G beta gamma mediates the interplay between tubulin dimers and microtubules in the modulation of Gq signaling

Pseudokinase NRP1 facilitates endocytosis of transferrin in the African trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT) and nagana in cattle. During infection of a vertebrate, endocytosis of host transferrin (Tf) is important for viability of the parasite. The majority of proteins involved in trypanosome endocytosis of Tf are unknown. Here we identify pseudokinase NRP1 (Tb427tmp.160.4770) as a regulator of Tf endocytosis. Genetic knockdown of NRP1 inhibited endocytosis of Tf without blocking uptake of bovine serum albumin. Binding of Tf to the flagellar pocket was not affected by knockdown of NRP1. However the quantity of Tf per endosome dropped significantly, consistent with NRP1 promoting robust capture and/or retention of Tf in vesicles. NRP1 is involved in motility of Tf-laden vesicles since distances between endosomes and the kinetoplast were reduced after knockdown of the gene. In search of possible mediators of NRP1 modulation of Tf endocytosis, the gene was knocked down and the phosphoproteome analyzed. Phosphorylation of protein kinases forkhead, NEK6, and MAPK10 was altered, in addition to EpsinR, synaptobrevin and other vesicle-associated proteins predicted to be involved in endocytosis. These candidate proteins may link NRP1 functionally either to protein kinases or to vesicle-associated proteins.

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

Gq Signaling in Autophagy Control: Between Chemical and Mechanical Cues

All processes in human physiology relies on homeostatic mechanisms which require the activation of specific control circuits to adapt the changes imposed by external stimuli. One of the critical modulators of homeostatic balance is autophagy, a catabolic process that is responsible of the destruction of long-lived proteins and organelles through a lysosome degradative pathway. Identification of the mechanism underlying autophagic flux is considered of great importance as both protective and detrimental functions are linked with deregulated autophagy. At the mechanistic and regulatory levels, autophagy is activated in response to diverse stress conditions (food deprivation, hyperthermia and hypoxia), even a novel perspective highlight the potential role of physical forces in autophagy modulation. To understand the crosstalk between all these controlling mechanisms could give us new clues about the specific contribution of autophagy in a wide range of diseases including vascular disorders, inflammation and cancer. Of note, any homeostatic control critically depends in at least two additional and poorly studied interdependent components: a receptor and its downstream effectors. Addressing the selective receptors involved in autophagy regulation is an open question and represents a new area of research in this field. G-protein coupled receptors (GPCRs) represent one of the largest and druggable targets membrane receptor protein superfamily. By exerting their action through G proteins, GPCRs play fundamental roles in the control of cellular homeostasis. Novel studies have shown Gαq, a subunit of heterotrimeric G proteins, as a core modulator of mTORC1 and autophagy, suggesting a fundamental contribution of Gαq-coupled GPCRs mechanisms in the control of this homeostatic feedback loop. To address how GPCR-G proteins machinery integrates the response to different stresses including oxidative conditions and mechanical stimuli, could provide deeper insight into new signaling pathways and open potential and novel therapeutic strategies in the modulation of different pathological conditions.

A Novel Classification of Glioma Subgroup, Which Is Highly Correlated With the Clinical Characteristics and Tumor Tissue Characteristics, Based on the Expression Levels of Gβ and Gγ Genes

Purpose

Glioma is a classical type of primary brain tumors that is most common seen in adults, and its high heterogeneity used to be a reference standard for subgroup classification. Glioma has been diagnosed based on histopathology, grade, and molecular markers including IDH mutation, chromosome 1p/19q loss, and H3K27M mutation. This subgroup classification cannot fully meet the current needs of clinicians and researchers. We, therefore, present a new subgroup classification for glioma based on the expression levels of Gβ and Gγ genes to complement studies on glioma and Gβγ subunits, and to support clinicians to assess a patient’s tumor status.

Methods

Glioma samples retrieved from the CGGA database and the TCGA database. We clustered the gliomas into different groups by using expression values of Gβ and Gγ genes extracted from RNA sequencing data. The Kaplan–Meier method with a two-sided log-rank test was adopted to compare the OS of the patients between GNB2 group and non-GNB2 group. Univariate Cox regression analysis was referred to in order to investigate the prognostic role of each Gβ and Gγ genes. KEGG and ssGSEA analysis were applied to identify highly activated pathways. The “estimate” package, “GSVA” package, and the online analytical tools CIBERSORTx were employed to evaluate immune cell infiltration in glioma samples.

Results

Three subgroups were identified. Each subgroup had its own specific pathway activation pattern and other biological characteristics. High M2 cell infiltration was observed in the GNB2 subgroup. Different subgroups displayed different sensitivities to chemotherapeutics. GNB2 subgroup predicted poor survival in patients with gliomas, especially in patients with LGG with mutation IDH and non-codeleted 1p19q.

Conclusion

The subgroup classification we proposed has great application value. It can be used to select chemotherapeutic drugs and the prognosis of patients with target gliomas. The unique relationships between subgroups and tumor-related pathways are worthy of further investigation to identify therapeutic Gβγ heterodimer targets.

Muscarinic Acetylcholine Type 1 Receptor Activity Constrains Neurite Outgrowth by Inhibiting Microtubule Polymerization and Mitochondrial Trafficking in Adult Sensory Neurons

The muscarinic acetylcholine type 1 receptor (M₁R) is a metabotropic G protein-coupled receptor. Knockout of M₁R or exposure to selective or specific receptor antagonists elevates neurite outgrowth in adult sensory neurons and is therapeutic in diverse models of peripheral neuropathy. We tested the hypothesis that endogenous M₁R activation constrained neurite outgrowth via a negative impact on the cytoskeleton and subsequent mitochondrial trafficking. We overexpressed M₁R in primary cultures of adult rat sensory neurons and cell lines and studied the physiological and molecular consequences related to regulation of cytoskeletal/mitochondrial dynamics and neurite outgrowth. In adult primary neurons, overexpression of M₁R caused disruption of the tubulin, but not actin, cytoskeleton and significantly reduced neurite outgrowth. Over-expression of a M₁R-DREADD mutant comparatively increased neurite outgrowth suggesting that acetylcholine released from cultured neurons interacts with M₁R to suppress neurite outgrowth. M₁R-dependent constraint on neurite outgrowth was removed by selective (pirenzepine) or specific (muscarinic toxin 7) M₁R antagonists. M₁R-dependent disruption of the cytoskeleton also diminished mitochondrial abundance and trafficking in distal neurites, a disorder that was also rescued by pirenzepine or muscarinic toxin 7. M₁R activation modulated cytoskeletal dynamics through activation of the G protein (Gα13) that inhibited tubulin polymerization and thus reduced neurite outgrowth. Our study provides a novel mechanism of M₁R control of Gα13 protein-dependent modulation of the tubulin cytoskeleton, mitochondrial trafficking and neurite outgrowth in axons of adult sensory neurons. This novel pathway could be harnessed to treat dying-back neuropathies since anti-muscarinic drugs are currently utilized for other clinical conditions.

Linoleic acid: Is this the key that unlocks the quantum brain? Insights linking broken symmetries in molecular biology, mood disorders and personalistic emergentism

In this paper we present a mechanistic model that integrates subneuronal structures, namely ion channels, membrane fatty acids, lipid rafts, G proteins and the cytoskeleton in a dynamic system that is finely tuned in a healthy brain. We also argue that subtle changes in the composition of the membrane's fatty acids may lead to down-stream effects causing dysregulation of the membrane, cytoskeleton and their interface. Such exquisite sensitivity to minor changes is known to occur in physical systems undergoing phase transitions, the simplest and most studied of them is the so-called Ising model, which exhibits a phase transition at a finite temperature between an ordered and disordered state in 2- or 3-dimensional space. We propose this model in the context of neuronal dynamics and further hypothesize that it may involve quantum degrees of freedom dependent upon variation in membrane domains associated with ion channels or microtubules. Finally, we provide a link between these physical characteristics of the dynamical mechanism to psychiatric disorders such as major depression and antidepressant action.

Arabidopsis G-Protein β Subunit AGB1 Interacts with BES1 to Regulate Brassinosteroid Signaling and Cell Elongation

In Arabidopsis, brassinosteroids (BR) are major growth-promoting hormones, which integrate with the heterotrimeric guanine nucleotide-binding protein (G-protein) signals and cooperatively modulate cell division and elongation. However, the mechanisms of interaction between BR and G-protein are not well understood. Here, we show that the G-protein β subunit AGB1 directly interacts with the BR transcription factor BES1 in vitro and in vivo. An AGB1-null mutant, agb1-2, displays BR hyposensitivity and brassinazole (BRZ, BR biosynthesis inhibitor) hypersensitivity, which suggests that AGB1 positively mediates the BR signaling pathway. Moreover, we demonstrate that AGB1 synergistically regulates expression of BES1 target genes, including the BR biosynthesis genes CPD and DWF4 and the SAUR family genes required for promoting cell elongation. Further, Western blot analysis of BES1 phosphorylation states indicates that the interaction between AGB1 and BES1 alters the phosphorylation status of BES1 and increases the ratio of dephosphorylated to phosphorylated BES1, which leads to accumulation of dephosphorylated BES1 in the nucleus. Finally, AGB1 promotes BES1 binding to BR target genes and stimulates the transcriptional activity of BES1. Taken together, our results demonstrate that AGB1 positively regulates cell elongation by affecting the phosphorylation status and transcriptional activity of BES1.

Activation of microtubule dynamics increases neuronal growth via the nerve growth factor (NGF)- and Gαs-mediated signaling pathways

Signals that activate the G protein Gαs and promote neuronal differentiation evoke Gαs internalization in rat pheochromocytoma (PC12) cells. These agents also significantly increase Gαs association with microtubules, resulting in an increase in microtubule dynamics because of the activation of tubulin GTPase by Gαs. To determine the function of Gαs/microtubule association in neuronal development, we used real-time trafficking of a GFP-Gαs fusion protein. GFP-Gαs concentrates at the distal end of the neurites in differentiated living PC12 cells as well as in cultured hippocampal neurons. Gαs translocates to specialized membrane compartments at tips of growing neurites. A dominant-negative Gα chimera that interferes with Gαs binding to tubulin and activation of tubulin GTPase attenuates neurite elongation and neurite number both in PC12 cells and primary hippocampal neurons. This effect is greatest on differentiation induced by activated Gαs. Together, these data suggest that activated Gαs translocates from the plasma membrane and, through interaction with tubulin/microtubules in the cytosol, is important for neurite formation, development, and outgrowth. Characterization of neuronal G protein dynamics and their contribution to microtubule dynamics is important for understanding the molecular mechanisms by which G protein-coupled receptor signaling orchestrates neuronal growth and differentiation.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Tubulin, actin and heterotrimeric G proteins: coordination of signaling and structure

G proteins mediate signals from membrane G protein coupled receptors to the cell interior, evoking significant regulation of cell physiology. The cytoskeleton contributes to cell morphology, motility, division, and transport functions. This review will discuss the interplay between heterotrimeric G protein signaling and elements of the cytoskeleton. Also described and discussed will be the interplay between tubulin and G proteins that results in atypical modulation of signaling pathways and cytoskeletal dynamics. This will be extended to describe how tubulin and G proteins act in concert to influence various aspects of cellular behavior. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Glutamine and alanyl-glutamine increase RhoA expression and reduce Clostridium difficile toxin-a-induced intestinal epithelial cell damage

Clostridium difficile is a major cause of antibiotic-associated colitis and is associated with significant morbidity and mortality. Glutamine (Gln) is a major fuel for the intestinal cell population. Alanyl-glutamine (Ala-Gln) is a dipeptide that is highly soluble and well tolerated. IEC-6 cells were used in the in vitro experiments. Cell morphology was evaluated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Cell proliferation was assessed by WST-1 and Ki-67 and apoptosis was assessed by TUNEL. Cytoskeleton was evaluated by immunofluorescence for RhoA and F-actin. RhoA was quantified by immunoblotting. TcdA induced cell shrinkage as observed by AFM, SEM, and fluorescent microscopy. Additionally, collapse of the F-actin cytoskeleton was demonstrated by immunofluorescence. TcdA decreased cell volume and area and increased cell height by 79%, 66.2%, and 58.9%, respectively. Following TcdA treatment, Ala-Gln and Gln supplementation, significantly increased RhoA by 65.5% and 89.7%, respectively at 24 h. Ala-Gln supplementation increased cell proliferation by 137.5% at 24 h and decreased cell apoptosis by 61.4% at 24 h following TcdA treatment. In conclusion, TcdA altered intestinal cell morphology and cytoskeleton organization, decreased cell proliferation, and increased cell apoptosis. Ala-Gln and Gln supplementation reduced intestinal epithelial cell damage and increased RhoA expression.

Postmortem proteomic analysis in human amygdala of drug addicts: possible impact of tubulin on drug-abusing behavior

Besides the ventral tegmental area and the nucleus accumbens as the most investigated brain reward structures, several reports about the relation between volume and activity of the amygdala and drug-seeking behavior have emphasized the central role of the amygdala in the etiology of addiction. Considering its proposed important role and the limited number of human protein expression studies with amygdala in drug addiction, we performed a human postmortem proteomic analysis of amygdala tissue obtained from 8 opiate addicts and 7 control individuals. Results were validated by Western blot in an independent postmortem replication sample from 12 opiate addicts compared to 12 controls and 12 suicide victims, as a second "control sample". Applying 2D-electrophoresis and MALDI-TOF-MS analysis, we detected alterations of beta-tubulin expression and decreased levels of the heat-shock protein HSP60 in drug addicts. Western blot analysis in the additional sample demonstrated significantly increased alpha- and beta-tubulin concentrations in the amygdala of drug abusers versus controls (P = 0.021, 0.029) and to suicide victims (P = 0.006, 0.002). Our results suggest that cytoskeletal alterations in the amygdala determined by tubulin seem to be involved in the pathophysiology of drug addiction, probably via a relation to neurotransmission and cellular signaling. Moreover, the loss of neuroprotection against stressors by chaperons as HSP60 might also contribute to structural alteration in the brain of drug addicts. Although further studies have to confirm our results, this might be a possible pathway that may increase our understanding of drug addiction.

Clostridium difficile toxin A decreases acetylation of tubulin, leading to microtubule depolymerization through activation of histone deacetylase 6, and this mediates acute inflammation

Clostridium difficile toxin A is known to cause actin disaggregation through the enzymatic inactivation of intracellular Rho proteins. Based on the rapid and severe cell rounding of toxin A-exposed cells, we speculated that toxin A may be involved in post-translational modification of tubulin, leading to microtubule instability. In the current study, we observed that toxin A strongly reduced α-tubulin acetylation in human colonocytes and mouse intestine. Fractionation analysis demonstrated that toxin A-induced α-tubulin deacetylation yielded monomeric tubulin, indicating the presence of microtubule depolymerization. Inhibition of the glucosyltransferase activity against Rho proteins of toxin A by UDP-2',3'-dialdehyde significantly abrogated toxin A-induced α-tubulin deacetylation. In colonocytes treated with trichostatin A (TSA), an inhibitor of the HDAC6 tubulin deacetylase, toxin A-induced α-tubulin deacetylation and loss of tight junction were completely blocked. Administration of TSA also attenuated proinflammatory cytokine production, mucosal damage, and epithelial cell apoptosis in mouse intestine exposed to toxin A. These results suggest that toxin A causes microtubule depolymerization by activation of HDAC6-mediated tubulin deacetylation. Indeed, blockage of HDAC6 by TSA markedly attenuates α-tubulin deacetylation, proinflammatory cytokine production, and mucosal damage in a toxin A-induced mouse enteritis model. Tubulin deacetylation is an important component of the intestinal inflammatory cascade following toxin A-mediated Rho inactivation in vitro and in vivo.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Heterotrimeric G-proteins interact directly with cytoskeletal components to modify microtubule-dependent cellular processes

A large percentage of current drugs target G-protein-coupled receptors, which couple to well-known signaling pathways involving cAMP or calcium. G-proteins themselves may subserve a second messenger function. Here, we review the role of tubulin and microtubules in directly mediating effects of heterotrimeric G-proteins on neuronal outgrowth, shape and differentiation. G-protein-tubulin interactions appear to be regulated by neurotransmitter activity, and, in turn, regulate the location of Galpha in membrane microdomains (such as lipid rafts) or the cytosol. Tubulin binds with nanomolar affinity to Gsalpha, Gialpha1 and Gqalpha (but not other Galpha subunits) as well as Gbeta(1)gamma(2) subunits. Galpha subunits destabilize microtubules by stimulating tubulin's GTPase, while Gbetagamma subunits promote microtubule stability. The same region on Gsalpha that binds adenylyl cyclase and Gbetagamma also interacts with tubulin, suggesting that cytoskeletal proteins are novel Galpha effectors. Additionally, intracellular Gialpha-GDP, in concert with other GTPase proteins and Gbetagamma, regulates the position of the mitotic spindle in mitosis. Thus, G-protein activation modulates cell growth and differentiation by directly altering microtubule stability. Further studies are needed to fully establish a structural mechanism of this interaction and its role in synaptic plasticity.

G protein βγ subunits: central mediators of G protein-coupled receptor signaling

G protein betagamma subunits are central participants in G protein-coupled receptor signaling pathways. They interact with receptors, G protein alpha subunits and downstream targets to coordinate multiple, different GPCR functions. Much is known about the biology of Gbetagamma subunits but mysteries remain. Here, we will review what is known about general aspects of structure and function of Gbetagamma as well as discuss emerging mechanisms for regulation of Gbetagamma signaling. Recent data suggest that Gbetagamma is a potential therapeutic drug target. Thus, a thorough understanding of the molecular and physiological functions of Gbetagamma has significant implications.

Structural model of a complex between the heterotrimeric G protein, Gsalpha, and tubulin

A number of studies have demonstrated interplay between the cytoskeleton and G protein signaling. Many of these studies have determined a specific interaction between tubulin, the building block of microtubules, and G proteins. The alpha subunits of some heterotrimeric G proteins, including Gsalpha, have been shown to interact strongly with tubulin. Binding of Galpha to tubulin results in increased dynamicity of microtubules due to activation of GTPase of tubulin. Tubulin also activates Gsalpha via a direct transfer of GTP between these molecules. Structural insight into the interaction between tubulin and Gsalpha was required, and was determined, in this report, through biochemical and molecular docking techniques. Solid phase peptide arrays suggested that a portion of the amino terminus, alpha2-beta4 (the region between switch II and switch III) and alpha3-beta5 (just distal to the switch III region) domains of Gsalpha are important for interaction with tubulin. Molecular docking studies revealed the best-fit models based on the biochemical data, showing an interface between the two molecules that includes the adenylyl cyclase/Gbetagamma interaction regions of Gsalpha and the exchangeable nucleotide-binding site of tubulin. These structural models explain the ability of tubulin to facilitate GTP exchange on Galpha and the ability of Galpha to activate tubulin GTPase.

Phospholipase C signaling involvement in macrotubule assembly and activation of the mechanism regulating protoplast volume in plasmolyzed root cells of Triticum turgidum

The role of phosphoinositide-specific phospholipase C (PI-PLC) signaling in the macrotubule-dependent protoplast volume regulation in plasmolyzed root cells of Triticum turgidum was investigated. At the onset of hyperosmotic stress, PI-PLC activation was documented. Inhibition of PI-PLC activity by U73122 blocked tubulin macrotubule formation in plasmolyzed cells and their protoplast volume regulatory mechanism. In neomycin-treated plasmolyzed cells, macrotubule formation and protoplast volume regulation were not affected. In these cells the PI-PLC pathway is down-regulated as neomycin sequesters the PI-PLC substrate, 4,5-diphosphate-phosphatidyl inositol (PtdInsP(2)). These phenomena were unaffected by R59022, an inhibitor of phosphatidic acic (PA) production via the PLC pathway. Taxol, a microtubule (MT) stabilizer, inhibited the hyperosmotic activation of PI-PLC, but oryzalin, which disorganized MTs, triggered PI-PLC activity. Taxol prevented macrotubule formation and inhibited the mechanism regulating the volume of the plasmolyzed protoplast. Neomycin partly relieved some of the taxol effects. These data suggest that PtdInspP(2) turnover via PI-PLC assists macrotubule formation and activation of the mechanism regulating the plasmolyzed protoplast volume; and the massive disorganization of MTs that is carried out at the onset of hyperosmotic treatment triggers the activation of this mechanism.

RGS proteins: Swiss army knives in seven-transmembrane domain receptor signaling networks

Coordinated regulation of heterotrimeric guanine nucleotide-binding protein (G protein) activity is critical for the integration of information from multiple intracellular signaling networks. The human regulator of G protein signaling (RGS) protein family contains more than 35 members that are well suited for this purpose. Although all RGS proteins contain a core ~120-amino acid Galpha-interacting domain (called the RGS domain), they differ widely in size and organization of other functional domains. Architecturally complex RGS proteins contain multiple modular protein-protein interaction domains that mediate their interaction with diverse signaling effectors. Architecturally simple RGS proteins contain small amino-terminal domains; however, they show surprising versatility in the number of intracellular partners with which they interact. This Perspective focuses on RGS2, a simple RGS protein with the potential to integrate multiple signaling networks. In three recent studies, the amino-terminal domain of RGS2 was shown to interact with and regulate three different effector proteins: adenylyl cyclase, tubulin, and the cation channel TRPV6. To explain this growing list of RGS2-interacting partners, we propose two models: (i) The amino-terminal domain of RGS2 comprises several short effector protein interaction motifs; (ii) the amino-terminal domain of RGS2 adopts distinct structures to bind various targets. Whatever the precise mechanism controlling its target interactions, these studies suggest that RGS2 is a key point of integration for multiple intracellular signaling pathways, and they highlight the role of RGS proteins as dynamic, multifunctional signaling centers that coordinate a diverse range of cellular functions.

G protein βγ subunits interact with αβ- and γ-tubulin and play a role in microtubule assembly in PC12 cells

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

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.

G protein activation is prerequisite for functional coupling between Gα/Gβγ and tubulin/microtubules

Heterotrimeric G proteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Interestingly, recent results suggest that G proteins also interact with microtubules and participate in cell division and differentiation. It has been shown earlier that both alpha and betagamma subunits of G proteins modulate microtubule assembly in vitro. Since G protein activation and subsequent dissociation of alpha and betagamma subunits are necessary for G proteins to participate in signaling processes, here we asked if similar activation is required for modulation of microtubule assembly by G proteins. We reconstituted Galphabetagamma heterotrimer from myristoylated-Galpha and prenylated-Gbetagamma, and found that the heterotrimer blocks Gi1alpha activation of tubulin GTPase and inhibits the ability of Gbeta1gamma2 to promote in vitro microtubule assembly. Results suggest that G protein activation is required for functional coupling between Galpha/Gbetagamma and tubulin/microtubules, and supports the notion that regulation of microtubules is an integral component of G protein mediated signaling.

Responsive microtubule dynamics promote cell invasion by Trypanosoma cruzi

The American trypanosome, Trypanosoma cruzi, can invade non-phagocytic cell types by a G-protein-mediated, calcium-dependent mechanism, in which the cell's natural puncture repair mechanism is usurped in order to recruit lysosomes to the parasite/host cell junction or 'parasite synapse.' The fusion of lysosomes necessary for construction of the nascent parasitophorous vacuole is achieved by directed trafficking along microtubules. We demonstrate altered host cell microtubule dynamics during the initial stages of the entry process involving de novo microtubule polymerization from the cytoplasmic face of the parasite synapse which appears to serve as a secondary microtubule organizing centre. The net result of these dynamic changes to the host cell's microtubule cytoskeleton is the development of the necessary infrastructure for transport of lysosomes to the parasite synapse.

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.

Pleckstrin homology domains of phospholipase C-gamma1 directly interact with beta-tubulin for activation of phospholipase C-gamma1 and reciprocal modulation of beta-tubulin function in microtubule assembly

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) has two pleckstrin homology (PH) domains, an N-terminal domain and a split PH domain. Here we show that pull down of NIH3T3 cell extracts with PLC-gamma1 PH domain-glutathione S-transferase fusion proteins, followed by matrix-assisted laser desorption ionization-time of flight-mass spectrometry, identified beta-tubulin as a binding protein of both PLC-gamma1 PH domains. Tubulin is a main component of microtubules and mitotic spindle fibers, which are composed of alpha- and beta-tubulin heterodimers in all eukaryotic cells. PLC-gamma1 and beta-tubulin colocalized in the perinuclear region in COS-7 cells and cotranslocated to the plasma membrane upon agonist stimulation. Membrane-targeted translocation of depolymerized tubulin by agonist stimulation was also supported by immunoprecipitation analyses. The phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolyzing activity of PLC-gamma1 was substantially increased in the presence of purified tubulin in vitro, whereas the activity was not promoted by bovine serum albumin, suggesting that beta-tubulin activates PLC-gamma1. Furthermore, indirect immunofluorescent microscopy showed that PLC-gamma1 was highly concentrated in mitotic spindle fibers, suggesting that PLC-gamma1 is involved in spindle fiber formation. The effect of PLC-gamma1 in microtubule formation was assessed by overexpression and silencing PLC-gamma1 in COS-7 cells, which resulted in altered microtubule dynamics in vivo. Cells overexpressing PLC-gamma1 showed higher microtubule densities than controls, whereas PLC-gamma1 silencing with small interfering RNAs led to decreased microtubule network densities as compared with control cells. Taken together, our results suggest that PLC-gamma1 and beta-tubulin transmodulate each other, i.e. that PLC-gamma1 modulates microtubule assembly by beta-tubulin, and beta-tubulin promotes PLC-gamma1 activity.

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.

Clathrin-mediated endocytosis of m3 muscarinic receptors. Roles for Gbetagamma and tubulin

Receptors as well as some G protein subunits internalize after agonist stimulation. It is not clear whether Galpha(q) or Gbetagamma undergo such regulated translocation. Recent studies demonstrate that m3 muscarinic receptor activation in SK-N-SH neuroblastoma cells causes recruitment of tubulin to the plasma membrane. This subsequently transactivates Galpha(q) and activates phospholipase Cbeta1. Interaction of tubulin-GDP with Gbetagamma at the offset of phospholipase Cbeta1 signaling appears involved in translocation of tubulin and Gbetagamma to vesicle-like structures in the cytosol (Popova, J. S., and Rasenick, M. M. (2003) J. Biol. Chem. 278, 34299-34308). The relationship of this internalization to the clathrin-mediated endocytosis of the activated m3 muscarinic receptors or Galpha(q) involvement in this process has not been clarified. To test this, SK-N-SH cells were treated with carbachol, and localization of Galpha(q), Gbetagamma, tubulin, clathrin, and m3 receptors were analyzed by both cellular imaging and biochemical techniques. Upon agonist stimulation both tubulin and clathrin translocated to the plasma membrane and co-localized with receptors, Galpha(q) and Gbetagamma. Fifteen minutes later receptors, Gbetagamma and tubulin, but not Galpha(q), internalized with the clathrin-coated vesicles. Coimmunoprecipitation of m3 receptors with Gbetagamma, tubulin, and clathrin from the cytosol of carbachol-treated cells was readily observed. These data suggested that Gbetagamma subunits might organize the formation of a multiprotein complex linking m3 receptors to tubulin since they interacted with both proteins. Such protein assemblies might explain the dynamin-dependent but beta-arrestin-independent endocytosis of m3 muscarinic receptors since tubulin interaction with dynamin might guide or insert the complex into clathrin-coated pits. This novel mechanism of internalization might prove important for other beta-arrestin-independent endocytic pathways. It also suggests cross-regulation between G protein-mediated signaling and the dynamics of the microtubule cytoskeleton.