Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge. Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task. Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action. We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system. FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention. Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.

It is now established that most of the ∼800 G-protein-coupled receptors (GPCRs) are regulated by phosphorylation in a process that results in the recruitment of arrestins, leading to receptor desensitization and the activation of arrestin-dependent processes. This generalized view of GPCR regulation, however, does not provide an adequate mechanism for the control of tissue-specific GPCR signalling. Here, we review the evidence that GPCR phosphorylation is, in fact, a flexible and dynamic regulatory process in which GPCRs are phosphorylated in a unique manner that is associated with the cell type in which the receptor is expressed. In this scenario, phosphorylation offers a mechanism of regulating the signalling outcome of GPCRs that can be tailored to meet a specific physiological role.

-2-yl)methoxy)phenyl)propanoic acid] was recently described as a potent and selective agonist for the long chain free fatty acid (LCFA) receptor 4 (FFA4; previously G protein-coupled receptor 120, or GPR120). Herein, we have used TUG-891 to further define the function of FFA4 and used this compound in proof of principle studies to indicate the therapeutic potential of this receptor. TUG-891 displayed similar signaling properties to the LCFA a-linolenic acid at human FFA4 across various assay end points, including stimulation of Ca 21 mobilization, b-arrestin-1 and b-arrestin-2 recruitment, and extracellular signal-regulated kinase phosphorylation. Activation of human FFA4 by TUG-891 also resulted in rapid phosphorylation and internalization of the receptor. While these latter events were associated with desensitization of the FFA4 signaling response, removal of TUG-891 allowed both rapid recycling of FFA4 back to the cell surface and resensitization of the FFA4 Ca 21 signaling response. TUG-891 was also a potent agonist of mouse FFA4, but it showed only limited selectivity over mouse FFA1, complicating its use in vivo in this species. Pharmacologic dissection of responses to TUG-891 in model murine cell systems indicated that activation of FFA4 was able to mimic many potentially beneficial therapeutic properties previously reported for LCFAs, including stimulating glucagon-like peptide-1 secretion from enteroendocrine cells, enhancing glucose uptake in 3T3-L1 adipocytes, and inhibiting release of proinflammatory mediators from RAW264.7 macrophages, which suggests promise for FFA4 as a therapeutic target for type 2 diabetes and obesity. Together, these results demonstrate both potential but also significant challenges that still need to be overcome to therapeutically target FFA4.

Phosphatidylinositol 3-kinase (PI3K) has been shown to enhance native voltage-dependent calcium channel (Ca(v)) currents both in myocytes and in neurons; however, the mechanism(s) responsible for this regulation were not known. Here we show that PI3K promotes the translocation of GFP-tagged Ca(v) channels to the plasma membrane in both COS-7 cells and neurons. We show that the effect of PI3K is mediated by Akt/PKB and specifically requires Ca(v)beta(2) subunits. The mutations S574A and S574E in Ca(v)beta(2a) prevented and mimicked, respectively, the effect of PI3K/Akt-PKB, indicating that phosphorylation of Ser574 on Ca(v)beta(2a) is necessary and sufficient to promote Ca(v) channel trafficking.

G-protein-coupled receptors are hyper-phosphorylated in a process that controls receptor coupling to downstream signaling pathways. The pattern of receptor phosphorylation has been proposed to generate a “bar code” that can be varied in a tissue-specific manner to direct physiologically relevant receptor signaling. If such a mechanism existed, receptors would be expected to be phosphorylated in a cell/tissue-specific manner. Using tryptic phosphopeptide maps, mass spectrometry, and phospho-specific antibodies, it was determined here that the prototypical Gq/11-coupled M3-muscarinic receptor was indeed differentially phosphorylated in various cell and tissue types supporting a role for differential receptor phosphorylation in directing tissue-specific signaling. Furthermore, the phosphorylation profile of the M3-muscarinic receptor was also dependent on the stimulus. Full and partial agonists to the M3-muscarinic receptor were observed to direct phosphorylation preferentially to specific sites. This hitherto unappreciated property of ligands raises the possibility that one mechanism underlying ligand bias/functional selectivity, a process where ligands direct receptors to preferred signaling pathways, may be centered on the capacity of ligands to promote receptor phosphorylation at specific sites.

beta-Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha1B (Ca(V)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (approximately 17 nM). This may represent the interaction of the beta-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1B without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gbetagamma dissociation from the beta-subunit-bound alpha1B channels. The data are discussed in terms of two hypotheses, either binding of two beta-subunits to the alpha1B channel or a state-dependent alteration in affinity of the channel for the beta-subunit.

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The Ca V ␤ subunits of voltage-gated calcium channels regulate these channels in several ways. Here we investigate the role of these auxiliary subunits in the expression of functional N-type channels at the plasma membrane and in the modulation by G-protein-coupled receptors of this neuronal channel. To do so, we mutated tryptophan 391 to an alanine within the ␣-interacting domain (AID) in the I-II linker of Ca V 2.2. We showed that the mutation W391 virtually abolishes the binding of Ca V ␤1b and Ca V ␤2a to the Ca V 2.2 I-II linker and strongly reduced current density and cell surface expression of both Ca V 2.2/␣2␦-2/␤1b and/␤2a channels. When associated with Ca V ␤1b, the W391A mutation also prevented the Ca V ␤1b-mediated hyperpolarization of Ca V 2.2 channel activation and steady-state inactivation. However, the mutated Ca V 2.2W391A/␤1b channels were still inhibited to a similar extent by activation of the D 2 dopamine receptor with the agonist quinpirole. Nevertheless, key hallmarks of G-protein modulation of N-type currents, such as slowed activation kinetics and prepulse facilitation, were not observed for the mutated channel. In contrast, Ca V ␤2a was still able to completely modulate the biophysical properties of Ca V 2.2W391A channel and allow voltage-dependent G-protein modulation of Ca V 2.2W391A. Additional data suggest that the concentration of Ca V ␤2a in the proximity of the channel is enhanced independently of its binding to the AID by its palmitoylation. This is essentially sufficient for all of the functional effects of Ca V ␤2a, which may occur via a second lower-affinity binding site, except trafficking the channel to the plasma membrane, which requires interaction with the AID region.