagonized somatostatin-induced increase in PIP 2 levels. In addition, somatostatin increased PLD activity. These results suggest that activation of somatostatin receptors that are coupled to the ␤␥ dimer of G i/o led to PLD1 activation, thus promoting the synthesis of phosphatidic acid. Phosphatidic acid activates PIP-5 kinase, which evokes an increase in PIP 2 synthesis. The PIP 2 generated by somatostatin administration increases substrate for preactivated phospholipase C-␤, which hydrolyzes PIP 2 to form IP 3 , leading to an increase in [Ca 2ϩ ] i .