The PH domain of Ras-GAP is sufficient for in vitro binding to beta gamma subunits of heterotrimeric G proteins

Structure and regulation of phospholipase Cβ and ε at the membrane

Phospholipase C (PLC) β and ε enzymes hydrolyze phosphatidylinositol (PI) lipids in response to direct interactions with heterotrimeric G protein subunits and small GTPases, which are activated downstream of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). PI hydrolysis generates second messengers that increase the intracellular Ca²⁺ concentration and activate protein kinase C (PKC), thereby regulating numerous physiological processes. PLCβ and PLCε share a highly conserved core required for lipase activity, but use different strategies and structural elements to autoinhibit basal activity, bind membranes, and engage G protein activators. In this review, we discuss recent structural insights into these enzymes and the implications for how they engage membranes alone or in complex with their G protein regulators.

NF1 Is a Direct G Protein Effector Essential for Opioid Signaling to Ras in the Striatum

It is well recognized that G-protein-coupled receptors (GPCRs) can activate Ras-regulated kinase pathways to produce lasting changes in neuronal function. Mechanisms by which GPCRs transduce these signals and their relevance to brain disorders are not well understood. Here, we identify a major Ras regulator, neurofibromin 1 (NF1), as a direct effector of GPCR signaling via Gβγ subunits in the striatum. We find that binding of Gβγ to NF1 inhibits its ability to inactivate Ras. Deletion of NF1 in striatal neurons prevents the opioid-receptor-induced activation of Ras and eliminates its coupling to Akt-mTOR-signaling pathway. By acting in the striatal medium spiny neurons of the direct pathway, NF1 regulates opioid-induced changes in Ras activity, thereby sensitizing mice to psychomotor and rewarding effects of morphine. These results delineate a novel mechanism of GPCR signaling to Ras pathways and establish a critical role of NF1 in opioid addiction.

The Rac/Cdc42 guanine nucleotide exchange factor beta1Pix enhances mastoparan-activated Gi-dependent pathway in mast cells

Carbachol stimulates granule exocytosis, phospholipase C (PLC), and phospholipase D (PLD) in RBL-2H3hm1 mast cells by a mechanism that involves Galphaq. However, mastoparan stimulates the same responses through Gi protein. Both Gi and Galphaq pathways are suppressed by Clostridium difficile toxin B, suggesting that Rac and Cdc42 small GTPases are also involved. Over-expression of beta1Pix, a guanine nucleotide exchange factor for Rac and Cdc42, enhances mastoparan-but not carbachol-induced hexosaminidase secretion and PLC and PLD activation. Furthermore, cells expressing beta1Pix exhibit elevated levels of mastoparan-stimulated IP3 production. Taken together, these findings implicate beta1Pix in regulating hexoasaminidase secretion and IP3 production in early stage upon mastoparan stimulation.

The pleckstrin homology domain of phospholipase C-beta2 as an effector site for Rac

Increasing evidence links the activation of Rho family GTPases to the stimulation of lipid hydrolysis catalyzed by phospholipase C (PLC)-beta isozymes. To better define this relationship, members of a library of recombinant Rho GTPases were screened for their capacity to directly engage various purified PLC-beta isozymes. Of the 17 tested members of the Rho family, only the active isoforms of Rac (Rac1, Rac2, and Rac3) both stimulate PLC-beta activity in vivo and bind PLC-beta2 and PLC-beta3, but not PLC-beta1, in vitro. Furthermore, the recognition site for Rac GTPases was localized to the pleckstrin homology (PH) domain of PLC-beta2, and this PH domain is fully sufficient to selectively interact with the active versions of the Rac GTPases, but not with other similar Rho GTPases. Together, these findings present a quantitative evaluation of the direct interactions between Rac GTPases and PLC-beta isozymes and define a novel role for the PH domain of PLC-beta2 as a putative effector site for Rac GTPases.

Regulation of myogenesis by fibroblast growth factors requires beta-gamma subunits of pertussis toxin-sensitive G proteins

Terminal differentiation of skeletal muscle cells in culture is inhibited by a number of different growth factors whose subsequent intracellular signaling events are poorly understood. In this study, we have investigated the role of heterotrimeric G proteins in mediating fibroblast growth factor (FGF)-dependent signals that regulate myogenic differentiation. Pertussis toxin, which ADP-ribosylates and inactivates susceptible G proteins, promotes terminal differentiation in the presence of FGF-2, suggesting that Galpha or Gbeta gamma subunits or both are involved in transducing the FGF-dependent signal(s) that inhibits myogenesis. We found that Gbetagamma subunits are likely to be involved since the expression of the C terminus of beta-adrenergic receptor kinase 1, a Gbetagamma subunit-sequestering agent, promotes differentiation in the presence of FGF-2, and expression of the free Gbeta gamma dimer can replace FGF-2, rescuing cells from pertussis toxin-induced differentiation. Addition of pertussis toxin also blocked FGF-2-mediated activation of mitogen-activated protein kinases (MAPKs). Ectopic expression of dominant active mutants in the Ras/MAPK pathway rescued cells from pertussis toxin-induced terminal differentiation, suggesting that the Gbeta gamma subunits act upstream of the Ras/MAPK pathway. It is unlikely that the pertussis toxin-sensitive pathway is activated by other, as yet unidentified FGF receptors since PDGF (platelet-derived growth factor)-stimulated MM14 cells expressing a chimeric receptor containing the FGF receptor-1 intracellular domain and the PDGF receptor extracellular domain were sensitive to pertussis toxin. Our data suggest that FGF-mediated signals involved in repression of myogenic differentiation are transduced by a pertussis toxin-sensitive G-protein-coupled mechanism. This signaling pathway requires the action of Gbeta gamma subunits and activation of MAPKs to repress skeletal muscle differentiation.

Regulation of rat neuronal voltage-dependent calcium channels by endogenous p21-ras

Influx of calcium through voltage-dependent calcium channels (VDCCs) has been implicated in the processes of cell growth and differentiation. Various signalling proteins, including nerve growth factor (NGF), p21-ras and src tyrosine kinases, have been suggested to have a role in the regulation of neuronal VDCCs. Using the whole-cell patch-clamp technique we have investigated the role of endogenous p21-ras in the regulation of VDCCs in primary cultured dorsal root ganglion (DRG) neurons obtained from neonatal rats. Neutralization of endogenous p21-ras by microinjection of p21-ras antibody (Y13-259) reduced the maximum peak barium current, I(max), whereas microinjection of oncogenic p21-K-ras increased the current. Thus, endogenous p21-ras is involved in the tonic regulation of calcium currents in these cells. Intracellular application of a phosphopeptide, Trk 490, which prevents the binding of the adaptor protein shc to the activated NGF receptor, so blocking p21-ras activation, reduced I(max). Similarly, deprivation of NGF by overnight incubation in NGF-free medium also reduced I(max). Together, these results suggest that NGF receptor tyrosine kinase activation of p21-ras is likely to be involved in the tonic regulation of VDCCs in DRG neurons. Deprivation of NGF combined with microinjection of p21-ras antibody (Y13-259), however, caused an even greater reduction of I(max). Thus, NGF activation can only partially explain the regulation of these currents by endogenous p21-ras. Src tyrosine kinases have been suggested to activate p21-ras. In DRG neurons, microinjection of purified src tyrosine kinase, pp60c-src, increased I(max) in these cells. However, co-microinjection of pp60c-src with Y13-259 antibody prevented the increase in I(max), implying that pp60c-src can also regulate calcium currents via the activation of endogenous p21-ras. Further support for the involvement of tyrosine kinases in VDCC regulation was provided by the application of the general tyrosine kinase inhibitor, genistein, which also reduced I(max). Thus, VDCCs in rat DRG neurons appear to be tonically up-regulated by endogenous p21-ras. This effect appears largely to involve NGF receptor tyrosine kinase activation of p21-ras. In addition, src tyrosine kinase may also regulate VDCCs, possibly via p21-ras.