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

The roles of RACK1 in the pathogenesis of Alzheimer's disease

The receptor for activated C kinase 1 (RACK1) is a protein that plays a crucial role in various signaling pathways and is involved in the pathogenesis of Alzheimer's disease (AD), a prevalent neurodegenerative disease. RACK1 is highly expressed in neuronal cells of the central nervous system and regulates the pathogenesis of AD. Specifically, RACK1 is involved in regulation of the amyloid-β precursor protein processing through α- or β-secretase by binding to different protein kinase C isoforms. Additionally, RACK1 promotes synaptogenesis and synaptic plasticity by inhibiting N-methyl-D-aspartate receptors and activating gamma-aminobutyric acid A receptors, thereby preventing neuronal excitotoxicity. RACK1 also assembles inflammasomes that are involved in various neuroinflammatory pathways, such as nuclear factor-kappa B, tumor necrosis factor-alpha, and NOD-like receptor family pyrin domain-containing 3 pathways. The potential to design therapeutics that block amyloid-β accumulation and inflammation or precisely regulate synaptic plasticity represents an attractive therapeutic strategy, in which RACK1 is a potential target. In this review, we summarize the contribution of RACK1 to the pathogenesis of AD and its potential as a therapeutic target.

The caleosin CLO7 and its role in the heterotrimeric G-protein signalling network

The investigation of the caleosin CLO7 in relation to heterotrimeric G-protein signalling in Arabidopsis showed that the gene plays a role in seed germination and embryo viability. The caleosin CLO7 belongs to a multi-gene family of calcium-binding proteins which are characterized by single EF-hand motifs. Other members of the caleosin gene family have been shown to affect transpiration and seed germination as well as play a role in both abiotic and biotic stress responses. The proteins are associated with lipid droplets/oil bodies and some members of the gene family have been shown to have peroxygenase activity. Members of the gene family have also been shown to interact with the α subunit of the heterotrimeric G protein complex. In this study, we further expand on the diversity of physiological responses in which members of this gene family play regulatory roles. Utilizing BiFC and Y2H protein-protein interaction assays, CLO7 is identified as an interactor of the heterotrimeric G protein α subunit, GPA1. The full-length CLO7 is shown to interact with both the wild-type GPA1 and its constitutively active form, GPA1^QL, at the plasma membrane. Point mutations to critical amino acids for calcium binding in the EF-hand of CLO7 indicate that the interaction with GPA1 is calcium-dependent and that the interaction with GPA1^QL is enhanced by calcium. Protein-protein interaction assays also show that CLO7 interacts with Pirin1, a member of the cupin gene superfamily and a known downstream effector of GPA1, and this interaction is calcium-dependent. The N-terminal portion of CLO7 is responsible for these interactions. GFP-tagged CLO7 protein localizes to the endoplasmic reticulum (ER) and to lipid bodies. Characterization of the clo7 mutant line has shown that CLO7 is implicated in the abscisic acid (ABA) and mannitol-mediated inhibition of seed germination, with the clo7 mutant displaying higher germination rates in response to osmotic stress and ABA hormone treatment. These results provide insight into the role of CLO7 in seed germination in response to abiotic stress as well as its interaction with GPA1 and Pirin1. CLO7 also plays a role in embryo viability with the clo7gpa1 double mutant displaying embryo lethality, and therefore the double mutant cannot be recovered.

Multi-omics analysis of glucose-mediated signaling by a moonlighting Gβ protein Asc1/RACK1

Heterotrimeric G proteins were originally discovered through efforts to understand the effects of hormones, such as glucagon and epinephrine, on glucose metabolism. On the other hand, many cellular metabolites, including glucose, serve as ligands for G protein-coupled receptors. Here we investigate the consequences of glucose-mediated receptor signaling, and in particular the role of a Gα subunit Gpa2 and a non-canonical Gβ subunit, known as Asc1 in yeast and RACK1 in animals. Asc1/RACK1 is of particular interest because it has multiple, seemingly unrelated, functions in the cell. The existence of such “moonlighting” operations has complicated the determination of phenotype from genotype. Through a comparative analysis of individual gene deletion mutants, and by integrating transcriptomics and metabolomics measurements, we have determined the relative contributions of the Gα and Gβ protein subunits to glucose-initiated processes in yeast. We determined that Gpa2 is primarily involved in regulating carbohydrate metabolism while Asc1 is primarily involved in amino acid metabolism. Both proteins are involved in regulating purine metabolism. Of the two subunits, Gpa2 regulates a greater number of gene transcripts and was particularly important in determining the amplitude of response to glucose addition. We conclude that the two G protein subunits regulate distinct but complementary processes downstream of the glucose-sensing receptor, as well as processes that lead ultimately to changes in cell growth and metabolism.

Despite the societal importance of glucose fermentation in yeast, the mechanisms by which these cells detect and respond to glucose have remained obscure. Glucose detection requires a cell surface receptor coupled to a G protein that is comprised of two subunits, rather than the more typical heterotrimer: an α subunit Gpa2 and the β subunit Asc1 (or RACK1 in humans). Asc1/RACK1 also serves as a subunit of the ribosome, where it regulates the synthesis of proteins involved in glucose fermentation. This manuscript uses global metabolomics and transcriptomics to demonstrate the distinct roles of each G protein subunit in transmitting the glucose signal. Whereas Gpa2 is primarily involved in the metabolism of carbohydrates, Asc1/RACK1 contributes to production of amino acids necessary for protein synthesis and cell division. These findings reveal the initial steps of glucose signaling and several unique and complementary functions of the G protein subunits. More broadly, the integrated approach used here is likely to guide efforts to determine the topology of complex G protein and metabolic signaling networks in humans.

Tensile force transmitted through LFA-1 bonds mechanoregulate neutrophil inflammatory response

Recruitment of leukocytes to sites of acute inflammation is guided by spatial and temporal cues that ensure appropriate cell numbers infiltrate the tissue at precise locations to protect it from infection and initiate repair. On inflamed endothelium, neutrophil rolling via selectins elicits cytosolic calcium release from endoplasmic reticulum (ER)-stores that are synergistic with chemokine signaling to activate formation of high affinity (HA) LFA-1 bonds to ICAM-1, which is necessary to anchor cells against the drag force of blood flow. Bond tension on LFA-1 within the area of adhesive contact with endothelium elicits calcium entry through calcium release-activated calcium channel protein 1 (Orai-1) membrane channels that in turn activate neutrophil shape change and migration. We hypothesized that mechanotransduction via LFA-1 is mediated by assembly of a cytosolic molecular complex consisting of Kindlin-3, receptor for activated C kinase 1 (RACK1), and Orai1. Initiation of Ca²⁺ flux at sites of adhesive contact required a threshold level of shear stress and increased with the magnitude of bond tension transduced across as few as 200 HA LFA-1. A sequential mechanism triggered by force acting on LFA-1/Kindlin-3 precipitated dissociation of RACK1, which formed a concentration gradient above LFA-1 bond clusters. This directed translocation of ER proximal to Orai1, where binding of inositol 1,4,5-triphosphate receptor type 1 and activation via stromal interaction molecule 1 elicited Ca flux and subsequent neutrophil shape change and motility. We conclude that neutrophils sense adhesive traction on LFA-1 bonds on a submicron scale to direct calcium influx, thereby ensuring sufficient shear stress of blood flow is present to trigger cell arrest and initiate transmigration at precise regions of vascular inflammation.

Functional analysis of drought and salt tolerance mechanisms of mulberry RACK1 gene

The receptor for activated C kinase 1 (RACK1) protein acts as a central hub for the integration of many physiological processes in eukaryotic organisms. Plant RACK1 is implicated in abiotic stress responses, but the underlying molecular mechanisms of stress adaptation remain largely unknown. Here, the overexpression of the mulberry (Morus alba L.) RACK1 gene in Arabidopsis decreased tolerance to drought and salt stresses and MaRACK1 overexpression changed expression levels of genes in response to stress and stimuli. We developed a simple and efficient transient transformation system in mulberry, and the mulberry seedlings transiently expressing MaRACK1 were hypersensitive to drought and salt stresses. The expression levels of guanine nucleotide-binding protein (G-protein) encoding genes in mulberry and Arabidopsis were not affected by MaRACK1 overexpression. The interactions between RACK1 and G-proteins were confirmed, and the RACK1 proteins from mulberry and Arabidopsis could not interact with their respective G-proteins, which indicated that RACK1 may regulate stress responses independently of G-proteins. Additionally, MaRACK1 may regulate drought and salt stress tolerances by interacting with a fructose 1, 6-bisphosphate aldolase. Our findings provide new insights into the mechanisms underlying RACK1 functions in abiotic stress responses and important information for their further characterization.

MoMip11, a MoRgs7-interacting protein, functions as a scaffolding protein to regulate cAMP signaling and pathogenicity in the rice blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae has eight regulators of G-protein signaling (RGS) and RGS-like proteins (MoRgs1 to MoRgs8) that exhibit both distinct and shared regulatory functions in the growth, differentiation and pathogenicity of the fungus. We found MoRgs7 with a unique RGS-seven transmembrane (7-TM) domain motif is localized to the highly dynamic tubule-vesicular compartments during early appressorium differentiation followed by gradually degradation. To explore whether this involves an active signal perception of MoRgs7, we identified a Gbeta-like/RACK1 protein homolog in M. oryzae MoMip11 that interacts with MoRgs7. Interestingly, MoMip11 selectively interacted with several components of the cAMP regulatory pathway, including Gα MoMagA and the high-affinity phosphodiesterase MoPdeH. We further showed that MoMip11 promotes MoMagA activation and suppresses MoPdeH activity thereby upregulating intracellular cAMP levels. Moreover, MoMip11 is required for the response to multiple stresses, a role also shared by Gbeta-like/RACK1 adaptor proteins. In summary, we revealed a unique mechanism by which MoMip11 links MoRgs7 and G-proteins to reugulate cAMP signaling, stress responses and pathogenicity of M. oryzae. Our studies revealed the multitude of regulatory networks that govern growth, development and pathogenicity in this important causal agent of rice blast.

The integration of Gβ and MAPK signaling cascade in zygote development

Cells respond to many signals with a limited number of signaling components. Heterotrimeric G proteins and MAPK cascades are universally used by eukaryotic cells to transduce signals in various developmental processes or stress responses by activating different effectors. MAPK cascade is integrated with G proteins by scaffold protein during plant immunity. However, the molecular relationship between G proteins and MAPK modules in plant development is still unclear. In this study, we demonstrate that Arabidopsis Gβ protein AGB1 interacts with MPK3 and 6, MKK4 and 5, as well as the regulatory domains of YODA (YDA), the upstream MEKK of MKK4/5. Remarkably, YDA interacts with the plasma membrane associated SHORT SUSPENSOR (SSP) through its N- and C-terminal region in vitro and in vivo. Additionally, genetic analysis shows that AGB1 functions together with MPK3/6 signaling cascade during the asymmetric division of the zygote. These data indicate that Gβ may function likely as a scaffold, through direct physical interaction with the components of the MPK signaling module in plant development. Our results provide new insights into the molecular functions of G protein and will advance the understanding of the complex mechanism of kinase signaling cascades.

The RNA-binding protein SERBP1 interacts selectively with the signaling protein RACK1

The RACK1 protein interacts with numerous proteins involved in signal transduction, the cytoskeleton, and mRNA splicing and translation. We used the 2-hybrid system to identify additional proteins interacting with RACK1 and isolated the RNA-binding protein SERBP1. SERPB1 shares amino acid sequence homology with HABP4 (also known as Ki-1/57), a component of the RNA spicing machinery that has been shown previously to interact with RACK1. Several different isoforms of SERBP1, generated by alternative mRNA splicing, interacted with RACK1 with indistinguishable interaction strength, as determined by a 2-hybrid beta-galactosidase assay. Analysis of deletion constructs of SERBP1 showed that the C-terminal third of the SERBP1 protein, which contains one of its two substrate sites for protein arginine N-methyltransferase 1 (PRMT1), is necessary and sufficient for it to interact with RACK1. Analysis of single amino acid substitutions in RACK1, identified in a reverse 2-hybrid screen, showed very substantial overlap with those implicated in the interaction of RACK1 with the cAMP-selective phosphodiesterase PDE4D5. These data are consistent with SERBP1 interacting selectively with RACK1, mediated by an extensive interaction surface on both proteins.

Structural analysis of ribosomal RACK1 and its role in translational control

Receptor for Activated C-Kinase 1 (RACK1) belongs to the WD40 family of proteins, known to act as scaffolding proteins in interaction networks. Accordingly, RACK1 is found to have numerous interacting partners ranging from kinases and signaling proteins to membrane bound receptors and ion channels. Interestingly, RACK1 has also been identified as a ribosomal protein present in all eukaryotic ribosomes. Structures of eukaryotic ribosomes have shown RACK1 to be located at the back of the head of the small ribosomal subunit. This suggests that RACK1 could act as a ribosomal scaffolding protein recruiting regulators of translation to the ribosome, and several studies have in fact found RACK1 to play a role in regulation of translation. To fully understand the role of RACK1 we need to understand whether the many reported interaction partners of RACK1 bind to free or ribosomal RACK1. In this review we provide a structural analysis of ribosome-bound RACK1 to provide a basis for answering this fundamental question. Our analysis shows that RACK1 is tightly bound to the ribosome through highly conserved and specific interactions confirming RACK1 as an integral ribosomal protein. Furthermore, we have analyzed whether reported binding sites for RACK1 interacting partners with a proposed role in translational control are accessible on ribosomal RACK1. Our analysis shows that most of the interaction partners with putative regulatory functions have binding sites that are available on ribosomal RACK1, supporting the role of RACK1 as a ribosomal signaling hub. We also discuss the possible role for RACK1 in recruitment of ribosomes to focal adhesion sites and regulation of local translation during cell spreading and migration.

A RACK1-like protein regulates hyphal morphogenesis, root entry and in vivo virulence in Verticillium dahliae

To identify key genes expressed in Verticillium dahliae in early stages of infection of cotton roots, spore suspensions of eight V. dahliae isolates with different virulence levels were induced by cotton roots and genes expressed in these isolates during the early stages of infection were profiled. A gene that was differentially expressed between highly and less virulent strains was identified. Cloning and bioinformatics analysis of the gene suggested that it belongs to the putative Gβ-like/RACK1 protein family, and has seven WD40 domains. Targeted deletion of the gene revealed that it controls a number of growth-related phenotypes, including conidia and microsclerotia production, normal spore germination and hyphal development. RACK1 is a component of eukaryotic ribosomes, and here we found by qRT-PCR that disruption of RACK1 in V. dahliae (designated VdRACK1) significantly altered the transcriptional levels of other ribosomal proteins, suggesting possible global effects of VdRACK1 deletion on the protein translation of other genes. VdRACK1-null mutants lost the ability to penetrate intact cotton roots. However, the mutant strain was able to infect root-wounded cotton plants and, intriguingly, resulted in a hypervirulent phenotype, implicating a role for VdRACK1 in the restriction of rampant growth within the plant.

The cyclic AMP phosphodiesterase 4D5 (PDE4D5)/receptor for activated C-kinase 1 (RACK1) signalling complex as a sensor of the extracellular nano-environment

The cyclic AMP and protein kinase C (PKC) signalling pathways regulate a wide range of cellular processes that require tight control, including cell proliferation and differentiation, metabolism and inflammation. The identification of a protein complex formed by receptor for activated C kinase 1 (RACK1), a scaffold protein for protein kinase C (PKC), and the cyclic AMP-specific phosphodiesterase, PDE4D5, demonstrates a potential mechanism for crosstalk between these two signalling routes. Indeed, RACK1-bound PDE4D5 is activated by PKCα, providing a route through which the PKC pathway can control cellular cyclic AMP levels. Although RACK1 does not appear to affect the intracellular localisation of PDE4D5, it does afford structural stability, providing protection against denaturation, and increases the susceptibility of PDE4D5 to inhibition by cyclic AMP-elevating pharmaceuticals, such as rolipram. In addition, RACK1 can recruit PDE4D5 and PKC to intracellular protein complexes that control diverse cellular functions, including activated G protein-coupled receptors (GPCRs) and integrins clustered at focal adhesions. Through its ability to regulate local cyclic AMP levels in the vicinity of these multimeric receptor complexes, the RACK1/PDE4D5 signalling unit therefore has the potential to modify the quality of incoming signals from diverse extracellular cues, ranging from neurotransmitters and hormones to nanometric topology. Indeed, PDE4D5 and RACK1 have been found to form a tertiary complex with integrin-activated focal adhesion kinase (FAK), which localises to cellular focal adhesion sites. This supports PDE4D5 and RACK1 as potential regulators of cell adhesion, spreading and migration through the non-classical exchange protein activated by cyclic AMP (EPAC1)/Rap1 signalling route.

Adenylyl Cyclase 5 Regulation by Gβγ Involves Isoform-Specific Use of Multiple Interaction Sites

Adenylyl cyclase (AC) converts ATP into cyclic AMP (cAMP), an important second messenger in cell signaling. Heterotrimeric G proteins and other regulators are important for control of AC activity. Depending on the AC isoform, Gβγ subunits can either conditionally stimulate or inhibit cAMP synthesis. We previously showed that the Gαs-βγ heterotrimer binds to the N terminus (NT) of type 5 AC (AC5). We now show that Gβγ binds to the NT of a wide variety of AC isoforms. We hypothesized that Gβγ/AC5 interactions involving inactive heterotrimer and Gβγ stimulation of AC5 were separable events. Mutations of the Gβγ "hotspot" show that this site is necessary for AC5 stimulation but not for interactions with the first 198 aa of AC5NT, which is a G protein scaffolding site. This contrasts with AC6, where the Gβγ hotspot is required for both interactions with AC6NT and for stimulation of AC6. Additionally, the SIGK hotspot peptide disrupts Gβγ regulation of AC isoforms 1, 2, and 6, but not AC5. Gβγ also binds the C1/C2 catalytic domains of AC5 and AC6. Finally, cellular interactions with full-length AC5 depend on multiple sites on Gβγ. This suggests an isoform-specific mechanism in which bound Gβγ at the AC5NT is ideally situated for spatiotemporal control of AC5. We propose Gβγ regulation of AC involves multiple binding events, and the role of the AC NT for mechanisms of regulation by heterotrimeric G protein subunits is isoform-specific.

Pathogen-secreted proteases activate a novel plant immune pathway

Mitogen-Activated Protein Kinase (MAPK) cascades play central roles in innate immune signaling networks in plants and animals^1,2. In plants, however, the molecular mechanisms of how signal perception is transduced to MAPK activation remain elusive¹. We report that pathogen-secreted proteases activate a previously unknown signaling pathway in Arabidopsis thaliana involving the Gα, Gβ and Gγ subunits of heterotrimeric G-protein complexes, which function upstream of a MAPK cascade. In this pathway, Receptor for Activated C Kinase 1 (RACK1) functions as a novel scaffold that binds to the Gβ subunit as well as to all three tiers of the MAPK cascade, thereby linking upstream G protein signaling to downstream activation of a MAPK cascade. The protease-G protein-RACK1-MAPK cascade modules identified in these studies are distinct from previously described plant immune signaling pathways such as the one elicited by bacterial flagellin, in which G proteins function downstream of or in parallel to a MAPK cascade without the involvement of the RACK1 scaffolding protein. The discovery of the novel protease-mediated immune signaling pathway described here was facilitated by the use of the broad host range, opportunistic bacterial pathogen Pseudomonas aeruginosa. The ability of P. aeruginosa to infect both plants and animals makes it an excellent model to identify novel types of immunoregulatory strategies that account for its niche adaptation to diverse host tissues and immune systems.

The Receptor for Activated C Kinase in Plant Signaling: Tale of a Promiscuous Little Molecule

Two decades after the first report of the plant homolog of the Receptor for Activated C Kinase 1 (RACK1) in cultured tobacco BY2 cells, a significant advancement has been made in the elucidation of its cellular and molecular role. The protein is now implicated in many biological functions including protein translation, multiple hormonal responses, developmental processes, pathogen infection resistance, environmental stress responses, and miRNA production. Such multiple functional roles are consistent with the scaffolding nature of the plant RACK1 protein. A significant advance was achieved when the β-propeller structure of the Arabidopsis RACK1A isoform was elucidated, thus revealing that its conserved seven WD repeats also assembled into this typical topology. From its crystal structure, it became apparent that it shares the structural platform for the interaction with ligands identified in other systems such as mammals. Although RACK1 proteins maintain conserved Protein Kinase C binding sites, the lack of a bona fide PKC adds complexity and enigma to the nature of the ligand partners with which RACK1 interacts in plants. Nevertheless, ligands recently identified using the split-ubiquitin based and conventional yeast two-hybrid assays, have revealed that plant RACK1 is involved in several processes that include defense response, drought and salt stress, ribosomal function, cell wall biogenesis, and photosynthesis. The information acquired indicates that, in spite of the high degree of conservation of its structure, the functions of the plant RACK1 homolog appear to be distinct and diverse from those in yeast, mammals, insects, etc. In this review, we take a critical look at the novel information regarding the many functions in which plant RACK1 has been reported to participate, with a special emphasis on the information on its currently identified and missing ligand partners.

The Dictyostelium discoideum RACK1 orthologue has roles in growth and development

Background

The receptor for activated C-kinase 1 (RACK1) is a conserved protein belonging to the WD40 repeat family of proteins. It folds into a beta propeller with seven blades which allow interactions with many proteins. Thus it can serve as a scaffolding protein and have roles in several cellular processes.

Results

We identified the product of the Dictyostelium discoideum gpbB gene as the Dictyostelium RACK1 homolog. The protein is mainly cytosolic but can also associate with cellular membranes. DdRACK1 binds to phosphoinositides (PIPs) in protein-lipid overlay and liposome-binding assays. The basis of this activity resides in a basic region located in the extended loop between blades 6 and 7 as revealed by mutational analysis. Similar to RACK1 proteins from other organisms DdRACK1 interacts with G protein subunits alpha, beta and gamma as shown by yeast two-hybrid, pulldown, and immunoprecipitation assays. Unlike the Saccharomyces cerevisiae and Cryptococcus neoformans RACK1 proteins it does not appear to take over Gβ function in D. discoideum as developmental and other defects were not rescued in Gβ null mutants overexpressing GFP-DdRACK1. Overexpression of GFP-tagged DdRACK1 and a mutant version (DdRACK1mut) which carried a charge-reversal mutation in the basic region in wild type cells led to changes during growth and development.

Conclusion

DdRACK1 interacts with heterotrimeric G proteins and can through these interactions impact on processes specifically regulated by these proteins.

Noncanonical Gβ Gib2 is a scaffolding protein promoting cAMP signaling through functions of Ras1 and Cac1 proteins in Cryptococcus neoformans

Gβ-like/RACK1 functions as a key mediator of various pathways and contributes to numerous cellular functions in eukaryotic organisms. In the pathogenic fungus Cryptococcus neoformans, noncanonical Gβ Gib2 promotes cAMP signaling in cells lacking normal Gpa1 function while displaying versatility in interactions with Gα Gpa1, protein kinase Pkc1, and endocytic intersectin Cin1. To elucidate the Gib2 functional mechanism(s), we demonstrate that Gib2 is required for normal growth and virulence. We show that Gib2 directly binds to Gpa1 and Gγ Gpg1/Gpg2 and that it interacts with phosphodiesterase Pde2 and monomeric GTPase Ras1. Pde2 remains functionally dispensable, but Ras1 is found to associate with adenylyl cyclase Cac1 through the conserved Ras association domain. In addition, the ras1 mutant exhibits normal capsule formation, whereas the ras1 gpa1 mutant displays enhanced capsule formation, and the ras1 gpa1 cac1 mutant is acapsular. Collectively, these findings suggest that Gib2 promotes cAMP levels by relieving an inhibitory function of Ras1 on Cac1 in the absence of Gpa1. In addition, using GST affinity purification combined with mass spectrometry, we identified 47 additional proteins that interact with Gib2. These proteins have putative functions ranging from signal transduction, energy generation, metabolism, and stress response to ribosomal function. After establishing and validating a protein-protein interactive network, we believe Gib2 to be a key adaptor/scaffolding protein that drives the formation of various protein complexes required for growth and virulence. Our study reveals Gib2 as an essential component in deciphering the complexity of regulatory networks that control growth and virulence in C. neoformans.

Extra-nuclear activity of INSM1 transcription factor enhances insulin receptor signaling pathway and Nkx6.1 expression through RACK1 interaction

INSM1 is an islet transcription factor essential for pancreas development. INSM1 functions as a transcriptional repressor of NeuroD/β2 and insulin gene in the pancreas. INSM1 also possesses extra-nuclear activities through binding to multiple cellular regulators such as cyclin D1 and RACK1. In this study, we report that the interaction of INSM1 and RACK1 is essential to enhance the insulin receptor (InR)-mediated signaling pathway. A proline-rich region in the N-terminus of INSM1 is required for RACK1 binding, which interrupts RACK1-InR interaction and enhances InR signal activation. Binding of INSM1 to RACK1 increases AKT phosphorylation. The INSM1-enhanced AKT phosphorylation can be inhibited by the PI3K inhibitor, LY294002. When INSM1 induces AR42J cell trans-differentiation, the Nkx6.1 gene is activated through the InR-mediated signaling pathway and an elevation of the acetyl-H4 modification on the Nkx6.1 gene promoter/enhancer is observed. The PI3K inhibitor interrupts Nkx6.1 and insulin gene expression. Therefore, we conclude that the extra-nuclear activity of INSM1 by enhancing the PI3K/AKT signaling pathway is important for pancreatic cell differentiation.

RACK1 (receptor for activated C-kinase 1) interacts with FBW2 (F-box and WD-repeat domain-containing 2) to up-regulate GCM1 (glial cell missing 1) stability and placental cell migration and invasion

GCM1 (glial cell missing 1) is a short-lived transcription factor essential for placental development. The F-box protein, FBW2 (F-box and WD-repeat domain-containing 2), which contains five WD (tryptophan-aspartate) repeats, recognizes GCM1 and mediates its ubiquitination via the SCFFBW2 E3 ligase complex. Although the interaction between GCM1 and FBW2 is facilitated by GCM1 phosphorylation, it is possible that this interaction might be regulated by additional cellular factors. In the present study, we perform tandem-affinity purification coupled with MS analysis identifying RACK1 (receptor for activated C-kinase 1) as an FBW2-interacting protein. RACK1 is a multifaceted scaffold protein containing seven WD repeats. We demonstrate that the WD repeats in both RACK1 and FBW2 are required for the interaction of RACK1 and FBW2. Furthermore, RACK1 competes with GCM1 for FBW2 and thereby prevents GCM1 ubiquitination, which is also supported by the observation that GCM1 is destabilized in RACK1-knockdown BeWo placental cells. Importantly, RACK1 knockdown leads to decreased expression of the GCM1 target gene HTRA4 (high-temperature requirement protein A4), which encodes a serine protease crucial for cell migration and invasion. As a result, migration and invasion activities are down-regulated in RACK1-knockdown BeWo cells. The present study reveals a novel function for RACK1 to regulate GCM1 activity and placental cell migration and invasion.

WD40-repeat proteins control the flow of Gβγ signaling for directional cell migration

The ability of cells to generate a highly polarized intracellular signal through G protein-coupled receptors (GPCRs) is essential for their migration toward chemoattractants. The Gβγ subunits of heterotrimeric G proteins play a critical role in transmitting chemotactic signals from GPCRs via the activation of diverse effectors, including PLCβ and PI3K, primarily at the leading edge of cells. Although Gβγ can directly activate many of these effectors through protein-protein interactions in vitro, it remains unclear how Gβγ spatially and temporally orchestrates the activation of these effectors in vivo. A yeast two-hybrid screen for Gβ interacting proteins identified two WD40-repeat domain containing proteins, RACK1 and WDR26, which are predicted to serve as scaffolding/adaptor proteins. Previous data indicates that RACK1 negatively regulates Gβγ-mediated leukocyte migration by inhibiting Gβγ-stimulated PLCβ and PI3K activities. In contrast, recently published work by Sun et al. indicates that WDR26 promotes leukocyte migration by enhancing Gβγ-mediated signal transduction. These findings reveal a novel mechanism regulating Gβγ signaling during chemotaxis, namely through the positive and negative regulation of WDR26 and RACK1 on Gβγ to promote and fine tune Gβγ-mediated effector activation, ultimately governing the ability of cells to polarize and migrate toward a chemoattractant gradient.

Pleiotropic roles of the Msi1-like protein Msl1 in Cryptococcus neoformans

Msi1-like (MSIL) proteins contain WD40 motifs and have a pleiotropic cellular function as negative regulators of the Ras/cyclic AMP (cAMP) pathway and components of chromatin assembly factor 1 (CAF-1), yet they have not been studied in fungal pathogens. Here we identified and characterized an MSIL protein, Msl1, in Cryptococcus neoformans, which causes life-threatening meningoencephalitis in humans. Notably, Msl1 plays pleiotropic roles in C. neoformans in both cAMP-dependent and -independent manners largely independent of Ras. Msl1 negatively controls antioxidant melanin production and sexual differentiation, and this was repressed by the inhibition of the cAMP-signaling pathway. In contrast, Msl1 controls thermotolerance, diverse stress responses, and antifungal drug resistance in a Ras/cAMP-independent manner. Cac2, which is the second CAF-1 component, appears to play both redundant and distinct functions compared to the functions of Msl1. Msl1 is required for the full virulence of C. neoformans. Transcriptome analysis identified a group of Msl1-regulated genes, which include stress-related genes such as HSP12 and HSP78. In conclusion, this study demonstrates pleiotropic roles of Msl1 in the human fungal pathogen C. neoformans, providing insight into a potential novel antifungal therapeutic target.

Protein kinase C (PKC) activity regulates functional effects of Kvβ1.3 subunit on KV1.5 channels: identification of a cardiac Kv1.5 channelosome

K(v)1.5 channels are the primary channels contributing to the ultrarapid outward potassium current (I(Kur)). The regulatory K(v)β1.3 subunit converts K(v)1.5 channels from delayed rectifiers with a modest degree of slow inactivation to channels with both fast and slow inactivation components. Previous studies have shown that inhibition of PKC with calphostin C abolishes the fast inactivation induced by K(v)β1.3. In this study, we investigated the mechanisms underlying this phenomenon using electrophysiological, biochemical, and confocal microscopy approaches. To achieve this, we used HEK293 cells (which lack K(v)β subunits) transiently cotransfected with K(v)1.5+K(v)β1.3 and also rat ventricular and atrial tissue to study native α-β subunit interactions. Immunocytochemistry assays demonstrated that these channel subunits colocalize in control conditions and after calphostin C treatment. Moreover, coimmunoprecipitation studies showed that K(v)1.5 and K(v)β1.3 remain associated after PKC inhibition. After knocking down all PKC isoforms by siRNA or inhibiting PKC with calphostin C, K(v)β1.3-induced fast inactivation at +60 mV was abolished. However, depolarization to +100 mV revealed K(v)β1.3-induced inactivation, indicating that PKC inhibition causes a dramatic positive shift of the inactivation curve. Our results demonstrate that calphostin C-mediated abolishment of fast inactivation is not due to the dissociation of K(v)1.5 and K(v)β1.3. Finally, immunoprecipitation and immunocytochemistry experiments revealed an association between K(v)1.5, K(v)β1.3, the receptor for activated C kinase (RACK1), PKCβI, PKCβII, and PKCθ in HEK293 cells. A very similar K(v)1.5 channelosome was found in rat ventricular tissue but not in atrial tissue.

The WD40 repeat protein WDR26 binds Gβγ and promotes Gβγ-dependent signal transduction and leukocyte migration

The Gβγ subunits of heterotrimeric G proteins transmit signals to control many cellular processes, including leukocyte migration. Gβγ signaling may regulate and be regulated by numerous signaling partners. Here, we reveal that WDR26, a member of the WD40 repeat protein family, directly bound free Gβγ in vitro, and formed a complex with endogenous Gβγ in Jurkat T cells stimulated by the chemokine SDF1α. Suppression of WDR26 by siRNAs selectively inhibited Gβγ-dependent phospholipase Cβ and PI3K activation, and attenuated chemotaxis in Jurkat T cells and differentiated HL60 cells in vitro and Jurkat T cell homing to lymphoid tissues in scid mice. Similarly, disruption of the WDR26/Gβγ interaction via expression of a WDR26 deletion mutant impaired Gβγ signaling and Jurkat T cell migration, indicating that the function of WDR26 depends on its binding to Gβγ. Additional data show that WDR26 also controlled RACK1, a negative regulator, in binding Gβγ and inhibiting leukocyte migration. Collectively, these experiments identify WDR26 as a novel Gβγ-binding protein that is required for the efficacy of Gβγ signaling and leukocyte migration.

RACK1, A multifaceted scaffolding protein: Structure and function

The Receptor for Activated C Kinase 1 (RACK1) is a member of the tryptophan-aspartate repeat (WD-repeat) family of proteins and shares significant homology to the β subunit of G-proteins (Gβ). RACK1 adopts a seven-bladed β-propeller structure which facilitates protein binding. RACK1 has a significant role to play in shuttling proteins around the cell, anchoring proteins at particular locations and in stabilising protein activity. It interacts with the ribosomal machinery, with several cell surface receptors and with proteins in the nucleus. As a result, RACK1 is a key mediator of various pathways and contributes to numerous aspects of cellular function. Here, we discuss RACK1 gene and structure and its role in specific signaling pathways, and address how posttranslational modifications facilitate subcellular location and translocation of RACK1. This review condenses several recent studies suggesting a role for RACK1 in physiological processes such as development, cell migration, central nervous system (CN) function and circadian rhythm as well as reviewing the role of RACK1 in disease.

Understanding molecular recognition by G protein βγ subunits on the path to pharmacological targeting

Heterotrimeric G proteins, composed of Gα and Gβγ subunits, transduce extracellular signals via G-protein-coupled receptors to modulate many important intracellular responses. The Gβγ subunits hold a central position in this signaling system and have been implicated in multiple aspects of physiology and the pathophysiology of disease. The Gβ subunit belongs to a large family of WD40 repeat proteins with a circular β-bladed propeller structure. This structure allows Gβγ to interact with a broad range of proteins to play diverse roles. How Gβγ interacts with and regulates such a wide variety of partners yet maintains specificity is an interesting problem in protein-protein molecular recognition in signal transduction, where signal transfer by proteins is often driven by modular conserved recognition motifs. Evidence has accumulated that one mechanism for Gβγ multitarget recognition is through an intrinsically flexible protein surface or "hot spot" that accommodates multiple modes of binding. Because each target has a unique recognition mode for Gβγ subunits, it suggests that these interactions could be selectively manipulated with small molecules, which could have significant therapeutic potential.

Involvement of Arabidopsis RACK1 in protein translation and its regulation by abscisic acid

Earlier studies have shown that RACK1 functions as a negative regulator of abscisic acid (ABA) responses in Arabidopsis (Arabidopsis thaliana), but the molecular mechanism of the action of RACK1 in these processes remains elusive. Global gene expression profiling revealed that approximately 40% of the genes affected by ABA treatment were affected in a similar manner by the rack1 mutation, supporting the view that RACK1 is an important regulator of ABA responses. On the other hand, coexpression analysis revealed that more than 80% of the genes coexpressed with RACK1 encode ribosome proteins, implying a close relationship between RACK1's function and the ribosome complex. These results implied that the regulatory role for RACK1 in ABA responses may be partially due to its putative function in protein translation, which is one of the major cellular processes that mammalian and Saccharomyces cerevisiae RACK1 is involved in. Consistently, all three Arabidopsis RACK1 homologous genes, namely RACK1A, RACK1B, and RACK1C, complemented the growth defects of the S. cerevisiae cross pathway control2/rack1 mutant. In addition, RACK1 physically interacts with Arabidopsis Eukaryotic Initiation Factor6 (eIF6), whose mammalian homolog is a key regulator of 80S ribosome assembly. Moreover, rack1 mutants displayed hypersensitivity to anisomycin, an inhibitor of protein translation, and displayed characteristics of impaired 80S functional ribosome assembly and 60S ribosomal subunit biogenesis in a ribosome profiling assay. Gene expression analysis revealed that ABA inhibits the expression of both RACK1 and eIF6. Taken together, these results suggest that RACK1 may be required for normal production of 60S and 80S ribosomes and that its action in these processes may be regulated by ABA.

RACK1 associates with muscarinic receptors and regulates M(2) receptor trafficking

Receptor internalization from the cell surface occurs through several mechanisms. Some of these mechanisms, such as clathrin coated pits, are well understood. The M₂ muscarinic acetylcholine receptor undergoes internalization via a poorly-defined clathrin-independent mechanism. We used isotope coded affinity tagging and mass spectrometry to identify the scaffolding protein, receptor for activated C kinase (RACK1) as a protein enriched in M₂-immunoprecipitates from M₂-expressing cells over those of non-M₂ expressing cells. Treatment of cells with the agonist carbachol disrupted the interaction of RACK1 with M₂. We further found that RACK1 overexpression inhibits the internalization and subsequent down regulation of the M₂ receptor in a receptor subtype-specific manner. Decreased RACK1 expression increases the rate of agonist internalization of the M₂ receptor, but decreases the extent of subsequent down-regulation. These results suggest that RACK1 may both interfere with agonist-induced sequestration and be required for subsequent targeting of internalized M₂ receptors to the degradative pathway.

G-protein β2 subunit interacts with mitofusin 1 to regulate mitochondrial fusion

Mitofusins (Mfns) mediate the fusion of mitochondrial membranes. However, little is known about how Mfns are regulated to control mitochondrial fusion, which is a multistep process requiring tethering and docking of the outer membranes of two mitochondria. In this study, we report that guanine nucleotide binding protein-β subunit 2 (Gβ2), a WD40 repeats protein and a member of the β-subunits of the heterotrimeric G proteins, has a crucial function in mitochondrial fusion. Gβ2 was found to be enriched on the surface of mitochondria and interacted with mitofusin 1 (Mfn1) specifically. Gβ2 also regulated the mobility of Mfn1 on the surface of the mitochondrial membrane and affected the mitochondrial fusion. Depletion of endogenous Gβ2 resulted in mitochondrial fragmentation, which could be rescued by exogenous Gβ2. These findings have thus uncovered a novel role of Gβ2 in regulating mitochondrial fusion through its interaction with Mfn1.

Epigenetic regulation of BDNF expression via the scaffolding protein RACK1

Scaffolding proteins are major contributors to the spatial and temporal orchestration of signaling cascades and hence cellular functions. RACK1 is a scaffolding protein that plays an important role in the regulation of, and cross-talk between, various signaling pathways. Here we report that RACK1 is a mediator of chromatin remodeling, resulting in an exon-specific expression of the brain-derived neurotrophic factor (BDNF) gene. Specifically, we found that following the activation of the cAMP pathway, nuclear RACK1 localizes at the promoter IV region of the BDNF gene by its association with histones H3 and H4, leading to the dissociation of the transcription repressor methyl-CpG-binding protein 2 (MeCP2) from the promoter, resulting in the acetylation of histone H4. These chromatin modifications lead to the activation of the promoter and to the subsequent promoter-controlled transcription of BDNF exon IV. Our findings expand our knowledge regarding the function of scaffolding proteins such as RACK1. Furthermore, this novel mechanism for the regulation of exon-specific expression of the BDNF gene by RACK1 could have implications on the neuronal functions of the growth factor including synaptic plasticity, learning, and memory.

RACK1 is involved in β-amyloid impairment of muscarinic regulation of GABAergic transmission

RACK1 (receptor for activated C-kinase 1), an anchoring protein that shuttles activated PKC to cellular membranes, plays an important role in PKC-mediated signal transduction pathways. A significant loss of RACK1 has been found in the brain of aging animals and Alzheimer's disease (AD) patients, which implicates the potential involvement of RACK1 in altered PKC activation associated with dementia. Our previous studies have demonstrated that GABAergic inhibition in prefrontal cortex, which is important for cognitive processes like "working memory", is regulated by muscarinic receptors via a PKC-dependent mechanism, and this effect is impaired by β-amyloid peptide (Aβ). In this study, we found that Aβ oligomers decreased RACK1 distribution in the membrane fraction of cortical neurons. Moreover, overexpression of RACK1 rescued the effect of muscarinic receptors on GABAergic transmission in Aβ-treated cortical cultures in vitro and Aβ-injected cortical neurons in vivo. These results suggest that the Aβ-induced loss of RACK1 distribution in the cell membrane may underlie the Aβ impairment of muscarinic regulation of PKC and GABAergic transmission. Thus, RACK1 provides a potential therapeutic target that can restore some of the impaired cellular processes by Aβ.

The G protein betagamma subunit mediates reannealing of adherens junctions to reverse endothelial permeability increase by thrombin

The inflammatory mediator thrombin proteolytically activates protease-activated receptor (PAR1) eliciting a transient, but reversible increase in vascular permeability. PAR1-induced dissociation of Gα subunit from heterotrimeric Gq and G12/G13 proteins is known to signal the increase in endothelial permeability. However, the role of released Gβγ is unknown. We now show that impairment of Gβγ function does not affect the permeability increase induced by PAR1, but prevents reannealing of adherens junctions (AJ), thereby persistently elevating endothelial permeability. We observed that in the naive endothelium Gβ1, the predominant Gβ isoform is sequestered by receptor for activated C kinase 1 (RACK1). Thrombin induced dissociation of Gβ1 from RACK1, resulting in Gβ1 interaction with Fyn and focal adhesion kinase (FAK) required for FAK activation. RACK1 depletion triggered Gβ1 activation of FAK and endothelial barrier recovery, whereas Fyn knockdown interrupted with Gβ1-induced barrier recovery indicating RACK1 negatively regulates Gβ1-Fyn signaling. Activated FAK associated with AJ and stimulated AJ reassembly in a Fyn-dependent manner. Fyn deletion prevented FAK activation and augmented lung vascular permeability increase induced by PAR1 agonist. Rescuing FAK activation in fyn^−/− mice attenuated the rise in lung vascular permeability. Our results demonstrate that Gβ1-mediated Fyn activation integrates FAK with AJ, preventing persistent endothelial barrier leakiness.

N terminus of type 5 adenylyl cyclase scaffolds Gs heterotrimer

According to accepted doctrine, agonist-bound G protein-coupled receptors catalyze the exchange of GDP for GTP and facilitate the dissociation of Galpha and Gbetagamma, which in turn regulate their respective effectors. More recently, the existence of preformed signaling complexes, which may include receptors, heterotrimeric G proteins, and/or effectors, is gaining acceptance. We show herein the existence of a preformed complex of inactive heterotrimer (Galpha(s) x betagamma) and the effector type 5 adenylyl cyclase (AC5), localized by the N terminus of AC5. GST fusions of AC5 N terminus (5NT) bind to purified G protein subunits (GDP-Galpha(s) and Gbetagamma) with apparent affinities of 270 +/- 21 and 190 +/- 7 nM, respectively. GDP-bound Galpha(s) and Gbetagamma did not compete, but rather facilitated their interaction with 5NT, consistent with the isolation of a ternary complex (5NT, Galpha(s), and Gbetagamma) by gel filtration. The AC5/Gbetagamma interaction was also demonstrated by immunoprecipitation and fluorescence resonance energy transfer (FRET) and the binding site of heterotrimer Galpha(s) x betagamma mapped to amino acids 60 to 129 of 5NT. Deletion of this region in full-length AC5 resulted in significant reduction of FRET between Gbetagamma and AC. 5NT also interacts with the catalytic core of AC, mainly via the C1 domain, to enhance Galpha(s)--and forskolin-stimulated activity of C1/C2 domains. The N terminus also serves to constrain Galpha(i)-mediated inhibition of AC5, which is relieved in the presence of Gbetagamma. These results reveal that 5NT plays a key regulatory role by interacting with the catalytic core and scaffolding inactive heterotrimeric G proteins, forming a preassembled complex that is potentially braced for GPCR activation.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

PKC and PLA2: probing the complexities of the calcium network

Lipid signaling and phosphorylation cascades are fundamental to calcium signaling networks. In this review, we will discuss the recent laboratory findings for the phospholipase A(2) (PLA(2))/protein kinase C (PKC) pathway within cellular calcium networks. The complexity and connectivity of these ubiquitous cellular signals make interpretation of experimental results extremely challenging. We present here computational methods which have been developed to conquer such complex data, and how they can be used to make models capable of accurately predicting cellular responses within multiple calcium signaling pathways. We propose that information obtained from network analysis and computational techniques provides a rich source of knowledge which can be directly translated to the laboratory benchtop.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

RACK-1 directs dynactin-dependent RAB-11 endosomal recycling during mitosis in Caenorhabditis elegans

Membrane trafficking pathways are necessary for the addition and removal of membrane during cytokinesis. In animal cells, recycling endosomes act as a major source of the additional membranes during furrow progression and abscission. However, the mechanisms and factors that regulate recycling endosomes during the cell cycle remain poorly understood. Here, we show that the Caenorhabditis elegans Receptor of Activated C Kinase 1 (RACK-1) is required for cytokinesis, germline membrane organization, and the recruitment of RAB-11-labeled recycling endosomes to the pericentrosomal region and spindle. RACK-1 is also required for proper chromosome separation and astral microtubule length. RACK-1 localizes to the centrosomes, kinetochores, the midbody, and nuclear envelopes during the cell cycle. We found that RACK-1 directly binds to DNC-2, the C. elegans p50/dynamitin subunit of the dynactin complex. Last, RACK-1 may facilitate the sequestration of recycling endosomes by targeting DNC-2 to centrosomes and the spindle. Our findings suggest a mechanism by which RACK-1 directs the dynactin-dependent redistribution of recycling endosomes during the cell cycle, thus ensuring proper membrane trafficking events during cytokinesis.

Differential gene expression in the bovine corpus luteum during transition from early phase to midphase and its potential role in acquisition of luteolytic sensitivity to prostaglandin F2 alpha

Prostaglandin F2 alpha (PGF(2alpha)) brings about regression of the bovine corpus luteum (CL). This luteolytic property of PGF(2alpha) is used in beef and dairy cattle to synchronize estrus. A limitation of this protocol is insensitivity of the early CL to luteolytic actions of PGF(2alpha). The mechanisms underlying this differential luteal sensitivity are poorly understood. The developing CL has a maximum number of PGF(2alpha) receptors; therefore, differences in signaling events may be responsible for luteal insensitivity. Hence, differential gene expression at two developmental stages of CL, Day 4 (D-4) and D-10 after estrus, might account for differences in signal transduction pathways associated with luteal sensitivity. This possibility was examined in these studies. Microarray analysis (n = 3 cows per stage) identified 167 genes that were differentially expressed (P < 0.05). These were categorized into genes involved in protein biosynthesis and modification (18.5%), transcription regulation and DNA biosynthesis (18.5%), miscellaneous (17.0%), cell signaling (12.0%), steroidogenesis and metabolism (10.2%), extracellular matrix and cytoskeletal proteins (9.5%), unknown functions (6.0%), protein degradation (5.3%), and antioxidant property (3.0%). Real-time PCR confirmed the differential expression of nine selected genes, including tyrosine 3-monooxygenase/tryptophan 5-monooxygense activation protein zeta polypeptide (YWHAZ) and regulator of G protein signaling 2 24-kDa (RGS2), observed in microarray. Furthermore, the in vivo effect of exogenous PGF(2alpha) (n = 3 cows per stage) on selected genes that were found to be differentially expressed during this developmental transition was examined. PGF(2alpha) increased the expression of a guanine nucleotide-binding protein (G protein) beta polypeptide 1 (GNB1) in D-4 CL and calcium/calmodulin-dependent kinase kinase 2 beta (CAMKK2) in D-10 CL. Therefore, GNB1, CAMKK2, YWHAZ, and RGS2 are candidate genes that may have a significant role in acquisition of luteal sensitivity to PGF(2alpha). Additional evidence supporting the significance of the microarray data was obtained from the observation that the amount of CAMKK2 paralleled the differential mRNA expression observed for this gene when examined by microarray analysis and by real-time RT-PCR. Furthermore, the two types of luteal steroidogenic cells known to be targets for PGF(2alpha) actions were demonstrated to be a cellular source for CAMKK2.

Structure of a signal transduction regulator, RACK1, from Arabidopsis thaliana

The receptor for activated C-kinase 1 (RACK1) is a highly conserved WD40 repeat scaffold protein found in a wide range of eukaryotic species from Chlamydymonas to plants and humans. In tissues of higher mammals, RACK1 is ubiquitously expressed and has been implicated in diverse signaling pathways involving neuropathology, cellular stress, protein translation, and developmental processes. RACK1 has established itself as a scaffold protein through physical interaction with a myriad of signaling proteins ranging from kinases, phosphatases, ion channels, membrane receptors, G proteins, IP3 receptor, and with widely conserved structural proteins associated with the ribosome. In the plant Arabidopsis thaliana, RACK1A is implicated in diverse developmental and environmental stress pathways. Despite the functional conservation of RACK1-mediated protein-protein interaction-regulated signaling modes, the structural basis of such interactions is largely unknown. Here we present the first crystal structure of a RACK1 protein, RACK1 isoform A from Arabidopsis thaliana, at 2.4 A resolution, as a C-terminal fusion of the maltose binding protein. The structure implicates highly conserved surface residues that could play critical roles in protein-protein interactions and reveals the surface location of proposed post-transcriptionally modified residues. The availability of this structure provides a structural basis for dissecting RACK1-mediated cellular signaling mechanisms in both plants and animals.

Scaffolding proteins in G-protein signaling

Heterotrimeric G proteins are ubiquitous signaling partners of seven transmembrane-domain G-protein-coupled receptors (GPCRs), the largest (and most important pharmacologically) receptor family in mammals. A number of scaffolding proteins have been identified that regulate various facets of GPCR signaling. In this review, we summarize current knowledge concerning those scaffolding proteins that are known to directly bind heterotrimeric G proteins, and discuss the composition of the protein complexes they assemble and their effects on signal transduction. Emerging evidence about possible ways of regulation of activity of these scaffolding proteins is also discussed.

The RACK1 ortholog Asc1 functions as a G-protein beta subunit coupled to glucose responsiveness in yeast

According to the prevailing paradigm, G-proteins are composed of three subunits, an alpha subunit with GTPase activity and a tightly associated betagamma subunit complex. In the yeast Saccharomyces cerevisiae there are two known Galpha proteins (Gpa1 and Gpa2) but only one Gbetagamma, which binds only to Gpa1. Here we show that the yeast ortholog of RACK1 (receptor for activated protein kinase C1) Asc1 functions as the Gbeta for Gpa2. As with other known Gbeta proteins, Asc1 has a 7-WD domain structure, interacts directly with the Galpha in a guanine nucleotide-dependent manner, and inhibits Galpha guanine nucleotide exchange activity. In addition, Asc1 binds to the effector enzyme adenylyl cyclase (Cyr1), and diminishes the production of cAMP in response to glucose stimulation. Thus, whereas Gpa2 promotes glucose signaling through elevated production of cAMP, Asc1 has opposing effects on these same processes. Our findings reveal the existence of an unusual Gbeta subunit, one having multiple functions within the cell in addition to serving as a signal transducer for cell surface receptors and intracellular effectors.

Intramolecular interaction between the DEP domain of RGS7 and the Gbeta5 subunit

The R7 family of RGS proteins (RGS6, -7, -9, -11) is characterized by the presence of three domains: DEP, GGL, and RGS. The RGS domain interacts with Galpha subunits and exhibits GAP activity. The GGL domain permanently associates with Gbeta5. The DEP domain interacts with the membrane anchoring protein, R7BP. Here we provide evidence for a novel interaction within this complex: between the DEP domain and Gbeta5. GST fusion of the RGS7 DEP domain (GST-R7DEP) binds to both native and recombinant Gbeta5-RGS7, recombinant Gbetagamma complexes, and monomeric Gbeta5 and Gbeta1 subunits. Co-immunoprecipitation and FRET assays supported the GST pull-down experiments. GST-R7DEP reduced FRET between CFP-Gbeta5 and YFP-RGS7, indicating that the DEP-Gbeta5 interaction is dynamic. In transfected cells, R7BP had no effect on the Gbeta5/RGS7 pull down by GST-R7DEP. The DEP domain of RGS9 did not bind to Gbeta5. Substitution of RGS7 Glu-73 and Asp-74 for the corresponding Ser and Gly residues (ED/SG mutation) of RGS9 diminished the DEP-Gbeta5 interaction. In the absence of R7BP both the wild-type RGS7 and the ED/SG mutant attenuated muscarinic M3 receptor-mediated Ca2+ mobilization. In the presence of R7BP, wild-type RGS7 lost this inhibitory activity, whereas the ED/SG mutant remained active. Taken together, our results are consistent with the following model. The Gbeta5-RGS7 molecule can exist in two conformations: "closed" and "open", when the DEP domain and Gbeta5 subunit either do or do not interact. The closed conformation appears to be less active with respect to its effect on Gq-mediated signaling than the open conformation.

A2A adenosine-receptor-mediated facilitation of noradrenaline release in rat tail artery involves protein kinase C activation and betagamma subunits formed after alpha2-adrenoceptor activation

This work aimed to investigate the molecular mechanisms involved in the interaction of alpha2-adrenoceptors and adenosine A2A-receptor-mediated facilitation of noradrenaline release in rat tail artery, namely the type of G-protein involved in this effect and the step or steps where the signalling cascades triggered by alpha2-adrenoceptors and A2A-receptors interact. The selective adenosine A2A-receptor agonist 2-p-(2-carboxy ethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 nM) enhanced tritium overflow evoked by trains of 100 pulses at 5 Hz. This effect was abolished by the selective adenosine A2A-receptor antagonist 5-amino-7-(2-phenyl ethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261; 20 nM) and by yohimbine (1 microM). CGS 21680-mediated effects were also abolished by drugs that disrupted G(i/o)-protein coupling with receptors, PTX (2 microg/ml) or NEM (40 microM), by the anti-G(salpha) peptide (2 microg/ml) anti-G(betagamma) peptide (10 microg/ml) indicating coupling of A2A-receptors to G(salpha) and suggesting a crucial role for G(betagamma) subunits in the A(2A)-receptor-mediated enhancement of tritium overflow. Furthermore, phorbol 12-myristate 13-acetate (PMA; 1 microM) or forskolin (1 microM), direct activators of protein kinase C and of adenylyl cyclase, respectively, also enhanced tritium overflow. In addition, PMA-mediated effects were not observed in the presence of either yohimbine or PTX. Results indicate that facilitatory adenosine A2A-receptors couple to G(salpha) subunits which is essential, but not sufficient, for the release facilitation to occur, requiring the involvement of G(i/o)-protein coupling (it disappears after disruption of G(i/o)-protein coupling, PTX or NEM) and/or G(betagamma) subunits (anti-G(betagamma)). We propose a mechanism for the interaction in study suggesting group 2 AC isoforms as a plausible candidate for the interaction site, as these isoforms can integrate inputs from G(salpha) subunits (released after adenosine A2A-receptor activation; prime-activation), G(betagamma) subunits (released after activation of G(i/o)-protein coupled receptors) which can directly synergistically stimulate the prime-activated AC or indirectly via G(betagamma) activation of the PLC-PKC pathway.

RACK1 regulates Ki-Ras-mediated signaling and morphological transformation of NIH 3T3 cells

Activating Ras mutations are involved in a significant fraction of human tumors. A suppressor screen using a retroviral mouse fibroblast cDNA library was performed to identify novel factors in Ras-mediated transformation. We identified a novel potent inhibitor of Ras-mediated morphological transformation encoded by a truncated version of the receptor for activated C-kinase (RACK1). The truncated protein, designated RACK1DeltaWD1, lacked the N-terminal 49 amino acids encoding the first of the 7 WD40 repeats in RACK1. RACK1DeltaWD1 expression restored contact inhibition, stress fiber formation and reduced ERK phosphorylation in Ki-Ras transformed NIH 3T3 cells. We demonstrate that truncated RACK1 is involved in complexes consisting of wild-type RACK1 and protein kinase C isoforms alpha, betaI and delta, compromising the transduction of an activated Ras signal to the Raf-MEK-ERK pathway. The cellular localization of RACK1DeltaWD1 differed from wtRACK1, indicating that signaling complexes containing the truncated version of RACK1 are incorrectly localized. Notably, 12-O-tetradecanoyl-13-phorbol acetate (TPA) mediated intracellular translocation of RACK1-interacting PKC alpha and delta was abrogated in RACK1DeltaWD1-expressing cells. Our data support a model where RACK1 acts as a key factor in Ki-Ras-mediated morphological transformation.

Gib2, a novel Gbeta-like/RACK1 homolog, functions as a Gbeta subunit in cAMP signaling and is essential in Cryptococcus neoformans

Canonical G proteins are heterotrimeric, consisting of alpha, beta, and gamma subunits. Despite multiple Galpha subunits functioning in fungi, only a single Gbeta subunit per species has been identified, suggesting that non-conventional G protein signaling exists in this diverse group of eukaryotic organisms. Using the Galpha subunit Gpa1 that functions in cAMP signaling as bait in a two-hybrid screen, we have identified a novel Gbeta-like/RACK1 protein homolog, Gib2, from the human pathogenic fungus Cryptococcus neoformans. Gib2 contains a seven WD-40 repeat motif and is predicted to form a seven-bladed beta propeller structure characteristic of beta transducins. Gib2 is also shown to interact, respectively, with two Ggamma subunit homologs, Gpg1 and Gpg2, similar to the conventional Gbeta subunit Gpb1. In contrast to Gpb1 whose overexpression promotes mating response, overproduction of Gib2 suppresses defects of gpa1 mutation in both melanization and capsule formation, the phenotypes regulated by cAMP signaling and associated with virulence. Furthermore, depletion of Gib2 by antisense suppression results in a severe growth defect, suggesting that Gib2 is essential. Finally, Gib2 is shown to also physically interact with a downstream target of Gpa1-cAMP signaling, Smg1, and the protein kinase C homolog Pkc1, indicating that Gib2 is also a multifunctional RACK1-like protein.

G protein beta2 subunit interacts directly with neuropathy target esterase and regulates its activity

Neuropathy target esterase (NTE) was identified as the primary target of organophosphate compounds that cause a delayed neuropathy with degeneration of nerve axons. NTE is a novel phospholipase B anchored to the cytoplasmic face of endoplasmic reticulum and essential for embryonic and nervous development. However, little is known about the regulation of NTE. A human fetal brain cDNA library was screened for proteins that interact with NTE, Gbeta2 and Gbeta2-like I subunits were found to be able to bind the C-terminal of NTE in yeast. The interaction of Gbeta2 and NTE was confirmed by in vivo co-immunoprecipitation analysis in COS7 cells. Furthermore, depletion of Gbeta2 by RNA interference down regulated the activity of NTE but not its expression level. In addition, the activity of NTE was down regulated by the G protein signal pathway influencing factor, pertussis toxin, treatment in vivo. These findings suggest that Gbeta2 may play a significant role in maintaining the activity of NTE.

Palmitoylcarnitine modulates interaction between protein kinase C betaII and its receptor RACK1

Palmitoylcarnitine, known to promote differentiation of neuroblastoma NB-2a cells as well as to inhibit protein kinase C (PKC) activity and to decrease phorbol ester binding, was shown previously to diminish the amount of complex formed between PKCdelta and its substrate GAP-43. In the present work we studied the effect of palmitoylcarnitine on the interaction between PKCbetaII and its receptor RACK1. Palmitoylcarnitine was found to decrease autophosphorylation of PKCbetaII on serine in a concentration-dependent manner and to decrease the amount of PKCbetaII/RACK1 complex. The effect of palmitoylcarnitine on cellular localization was found to be dependent on the presence of ATP; palmitoylcarnitine lowered the amount of PKCbetaII in cytosol and decreased the amount of PKCbetaII-RACK1 complex in membrane in the absence of ATP. Palmitoylcarnitine also reversed the effect of phorbol ester on the increase in the amount of PKCbetaII in membrane. Palmitoylcarnitine binds to PKCbetaII through hydrophobic interactions, although acylation of PKCbetaII by the palmitate moiety has been excluded. The presence of palmitoylcarnitine did not have any additive effect on the diminution of PKCbetaII-RACK1 complex formation in the presence of a RACK1-binding peptide from within the C2 region of PKCbetaII. These results rather exclude a possibility of interaction of palmitoylcarnitine with the C2 domain and suggest a possible interaction with the V5 domain and a conformational change affecting the C1 region.

Genome-scale analysis reveals Sst2 as the principal regulator of mating pheromone signaling in the yeast Saccharomyces cerevisiae

A common property of G protein-coupled receptors is that they become less responsive with prolonged stimulation. Regulators of G protein signaling (RGS proteins) are well known to accelerate G protein GTPase activity and do so by stabilizing the transition state conformation of the G protein alpha subunit. In the yeast Saccharomyces cerevisiae there are four RGS-homologous proteins (Sst2, Rgs2, Rax1, and Mdm1) and two Galpha proteins (Gpa1 and Gpa2). We show that Sst2 is the only RGS protein that binds selectively to the transition state conformation of Gpa1. The other RGS proteins also bind Gpa1 and modulate pheromone signaling, but to a lesser extent and in a manner clearly distinct from Sst2. To identify other candidate pathway regulators, we compared pheromone responses in 4,349 gene deletion mutants representing nearly all nonessential genes in yeast. A number of mutants produced an increase (sst2, bar1, asc1, and ygl024w) or decrease (cla4) in pheromone sensitivity or resulted in pheromone-independent signaling (sst2, pbs2, gas1, and ygl024w). These findings suggest that Sst2 is the principal regulator of Gpa1-mediated signaling in vivo but that other proteins also contribute in distinct ways to pathway regulation.