In multicellular organisms from Caenorhabditis elegans to Homo sapiens, the maintenance of homeostasis is dependent on the continual flow and processing of information through a complex network of cells. Moreover, in order for the organism to respond to an ever-changing environment, intercellular signals must be transduced, amplified, and ultimately converted to the appropriate physiological response. The resolution of the molecular events underlying signal response and integration forms the basis of the signal transduction field of research. An evolutionarily highly conserved group of molecules known as heterotrimeric guanine nucleotide-binding proteins (G proteins) are key determinants of the specificity and temporal characteristics of many signaling processes and are the topic of this review. Numerous hormones, neurotransmitters, chemokines, local mediators, and sensory stimuli exert their effects on cells by binding to heptahelical membrane receptors coupled to heterotrimeric G proteins. These highly specialized transducers can modulate the activity of multiple signaling pathways leading to diverse biological responses. In vivo, specific combinations of G alpha- and G beta gamma-subunits are likely required for connecting individual receptors to signaling pathways. The structural determinants of receptor-G protein-effector specificity are not completely understood and, in addition to involving interaction domains of these primary acting proteins, also require the participation of scaffolding and regulatory proteins.

The p t -differential inclusive production cross sections of the prompt charmed mesons D 0 , D + , and D * + in the rapidity range |y| < 0.5 were measured in proton-proton collisions at √ s = 7 TeV at the LHC using the ALICE detector. Reconstructing the decaysand their charge conjugates, about 8,400 D 0 , 2,900 D + , and 2,600 D * + mesons with 1 < p t < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14×10 8 events collected with a minimum-bias trigger (integrated luminosity L int = 5 nb −1 ). The results are described within uncertainties by predictions based on perturbative QCD.

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The carboxyl terminus of heterotrimeric G protein ␣ subunits plays an important role in receptor interaction. We demonstrate that peptides corresponding to the last 11 residues of G␣ i1/2 or G␣ o1 impair agonist binding to A 1 adenosine receptors, whereas G␣ s or G␣ t peptides have no effect. Previously, by using a combinatorial library we identified a series of G␣ t peptide analogs that bind rhodopsin with high affinity (Martin, E. L., RensDomiano, S., Schatz, P. J., and Hamm, H. E. (1996) J. Biol. Chem. 271, 361-366). Native G␣ i1/2 peptide as well as several analogs were tested for their ability to modulate agonist binding or antagonist-agonist competition using cells overexpressing human A 1 adenosine receptors. Three peptide analogs decreased the K i , suggesting that they disrupt the high affinity receptor-G protein interaction and stabilize an intermediate affinity state. To study the ability of the peptides to compete with endogenous G␣ i proteins and block signal transduction in a native setting, we measured activation of G proteincoupled K ؉ channels through A 1 adenosine or ␥-aminobutyric acid, type B, receptors in hippocampal CA1 pyramidal neurons. Native G␣ i1/2 , peptide, and certain analog peptides inhibited receptor-mediated K ؉ channel gating, dependent on which receptor was activated. This differential perturbation of receptor-G protein interaction suggests that receptors that act on the same G protein can be selectively disrupted.Hormones and neurotransmitters that bind to G proteincoupled receptors control a myriad of physiological functions. Transduction of these extracellular signals involves receptormediated activation of specific G proteins by catalysis of GDP/ GTP exchange. These receptors are the target for many pharmaceutical products and are the focus of intense drug discovery efforts. Traditionally, the agonist binding site is the point of intervention, but in some cases receptor subtype-selective drugs have been difficult to achieve. Another possible target for inhibition is the receptor-G protein interface, which has been defined in some detail and involves the intracellular loops of the seven-transmembrane helix receptors with several regions on heterotrimeric G proteins (1-3). It is important to assess whether inhibitors of this interface can be found or designed and whether specific inhibition can be achieved.The carboxyl-terminal region of the G␣ subunits represents an important site of interaction between heterotrimeric G proteins and their cognate receptors. Within this region mutations (4 -6), covalent modification by pertussis toxin-catalyzed ADPribosylation (7), or binding of specific antibodies (8) all uncouple G proteins from their receptors. In particular, the last 4 residues of the G␣ carboxyl terminus play an important role in determining the fidelity of receptor activation (9, 10). Moreover, synthetic peptides from various portions of the G␣ s carboxyl terminus inhibit ␤-adrenergic receptor-G s coupling (11,12). Two of these peptides, G␣ s -(384 -394) and G␣ s -(354 ...

2'-C-Methyl analogues of selective adenosine receptor agonists such as (R)-PIA, CPA, CCPA, NECA, and IB-MECA were synthesized in order to further investigate the subdomain that binds the ribose moiety. Binding affinities of these new compounds at A1 and A2A receptors in bovine brain membranes and at A3 in rat testis membranes were determined and compared. It was found that the 2'-C-methyl modification resulted in a decrease of the affinity, particularly at A2A and A3 receptors. When such modification was combined with N6-substitutions with groups which induce high potency and selectivity at A1 receptors, the high affinity was retained and the selectivity was increased. Thus, 2-chloro-2'-C-methyl-N6-cyclopentyladenosine (2'-Me-CCPA), which displayed a Ki value of 1.8 nM at A1 receptors, was selective for A1 vs A2A and A3 receptors by 2166- and 2777-fold, respectively, resulting in one of the most potent and A1-selective agonists so far known. In functional assay, this compound inhibited forskolin-stimulated adenylyl cyclase activity with an IC50 value of 13.1 nM, acting as a full agonist.

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The tryptic cleavage pattern of transducin (G t ) in solution was compared with that in the presence of phospholipid vesicles, rod outer segment (ROS) membranes kept in the dark, or ROS membranes containing lightactivated rhodopsin, metarhodopsin II (Rh*). When G t was in the high affinity complex with Rh*, the ␣ t subunit was almost completely protected from proteolysis. , only a few angstroms away from the carboxyl terminus of ␣ t , which is known to directly bind to Rh*, is likely to also be a part of the Rh* binding site. This is in agreement with other studies and has implications for the mechanism by which receptors catalyze GDP release from G proteins. The protection of Lys 18 in the presence of phospholipid vesicles suggests that the amino-terminal region is in contact with the membrane, consistent with the crystal structure of the heterotrimer (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319).Certain extracellular signals, including hormones, neurotransmitters, neuromodulators, chemokines, odorants, and light, activate a class of receptors that initiate cellular effects via activation of heterotrimeric G proteins. Agonist binding to the G protein-coupled receptors leads to conformational changes that promote a tighter interaction with specific G proteins, catalysis of GDP release, and subsequent G protein activation. In the absence of guanine nucleotides, agonist binding to the receptor is stabilized by the bound G protein. The structural basis of the ternary complex among agonist, receptor, and heterotrimeric G protein is an active area of study. Extensive mutagenesis experiments, as well as peptide competition investigations for a variety of G protein-coupled receptors, have led to an understanding that the second and third cytoplasmic loops and, in some circumstances, the putative fourth loop, as well as portions of ␣ helices VI and VII, are important in recognition of cognate G proteins (1-3). It has been shown that the heterotrimeric G protein, rather than just the ␣ or ␤␥ subunits, is required for the interaction, but studies pointing out the importance of specific regions are thus far limited to the ␣ subunits (4 -7).The crystal structures of the active (GTP␥S 1 and GDP AlF 4 Ϫ -bound) (8 -10) and inactive (GDP-bound) (11, 12) forms of the ␣ subunits of transducin (G t ) and G i1 have been reported. Analysis of the two crystal forms has established the nature of the conformational change induced by the exchange of GTP for GDP and the switch mechanism by which the presence or absence of the ␥-phosphate defines the active or inactive state of the ␣ t subunit (9, 11). The high resolution crystal structures of the heterotrimeric G proteins, G t and G i1 , provide a fundamental context for understanding how a heterotrimer interacts with the membrane and with activated receptors (13,14). The molecular mechanisms involved in the conformational changes of the ␣ subunit and the nucleotide exchange induced by the heterotrimeric G protein inter...

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