Functional interaction between Galpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

Conformation- and activation-based BRET sensors differentially report on GPCR-G protein coupling

G protein-coupled receptors (GPCRs) regulate cellular signaling processes by coupling to diverse combinations of heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits. Biosensors based on bioluminescence resonance energy transfer (BRET) have advanced our understanding of GPCR functional selectivity. Some BRET biosensors monitor ligand-induced conformational changes in the receptor or G proteins, whereas others monitor the recruitment of downstream effectors to sites of G protein activation. Here, we compared the ability of conformation-and activation-based BRET biosensors to assess the coupling of various class A and B GPCRs to specific Gα proteins in cultured cells. These GPCRs included serotonin 5-HT2A and 5-HT7 receptors, the GLP-1 receptor (GLP-1R), and the M3 muscarinic receptor. We observed different signaling profiles between the two types of sensors, highlighting how data interpretation could be affected by the nature of the biosensor. We also found that the identity of the Gβγ subunits used in the assay could differentially influence the selectivity of a receptor toward Gα subtypes, emphasizing the importance of the receptor-Gβγ pairing in determining Gα coupling specificity. Last, the addition of epitope tags to the receptor could affect stoichiometry and coupling selectivity and yield artifactual findings. These results highlight the need for careful sensor selection and experimental design when probing GPCR-G protein coupling.

Effector membrane translocation biosensors reveal G protein and βarrestin coupling profiles of 100 therapeutically relevant GPCRs

The recognition that individual GPCRs can activate multiple signaling pathways has raised the possibility of developing drugs selectively targeting therapeutically relevant ones. This requires tools to determine which G proteins and βarrestins are activated by a given receptor. Here, we present a set of BRET sensors monitoring the activation of the 12 G protein subtypes based on the translocation of their effectors to the plasma membrane (EMTA). Unlike most of the existing detection systems, EMTA does not require modification of receptors or G proteins (except for G_s). EMTA was found to be suitable for the detection of constitutive activity, inverse agonism, biased signaling and polypharmacology. Profiling of 100 therapeutically relevant human GPCRs resulted in 1500 pathway-specific concentration-response curves and revealed a great diversity of coupling profiles ranging from exquisite selectivity to broad promiscuity. Overall, this work describes unique resources for studying the complexities underlying GPCR signaling and pharmacology.

Cannabinoid receptor subtype influence on neuritogenesis in human SH-SY5Y cells

Human SH-SY5Y neuroblastoma cells stably expressing exogenous CB₁ (CB₁XS) or CB₂ (CB₂XS) receptors were developed to investigate endocannabinoid signaling in the extension of neuronal projections. Expression of cannabinoid receptors did not alter proliferation rate, viability, or apoptosis relative to parental SH-SY5Y. Transcripts for endogenous cannabinoid system enzymes (diacylglycerol lipase, monoacylglycerol lipase, α/β-hydrolase domain containing proteins 6 and 12, N-acyl phosphatidylethanolamine-phospholipase D, and fatty acid amide hydrolase) were not altered by CB₁ or CB₂ expression. Endocannabinoid ligands 2-arachidonoylglycerol (2-AG) and anandamide were quantitated in SH-SY5Y cells, and diacylglycerol lipase inhibitor tetrahydrolipstatin decreased 2-AG abundance by 90% but did not alter anandamide abundance. M3 muscarinic agonist oxotremorine M, and inhibitors of monoacylglycerol lipase and α/β hydrolase domain containing proteins 6 &12 increased 2-AG abundance. CB₁ receptor expression increased lengths of short (<30 μm) and long (>30 μm) projections, and this effect was significantly reduced by tetrahydrolipstatin, indicative of stimulation by endogenously produced 2-AG. Pertussis toxin, Gβγ inhibitor gallein, and β-arrestin inhibitor barbadin did not significantly alter long projection length in CB₁XS, but significantly reduced short projections, with gallein having the greatest inhibition. The rho kinase inhibitor Y27632 increased CB₁ receptor-mediated long projection extension, indicative of actin cytoskeleton involvement. CB₁ receptor expression increased GAP43 and ST8SIA2 mRNA and decreased ITGA1 mRNA, whereas CB₂ receptor expression increased NCAM and SYT mRNA. We propose that basal endogenous production of 2-AG provides autocrine stimulation of CB₁ receptor signaling through Gi/o, Gβγ, and β-arrestin mechanisms to promote neuritogenesis, and rho kinase influences process extension.

Myelin-associated glycoprotein inhibits neurite outgrowth through inactivation of the small GTPase Rap1

Rap1 is a small GTPase that has been implicated in dendritic development and plasticity. In this study, we investigated the role of Rap1 in axonal growth and its activation in response to neurotrophins and myelin-associated inhibitors. We report that Rap1 is activated by brain-derived neurotrophic factor and that this activation can be blocked by myelin-associated glycoprotein (MAG) or central nervous system myelin, which also induced increases in Rap1GAP1 levels. In addition, we demonstrate that adenoviral overexpression of Rap1 enhances neurite outgrowth in the presence of MAG and myelin, while inhibition of Rap1 activity through overexpression of Rap1GAP1 blocks neurite outgrowth. These findings suggest that Rap1GAP1 negatively regulates neurite outgrowth, making it a potential therapeutic target to promote axonal regeneration.

Targeting dysfunctional beta-cell signaling for the potential treatment of type 1 diabetes mellitus

Since its discovery and purification by Frederick Banting in 1921, exogenous insulin has remained almost the sole therapy for type 1 diabetes mellitus. While insulin alleviates the primary dysfunction of the disease, many other aspects of the pathophysiology of type 1 diabetes mellitus are unaffected. Research aimed towards the discovery of novel type 1 diabetes mellitus therapeutics targeting different cell signaling pathways is gaining momentum. The focus of these efforts has been almost entirely on the impact of immunomodulatory drugs, particularly those that have already received FDA-approval for other autoimmune diseases. However, these drugs can often have severe side effects, while also putting already immunocompromised individuals at an increased risk for other infections. Potential therapeutic targets in the insulin-producing beta-cell have been largely ignored by the type 1 diabetes mellitus field, save the glucagon-like peptide 1 receptor. While there is preliminary evidence to support the clinical exploration of glucagon-like peptide 1 receptor-based drugs as type 1 diabetes mellitus adjuvant therapeutics, there is a vast space for other putative therapeutic targets to be explored. The alpha subunit of the heterotrimeric G_z protein (Gα_z) has been shown to promote beta-cell inflammation, dysfunction, death, and failure to replicate in the context of diabetes in a number of mouse models. Genetic loss of Gα_z or inhibition of the Gα_z signaling pathway through dietary interventions is protective against the development of insulitis and hyperglycemia. The multifaceted effects of Gα_z in regards to beta-cell health in the context of diabetes make it an ideal therapeutic target for further study. It is our belief that a low-risk, effective therapy for type 1 diabetes mellitus will involve a multidimensional approach targeting a number of regulatory systems, not the least of which is the insulin-producing beta-cell. Impact statement The expanding investigation of beta-cell therapeutic targets for the treatment and prevention of type 1 diabetes mellitus is fundamentally relevant and timely. This review summarizes the overall scope of research into novel type 1 diabetes mellitus therapeutics, highlighting weaknesses or caveats in current clinical trials as well as describing potential new targets to pursue. More specifically, signaling proteins that act as modulators of beta-cell function, survival, and replication, as well as immune infiltration may need to be targeted to develop the most efficient pharmaceutical interventions for type 1 diabetes mellitus. One such beta-cell signaling pathway, mediated by the alpha subunit of the heterotrimeric G_z protein (Gα_z), is discussed in more detail. The work described here will be critical in moving the field forward as it emphasizes the central role of the beta-cell in type 1 diabetes mellitus disease pathology.

An investigation into the potential role of brain angiogenesis inhibitor protein 3 (BAI3) in the tumorigenesis of small-cell carcinoma: a review of the surrounding literature

Brain angiogenesis inhibitor protein 3 (BAI3) is from the adhesion group of seven-transmembrane spanning G protein-coupled receptors (GPCRs) and has been identified via gene expression profiling as being upregulated in small-cell lung cancer (SCLC) tumors. It has subsequently been validated as a sensitive and specific immunohistochemical marker for SCLC, helping to differentiate these tumors from morphologically similar large-cell neuroendocrine (LCNEC) malignancies. It is, however, still unclear as to the role BAI3 proteins might play in SCLC and indeed how they might contribute to tumorigenesis. Interestingly, the pattern of staining observed on immunohistochemistry was in fact nuclear as opposed to the membranous staining pattern expected of transmembrane-bound molecules. This fact has lead the authors to believe that the protein receptor is structurally altered in SCLC and that this modification may confer different behavioral properties that contribute toward tumorigenesis. Nuclear localization is not unique to BAI3 and has been reported in a number of GPCRs and frequently correlates with survival outcomes. BAI3 has the potential to act as target for pharmaceutical intervention inline with developing trends in molecular pathology aiming to provide personalized, treatment regimes based on tumor-specific mutation profiles. The adhesion group of the GPCR superfamily is still poorly understood. We present a review of the existing literature regarding the role they play in both physiological and disease states and the mechanisms by which they influence a range of cellular processes.

Roles of Rap1 signaling in tumor cell migration and invasion

Ras-associated protein-1 (Rap1), a small GTPase in the Ras-related protein family, is an important regulator of basic cellular functions (e.g., formation and control of cell adhesions and junctions), cellular migration, and polarization. Through its interaction with other proteins, Rap1 plays many roles during cell invasion and metastasis in different cancers. The basic function of Rap1 is straightforward; it acts as a switch during cellular signaling transduction and regulated by its binding to either guanosine triphosphate (GTP) or guanosine diphosphate (GDP). However, its remarkably diverse function is rendered by its interplay with a large number of distinct Rap guanine nucleotide exchange factors and Rap GTPase activating proteins. This review summarizes the mechanisms by which Rap1 signaling can regulate cell invasion and metastasis, focusing on its roles in integrin and cadherin regulation, Rho GTPase control, and matrix metalloproteinase expression.

Function and Regulation of Heterotrimeric G Proteins during Chemotaxis

Chemotaxis, or directional movement towards an extracellular gradient of chemicals, is necessary for processes as diverse as finding nutrients, the immune response, metastasis and wound healing. Activation of G-protein coupled receptors (GPCRs) is at the very base of the chemotactic signaling pathway. Chemotaxis starts with binding of the chemoattractant to GPCRs at the cell-surface, which finally leads to major changes in the cytoskeleton and directional cell movement towards the chemoattractant. Many chemotaxis pathways that are directly regulated by Gβγ have been identified and studied extensively; however, whether Gα is just a handle that regulates the release of Gβγ or whether Gα has its own set of distinct chemotactic effectors, is only beginning to be understood. In this review, we will discuss the different levels of regulation in GPCR signaling and the downstream pathways that are essential for proper chemotaxis.

Gαz regulates BDNF-induction of axon growth in cortical neurons

The disruption of neurotransmitter and neurotrophic factor signaling in the central nervous system (CNS) is implicated as the root cause of neuropsychiatric disorders, including schizophrenia, epilepsy, chronic pain, and depression. Therefore, identifying the underlying molecular mechanisms by which neurotransmitter and neurotrophic factor signaling regulates neuronal survival or growth may facilitate identification of more effective therapies for these disorders. Previously, our lab found that the heterotrimeric G protein, Gz, mediates crosstalk between G protein-coupled receptors and neurotrophin signaling in the neural cell line PC12. These data, combined with Gαz expression profiles--predominantly in neuronal cells with higher expression levels corresponding to developmental times of target tissue innervation--suggested that Gαz may play an important role in neurotrophin signaling and neuronal development. Here, we provide evidence in cortical neurons, both manipulated ex vivo and those cultured from Gz knockout mice, that Gαz is localized to axonal growth cones and plays a significant role in the development of axons of cortical neurons in the CNS. Our findings indicate that Gαz inhibits BDNF-stimulated axon growth in cortical neurons, establishing an endogenous role for Gαz in regulating neurotrophin signaling in the CNS.

The α-subunit of the trimeric GTPase Go2 regulates axonal growth

The Goα splice variants Go1α and Go2α are subunits of the most abundant G-proteins in brain, Go1 and Go2. Only a few interacting partners binding to Go1α have been described so far and splice variant-specific differences are not known. Using a yeast two-hybrid screen with constitutively active Go2α as bait, we identified Rap1GTPase activating protein (Rap1GAP) and Girdin as interacting partners of Go2α, which was confirmed by co-immunoprecipitation. Comparison of subcellular fractions from brains of wild type and Go2α-/- mice revealed no differences in the overall expression level of Girdin or Rap1GAP. However, we found higher amounts of active Rap1-GTP in brains of Go2α deficient mutants, indicating that Go2α may increase Rap1GAP activity, thereby effecting the Rap1 activation/deactivation cycle. Rap1 has been shown to be involved in neurite outgrowth and given a Rap1GAP-Go2α interaction, we found that the loss of Go2α affected axonal outgrowth. Axons of cultured cortical and hippocampal neurons prepared from embryonic Go2α-/- mice grew longer and developed more branches than those from wild-type mice. Taken together, we provide evidence that Go2α regulates axonal outgrowth and branching.

Rap1GAP alters leukemia cell differentiation, apoptosis and invasion in vitro

Rap1GAP which regulates the GTP-GDP form switch of Rap1 is a member of the GTPase-activating protein (GAP) family and has recently received substantial attention. Rap1GAP is thought of as a putative tumor suppressor gene and plays an important role in human tumor progression including pancreatic cancer, thyroid cancer and melanoma. In the current study, we found that the expression of Rap1GAP was lower in acute myeloid leukemia (AML) patients compared to non-malignant blood disease patients. The expression of Rap1GAP was also low in HL-60, NB4, U937 and SHI-1 myeloid leukemia cell lines. Upregulated Rap1GAP in NB4 and HL-60 cells promoted cell differentiation induced by ATRA or TPA compared to the empty vector control cells. Furthermore, Rap1GAP-transfected cells also showed a higher rate of apoptosis in response to arsenic trioxide compared to the control counterpart cells. In addition, we found that increased expression of Rap1GAP promoted leukemia cell invasion may be due to matrix metalloproteinase 9 (MMP9). In conclusion, these results demonstrated that Rap1GAP promoted leukemia cell differentiation and apoptosis, but increased leukemia cell invasion in vitro.

Group II activators of G-protein signalling and proteins containing a G-protein regulatory motif

Beyond the core triad of receptor, Gαβγ and effector, there are multiple accessory proteins that provide alternative modes of signal input and regulatory adaptability to G-protein signalling systems. Such accessory proteins may segregate a signalling complex to microdomains of the cell, regulate the basal activity, efficiency and specificity of signal propagation and/or serve as alternative binding partners for Gα or Gβγ independent of the classical heterotrimeric Gαβγ complex. The latter concept led to the postulate that Gα and Gβγ regulate intracellular events distinct from their role as transducers for cell surface seven-transmembrane span receptors. One general class of such accessory proteins is defined by AGS proteins or activators of G-protein signalling that refer to mammalian cDNAs identified in a specific yeast-based functional screen. The discovery of AGS proteins and related entities revealed a number of unexpected mechanisms for regulation of G-protein signalling systems and expanded functional roles for this important signalling system.

Tre1 GPCR signaling orients stem cell divisions in the Drosophila central nervous system

During development, directional cell division is a major mechanism for establishing the orientation of tissue growth. Drosophila neuroblasts undergo asymmetric divisions perpendicular to the overlying epithelium to produce descendant neurons on the opposite side, thereby orienting initial neural tissue growth. However, the mechanism remains elusive. We provide genetic evidence that extrinsic GPCR signaling determines the orientation of cortical polarity underlying asymmetric divisions of neuroblasts relative to the epithelium. The GPCR Tre1 activates the G protein oα subunit in neuroblasts by interacting with the epithelium to recruit Pins, which regulates spindle orientation. Because Pins associates with the Par-complex via Inscuteable, Tre1 consequently recruits the polarity complex to orthogonally orient the polarity axis to the epithelium. Given the universal role of the Par complex in cellular polarization, we propose that the GPCR-Pins system is a comprehensive mechanism controlling tissue polarity by orienting polarized stem cells and their divisions.

Prostaglandin E2 regulates renal cell carcinoma invasion through the EP4 receptor-Rap GTPase signal transduction pathway

Prognosis for patients with early stage kidney cancer has improved, but the treatment options for patients with locally advanced disease and metastasis remain few. Understanding the molecular mechanisms that regulate invasion and metastasis is critical for developing successful therapies to treat these patients. Proinflammatory prostaglandin E(2) plays an important role in cancer initiation and progression via activation of cognate EP receptors that belong to the superfamily of G protein-coupled receptors. Here we report that prostaglandin E(2) promotes renal cancer cell invasion through a signal transduction pathway that encompasses EP4 and small GTPase Rap. Inactivation of Rap signaling with Rap1GAP, like inhibition of EP4 signaling with ligand antagonist or knockdown with shRNA, reduces the kidney cancer cell invasion. Human kidney cells evidence increased EP4 and decreased Rap1GAP expression levels in the malignant compared with benign samples. These results support the idea that targeted inhibition of EP4 signaling and restoration of Rap1GAP expression constitute a new strategy to control kidney cancer progression.

Regulating Rap small G-proteins in time and space

Signaling by the small G-protein Rap is under tight regulation by its GEFs and GAPs. These are multi-domain proteins that are themselves controlled by distinct upstream pathways, and thus couple different extra- and intracellular cues to Rap. The individual RapGEFs and RapGAPs are, in addition, targeted to specific cellular locations by numerous anchoring mechanisms and, consequently, may control different pools of Rap. Here, we review the various activating signals and targeting mechanisms of these proteins and discuss their contribution to the spatiotemporal regulation and biological functions of the Rap proteins.

Resistance to inhibitors of cholinesterase-8A (Ric-8A) is critical for growth factor receptor-induced actin cytoskeletal reorganization

Heterotrimeric G proteins are critical transducers of cellular signaling. In addition to their classic roles in relaying signals from G protein-coupled receptors (GPCRs), heterotrimeric G proteins also mediate physiological functions from non-GPCRs. Previously, we have shown that Gα(13), a member of the heterotrimeric G proteins, is essential for growth factor receptor-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. These Gα(13)-mediated dorsal ruffle turnover and cell migration by growth factors acting on their receptor tyrosine kinases (RTKs) are independent of GPCRs. However, the mechanism by which RTKs signal to Gα(13) is not known. Here, we show that cholinesterase-8A (Ric-8A), a nonreceptor guanine nucleotide exchange factor for some heterotrimeric G proteins, is critical for coupling RTKs to Gα(13). Down-regulation of Ric-8A protein levels in cells by RNA interference slowed down platelet-derived growth factor (PDGF)-induced dorsal ruffle turnover and inhibited PDGF-initiated cell migration. PDGF was able to increase the activity of Ric-8A in cells. Furthermore, purified Ric-8A proteins interact directly with purified Gα(13) protein in a nucleotide-dependent manner. Deficiency of Ric-8A prevented the translocation of Gα(13) to the cell cortex. Hence, Ric-8A is critical for growth factor receptor-induced actin cytoskeletal reorganization.

Rap1GAP impairs cell-matrix adhesion in the absence of effects on cell-cell adhesion

The significance of the widespread downregulation of Rap1GAP in human tumors is unknown. In previous studies we demonstrated that silencing Rap1GAP expression in human colon cancer cells resulted in sustained increases in Rap activity, enhanced spreading on collagen and the weakening of cell-cell contacts. The latter finding was unexpected based on the role of Rap1 in strengthening cell-cell adhesion and reports that Rap1GAP impairs cell-cell adhesion. We now show that Rap1GAP is a more effective inhibitor of cell-matrix compared to cell-cell adhesion. Overexpression of Rap1GAP in human colon cancer cells impaired Rap2 activity and the ability of cells to spread and migrate on collagen IV. Under the same conditions, Rap1GAP had no effect on cell-cell adhesion. Overexpression of Rap1GAP did not enhance the dissociation of cell aggregates nor did it impair the accumulation of β-catenin and E-cadherin at cell-cell contacts. To further explore the role of Rap1GAP in the regulation of cell-cell adhesion, Rap1GAP was overexpressed in non-transformed thyroid epithelial cells. Although the formation of cell-cell contacts required Rap1, overexpression of Rap1GAP did not impair cell-cell adhesion. These data indicate that transient, modest expression of Rap1GAP is compatible with cell-cell adhesion and that the role of Rap1GAP in the regulation of cell-cell adhesion may be more complex than is currently appreciated.

RapGAPs in brain: multipurpose players in neuronal Rap signalling

Small Rap guanosine-tri-phosphate (GTP)ases are crucially involved in many cellular processes, including cell proliferation, differentiation, survival, adhesion and movement. In line, it has been shown that Rap signalling is involved in various aspects of neuronal differentiation, like the establishment of neuronal polarity or axonal growth cone movement. Rap GTPases can be activated by a wide variety of external stimuli, and this is mediated by specific guanine nucleotide exchange factors (RapGEFs). Inactivation of RapGTP can be achieved with the aid of specific GTPase-activating proteins (RapGAPs). In the brain, the most prominent RapGAPs are Rap1GAP and those of the spine-associated RapGAP (SPAR) family. This latter family consists of three members (SPAR1-3), from which two of them, namely SPAR1 and 2, have been investigated in more detail. As such, the localization of RapGAPs is crucially important in regulating Rap signalling at various sites in the cell and, for both SPAR1 and 2, enrichment at synaptic sites has been demonstrated. In recent years particularly the role of SPAR1 in shaping dendritic spine morphology has attracted considerable interest. In this review we will summarize the described actions of different RapGAPs expressed in the brain, and we will focus in particular on the SPAR family members.

Downregulation of Rap1GAP in human tumor cells alters cell/matrix and cell/cell adhesion

Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas. To elucidate the advantages conferred on tumor cells by loss of Rap1GAP, Rap1GAP expression was silenced in human colon carcinoma cells. Suppressing Rap1GAP induced profound alterations in cell adhesion. Rap1GAP-depleted cells exhibited defects in cell/cell adhesion that included an aberrant distribution of adherens junction proteins. Depletion of Rap1GAP enhanced adhesion and spreading on collagen. Silencing of Rap expression normalized spreading and restored E-cadherin, beta-catenin, and p120-catenin to cell/cell contacts, indicating that unrestrained Rap activity underlies the alterations in cell adhesion. The defects in adherens junction protein distribution required integrin signaling as E-cadherin and p120-catenin were restored at cell/cell contacts when cells were plated on poly-l-lysine. Unexpectedly, Src activity was increased in Rap1GAP-depleted cells. Inhibition of Src impaired spreading and restored E-cadherin at cell/cell contacts. These findings provide the first evidence that Rap1GAP contributes to cell/cell adhesion and highlight a role for Rap1GAP in regulating cell/matrix and cell/cell adhesion. The frequent downregulation of Rap1GAP in epithelial tumors where alterations in cell/cell and cell/matrix adhesion are early steps in tumor dissemination supports a role for Rap1GAP depletion in tumor progression.

Drosophila GoLoco-protein Pins is a target of Galpha(o)-mediated G protein-coupled receptor signaling

G protein-coupled receptors (GPCRs) transduce their signals through trimeric G proteins, inducing guanine nucleotide exchange on their Galpha-subunits; the resulting Galpha-GTP transmits the signal further inside the cell. GoLoco domains present in many proteins play important roles in multiple trimeric G protein-dependent activities, physically binding Galpha-subunits of the Galpha(i/o) class. In most cases GoLoco binds exclusively to the GDP-loaded form of the Galpha-subunits. Here we demonstrate that the poly-GoLoco-containing protein Pins of Drosophila can bind to both GDP- and GTP-forms of Drosophila Galpha(o). We identify Pins GoLoco domain 1 as necessary and sufficient for this unusual interaction with Galpha(o)-GTP. We further pinpoint a lysine residue located centrally in this domain as necessary for the interaction. Our studies thus identify Drosophila Pins as a target of Galpha(o)-mediated GPCR receptor signaling, e.g., in the context of the nervous system development, where Galpha(o) acts downstream from Frizzled and redundantly with Galpha(i) to control the asymmetry of cell divisions.

A point mutation to Galphai selectively blocks GoLoco motif binding: direct evidence for Galpha.GoLoco complexes in mitotic spindle dynamics

Heterotrimeric G-protein Galpha subunits and GoLoco motif proteins are key members of a conserved set of regulatory proteins that influence invertebrate asymmetric cell division and vertebrate neuroepithelium and epithelial progenitor differentiation. GoLoco motif proteins bind selectively to the inhibitory subclass (Galphai) of Galpha subunits, and thus it is assumed that a Galphai.GoLoco motif protein complex plays a direct functional role in microtubule dynamics underlying spindle orientation and metaphase chromosomal segregation during cell division. To address this hypothesis directly, we rationally identified a point mutation to Galphai subunits that renders a selective loss-of-function for GoLoco motif binding, namely an asparagine-to-isoleucine substitution in the alphaD-alphaE loop of the Galpha helical domain. This GoLoco-insensitivity ("GLi") mutation prevented Galphai1 association with all human GoLoco motif proteins and abrogated interaction between the Caenorhabditis elegans Galpha subunit GOA-1 and the GPR-1 GoLoco motif. In contrast, the GLi mutation did not perturb any other biochemical or signaling properties of Galphai subunits, including nucleotide binding, intrinsic and RGS protein-accelerated GTP hydrolysis, and interactions with Gbetagamma dimers, adenylyl cyclase, and seven transmembrane-domain receptors. GoLoco insensitivity rendered Galphai subunits unable to recruit GoLoco motif proteins such as GPSM2/LGN and GPSM3 to the plasma membrane, and abrogated the exaggerated mitotic spindle rocking normally seen upon ectopic expression of wild type Galphai subunits in kidney epithelial cells. This GLi mutation should prove valuable in establishing the physiological roles of Galphai.GoLoco motif protein complexes in microtubule dynamics and spindle function during cell division as well as to delineate potential roles for GoLoco motifs in receptor-mediated signal transduction.

Membrane interactions of G proteins and other related proteins

Guanine nucleotide-binding proteins, G proteins, propagate incoming messages from receptors to effector proteins. They switch from an inactive to active state by exchanging a GDP molecule for GTP, and they return to the inactive form by hydrolyzing GTP to GDP. Small monomeric G proteins, such as Ras, are involved in controlling cell proliferation, differentiation and apoptosis, and they interact with membranes through isoprenyl moieties, fatty acyl moieties, and electrostatic interactions. This protein-lipid binding facilitates productive encounters of Ras and Raf proteins in defined membrane regions, so that signals can subsequently proceed through MEK and ERK kinases, which constitute the canonical MAP kinase signaling cassette. On the other hand, heterotrimeric G proteins undergo co/post-translational modifications in the alpha (myristic and/or palmitic acid) and the gamma (farnesol or geranylgeraniol) subunits. These modifications not only assist the G protein to localize to the membrane but they also help distribute the heterotrimer (Galphabetagamma) and the subunits generated upon activation (Galpha and Gbetagamma) to appropriate membrane microdomains. These proteins transduce messages from ubiquitous serpentine receptors, which control important functions such as taste, vision, blood pressure, body weight, cell proliferation, mood, etc. Moreover, the exchange of GDP by GTP is triggered by nucleotide exchange factors. Membrane receptors that activate G proteins can be considered as such, but other cytosolic, membranal or amphitropic proteins can accelerate the rate of G protein exchange or even activate this process in the absence of receptor-mediated activation. These and other protein-protein interactions of G proteins with other signaling proteins are regulated by their lipid preferences. Thus, G protein-lipid interactions control the features of messages and cell physiology.

Independent regulation of Rap1 and mitogen-activated protein kinase by the alpha chain of Go

Receptors coupled to G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) cascade. The intracellular pathways linking the alpha chains of these G proteins to MAPK activation are not completely understood. One of the signaling molecules which has been suggested to act downstream of Galpha(i/o) is the small G protein Rap1. We investigated the role of Rap1 in MAPK stimulation by Galpha(o) in Chinese hamster ovary (CHO) cells. Our previous results have shown that in this cell system activated Galpha(o) strongly potentiates the MAPK response to the epidermal growth factor (EGF) receptor. Rap1 regulation was examined in cells transfected with Rap1 and wild-type Galpha(o) or the activated mutant Galpha(o)-Q205L. Immunocytochemical analysis detected both Rap1 and the Galpha(o) subunit at the plasma membrane as well as on perinuclear cytoplasmic vesicles. Expression of wild-type Galpha(o) had no significant effect on the levels of activated Rap1. In contrast, Galpha(o)-Q205L virtually abolished the activation of Rap1 induced by EGF. Further experiments showed that MAPK stimulation by EGF was greatly inhibited by expression of activated Rap1, suggesting that Rap1 inhibition could mediate the effect of Galpha(o) on the MAPK cascade. However, Galpha(o)-Q205L efficiently inhibited the activation of Rap1 induced by fibroblast growth factor (FGF). We have previously found that the ability of FGF to activate MAPK is not modified by Galpha(o). In addition, expression of the GAP protein RAP1GAPII blocked Rap1 activation without affecting EGF- or FGF-dependent MAPK stimulation. These findings provide evidence for independent regulation of Rap1 and MAPK by the G(o )alpha chain.

G Proteins G12 and G13 Control the Dynamic Turnover of Growth Factor-induced Dorsal Ruffles

Growth factors induce massive actin cytoskeletal remodeling in cells. These reorganization events underlie various cellular responses such as cell migration and morphological changes. One major form of actin reorganization is the formation and disassembly of dorsal ruffles (also named waves, dorsal rings, or circular ruffles). Dorsal ruffles are involved in physiological functions including cell migration, invasion, macropinocytosis, plasma membrane recycling, and others. Growth factors initiate rapid formation (within 5 min) of circular membrane ruffles, and these ruffles move along the dorsal side of the cells, constrict, close, and eventually disassemble ( approximately 20 min). Considerable attention has been devoted to the mechanism by which growth factors induce the formation of dorsal ruffles. However, little is known of the mechanism by which these ruffles are disassembled. Here we have shown that G proteins G(12) and G(13) control the rate of disassembly of dorsal ruffles. In Galpha(12)(-/-)Galpha(13)(-/-) fibroblast cells, dorsal ruffles induced by growth factor treatment remain visible substantially longer ( approximately 60 min) than in wild-type cells, whereas the rate of formation of these ruffles was the same with or without Galpha(12) and Galpha(13). Thus, Galpha(12)/Galpha(13) critically regulate dorsal ruffle turnover.

Differential G-alpha interaction capacities of the GoLoco motifs in Rap GTPase activating proteins

GoLoco motif proteins act as guanine nucleotide dissociation inhibitors (GDIs) for G-protein alpha subunits of the adenylyl cyclase-inhibitory (Galpha(i/o)) class. Rap1GAP2 is a newly identified GoLoco motif- and RapGAP domain-containing protein, and thus is considered a potential integrator of heterotrimeric and monomeric GTPase signaling. Primary sequence analysis indicated that the Rap1GAP2 GoLoco motif contains a lysine (Lys-75), rather than an arginine, at the crucial residue responsible for binding the alpha and beta phosphates of GDP and exerting GDI activity. To determine the functional outcome of this sequence variation we conducted a biophysical analysis of the human Rap1GAP2b/c GoLoco motif. We found that human Rap1GAP2b/c was deficient in GDI activity and Galpha interaction capability. Mutation of lysine-75 to arginine could not regain functional activity of the Rap1GAP2b/c GoLoco motif. Thus, the Rap1GAP2b/c GoLoco motif can be classed as inactive towards Galpha subunits. We also found that the Rap1GAP1a GoLoco motif, which lacks seven N-terminal amino acid residues present in canonical GoLoco motifs, does not interact with Galpha(i1). In contrast, the GoLoco motif of Rap1GAP1b, which is canonical in primary sequence, was found to interact with Galpha(i1).GDP.

The G protein G alpha(13) is required for growth factor-induced cell migration

Heterotrimeric G proteins are critical cellular signal transducers. They are known to directly relay signals from seven-transmembrane G protein-coupled receptors (GPCRs) to downstream effectors. On the other hand, receptor tyrosine kinases (RTKs), a different family of membrane receptors, signal through docking sites in their carboxy-terminal tails created by autophosphorylated tyrosine residues. Here we show that a heterotrimeric G protein, G alpha(13), is essential for RTK-induced migration of mouse fibroblast and endothelial cells. G alpha(13) activity in cell migration is retained in a C-terminal mutant that is defective in GPCR coupling, suggesting that the migration function is independent of GPCR signaling. Thus, G alpha(13) appears to be a critical signal transducer for RTKs as well as GPCRs. This broader role of G alpha(13) in cell migration initiated by two types of receptors could provide a molecular basis for the vascular system defects exhibited by G alpha(13) knockout mice.

Non-receptor activators of heterotrimeric G-protein signaling (AGS proteins)

G-protein coupled receptor (GPCR) signaling represents one of the most conserved and ubiquitous means in mammalian cells for transferring information across the plasma membrane to the intracellular environment. Heterotrimeric G-protein subunits play key roles in transducing these signals, and intracellular regulators influencing the activation state and interaction of these subunits regulate the extent and duration of GPCR signaling. One class of intracellular regulator, the non-receptor activators of G-protein signaling (or AGS proteins), are the major focus of this review. AGS proteins provide a basis for understanding the function of heterotrimeric G-proteins in both GPCR-driven and GPCR independent cellular signaling pathways.

Activation of heterotrimeric G-proteins independent of a G-protein coupled receptor and the implications for signal processing

Heterotrimeric G-proteins are key transducers for signal transfer from outside the cell, mediating signals emanating from cell-surface G-protein coupled receptors (GPCR). Many, if not all, subtypes of heterotrimeric G-proteins are also regulated by accessory proteins that influence guanine nucleotide binding, guanosine triphosphate (GTP) hydrolysis, or subunit interactions. One subgroup of such accessory proteins (activators of G-protein signaling; AGS proteins) refer to a functionally defined group of proteins that activate selected G-protein signaring systems in the absence of classical G-protein coupled receptors. AGS and related proteins provide unexpected insights into the regulation of the G-protein activation-deactivation cycle. Different AGS proteins function as guanine nucleotide exchange factors or guanine nucleotide dissociation inhibitors and may also influence subunit interactions by interaction with GBgamma. These proteins play important roles in the generation or positioning of signaling complexes and of the regulation of GPCR signaling, and as alternative binding partners for G-protein subunits. Perhaps of even broader impact is the discovery that AGS proteins provide a foundation for the concept that heterotrimeric G-protein subunits are processing signals within the cell involving intrinsic cues that do not involve the classical signal input from a cell surface GPCR.

Dual roles for the trimeric G protein Go in asymmetric cell division in Drosophila

During asymmetric division, a cell polarizes and differentially distributes components to its opposite ends. The subsequent division differentially segregates the two component pools to the daughters, which thereby inherit different developmental directives. In Drosophila sensory organ precursor cells, the localization of Numb protein to the cell's anterior cortex is a key patterning event and is achieved by the combined action of many proteins, including Pins, which itself is localized anteriorly. Here, a role is described for the trimeric G protein Go in the anterior localization of Numb and daughter cell fate specification. Go is shown to interact with Pins. In addition to a role in recruiting Numb to an asymmetric location in the cell's cortex, Go transduces a signal from the Frizzled receptor that directs the position in which the complex forms. Thus, Go likely integrates the signaling that directs the formation of the complex with the signaling that directs where the complex forms.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Identification of a putative tumor suppressor gene Rap1GAP in pancreatic cancer

Human chromosome 1p35-p36 has long been suspected to harbor a tumor suppressor gene in pancreatic cancer and other tumors. We found that expression of rap1GAP, a gene located in this chromosomal region, is significantly down-regulated in pancreatic cancer. Only a small percentage of preneoplastic pancreatic intraductal neoplasia lesions lost rap1GAP expression, whereas loss of rap1GAP expression occurred in 60% of invasive pancreatic cancers, suggesting that rap1GAP contributes to pancreatic cancer progression. In vitro and in vivo studies showed that loss of rap1GAP promotes pancreatic cancer growth, survival, and invasion, and may function through modulation of integrin activity. Furthermore, we showed a high frequency of loss of heterozygosity of rap1GAP in pancreatic cancer. Collectively, our data identify rap1GAP as a putative tumor suppressor gene in pancreatic cancer.

G-protein alpha subunit interaction and guanine nucleotide dissociation inhibitor activity of the dual GoLoco motif protein PCP-2 (Purkinje cell protein-2)

Purkinje cell protein-2 (PCP-2; L7/GPSM4) is a GoLoco motif-containing protein that is specifically expressed in Purkinje and retinal ON bipolar cells. An alternative splice variant of PCP-2 has recently been isolated which contains two GoLoco motifs. Although the second GoLoco motif (GL2) of PCP-2 has been reported to interact with Galpha-subunits, a complete biochemical analysis of each individual motif of PCP-2 has not been performed. We demonstrate that the first GoLoco motif (GL1) of PCP-2 is equipotent as a guanine nucleotide dissociation inhibitor (GDI) towards Galphai1 and Galphai2, while it has sevenfold lower GDI activity for Galphai3 and greater than 20-fold lower GDI activity against Galphao. In contrast we found PCP-2 GL2 to be essentially equipotent as a GDI for all Galphai subunits, but it had negligible activity toward Galphao. Using co-immunoprecipitation from COS-7 cells, we found that PCP-2 was only able to interact with Galphai1 but not Galphao nor Galpha-subunits from other families (Galphas, Galphaq, or Galpha12). Mutational analysis of a non-canonical residue (glycine 24) in human PCP-2 GL1 provided evidence for heterogeneity in mechanisms of Galphai interactions with GoLoco motifs. Collectively, the data demonstrate that PCP-2 is a comparatively weak GoLoco motif protein that exhibits highest affinity interactions and GDI activity toward Galphai1, Galphai2, and Galphai3 subunits.

G alpha selectivity and inhibitor function of the multiple GoLoco motif protein GPSM2/LGN

GPSM2 (G-protein signalling modulator 2; also known as LGN or mammalian Pins) is a protein that regulates mitotic spindle organization and cell division. GPSM2 contains seven tetratricopeptide repeats (TPR) and four Galpha(i/o)-Loco (GoLoco) motifs. GPSM2 has guanine nucleotide dissociation inhibitor (GDI) activity towards both Galpha(o)- and Galpha(i)-subunits; however, a systematic analysis of its individual GoLoco motifs has not been described. We analyzed each of the four individual GoLoco motifs from GPSM2, assessing their relative binding affinities and GDI potencies for Galpha(i1), Galpha(i2), and Galpha(i3) and Galpha(o). Each of the four GPSM2 GoLoco motifs (36-43 amino acids in length) was expressed in bacteria as a GST-fusion protein and purified to homogeneity. The binding of each of the four GST-GoLoco motifs to Galpha(i1)-, Galpha(o)-, and Galpha(s)-subunits was assessed by surface plasmon resonance; all of the motifs bound Galpha(i1), but exhibited low affinity towards Galpha(o). GDI activity was assessed by a fluorescence-based nucleotide-binding assay, revealing that all four GoLoco motifs are functional as GDIs for Galpha(i1), Galpha(i2), and Galpha(i3). Consistent with our binding studies, the GDI activity of GPSM2 GoLoco motifs on Galpha(o) was significantly lower than that toward Galpha(i1), suggesting that the in vivo targets of GPSM2 are most likely to be Galpha(i)-subunits.

A Role for Gz in Pancreatic Islet β-Cell Biology

Glucose-stimulated insulin secretion and beta-cell growth are important facets of pancreatic islet beta-cell biology. As a result, factors that modulate these processes are of great interest for the potential treatment of Type 2 diabetes. Here, we present evidence that the heterotrimeric G protein G(z) and its effectors, including some previously thought to be confined in expression to neuronal cells, are present in pancreatic beta-cells, the largest cellular constituent of the islets of Langerhans. Furthermore, signaling pathways upon which G alpha(z) impacts are intact in beta-cells, and G alpha(z) activation inhibits both cAMP production and glucose-stimulated insulin secretion in the Ins-1(832/13) beta-cell-derived line. Inhibition of glucose-stimulated insulin secretion by prostaglandin E (PGE1) is pertussis-toxin insensitive, indicating that other G alpha(i) family members are not involved in this process in this beta-cell line. Indeed, overexpression of a selective deactivator of G alpha(z), the RGS domain of RGSZ1, blocks the inhibitory effect of PGE1 on glucose-stimulated insulin secretion. Finally, the inhibition of glucose-stimulated insulin secretion by PGE1 is substantially blunted by small interfering RNA-mediated knockdown of G alpha(z) expression. Taken together, these data strongly imply that the endogenous E prostanoid receptor in the Ins-1(832/13) beta-cell line couples to G(z) predominantly and perhaps even exclusively. These data provide the first evidence for G(z) signaling in pancreatic beta-cells, and identify an endogenous receptor-mediated signaling process in beta-cells that is dependent on G alpha(z) function.

Regulation of the small GTPase Rap1 and extracellular signal-regulated kinases by the costimulatory molecule CTLA-4

The mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) is activated following engagement of the T-cell receptor and is required for interleukin 2 (IL-2) production and T-cell proliferation. This activation is enhanced by stimulation of the coreceptor CD28 and inhibited by the coreceptor CTLA-4. We show that the small G protein Rap1 is regulated in the opposite manner; it is inhibited by CD28 and activated by CTLA-4. Together, CD3 and CTLA-4 activate Rap1 in a sustained manner. To delineate T-cell function in the absence of Rap1 activity, we generated transgenic mice expressing Rap1GAP1, a Rap1-specific GTPase-activating protein. Transgenic mice showed lymphadenopathy, and transgenic T cells displayed increased ERK activation, proliferation, and IL-2 production. More significantly, the inhibitory effect of CTLA-4 on T-cell function in Rap1GAP1-transgenic T cells was reduced. We demonstrate that CTLA-4 activates Rap1, and we propose that intracellular signals from CTLA-4 antagonize CD28, at least in part, at the level of Rap1.

The GAPs, GEFs, and GDIs of heterotrimeric G-protein alpha subunits

The heterotrimeric G-protein alpha subunit has long been considered a bimodal, GTP-hydrolyzing switch controlling the duration of signal transduction by seven-transmembrane domain (7TM) cell-surface receptors. In 1996, we and others identified a superfamily of “regulator of G-protein signaling” (RGS) proteins that accelerate the rate of GTP hydrolysis by Gα subunits (dubbed GTPase-accelerating protein or “GAP” activity). This discovery resolved the paradox between the rapid physiological timing seen for 7TM receptor signal transduction in vivo and the slow rates of GTP hydrolysis exhibited by purified Gα subunits in vitro. Here, we review more recent discoveries that have highlighted newly-appreciated roles for RGS proteins beyond mere negative regulators of 7TM signaling. These new roles include the RGS-box-containing, RhoA-specific guanine nucleotide exchange factors (RGS-RhoGEFs) that serve as Gα effectors to couple 7TM and semaphorin receptor signaling to RhoA activation, the potential for RGS12 to serve as a nexus for signaling from tyrosine kinases and G-proteins of both the Gα and Ras-superfamilies, the potential for R7-subfamily RGS proteins to couple Gα subunits to 7TM receptors in the absence of conventional Gβγ dimers, and the potential for the conjoint 7TM/RGS-box Arabidopsis protein AtRGS1 to serve as a ligand-operated GAP for the plant Gα AtGPA1. Moreover, we review the discovery of novel biochemical activities that also impinge on the guanine nucleotide binding and hydrolysis cycle of Gα subunits: namely, the guanine nucleotide dissociation inhibitor (GDI) activity of the GoLoco motif-containing proteins and the 7TM receptor-independent guanine nucleotide exchange factor (GEF) activity of Ric‑8/synembryn. Discovery of these novel GAP, GDI, and GEF activities have helped to illuminate a new role for Gα subunit GDP/GTP cycling required for microtubule force generation and mitotic spindle function in chromosomal segregation.

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Galpha.GDP/Gbetagamma heterotrimers to promote GDP release and GTP binding, resulting in liberation of Galpha from Gbetagamma. Galpha.GTP and Gbetagamma target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Galpha and heterotrimer reformation - a cycle accelerated by 'regulators of G-protein signaling' (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) beta is activated by Galpha(q) and Gbetagamma, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Galpha nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways.

GAP control: regulating the regulators of small GTPases

The small GTPases of the Ras superfamily mediate numerous biological processes through their ability to cycle between an inactive GDP-bound and an active GTP-bound form. Among the key regulators of GTPase cycling are the GTPase-activating proteins (GAPs), which stimulate the weak intrinsic GTP-hydrolysis activity of the GTPases, thereby inactivating them. Despite the abundance of GAPs and the fact that mutations in GAP-encoding genes underlie several human diseases, these proteins have received relatively little attention. Recent studies have addressed the regulatory mechanisms that influence GAP activity. So far, findings suggest that GAP activity is regulated by several mechanisms, including protein-protein interactions, phospholipid interactions, phosphorylation, subcellular translocation and proteolytic degradation.

Rap1GAP2 is a new GTPase-activating protein of Rap1 expressed in human platelets

The Ras-like guanine-nucleotide-binding protein Rap1 controls integrin alpha(IIb)beta3 activity and platelet aggregation. Recently, we have found that Rap1 activation can be blocked by the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway by type 1 cGMP-dependent protein kinase (cGKI). In search of possible targets of NO/cGMP/cGKI, we studied the expression of Rap1-specific GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in platelets. We could detect mRNAs for a new protein most closely related to Rap1GAP and for postsynaptic density-95 discs-large and zona occludens protein 1 (PDZ)-GEF1 and CalDAG-GEFs I and III. Using 5'-rapid amplification of cDNA ends (RACE), we isolated the complete cDNA of the new GAP encoding a 715-amino acid protein, which we have termed Rap1GAP2. Rap1GAP2 is expressed in at least 3 splice variants, 2 of which are detectable in platelets. Endogenous Rap1GAP2 protein partially colocalizes with Rap1 in human platelets. In transfected cells, we show that Rap1GAP2 exhibits strong GTPase-stimulating activity toward Rap1. Rap1GAP2 is highly phosphorylated, and we have identified cGKI as a Rap1GAP2 kinase. cGKI phosphorylates Rap1GAP2 exclusively on serine 7, a residue present only in the platelet splice variants of Rap1GAP2. Phosphorylation of Rap1GAP2 by cGKI might mediate inhibitory effects of NO/cGMP on Rap1. Rap1GAP2 is the first GTPase-activating protein of Rap1 found in platelets and is likely to have an important regulatory role in platelet aggregation.

Return of the GDI: the GoLoco motif in cell division

The GoLoco motif is a 19-amino-acid sequence with guanine nucleotide dissociation inhibitor activity against G-alpha subunits of the adenylyl-cyclase-inhibitory subclass. The GoLoco motif is present as an independent element within multidomain signaling regulators, such as Loco, RGS12, RGS14, and Rap1GAP, as well as in tandem arrays in proteins, such as AGS3, G18, LGN, Pcp-2/L7, and Partner of Inscuteable (Pins/Rapsynoid). Here we discuss the biochemical mechanisms of GoLoco motif action on G-alpha subunits in light of the recent crystal structure of G-alpha-i1 bound to the RGS14 GoLoco motif. Currently, there is sparse evidence for GoLoco motif regulation of canonical G-protein-coupled receptor signaling. Rather, studies of asymmetric cell division in Drosophila and Caenorhabditis elegans, as well as mammalian mitosis, implicate GoLoco proteins, such as Pins, GPR-1/GPR-2, LGN, and RGS14, in mitotic spindle organization and force generation. We discuss potential mechanisms by which GoLoco/Galpha complexes might modulate spindle dynamics.

Galphaz inhibits serum response factor-dependent transcription by inhibiting Rho signaling

Galpha12/13 or Galphaq signals induce activation of Rho GTPase, leading to serum response factor (SRF)-mediated gene transcription and actin cytoskeletal organization; however, less is known regarding how Rho pathway signals are down-regulated. Here we report that Galphaz signals inhibit serum response factor (SRF)-dependent transcription. Galphaz expression inhibits Galpha12/13-, Galphaq-, and Rho guanine nucleotide exchange factor (GEF)-induced serum response element (SRE) reporter activation in human embryonic kidney 293T and PC-12 cells. Expression of Galphaz mutants with defective fatty acylation has no inhibitory effect. Expression of Galphaz, but not Galphai, attenuates serum-induced SRE reporter activation, suggesting that Galphaz can down-regulate endogenous signals leading to SRF. Whereas Galphaz also blocks SRE reporter induction by the activated mutant RhoAL63, it does not affect Galpha12- or Rho GEF-induced RhoA activation or RhoAL63-GTP binding in vivo. Moreover, Galphaz does not inhibit SRE reporter induction by an activated form of Rho kinase. Because Galphaz inhibits RhoAL63/A188-induced reporter activation, phosphorylation of RhoA on serine 188 does not seem to be involved; furthermore, RhoA subcellular localization was not affected. Use of pharmacologic inhibitors implies that Galphaz-induced reduction of SRE reporter activation occurs via a mechanism other than adenylate cyclase modulation. These findings suggest that Galphaz signals may attenuate Rho-induced stimulation of SRF-mediated transcription.

Reciprocal Signaling between the Transcriptional Co-Factor Eya2 and Specific Members of the Gαi Family

As part of a program to elucidate signaling processes controlled by the heterotrimeric G protein Galphaz, a human fetal brain cDNA library was screened for proteins that specifically interact with the activated form of Galphaz. One of the most-encountered molecules in this screen was Eya2, a member of the Eyes absent family of proteins. Mammalian Eya proteins are predominantly cytosolic proteins that are known to interact with members of the Sine oculis (Six) family of homeodomain transcription factors. This interaction facilitates the translocation of Eya into the nucleus, where the Eya/Six complex regulates transcription during critical stages of embryonic development. In vitro binding studies confirmed that Galphaz interacts with Eya2 in an activation-dependent fashion; furthermore, most other members of the Galphai family including Galphai1, Galphai2, and Galphai3 were found to interact with Eya2. It is interesting that one of the most abundant Galphai proteins, Galphao, did not interact with Eya2. Coexpression of the activated forms of Galphai1, Galphai2, and Galphai3, but not Galphao, with Eya2 recruited Eya2 to the plasma membrane, prevented Eya2 translocation into the nucleus, and abrogated Eya2/Six4-mediated transcription. In addition, Eya2 impinged on G protein-mediated signaling, as evidenced by its ability to relieve Galphai2-mediated inhibition of adenylyl cyclase. These results demonstrate that the interaction between the Galphai proteins and Eya2 may impact on seemingly disparate regulatory events involving both classes of proteins.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether Rap1GAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3beta (GSK3beta) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3beta inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3beta and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMP-stimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor.

Phosphorylation-regulated inhibition of the Gz GTPase-activating protein activity of RGS proteins by synapsin I

Synapsins are neuronal proteins that bind and cluster synaptic vesicles in the presynaptic space, presumably by anchoring to actin filaments, but specific regulatory functions of the synapsins are unknown. We found that a sub-population of brain synapsin Ia, a splice variant of one of three synapsin isoforms, inhibits the GTPase-activating protein (GAP) activity of several RGS proteins. Inhibition is highly selective for Galphaz, a member of the Gi family that is found in neurons, platelets, adrenal chromaffin cells, and a few other neurosecretory cells. Gz has been indirectly implicated in the regulation of secretion. Synapsin Ia constitutes a major fraction of the total GAP-inhibitory activity in brain, and its inhibitory activity is absent from the brains of synapsin I(-/-)/II(-/-) mice. Inhibition depends on the cationic D/E domain of synapsin. Phosphorylation of synapsin Ia at serine 9 by either cyclic AMP-dependent protein kinase or p21-activated protein kinase (PAK1) attenuates its potency as a GAP inhibitor more than 7-fold. Synapsin can thus act as a phosphorylation-modulated mediator of feedback regulation of Gz signaling by the synaptic machinery.

The Yeast G Protein α Subunit Gpa1 Transmits a Signal through an RNA Binding Effector Protein Scp160

In yeast Saccharomyces cerevisiae the G protein betagamma subunits (Ste4/Ste18) have long been known to transmit the signal required for mating. Here we demonstrate that GTPase-deficient mutants of Galpha (Gpa1) directly activate the mating response pathway. We also show that signaling by activated Gpa1 requires direct coupling to an RNA binding protein Scp160. These findings suggest an additional role for Gpa1 and reveal Scp160 as a component of the mating response pathway in yeast.

The small GTPase Rap1 is activated by turbulence and is involved in integrin [alpha]IIb[beta]3-mediated cell adhesion in human megakaryocytes

The small GTPase Rap1, which is activated by a large variety of stimuli, functions in the control of integrin-mediated cell adhesion. Here we show that in human megakaryocytes and several other commonly used hematopoietic cell lines such as K562, Jurkat, and THP-1, stress induced by gentle tumbling of the samples resulted in rapid and strong activation of Rap1. This turbulence-induced activation could not be blocked by inhibitors previously shown to affect Rap1 activation in human platelets, such as the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) and various protein kinase C inhibitors. Also inhibition of actin cytoskeleton dynamics did not influence this activation of Rap1, suggesting that this activation is mediated by cell surface receptors. Human platelets, however, were refractory to turbulence-induced activation of Rap1. To determine the consequences of Rap1 activation we measured adhesion of megakaryocytes to fibrinogen, which is mediated by the integrin alphaIIbbeta3, in the presence of inhibitors of Rap1 signaling. Introduction of both Rap1GAP and RalGDS-RBD in the megakaryoblastic cell line DAMI strongly reduced basal adhesion to immobilized fibrinogen. This inhibition was partially rescued by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate but not by alpha-thrombin. From these results we conclude that in megakaryocytes turbulence induces Rap1 activation that controls alphaIIbbeta3-mediated cell adhesion.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.