Sequestration of the G protein beta gamma subunit complex inhibits receptor-mediated endocytosis

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.

Investigating the Role of G Protein βγ in Kv7-Dependent Relaxations of the Rat Vasculature

Objective- In renal arteries, inhibitors of G protein βγ subunits (Gβγ) reduce Kv7 activity and inhibit Kv7-dependent receptor-mediated vasorelaxations. However, the mechanisms underlying receptor-mediated relaxation are artery specific. Consequently, the aim of this study was to ascertain the role of Gβγ in Kv7-dependent vasorelaxations of the rat vasculature. Approach and Results- Isometric tension recording was performed in isolated rat renal, mesenteric, and cerebral arteries to study isoproterenol and calcitonin gene-related peptide relaxations. Kv7.4 was knocked down via morpholino transfection while inhibition of Gβγ was investigated with gallein and M119K. Proximity ligation assay was performed on isolated myocytes to study the association between Kv7.4 and G protein β subunits or signaling intermediaries. Isoproterenol or calcitonin gene-related peptide-induced relaxations were attenuated by Kv7.4 knockdown in all arteries studied. Inhibition of Gβγ with gallein or M119K had no effect on isoproterenol-mediated relaxations in mesenteric artery but had a marked effect on calcitonin gene-related peptide-induced responses in mesenteric artery and cerebral artery and isoproterenol responses in renal artery. Isoproterenol increased association with Kv7.4 and Rap1a in mesenteric artery which were not sensitive to gallein, whereas in renal artery, isoproterenol increased Kv7.4-AKAP (A-kinase anchoring protein) associations in a gallein-sensitive manner. Conclusions- The Gβγ-Kv7 relationship differs between vessels and is an essential requirement for AKAP, but not Rap-mediated regulation of the channel.

Biochemical, Biophysical and Cellular Techniques to Study the Guanine Nucleotide Exchange Factor, GIV/Girdin

Canonical signal transduction via heterotrimeric G proteins is spatiotemporally restricted, i.e., triggered exclusively at the plasma membrane, only by agonist activation of G protein-coupled receptors via a finite process that is terminated within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a noncanonical pathway for activation of heterotrimeric G proteins via the nonreceptor guanidine-nucleotide exchange factor, GIV/Girdin. Biochemical, biophysical, and functional studies evaluating this pathway have unraveled its unique properties and distinctive spatiotemporal features. As in the case of any new pathway/paradigm, these studies first required an in-depth optimization of tools/techniques and protocols, governed by rationale and fundamentals unique to the pathway, and more specifically to the large multimodular GIV protein. Here we provide the most up-to-date overview of protocols that have generated most of what we know today about noncanonical G protein activation by GIV and its relevance in health and disease. © 2016 by John Wiley & Sons, Inc.

Ongoing activation of sphingosine 1-phosphate receptors mediates maturation of exosomal multivesicular endosomes

During late endosome maturation, cargo molecules are sorted into intralumenal vesicles (ILVs) of multivesicular endosomes (MVEs), and are either delivered to lysosomes for degradation or fused with the plasma membranes for exosome release. The mechanism underlying formation of exosomal ILVs and cargo sorting into ILVs destined for exosome release is still unclear. Here we show that inhibitory G protein (Gi)-coupled sphingosine 1-phosphate (S1P) receptors regulate exosomal MVE maturation. Gi-coupled S1P receptors on MVEs are constitutively activated through a constant supply of S1P via autocrine activation within organelles. We also found that the continuous activation of Gi-coupled S1P receptors on MVEs is essential for cargo sorting into ILVs destined for exosome release. Our results reveal a mechanism underlying ESCRT-independent maturation of exosomal MVEs.

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.

G protein-coupled receptor-promoted trafficking of Gbeta1gamma2 leads to AKT activation at endosomes via a mechanism mediated by Gbeta1gamma2-Rab11a interaction

G-protein coupled receptors activate heterotrimeric G proteins at the plasma membrane in which most of their effectors are intrinsically located or transiently associated as the external signal is being transduced. This paradigm has been extended to the intracellular compartments by studies in yeast showing that trafficking of Galpha activates phosphatidylinositol 3-kinase (PI3K) at endosomal compartments, suggesting that vesicle trafficking regulates potential actions of Galpha and possibly Gbetagamma at the level of endosomes. Here, we show that Gbetagamma interacts with Rab11a and that the two proteins colocalize at early and recycling endosomes in response to activation of lysophosphatidic acid (LPA) receptors. This agonist-dependent association of Gbetagamma to Rab11a-positive endosomes contributes to the recruitment of PI3K and phosphorylation of AKT at this intracellular compartment. These events are sensitive to the expression of a dominant-negative Rab11a mutant or treatment with wortmannin, suggesting that Rab11a-dependent Gbetagamma trafficking promotes the activation of the PI3K/AKT signaling pathway associated with endosomal compartments. In addition, RNA interference-mediated Rab11a depletion, or expression of a dominant-negative Rab11a mutant attenuated LPA-dependent cell survival and proliferation, suggesting that endosomal activation of the PI3K/AKT signaling pathway in response to Gbetagamma trafficking, via its interaction with Rab11, is a relevant step in the mechanism controlling these fundamental events.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

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.

Transport across the endothelium: regulation of endothelial permeability

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways controlling endothelial barrier function have been identified. The transcellular pathway transports plasma proteins of the size of albumin or greater via the process of transcytosis in vesicle carriers originating from cell surface caveolae. Specific signalling cues are able to induce the internalisation of caveolae and their movement to the basal side of the endothelium. Caveolin-1, the primary structural protein required for the formation of caveolae, is also important in regulating vesicle trafficking through the cell by controlling the activity and localisation of signalling molecules that mediate vesicle fission, endocytosis, fusion and finally exocytosis. An important function of the transcytotic pathways is to regulate the delivery of albumin and immunoglobulins, thereby controlling tissue oncotic pressure and host-defence. The paracellular pathway induced during inflammation is formed by gaps between endothelial cells at the level of adherens and tight junctional complexes. Paracellular permeability is increased by second messenger signalling pathways involving Ca2+ influx via activation of store-operated channels, protein kinase Calpha (PKCalpha), and Rho kinase that together participate in the stimulation of myosin light chain phosphorylation, actin-myosin contraction, and disruption of the junctions. In this review of the field, we discuss the current understanding of the signalling pathways regulating paracellular and transcellular endothelial permeability.

Proteomic analysis of native metabotropic glutamate receptor 5 protein complexes reveals novel molecular constituents

We used a proteomic approach to identify novel proteins that may regulate metabotropic glutamate receptor 5 (mGluR5) responses by direct or indirect protein interactions. This approach does not rely on the heterologous expression of proteins and offers the advantage of identifying protein interactions in a native environment. The mGluR5 protein was immunoprecipitated from rat brain lysates; co-immunoprecipitating proteins were analyzed by mass spectrometry and identified peptides were matched to protein databases to determine the correlating parent proteins. This proteomic approach revealed the interaction of mGluR5 with known regulatory proteins, as well as novel proteins that reflect previously unidentified molecular constituents of the mGluR5-signaling complex. Immunoblot analysis confirmed the interaction of high confidence proteins, such as phosphofurin acidic cluster sorting protein 1, microtubule-associated protein 2a and dynamin 1, as mGluR5-interacting proteins. These studies show that a proteomic approach can be used to identify candidate interacting proteins. This approach may be particularly useful for neurobiology applications where distinct protein interactions within a signaling complex can dramatically alter the outcome of the response to neurotransmitter release, or the disruption of normal protein interactions can lead to severe neurological and psychiatric disorders.

Gβγ Activation of Src Induces Caveolae-mediated Endocytosis in Endothelial Cells

Caveolae-mediated endocytosis in endothelial cells is stimulated by the binding of albumin to gp60, a specific albumin-binding protein localized in caveolae. The activation of gp60 induces its cell surface clustering and association with caveolin-1, the caveolar-scaffolding protein. This interaction leads to G(i)-induced Src kinase activation, which in turn signals dynamin-2-mediated fission and directed migration of caveolae-derived vesicles from apical to basal membrane. In this study, we investigated the possible role of the Gbetagamma heterodimer in signaling G(i)-induced Src activation and subsequent caveolae-mediated endocytosis. We observed using rat lung microvascular endothelial cells that expression of the C terminus of beta-adrenergic receptor kinase (ct-betaARK), an inhibitor Gbetagamma signaling, prevented gp60-dependent Src activation as well as caveolae-mediated endocytosis and transcellular transport of albumin and uptake of cholera toxin subunit B, a specific marker of caveolae internalization. Expression of ct-betaARK also prevented Src-mediated tyrosine phosphorylation of caveolin-1 and dynamin-2 and the resultant phosphorylation-dependent association of dynamin-2 and caveolin-1. Also, the direct activation of Gbetagamma using a specific cell-permeant activating peptide (myristoylated-SIRKALNILGYPDYD) simulated the effects of gp60 in inducing Src activation, caveolin-1, and dynamin-2 phosphorylation as well as caveolae-mediated endocytosis of cholera toxin subunit B. The myristoylated-SIRKALNILGYPDYD peptide-induced responses were inhibited by the expression of ct-betaARK. Taken together, our results demonstrate that Gbetagamma activation of Src signals caveolae-mediated endocytosis and transendothelial albumin transport via transcytosis.

Regulation of epidermal growth factor receptor degradation by heterotrimeric Galphas protein

Heterotrimeric G proteins have been implicated in the regulation of membrane trafficking, but the mechanisms involved are not well understood. Here, we report that overexpression of the stimulatory G protein subunit (Galphas) promotes ligand-dependent degradation of epidermal growth factor (EGF) receptors and Texas Red EGF, and knock-down of Galphas expression by RNA interference (RNAi) delays receptor degradation. We also show that Galphas and its GTPase activating protein (GAP), RGS-PX1, interact with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), a critical component of the endosomal sorting machinery. Galphas coimmunoprecipitates with Hrs and binds Hrs in pull-down assays. By immunofluorescence, exogenously expressed Galphas colocalizes with myc-Hrs and GFP-RGS-PX1 on early endosomes, and expression of either Hrs or RGS-PX1 increases the localization of Galphas on endosomes. Furthermore, knock-down of both Hrs and Galphas by double RNAi causes greater inhibition of EGF receptor degradation than knock-down of either protein alone, suggesting that Galphas and Hrs have cooperative effects on regulating EGF receptor degradation. These observations define a novel regulatory role for Galphas in EGF receptor degradation and provide mechanistic insights into the function of Galphas in endocytic sorting.

Role of Src-induced Dynamin-2 Phosphorylation in Caveolae-mediated Endocytosis in Endothelial Cells

Albumin transcytosis, a determinant of transendothelial permeability, is mediated by the release of caveolae from the plasma membrane. We addressed the role of Src phosphorylation of the GTPase dynamin-2 in the mechanism of caveolae release and albumin transport. Studies were made in microvascular endothelial cells in which the uptake of cholera toxin subunit B, a marker of caveolae, and (125)I-albumin was used to assess caveolae-mediated endocytosis. Albumin binding to the 60-kDa cell surface albumin-binding protein, gp60, induced Src activation (phosphorylation on Tyr(416)) within 1 min and resulted in Src-dependent tyrosine phosphorylation of dynamin-2, which increased its association with caveolin-1, the caveolae scaffold protein. Expression of kinase-defective Src mutant interfered with the association between dynamin-2, which caveolin-1 and prevented the uptake of albumin. Expression of non-Src-phosphorylatable dynamin (Y231F/Y597F) resulted in reduced association with caveolin-1, and in contrast to WT-dynamin-2, the mutant failed to translocate to the caveolin-rich membrane fraction. The Y231F/Y597F dynamin-2 mutant expression also resulted in impaired albumin and cholera toxin subunit B uptake and reduced transendothelial albumin transport. Thus, Src-mediated phosphorylation of dynamin-2 is an essential requirement for scission of caveolae and the resultant transendothelial transport of albumin.

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.

Phosphoinositide 3-kinase regulates excitation-contraction coupling in neonatal cardiomyocytes

The phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 decreased steady-state contraction in neonatal rat ventricular myocytes (NRVM). To determine whether the effect on steady-state contraction could be due to decreased intracellular Ca2+content, Ca2+content was assessed with fluorescent plate reader analysis by using the caffeine-releasable Ca2+stores as an index of sarcoplasmic reticulum (SR) Ca2+content. Caffeine-releasable Ca2+content was diminished in a dose-dependent manner with LY-294002, suggesting that the decrease in steady-state contraction was due to diminished intracellular Ca2+content. Activation of the L-type Ca2+channel by BAY K 8644 was attenuated by LY-294002, suggesting the effect of LY-294002 is to reduce Ca2+influx at this channel. To investigate whether additional proteins involved in excitation-contraction (EC) coupling are likewise regulated by PI3K activity, the effects of compounds acting at sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), the ryanodine receptor, and the Na/Ca exchanger (NCX) were compared with LY-294002. Inhibition of SERCA2a by thapsigargin increased basal Ca2+levels in contrast to LY-294002, indicating that SERCA2a activity is sustained in the presence of LY-294002. Ryanodine decreased SR Ca2+content. The additive effect with coadministration of LY-294002 could be attributed to a decrease in Ca2+influx at the L-type Ca2+channel. The NCX inhibitor Ni2+was used to investigate whether the decrease in intracellular Ca2+content with LY-294002 could be due to inhibition of the NCX reverse-mode activity. The minimal effect of LY-294002 with Ni2+suggests that the primary effect of LY-294002 on EC coupling occurs through inhibition of PI3K-mediated L-type Ca2+channel activity.

Heterotrimeric G protein subunits are located on rat liver endosomes

Background

Rat liver endosomes contain activated insulin receptors and downstream signal transduction molecules. We undertook these studies to determine whether endosomes also contain heterotrimeric G proteins that may be involved in signal transduction from G protein-coupled receptors.

Results

By Western blotting G_sα, G_iα1,2, G_iα3 and G_β were enriched in both canalicular (CM) and basolateral (BLM) membranes but also readily detectable on three types of purified rat liver endosomes in the order recycling receptor compartment (RRC) > compartment for uncoupling of receptor and ligand (CURL) > multivesicular bodies (MVB) >> purified secondary lysosomes. Western blotting with antibodies to Na, K-ATPase and to other proteins associated with plasma membranes and intracellular organelles indicated this was not due to contamination of endosome preparations by CM or BLM. Adenylate cyclase (AC) was also identified on purified CM, BLM, RRC, CURL and MVB. Percoll gradient fractionation of liver postnuclear supernatants demonstrated co-occurrence of endosomes and heterotrimeric G protein subunits in fractions with little plasma membrane markers. By confocal microscopy, punctate staining for G_sα, G_iα3 and G_β corresponded to punctate areas of endocytosed Texas red-dextran in hepatocytes from control and cholera toxin-treated livers.

Conclusion

We conclude that heterotrimeric G protein subunits as well as AC likely traffic into hepatocytes on endosome membranes, possibly generating downstream signals spatially separate from signalling generated at the plasma membrane, analogous to the role(s) of internalized insulin receptors.

The betagamma subunit of heterotrimeric G proteins interacts with RACK1 and two other WD repeat proteins

A yeast two-hybrid approach was used to discern possible new effectors for the betagamma subunit of heterotrimeric G proteins. Three of the clones isolated are structurally similar to Gbeta, each exhibiting the WD40 repeat motif. Two of these proteins, the receptor for activated C kinase 1 (RACK1) and the dynein intermediate chain, co-immunoprecipitate with Gbetagamma using an anti-Gbeta antibody. The third protein, AAH20044, has no known function; however, sequence analysis indicates that it is a WD40 repeat protein. Further investigation with RACK1 shows that it not only interacts with Gbeta(1)gamma(1) but also unexpectedly with the transducin heterotrimer Galpha(t)beta(1)gamma(1). Galpha(t) alone does not interact, but it must contribute to the interaction because the apparent EC(50) value of RACK1 for Galpha(t)beta(1)gamma(1) is 3-fold greater than that for Gbeta(1)gamma(1) (0.1 versus 0.3 microm). RACK1 is a scaffold that interacts with several proteins, among which are activated betaIIPKC and dynamin-1 (1). betaIIPKC and dynamin-1 compete with Gbeta(1)gamma(1) and Galpha(t)beta(1)gamma(1) for interaction with RACK1. These findings have several implications: 1) that WD40 repeat proteins may interact with each other; 2) that Gbetagamma interacts differently with RACK1 than with its other known effectors; and/or 3) that the G protein-RACK1 complex may constitute a signaling scaffold important for intracellular responses.

Subunit composition and functional properties of G-protein heterotrimers on rat chromaffin granules

Heterotrimeric G-proteins at the plasma membrane serve as switches between heptahelical receptors and intracellular signal cascades. Likewise endomembrane associated G-proteins may transduce signals from intracellular compartments provided they consist of a functional trimer. Using quantitative immunoelectron microscopy we found heterotrimeric G-protein subunits Galpha2, Galpha(q/11), Gbeta2 and Gbeta5 to reside on secretory granules in chromaffin cells of rat adrenal glands. Thus rat chromaffin granules are equipped with functional G-proteins that consist of a specific alpha-, beta- and probably gamma-subunit combination. Serotonin uptake into a crude rat chromaffin granule preparation was inhibited by activated Galphao2 (10 nM) to nearly the same extent as by GMppNp (50 microM) whereas GDPbetaS was ineffective. The data support the idea that vesicular G-proteins directly regulate the transmitter content of secretory vesicles. In this respect Galphao2 appears to be the main regulator of vesicular momoamine transporter activity.

GAIP, GIPC and Galphai3 are concentrated in endocytic compartments of proximal tubule cells: putative role in regulating megalin's function

Megalin is the most abundant endocytic receptor in the proximal tubule epithelium (PTE), where it is concentrated in clathrin-coated pits (CCPs) and vesicles in the brush border region. The heterotrimeric G protein alpha subunit, Galphai3, has also been localized to the brush border region of PTE. By immunofluorescence GIPC and GAIP, components of G protein-mediated signaling pathways, are also concentrated in the brush border region of PTE and are present in megalin-expressing cell lines. By cell fractionation, these signaling molecules cosediment with megalin in brush border and microvillar fractions. GAIP is found by immunoelectron microscopy in CCPs, and GIPC is found in CCPs and apical tubules of endocytic compartments in the renal brush border. In precipitation assays, GST-GIPC specifically binds megalin. The concentration of Galphai3, GIPC, and GAIP with megalin in endocytic compartments of the proximal tubule, where extensive endocytosis occurs, and the interaction between GIPC and the cytoplasmic tail of megalin suggest a model whereby G protein-mediated signaling may regulate megalin's endocytic function and/or trafficking.

Chemokine signaling and functional responses: the role of receptor dimerization and TK pathway activation

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

Regulator of G-protein signaling 3 (RGS3) inhibits Gbeta1gamma 2-induced inositol phosphate production, mitogen-activated protein kinase activation, and Akt activation

Regulator of G-protein signaling 3 (RGS3) enhances the intrinsic rate at which Galpha(i) and Galpha(q) hydrolyze GTP to GDP, thereby limiting the duration in which GTP-Galpha(i) and GTP-Galpha(q) can activate effectors. Since GDP-Galpha subunits rapidly combine with free Gbetagamma subunits to reform inactive heterotrimeric G-proteins, RGS3 and other RGS proteins may also reduce the amount of Gbetagamma subunits available for effector interactions. Although RGS6, RGS7, and RGS11 bind Gbeta(5) in the absence of a Ggamma subunit, RGS proteins are not known to directly influence Gbetagamma signaling. Here we show that RGS3 binds Gbeta(1)gamma(2) subunits and limits their ability to trigger the production of inositol phosphates and the activation of Akt and mitogen-activated protein kinase. Co-expression of RGS3 with Gbeta(1)gamma(2) inhibits Gbeta(1)gamma(2)-induced inositol phosphate production and Akt activation in COS-7 cells and mitogen-activated protein kinase activation in HEK 293 cells. The inhibition of Gbeta(1)gamma(2) signaling does not require an intact RGS domain but depends upon two regions in RGS3 located between acids 313 and 390 and between 391 and 458. Several other RGS proteins do not affect Gbeta(1)gamma(2) signaling in these assays. Consistent with the in vivo results, RGS3 inhibits Gbetagamma-mediated activation of phospholipase Cbeta in vitro. Thus, RGS3 may limit Gbetagamma signaling not only by virtue of its GTPase-activating protein activity for Galpha subunits, but also by directly interfering with the activation of effectors.

Clathrin

Clathrin was discovered nearly 25 years ago. Since then, a large number of other proteins that participate in the process by which clathrin-coated vesicles retrieve synaptic membranes or take up endocytic receptors have been identified. The functional relationships among these disparate components remain, in many cases, obscure. High-resolution structures of parts of clathrin, determined by X-ray crystallography, and lower-resolution images of assembled coats, determined by electron cryomicroscopy, now provide the information necessary to integrate various lines of evidence and to design experiments that test specific mechanistic notions. This review summarizes and illustrates the recent structural results and outlines what is known about coated-vesicle assembly in the context of this information.

Dynamin inhibits phosphatidylinositol 3-kinase in hematopoietic cells

Phosphatidylinositol 3-kinase (PI 3-kinase) plays a role in late stages of endocytosis as well as in cellular proliferation and transformation. The SH3 domain of its regulatory p85 subunit stimulates the GTPase activity of dynamin in vitro. Dynamin is a GTPase enzyme required for endocytosis of activated growth factor receptors. An interaction between these proteins has not been demonstrated in vivo. Here, we report that dynamin associates with PI 3-kinase in hematopoietic cells. We detected both p85 and PI 3-kinase activity in dynamin immune complexes from IL-3-dependent BaF3 cells. However, this association was significantly reduced in BaF3 cells transformed with the BCR/abl oncogene. After transformation only a 4-fold increase in PI 3-kinase activity was detected in dynamin immune complexes, whereas grb2 associated activity was elevated 20-fold. Furthermore, dynamin inhibited the activity of both purified recombinant and immunoprecipitated PI 3-kinase. In BaF3 cells expressing a temperature-sensitive mutant of BCR/abl, a significant decrease in p85 and dynamin association was observed 4 h after the induction of BCR/abl activity. In contrast, in IL-3-stimulated parental BaF3 cells, this association was increased. Our results demonstrate an in vivo association of PI 3-kinase with dynamin and this interaction regulates the activity of PI 3-kinase.

G protein beta gamma subunits inhibit nongenomic progesterone-induced signaling and maturation in Xenopus laevis oocytes. Evidence for a release of inhibition mechanism for cell cycle progression

Progesterone-induced maturation of Xenopus oocytes is a well known example of nongenomic signaling by steroids; however, little is known about the early signaling events involved in this process. Previous work has suggested that G proteins and G protein-coupled receptors may be involved in progesterone-mediated oocyte maturation as well as in other nongenomic steroid-induced signaling events. To investigate the role of G proteins in nongenomic signaling by progesterone, the effects of modulating Galpha and Gbetagamma levels in Xenopus oocytes on progesterone-induced signaling and maturation were examined. Our results demonstrate that Gbetagamma subunits, rather than Galpha, are the principal mediators of progesterone action in this system. We show that overexpression of Gbetagamma inhibits both progesterone-induced maturation and activation of the MAPK pathway, whereas sequestration of endogenous Gbetagamma subunits enhances progesterone-mediated signaling and maturation. These data are consistent with a model whereby endogenous free Xenopus Gbetagamma subunits constitutively inhibit oocyte maturation. Progesterone may induce maturation by antagonizing this inhibition and therefore allowing cell cycle progression to occur. These studies offer new insight into the early signaling events mediated by progesterone and may be useful in characterizing and identifying the membrane progesterone receptor in oocytes.

Endocytosis of FcgammaRI is regulated by two distinct signalling pathways

Aggregation by immune complexes of receptors specific for the Fc region of IgG results in their internalisation and disposal by trafficking to lysosomes. We show here that internalisation of FcgammaRI by IFN-gamma treated U937 cells following receptor aggregation by cross-linking antibodies requires the activation of two distinct signalling pathways. The pathways were functionally dissected in streptolysin-O-permeabilised cells by capitalising on their relative dependence on active GTP binding proteins. One pathway required the presence of GTP-gammaS or active betagamma subunits, the other did not. Use of inhibitors revealed that the betagamma-independent pathway required activation of PI 3-kinases and was PKC-independent In contrast, the betagamma-dependent pathway involved activation of phospholipase C-beta and PKC, but was PI 3-kinase-independent. Both these pathways were found to be active in intact cells and are likely to determine receptor trafficking following internalisation.

The role of Rho in G protein-coupled receptor signal transduction

Low molecular weight G proteins of the Rho subfamily are regulators of actin cytoskeletal organization. In contrast to the heterotrimeric G proteins, the small GTPases are not directly activated through ligand binding to G protein-coupled receptors (GPCRs). However, a subset of GPCRs, including those for lysophosphatidic acid and thrombin, induce stress fibers, focal adhesions, and cell rounding through Rho-dependent pathways. C3 exoenzyme has been a useful tool for demonstrating Rho involvement in these and other responses, including Ca2+ sensitization of smooth muscle contraction, cell migration, transformation, and serum response element-mediated gene expression. Most of the GPCRs that induce Rho-dependent responses can activate Gq, but this is not a sufficient signal. Recent data demonstrate that G alpha 12/13 can induce Rho-dependent responses. Furthermore, G alpha 12/13 can bind and activate Rho-specific guanine nucleotide exchange factors, providing a mechanism by which GPCRs that couple to G alpha 12/13 could activate Rho and its downstream responses.

Complex involvement of pertussis toxin-sensitive G proteins in the regulation of type 1alpha metabotropic glutamate receptor signaling in baby hamster kidney cells

Previously, we demonstrated that the coupling of the metabotropic glutamate receptor mGlu1alpha to phosphoinositide hydrolysis is enhanced by pertussis toxin (PTX) in stably transfected baby hamster kidney cells (BHK). Here, we show that the PTX effect on agonist-stimulated [(3)H]inositol phosphate accumulation can be resolved into two components: an immediate increase in agonist potency, and a more slowly developing increase in the magnitude of the response observed at maximally effective agonist concentrations. Using G(q/11)alpha- and G(i/o)alpha-selective antibodies to immunoprecipitate [(35)S]guanosine-5'-O-(3-thio)triphosphate-bound Galpha proteins, we also show that agonist stimulation of mGlu1alpha in BHK membranes increases specific [(35)S]guanosine-5'-O-(3-thio)triphosphate binding to both G(q/11) and G(i/o) proteins. Preincubation of BHK-mGlu1alpha with L-glutamate (300 microM) results in a progressive loss (60% in 30 min) of L-quisqualate-induced [(3)H]inositol phosphate accumulation (without a change in potency), providing evidence for agonist-induced receptor desensitization. Although such desensitization of mGlu receptor signaling was mimicked by a phorbol ester, agonist-induced phosphorylation of the receptor was not observed and protein kinase C inhibition by Ro 31-8220 did not prevent L-glutamate-mediated desensitization. In contrast, PTX treatment of the cells almost completely prevented L-glutamate-mediated desensitization. Together, these data provide evidence for a multifunctional coupling of mGlu1alpha to different types of G proteins, including PTX-sensitive G(i)-type G proteins. The latter are involved in the negative control of phospholipase C activity while also influencing the rate of desensitization of the mGlu1alpha receptor.

Studying heterotrimeric G-protein-linked signal transduction using replication-deficient adenoviruses

Plasma membrane-spanning G-protein-linked receptors transduce approximately 60% of all extracellular stimuli in higher animals. Many G-protein-linked receptor pathways are yet to be elucidated, with the receptor, G-protein or effector system as yet unidentified. In addition, many fundamental issues pertaining to G-protein signalling remain unresolved, such as the factors governing the specificity of G-protein receptor coupling and the control of signal amplitude in response to G-protein activation. In order to address some of these issues, the use of replication-deficient adenoviruses as gene transfer vectors for investigations of G-protein signalling has been developed, facilitating dissection of G-protein-linked signal transduction pathways in an extensive range of cultured cells, as well as in vivo. The present review focuses on the versatility and utility of adenoviruses for the investigation of signalling by heterotrimeric G-proteins and explores some of the recent advances in adenoviral technology as they relate to the study of signal transduction.

Identification of Gbetagamma binding sites in the third intracellular loop of the M(3)-muscarinic receptor and their role in receptor regulation

Gbetagamma binds directly to the third intracellular (i3) loop subdomain of the M(3)-muscarinic receptor (MR). In this report, we identified the Gbetagamma binding motif and G-protein-coupled receptor kinase (GRK2) phosphorylation sites in the M(3)-MR i3 loop via a strategy of deletional and site-directed mutagenesis. The Gbetagamma binding domain was localized to Cys(289)-His(330) within the M(3)-MR-Arg(252)-Gln(490) i3 loop, and the binding properties (affinity, influence of ionic strength) of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain were similar to those observed for the entire i3 loop. Site-directed mutagenesis of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain indicated that Phe(312), Phe(314), and a negatively charged region (Glu(324)-Asp(329)) were required for interaction with Gbetagamma. Generation of the full-length M(3)-MR-Arg(252)-Gln(490) i3 peptides containing the F312A mutation were also deficient in Gbetagamma binding and exhibited a reduced capacity for phosphorylation by GRK2. A similar, parallel strategy resulted in identification of major residues ((331)SSS(333) and (348)SASS(351)) phosphorylated by GRK2, which were just downstream of the Gbetagamma binding motif. Full-length M(3)-MR constructs lacking the 42-amino acid Gbetagamma binding domain (Cys(289)-His(330)) or containing the F312A mutation exhibited ligand recognition properties similar to wild type receptor and also effectively mediated agonist-induced increases in intracellular calcium following receptor expression in Chinese hamster ovary and/or COS 7 cells. However, the M(3)-MRDeltaCys(289)-His(330) and M(3)-MR(F312A) constructs were deficient in agonist-induced sequestration, indicating a key role for the Gbetagamma-M(3)-MR i3 loop interaction in receptor regulation and signal processing.

G protein betagamma subunits induce stress fiber formation and focal adhesion assembly in a Rho-dependent manner in HeLa cells

In fibroblasts, the G protein alpha subunits Galpha(12) and Galpha(13) stimulate Rho-dependent stress fiber formation and focal adhesion assembly, whereas G protein betagamma subunits instead exert a disruptive influence. We show here that the latter can, however, stimulate the formation of stress fibers and focal adhesions in epithelial-like HeLa cells. Transient expression of beta(1) with gamma(2), gamma(5), gamma(7), and gamma(12) in quiescent HeLa cells induced stress fiber formation and focal adhesion assembly as did expression of the constitutively active Galpha(12). Co-expression of betagamma with Galpha(i2) and the C-terminal fragment of the beta-adrenergic receptor kinase, both of which are known to bind and sequester free betagamma, blocked betagamma-induced stress fiber and focal adhesion formation. Inhibition was also noted with co-expression of a dominant negative mutant of Rho. Botulinum C3 exoenzyme, which ADP-ribosylates and inactivates Rho, and a Rho-associated protein kinase inhibitor, Y-27632, similarly inhibited betagamma-induced stress fiber and focal adhesion assembly. These results indicate that G protein betagamma subunits regulate Rho-dependent actin polymerization in HeLa cells.

Dynamin and its role in membrane fission

Dynamin, a 100-kDa GTPase, is an essential component of vesicle formation in receptor-mediated endocytosis, synaptic vesicle recycling, caveolae internalization, and possibly vesicle trafficking in and out of the Golgi. In addition to the GTPase domain, dynamin also contains a pleckstrin homology domain (PH) implicated in membrane binding, a GTPase effector domain (GED) shown to be essential for self-assembly and stimulated GTPase activity, and a C-terminal proline-rich domain (PRD), which contains several SH3-binding sites. Dynamin partners bind to the PRD and may either stimulate dynamin's GTPase activity or target dynamin to the plasma membrane. Purified dynamin readily self-assembles into rings or spirals. This striking structural property supports the hypothesis that dynamin wraps around the necks of budding vesicles where it plays a key role in membrane fission. The focus of this review is on the relationship between the GTPase and self-assembly properties of dynamin and its cellular function.

Receptor-independent activators of heterotrimeric G-protein signaling pathways

Heterotrimeric G-protein signaling systems are activated via cell surface receptors possessing the seven-membrane span motif. Several observations suggest the existence of other modes of stimulus input to heterotrimeric G-proteins. As part of an overall effort to identify such proteins we developed a functional screen based upon the pheromone response pathway in Saccharomyces cerevisiae. We identified two mammalian proteins, AGS2 and AGS3 (activators of G-protein signaling), that activated the pheromone response pathway at the level of heterotrimeric G-proteins in the absence of a typical receptor. beta-galactosidase reporter assays in yeast strains expressing different Galpha subunits (Gpa1, G(s)alpha, G(i)alpha(2(Gpa1(1-41))), G(i)alpha(3(Gpa1(1-41))), Galpha(16(Gpa1(1-41)))) indicated that AGS proteins selectively activated G-protein heterotrimers. AGS3 was only active in the G(i)alpha(2) and G(i)alpha(3) genetic backgrounds, whereas AGS2 was active in each of the genetic backgrounds except Gpa1. In protein interaction studies, AGS2 selectively associated with Gbetagamma, whereas AGS3 bound Galpha and exhibited a preference for GalphaGDP versus GalphaGTPgammaS. Subsequent studies indicated that the mechanisms of G-protein activation by AGS2 and AGS3 were distinct from that of a typical G-protein-coupled receptor. AGS proteins provide unexpected mechanisms for input to heterotrimeric G-protein signaling pathways. AGS2 and AGS3 may also serve as novel binding partners for Galpha and Gbetagamma that allow the subunits to subserve functions that do not require initial heterotrimer formation.

Structure-activity relationships of G protein-coupled receptors

The primary function of cell-surface receptors is to discriminate the specific signaling molecule or ligand from a large array of chemically diverse extracellular substances and to activate an effector signaling cascade that triggers an intracellular response and eventually a biological effect. G protein-coupled cell-surface receptors (GPCRs) mediate their intracellular actions through the activation of guanine nucleotide-binding signal-transducing proteins (G proteins), which form a diverse family of regulatory GTPases that, in the GTP-bound state, bind and activate downstream membrane-localized effectors. Hundreds of GPCRs signal through one or more of these G proteins in response to a large variety of stimuli including photons, neurotransmitters, and hormones of variable molecular structure. The mechanisms by which these ligands provoke activation of the receptor/G-protein system are highly complex and multifactorial. Knowledge and mapping of the structural determinants and requirements for optimal GPCR function are of paramount importance, not only for a better and more detailed understanding of the molecular basis of ligand action and receptor function in normal and abnormal conditions, but also for a rational design of early diagnostic and therapeutic tools that may allow exogenous regulation of receptor and G protein function in disease processes.

Phosphorylation of F-actin-associating G protein gamma12 subunit enhances fibroblast motility

Eleven isoforms of G protein gamma subunit have been found thus far, but the precise roles of individual gamma subunits are not known. The gamma12 subunit has two unique properties: phosphorylation by protein kinase C and association with F-actin. To elucidate the role of gamma12, we overexpressed gamma12 and other gamma subunits in NIH 3T3 cells together with the beta1 subunit. The overexpressed gamma12 as well as endogenous gamma12, but not gamma2, gamma5, and gamma7 subunits, associated with cytoskeletal components. Expression of gamma12 induced remarkable changes including cell rounding, disruption of stress fibers, and enhancement of cell migration, but expression of other gamma subunits did not induce significant changes. Deletion of the N-terminal region of gamma12 decreased the abilities of gamma12 to associate with cytoskeletal fractions, to induce cell rounding, and to increase cell motility. Replacement by alanine of Ser2 of gamma12 (Ser1 of a mature gamma12 protein), a phosphorylation site for protein kinase C, eliminated these effects of gamma12, whereas a mutant in which Ser2 was replaced with glutamic acid showed effects equivalent to wild-type gamma12. These results indicate that phosphorylation of gamma12 at Ser2 enhances the motility of cells.

Gbetagamma dimers stimulate vascular L-type Ca2+ channels via phosphoinositide 3-kinase

We have previously reported that, in venous myocytes, Gbetagamma scavengers inhibit angiotensin AT1A receptor-induced stimulation of L-type Ca2+ channels (1). Here, we demonstrate that intracellular infusion of purified Gbetagamma complexes stimulates the L-type Ca2+ channel current in a concentration-dependent manner. Additional intracellular dialysis of GDP-bound inactive Galphao or of a peptide corresponding to the Gbetagamma binding region of the beta-adrenergic receptor kinase completely inhibited the Gbetagamma-induced stimulation of Ca2+ channel currents. The gating properties of the channel were not affected by intracellular application of Gbetagamma, suggesting that Gbetagamma increased the whole-cell calcium conductance. In addition, both the angiotensin AT1A receptor- and the Gbetagamma-induced stimulation of L-type Ca2+ channels were blocked by pretreatment of the cells with wortmannin, at nanomolar concentrations. Correspondingly, intracellular infusion of an enzymatically active purified recombinant Gbetagamma-sensitive phosphoinositide 3-kinase, PI3Kgamma, mimicked Gbetagamma-induced stimulation of Ca2+ channels. Both Gbetagamma- and PI3Kgamma-induced stimulations of Ca2+ channel currents were reduced by protein kinase C inhibitors suggesting that the Gbetagamma/PI3Kgamma-activated transduction pathway involves a protein kinase C. These results indicate for the first time that Gbetagamma dimers stimulate the vascular L-type Ca2+ channels through a Gbetagamma-sensitive PI3K.

Importance of the pleckstrin homology domain of dynamin in clathrin-mediated endocytosis

The GTPase dynamin plays an essential role in clathrin-mediated endocytosis [1] [2] [3]. Substantial evidence suggests that dynamin oligomerisation around the necks of endocytosing vesicles and subsequent dynamin-catalysed GTP hydrolysis is responsible for membrane fission [4] [5]. The pleckstrin homology (PH) domain of dynamin has previously been shown to interact with phosphoinositides, but it has not been determined whether this interaction is essential for dynamin's function in endocytosis [6] [7] [8] [9]. In this study, we address the in vivo function of the PH domain of dynamin by assaying the effects of deletions and point mutations in this region on transferrin uptake in COS-7 fibroblasts. Overexpression of a dynamin construct lacking its entire PH domain potently blocked transferrin uptake, as did overexpression of a dynamin construct containing a mutation in the first variable loop of the PH domain. Structural modelling of this latter mutant suggested that the lysine residue at position 535 (Lys535) may be critical in the coordination of phosphoinositides, and indeed, the purified mutant no longer interacted with lipid nanotubes. Interestingly, the inhibitory phenotype of cells expressing this dynamin mutant was partially relieved by a second mutation in the carboxy-terminal proline-rich domain (PRD), one that prevents dynamin from binding to the Src homology 3 (SH3) domain of amphiphysin. These data demonstrate that dynamin's interaction with phosphoinositides through its PH domain is essential for endocytosis. These findings also support our hypothesis that PRD-SH3 domain interactions are important in the recruitment of dynamin to sites of endocytosis.

Beta-arrestin-dependent formation of beta2 adrenergic receptor-Src protein kinase complexes

The Ras-dependent activation of mitogen-activated protein (MAP) kinase pathways by many receptors coupled to heterotrimeric guanine nucleotide binding proteins (G proteins) requires the activation of Src family tyrosine kinases. Stimulation of beta2 adrenergic receptors resulted in the assembly of a protein complex containing activated c-Src and the receptor. Src recruitment was mediated by beta-arrestin, which functions as an adapter protein, binding both c-Src and the agonist-occupied receptor. beta-Arrestin 1 mutants, impaired either in c-Src binding or in the ability to target receptors to clathrin-coated pits, acted as dominant negative inhibitors of beta2 adrenergic receptor-mediated activation of the MAP kinases Erk1 and Erk2. These data suggest that beta-arrestin binding, which terminates receptor-G protein coupling, also initiates a second wave of signal transduction in which the "desensitized" receptor functions as a critical structural component of a mitogenic signaling complex.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

Dynamin and its partners: a progress report

Dynamin's role in clathrin-mediated endocytosis is now well established. Here we review new evidence from the past two years for the function of dynamin and related GTPases in other Intracellular trafficking events. We then summarize current information on the domain structure and function of this multidomain GTPase. Finally, we describe dynamin partners and their function in the context of clathrin-mediated endocytosis.