A specific domain of Gialpha required for the transactivation of Gialpha by tubulin is implicated in the organization of cellular microtubules

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.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

Integration of intracellular signaling: Biological analogues of wires, processors and memories organized by a centrosome 3D reference system

BACKGROUND

Myriads of signaling pathways in a single cell function to achieve the highest spatio-temporal integration. Data are accumulating on the role of electromechanical soliton-like waves in signal transduction processes. Theoretical studies strongly suggest feasibility of both classical and quantum computing involving microtubules.

AIM

A theoretical study of the role of the complex composed of the plasma membrane and the microtubule-based cytoskeleton as a system that transmits, stores and processes information.

METHODS

Theoretical analysis presented here refers to (i) the Penrose-Hameroff theory of consciousness (Orchestrated Objective Reduction; Orch OR), (ii) the description of the centrosome as a reference system for construction of the 3D map of the cell proposed by Regolini, (iii) the Heimburg-Jackson model of the nerve pulse propagation along axons' lipid bilayer as soliton-like electro-mechanical waves.

RESULTS AND CONCLUSION

The ideas presented in this paper provide a qualitative model for the decision-making processes in a living cell undergoing a differentiation process.

OUTLOOK

This paper paves the way for the real-time live-cell observation of information processing by microtubule-based cytoskeleton and cell fate decision making.

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.

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.

A molecular and structural mechanism for G protein-mediated microtubule destabilization

The heterotrimeric, G protein-coupled receptor-associated G protein, Gα(s), binds tubulin with nanomolar affinity and disrupts microtubules in cells and in vitro. Here we determine that the activated form of Gα(s) binds tubulin with a K(D) of 100 nm, stimulates tubulin GTPase, and promotes microtubule dynamic instability. Moreover, the data reveal that the α3-β5 region of Gα(s) is a functionally important motif in the Gα(s)-mediated microtubule destabilization. Indeed, peptides corresponding to that region of Gα(s) mimic Gα(s) protein in activating tubulin GTPase and increase microtubule dynamic instability. We have identified specific mutations in peptides or proteins that interfere with this process. The data allow for a model of the Gα(s)/tubulin interface in which Gα(s) binds to the microtubule plus-end and activates the intrinsic tubulin GTPase. This model illuminates both the role of tubulin as an "effector" (e.g. adenylyl cyclase) for Gα(s) and the role of Gα(s) as a GTPase activator for tubulin. Given the ability of Gα(s) to translocate intracellularly in response to agonist activation, Gα(s) may play a role in hormone- or neurotransmitter-induced regulation of cellular morphology.

In vitro interaction of tubulin with the photoreceptor cGMP phosphodiesterase gamma-subunit

The alpha and beta tubulins compose the microtubule cytoskeleton which is involved in many cellular processes such as vesicular transport. The photoreceptor cells in the retina are neurons specialized for phototransduction. Here we report a novel interaction between tubulin and the photoreceptor cGMP phosphodiesterase (PDE6) gamma subunit (PDE gamma). The specificity and molecular details of the PDE gamma:tubulin interaction were analyzed through the experiments of pull down, microtubule co-sedimentation, and NMR spectroscopy. The tubulin-interacting site was identified to be in the PDE gamma C-terminal I67-G85 region, and the interaction interface appeared to be distinct from those with the other PDE gamma targets in phototransduction. We also observed that PDE gamma interacted with tubulin in a GTP-dependent manner. Our findings offer implications for non-phototransduction role(s) of PDE gamma in the photoreceptor neurons.

Girding for migratory cues: roles of the Akt substrate Girdin in cancer progression and angiogenesis

Cell migration is a fundamental aspect of a multitude of physiological and pathological processes, including embryonic development, inflammation, angiogenesis, and cancer progression. A variety of proteins are essential for cell migration, but context-specific signaling pathways and promigratory proteins must now be identified for our understanding of cancer biology to continue to advance. In this review, we focus on the emerging roles of Girdin (also designated KIAA1212, APE, GIV, and HkRP1), a novel component of the phosphatidylinositol 3-kinase (PI3-K)/Akt signaling pathway that is a core-signaling transduction pathway in cancer progression. Girdin is expressed in some types of cancer cells and immature endothelial cells, and is therefore at the crossroads of multiple intracellular processes, including reorganization of the actin cytoskeleton, endocytosis, and modulation of Akt activity, which ultimately lead to cancer invasion and angiogenesis. It also acts as a nonreceptor guanine nucleotide exchange factor (GEF) for Galphai proteins. A significant observation is that Girdin, although vital for cancer progression and postnatal vascular remodelling, is dispensable for cell migratory events during embryonic development. These findings suggest that Girdin and its interacting proteins are potential pharmaceutical targets for cancer therapies and pathological anigiogenesis, including tumor angiogenesis.

The mating-specific Galpha interacts with a kinesin-14 and regulates pheromone-induced nuclear migration in budding yeast

As a budding yeast cell elongates toward its mating partner, cytoplasmic microtubules connect the nucleus to the cell cortex at the growth tip. The Kar3 kinesin-like motor protein is then thought to stimulate plus-end depolymerization of these microtubules, thus drawing the nucleus closer to the site where cell fusion and karyogamy will occur. Here, we show that pheromone stimulates a microtubule-independent interaction between Kar3 and the mating-specific Galpha protein Gpa1 and that Gpa1 affects both microtubule orientation and cortical contact. The membrane localization of Gpa1 was found to polarize early in the mating response, at about the same time that the microtubules begin to attach to the incipient growth site. In the absence of Gpa1, microtubules lose contact with the cortex upon shrinking and Kar3 is improperly localized, suggesting that Gpa1 is a cortical anchor for Kar3. We infer that Gpa1 serves as a positional determinant for Kar3-bound microtubule plus ends during mating.

Cytosolic G{alpha}s acts as an intracellular messenger to increase microtubule dynamics and promote neurite outgrowth

It is now evident that Galpha(s) traffics into cytosol following G protein-coupled receptor activation, and alpha subunits of some heterotrimeric G-proteins, including Galpha(s) bind to tubulin in vitro. Nevertheless, many features of G-protein-microtubule interaction and possible intracellular effects of G protein alpha subunits remain unclear. In this study, several biochemical approaches demonstrated that activated Galpha(s) directly bound to tubulin and cellular microtubules, and fluorescence microscopy showed that cholera toxin-activated Galpha(s) colocalized with microtubules. The activated, GTP-bound, Galpha(s) mimicked tubulin in serving as a GTPase activator for beta-tubulin. As a result, activated Galpha(s) made microtubules more dynamic, both in vitro and in cells, decreasing the pool of insoluble microtubules without changing total cellular tubulin content. The amount of acetylated tubulin (an indicator of microtubule stability) was reduced in the presence of Galpha(s) activated by mutation. Previous studies showed that cholera toxin and cAMP analogs may stimulate neurite outgrowth in PC12 cells. However, in this study, overexpression of a constitutively activated Galpha(s) or activation of Galpha(s) with cholera toxin in protein kinase A-deficient PC12 cells promoted neurite outgrowth in a cAMP-independent manner. Thus, it is suggested that activated Galpha(s) acts as an intracellular messenger to regulate directly microtubule dynamics and promote neurite outgrowth. These data serve to link G-protein signaling with modulation of the cytoskeleton and cell morphology.

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.

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

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.

Simple alignment-free methods for protein classification: a case study from G-protein-coupled receptors

Computational methods of predicting protein functions rely on detecting similarities among proteins. However, sufficient sequence information is not always available for some protein families. For example, proteins of interest may be new members of a divergent protein family. The performance of protein classification methods could vary in such challenging situations. Using the G-protein-coupled receptor superfamily as an example, we investigated the performance of several protein classifiers. Alignment-free classifiers based on support vector machines using simple amino acid compositions were effective in remote-similarity detection even from short fragmented sequences. Although it is computationally expensive, a support vector machine classifier using local pairwise alignment scores showed very good balanced performance. More commonly used profile hidden Markov models were generally highly specific and well suited to classifying well-established protein family members. It is suggested that different types of protein classifiers should be applied to gain the optimal mining power.

Identification and characterisation of novel tubulin-binding motifs located within the C-terminus of TRPV1

Previously, we reported that TRPV1, the vanilloid receptor, interacts with soluble alphabeta-tubulin dimers as well as microtubules via its C-terminal cytoplasmic domain. The interacting region of TRPV1, however, has not been defined. We found that the TRPV1 C-terminus preferably interacts with beta-tubulin and less with alpha-tubulin. Using a systematic deletion approach and biotinylated-peptides we identified two tubulin-binding sites present in TRPV1. These two sequence stretches are highly conserved in all known mammalian TRPV1 orthologues and partially conserved in some of the TRPV1 homologues. As these sequence stretches are not similar to any known tubulin-binding sequences, we conclude that TRPV1 interacts with tubulin and microtubule through two novel tubulin-binding motifs.

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.

Selective interactions between Giα1 and Giα3 and the GoLoco/GPR domain of RGS14 influence its dynamic subcellular localization

RGS14 is a multifunctional protein that contains an RGS domain, which binds active Gi/o alpha-GTP, a GoLoco/GPR domain, which binds inactive Gi alpha-GDP, and a tandem Rap1/2 binding domain (RBD). Studies were initiated to determine the roles of these domains and their interactions with Gi alpha on RGS14 subcellular localization. We report that RGS14 dynamic subcellular localization in HeLa cells depends on distinct domains and selective interactions with preferred Gi alpha isoforms. RGS14 shuttles rapidly between the nucleus and cytoplasm, and associates with centrosomes during interphase and mitosis. RGS14 localization to the nucleus depends on the RGS and RBD domains, its translocation out of the nucleus depends on the GoLoco/GPR domain, and its localization to centrosomes depends on the RBD domain. Gi alpha subunits (Gi alpha1, 2 and 3) localize predominantly at the plasma membrane. RGS14 binds directly to inactive and active forms of Gi alpha1 and Gi alpha3, but not Gi alpha2, both as a purified protein and when recovered from cells. RGS14 localizes predominantly at the plasma membrane in cells with inactive Gi alpha1 and Gi alpha3, but not Gi alpha2, whereas less RGS14 associates with active Gi alpha1/3 at the plasma membrane. RGS14 binding to inactive, but not active Gi alpha1/3 also prevents association with centrosomes or nuclear localization. Removal or functional inactivation of the GoLoco/GPR domain causes RGS14 to accumulate at centrosomes and in the nucleus, but renders it insensitive to recruitment to the plasma membrane by Gi alpha1/3. These findings highlight the importance of the GoLoco/GPR domain and its interactions with Gi alpha1/3 in determining RGS14 subcellular localization and linked functions.

Identification and Characterization of GIV, a Novel Gαi/s -interacting Protein Found on COPI, Endoplasmic Reticulum-Golgi Transport Vesicles

In this report, we characterize GIV (Galpha-interacting vesicle-associated protein), a novel protein that binds members of the Galpha(i) and Galpha subfamilies of heterotrimeric G proteins. The Galpha(s) interaction site was mapped to an 83-amino acid region of GIV that is enriched in highly charged amino acids. BLAST searches revealed two additional mammalian family members, Daple and an uncharacterized protein, FLJ00354. These family members share the highest homology at the Galpha binding domain, are homologous at the N terminus and central coiled coil domain but diverge at the C terminus. Using affinity-purified IgG made against two different regions of the protein, we localized GIV to COPI, endoplasmic reticulum (ER)-Golgi transport vesicles concentrated in the Golgi region in GH3 pituitary cells and COS7 cells. Identification as COPI vesicles was based on colocalization with beta-COP, a marker for these vesicles. GIV also codistributes in the Golgi region with endogenous calnuc and the KDEL receptor, which are cis Golgi markers and with Galpha(i3)-yellow fluorescent protein expressed in COS7 cells. By immunoelectron microscopy, GIV colocalizes with beta-COP and Galpha(i3) on vesicles found in close proximity to ER exit sites and to cis Golgi cisternae. In cell fractions prepared from rat liver, GIV is concentrated in a carrier vesicle fraction (CV2) enriched in ER-Golgi transport vesicles. beta-COP and several Galpha subunits (Galpha(i1-3), Galpha(s)) are also most enriched in CV2. Our results demonstrate the existence of a novel Galpha-interacting protein associated with COPI transport vesicles that may play a role in Galpha-mediated effects on vesicle trafficking within the Golgi and/or between the ER and the Golgi.

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.

Knock-down of RGS4 and beta tubulin in CHO cells expressing the human MT1 melatonin receptor prevents melatonin-induced receptor desensitization

Previously, it has been shown that chronic melatonin exposure in MT1-CHO cells results in receptor desensitization while at the same time producing drastic morphological changes. The addition of a depolymerizing agent during the melatonin pretreatment period prevents MT1 receptor desensitization and the changes in cellular morphology. The lack of morphological change in the presence of a depolymerizing agent is easily explained by the inability of the microtubules to polymerize, however, the prevention of receptor desensitization is a little more complex and may involve G-protein activation. The goal of this study was to determine whether melatonin-induced MT1 receptor desensitization is regulated by proteins known to regulate G-protein activation states, beta-tubulin and RGS4,using anti sense knockdown approaches. The expression of RGS4 mRNA in CHO cells was confirmed using RT PCR and successful knockdown of each was confirmed by western blot analysis or quantitative PCR. Pretreatment of MT1-CHO cells, transfected with the nonsense probes and exposed to melatonin, resulted in a desensitization of the receptor, an increase in forskolin-induced cAMP accumulation, an increase in 2-[125I]-iodomelatonin binding and no change in the affinity of melatonin for the MT1 receptor. However, knockdown of either beta-tubulin or RGS4 in MT1-CHO cells followed by pretreatment with melatonin attenuated the desensitization of melatonin receptors, decreased total 2-[125I]-iodomelatonin binding, and did not affect neither the forskolin response nor the affinity of melatonin for the MT1 receptor. Perhaps RGS4 and beta-tubulin modulate Galpha-GDP and Galpha-GTP states thus modulating MT1 melatonin receptor function.

Tubulin as a regulator of G-protein signaling

Tubulin is known to form high-affinity complexes with certain G proteins. The formation of such complexes allows tubulin to activate Galpha and fosters a system whereby elements of the cytoskeleton can influence G-protein signaling. This article describes the interaction between tubulin and G proteins and discusses methods for examining this interaction.

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

Agonist stimulation causes tubulin association with the plasma membrane and activation of PLC beta 1 through direct interaction with, and transactivation of, G alpha q. Here we demonstrate that G beta gamma interaction with tubulin down-regulates this signaling pathway. Purified G beta gamma, alone or with phosphatidylinositol 4,5-bisphosphate (PIP2), inhibited carbachol-evoked membrane recruitment of tubulin and G alpha q transactivation by tubulin. Polymerization of microtubules elicited by G beta gamma overrode tubulin translocation to the membrane in response to carbachol stimulation. G beta gamma sequestration of tubulin reduced the inhibition of PLC beta 1 observed at high tubulin concentration. G beta 1 gamma 2 interacted preferentially with tubulin-GDP, whereas G alpha q was transactivated by tubulin-GTP. Prenylation of the gamma 2 polypeptide was required for G beta gamma/tubulin interaction. Both confocal microscopy and coimmunoprecipitation studies revealed the spatiotemporal pattern of G beta gamma/tubulin interaction during carbachol stimulation of neuroblastoma SK-N-SH cells. In resting cells G beta gamma localized predominantly at the cell membrane, whereas tubulin was found in well defined microtubules in the cytosol. Within 2 min of agonist exposure, a subset of tubulin translocated to the plasma membrane and colocalized with G beta. Fifteen min post-carbachol addition, tubulin and G beta colocalized in vesicle-like structures in the cytosol. G beta/tubulin colocalization increased after pretreatment of cells with the microtubule-depolymerizing agent, colchicine, and was inhibited by taxol. Taxol also inhibited carbachol-induced PIP2 hydrolysis. It is suggested that G beta gamma/tubulin interaction mediates internalization of membrane-associated tubulin at the offset of PLC beta 1 signaling. Newly cytosolic G beta gamma/tubulin complexes might promote microtubule polymerization attenuating further tubulin association with the plasma membrane. Thus G protein-coupled receptors might evoke G alpha and G beta gamma to orchestrate regulation of phospholipase signaling by tubulin dimers and control of cell shape by microtubules.