GTP-dependent segregation of H-ras from lipid rafts is required for biological activity

A single method for cryofixation and correlative light, electron microscopy and tomography of zebrafish embryos

The zebrafish is a powerful vertebrate system for cell and developmental studies. In this study, we have optimized methods for fast freezing and processing of zebrafish embryos for electron microscopy (EM). We show that in the absence of primary chemical fixation, excellent ultrastructure, preservation of green fluorescent protein (GFP) fluorescence, immunogold labelling and electron tomography can be obtained using a single technique involving high-pressure freezing and embedding in Lowicryl resins at low temperature. As well as being an important new tool for zebrafish research, the maintenance of GFP fluorescence after fast freezing, freeze substitution and resin embedding will be of general use for correlative light and EM of biological samples.

Identification of G protein alpha subunit-palmitoylating enzyme

The heterotrimeric G protein alpha subunit (Galpha) is targeted to the cytoplasmic face of the plasma membrane through reversible lipid palmitoylation and relays signals from G-protein-coupled receptors (GPCRs) to its effectors. By screening 23 DHHC motif (Asp-His-His-Cys) palmitoyl acyl-transferases, we identified DHHC3 and DHHC7 as Galpha palmitoylating enzymes. DHHC3 and DHHC7 robustly palmitoylated Galpha(q), Galpha(s), and Galpha(i2) in HEK293T cells. Knockdown of DHHC3 and DHHC7 decreased Galpha(q/11) palmitoylation and relocalized it from the plasma membrane into the cytoplasm. Photoconversion analysis revealed that Galpha(q) rapidly shuttles between the plasma membrane and the Golgi apparatus, where DHHC3 specifically localizes. Fluorescence recovery after photobleaching studies showed that DHHC3 and DHHC7 are necessary for this continuous Galpha(q) shuttling. Furthermore, DHHC3 and DHHC7 knockdown blocked the alpha(1A)-adrenergic receptor/Galpha(q/11)-mediated signaling pathway. Together, our findings revealed that DHHC3 and DHHC7 regulate GPCR-mediated signal transduction by controlling Galpha localization to the plasma membrane.

Activation of the MAPK module from different spatial locations generates distinct system outputs

The Ras/Raf/MEK/ERK (MAPK) pathway directs multiple cell fate decisions within a single cell. How different system outputs are generated is unknown. Here we explore whether activating the MAPK module from different membrane environments can rewire system output. We identify two classes of nanoscale environment within the plasma membrane. The first, which corresponds to nanoclusters occupied by GTP-loaded H-, N- or K-Ras, supports Raf activation and amplifies low Raf kinase input to generate a digital ERKpp output. The second class, which corresponds to nanoclusters occupied by GDP-loaded Ras, cannot activate Raf and therefore does not activate the MAPK module, illustrating how lateral segregation on plasma membrane influences signal output. The MAPK module is activated at the Golgi, but in striking contrast to the plasma membrane, ERKpp output is analog. Different modes of Raf activation precisely correlate with these different ERKpp system outputs. Intriguingly, the Golgi contains two distinct membrane environments that generate ERKpp, but only one is competent to drive PC12 cell differentiation. The MAPK module is not activated from the ER. Taken together these data clearly demonstrate that the different nanoscale environments available to Ras generate distinct circuit configurations for the MAPK module, bestowing cells with a simple mechanism to generate multiple system outputs from a single cascade.

Clinical target promiscuity: lessons from ras molecular trials

Mutated ras has been identified in approximately 30% of human tumors, and dysregulation of ras function and signal transduction pathways is a critical step in tumorigenesis. Herein, we review the early data that supports the concept that the intrinsic radiosensitivity of tumor cells can be altered by oncogenic ras expression and that this impacts the PI3K-dependent signaling cascade. This ras-induced radioresistance can be reversed using prenyl transferase inhibitors (PTIs.). We discuss the effects of PTIs as a radiosensitizer in both in vivo and in vitro studies and show that PTIs can lead to increased radiosensitization in vivo through a variety of potential mechanisms that enhance radiation-induced cell kill. We critically evaluate the use of ras biomarkers in predicting the clinical response to PTIs that may explain the mixed results seen thus far in clinical trials using PTIs as a clinical radiosensitizer. We conclude that Ras-mediated radioresistance is the result of multiple intercommunicating pathways functioning against a complex genetic background and a solitary biomarker may not be adequate to predict for PTI-mediated radiosensitization. Nonetheless, our knowledge of the ras-signaling pathway has led to development and testing of specific therapies directed against PI3K-AKT signaling pathways as a future approach towards clinical radiosensitization.

Mechanisms of Ras membrane organization and signalling: Ras on a rocker

Understanding the signalling function of Ras GTPases has been the focus of much research for over 20 years. Both the catalytic domain and the membrane anchoring C terminal hypervariable region (HVR) of Ras are necessary for its cellular function. However, while the highly conserved catalytic domain has been characterized in atomic detail, the structure of the full-length membrane-bound Ras has remained elusive. Lack of structural knowledge on the full-length protein limited our understanding of Ras signalling. For example, structures of the Ras catalytic domain solved in complex with effectors do not provide a basis for the functional specificity of different Ras isoforms. Recent molecular dynamics simulations in combination with biophysical and cell biological experiments have shown that the HVR and parts of the G domain cofunction with the lipid tails to anchor H-ras to the plasma membrane. In the GTP-bound state, H-ras adopts an orientation that allows read out by Ras effectors and translation into corresponding MAPK signalling. Here we discuss details of an analysis that suggests a novel balance model for Ras functioning. The balance model rationalizes Ras membrane orientation and may help explain isoform specific interactions of Ras with its effectors and modulators.

Uptake of long chain fatty acids is regulated by dynamic interaction of FAT/CD36 with cholesterol/sphingolipid enriched microdomains (lipid rafts)

Background

Mechanisms of long chain fatty acid uptake across the plasma membrane are important targets in treatment of many human diseases like obesity or hepatic steatosis. Long chain fatty acid translocation is achieved by a concert of co-existing mechanisms. These lipids can passively diffuse, but certain membrane proteins can also accelerate the transport. However, we now can provide further evidence that not only proteins but also lipid microdomains play an important part in the regulation of the facilitated uptake process.

Methods

Dynamic association of FAT/CD36 a candidate fatty acid transporter with lipid rafts was analysed by isolation of detergent resistant membranes (DRMs) and by clustering of lipid rafts with antibodies on living cells. Lipid raft integrity was modulated by cholesterol depletion using methyl-β-cyclodextrin and sphingolipid depletion using myriocin and sphingomyelinase. Functional analyses were performed using an [3H]-oleate uptake assay.

Results

Overexpression of FAT/CD36 and FATP4 increased long chain fatty acid uptake. The uptake of long chain fatty acids was cholesterol and sphingolipid dependent. Floating experiments showed that there are two pools of FAT/CD36, one found in DRMs and another outside of these domains. FAT/CD36 co-localized with the lipid raft marker PLAP in antibody-clustered domains at the plasma membrane and segregated away from the non-raft marker GFP-TMD. Antibody cross-linking increased DRM association of FAT/CD36 and accelerated the overall fatty acid uptake in a cholesterol dependent manner. Another candidate transporter, FATP4, was neither present in DRMs nor co-localized with FAT/CD36 at the plasma membrane.

Conclusion

Our observations suggest the existence of two pools of FAT/CD36 within cellular membranes. As increased raft association of FAT/CD36 leads to an increased fatty acid uptake, dynamic association of FAT/CD36 with lipid rafts might regulate the process. There is no direct interaction of FATP4 with lipid rafts or raft associated FAT/CD36. Thus, lipid rafts have to be considered as targets for the treatment of lipid disorders.

Compartmentalised MAPK pathways

The mitogen-activated protein kinase (MAPK) pathway provides cells with the means to interpret external signal cues or conditions, and respond accordingly. This cascade regulates many cell functions such as differentiation, proliferation and migration. Through modulation of both the amplitude and duration of MAPK signalling, cells can control their responses to the multiple activators of the pathway. In addition, recent work has highlighted the importance of the cellular compartment from which the signalling occurs. Cells have developed intricate systems that enable them to localise MAPK components to specific subcellular domains in response to a particular stimulus. Consequently, different factors can activate the same kinase in separate locations. Crucial to this ability are molecular scaffolds, which act as signalling modules for MAPKs, confining them to the desired compartment. The participation of the MAPK network in fundamental physiological processes, such as cell proliferation and inflammation, and the derangement of the homeostasis that occurs in disease processes, renders MAPK a highly desirable target for therapeutic intervention. As we enhance our comprehension of scaffolds and other regulatory molecules, novel targets for drug design may be discovered that will afford selective and specific MAPK modulation.

GFP-tagged proteins visualized by freeze-fracture immuno-electron microscopy: a new tool in cellular and molecular medicine

GFP-tagging is widely used as a molecular tool to localize and visualize the trafficking of proteins in cells but interpretation is frequently limited by the low resolution afforded by fluorescence light microscopy. Although complementary thin-section immunogold electron microscopic techniques go some way in aiding interpretation, major limitations, such as relatively poor structural preservation of membrane systems, low labelling efficiency and the two-dimensional nature of the images, remain. Here we demonstrate that the electron microscopic technique freeze-fracture replica immunogold labelling overcomes these disadvantages and can be used to define, at high resolution, the precise location of GFP-tagged proteins in specific membrane systems and organelles of the cell. Moreover, this technique provides information on the location of the protein within the phospholipid bilayer, potentially providing insight into mis-orientation of tagged proteins compared to their untagged counterparts. Complementary application of the freeze-fracture replica immunogold labelling technique alongside conventional fluorescence microscopy is seen as a novel and valuable approach to verification, clarification and extension of the data obtained using fluorescent-tagged proteins. The application of this approach is illustrated by new findings on PAT-family proteins tagged with GFP transfected into fibroblasts from patients with Niemann-Pick type C disease.

The potential for treatment with dietary long-chain polyunsaturated n-3 fatty acids during chemotherapy

Dietary intake of long-chain omega-3 (or n-3) polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) can affect numerous processes in the body, including cardiovascular, neurological and immune functions, as well as cancer. Studies on human cancer cell lines, animal models and preliminary trials with human subjects suggest that administration of EPA and DHA, found naturally in our diet in fatty fish, can alter toxicities and/or activity of many drugs used to treat cancer. Multiple mechanisms are proposed to explain how n-3 PUFA modulate the tumor cell response to chemotherapeutic drugs. n-3 PUFA are readily incorporated into cell membranes and lipid rafts, and their incorporation may affect membrane-associated signaling proteins such as Ras, Akt and Her-2/neu. Due to their high susceptibility to oxidation, it has also been proposed that n-3 PUFA may cause irreversible tumor cell damage through increased lipid peroxidation. n-3 PUFA may increase tumor cell susceptibility to apoptosis by altering expression or function of apoptotic proteins, or by modulating activity of survival-related transcription factors such as nuclear factor-kappaB. Some studies suggest n-3 PUFA may increase drug uptake or even enhance drug activation (e.g., in the case of some nucleoside analogue drugs). Further research is warranted to identify specific mechanisms by which n-3 PUFA increase chemotherapy efficacy and to determine the optimal cellular/membrane levels of n-3 PUFA required to promote these mechanisms, such that these fatty acids may be prescribed as adjuvants to chemotherapy.

Membrane interactions of G proteins and other related proteins

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

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Mapping the nucleotide and isoform-dependent structural and dynamical features of Ras proteins

Ras GTPases are conformational switches controlling cell proliferation, differentiation, and development. Despite their prominent role in many forms of cancer, the mechanism of conformational transition between inactive GDP-bound and active GTP-bound states remains unclear. Here we describe a detailed analysis of available experimental structures and molecular dynamics simulations to quantitatively assess the structural and dynamical features of active and inactive states and their interconversion. We demonstrate that GTP-bound and nucleotide-free G12V H-ras sample a wide region of conformational space, and show that the inactive-to-active transition is a multiphase process defined by the relative rearrangement of the two switches and the orientation of Tyr32. We also modeled and simulated N- and K-ras proteins and found that K-ras is more flexible than N- and H-ras. We identified a number of isoform-specific, long-range side chain interactions that define unique pathways of communication between the nucleotide binding site and the C terminus.

Ras isoform abundance and signalling in human cancer cell lines

The ubiquitously expressed major Ras isoforms: H-, K- and N-Ras, are highly conserved, yet exhibit different biological outputs. We have compared the relative efficiencies with which epidermal or hepatocyte growth factor activates Ras isoforms and the requirement for specific isoforms in the activation of downstream pathways. We find that the relative coupling efficiencies to each Ras isoform are conserved between stimuli. Furthermore, in both cases, inhibition of receptor endocytosis led to reduced N- and H-Ras activation, but K-Ras was unaffected. Acute knockdown of each isoform with siRNA allows endogenous Ras isoform function and abundance to be probed. This revealed that there is significant variation in the contribution of individual isoforms to total Ras across a panel of cancer cell lines although typically K> or =N>>H. Intriguingly, cancer cell lines where a significant fraction of endogenous Ras is oncogenically mutated showed attenuated activation of canonical Ras effector pathways. We profiled the contribution of each Ras isoform to the total Ras pool allowing interpretation of the effect of isoform-specific knockdown on signalling outcomes. In contrast to previous studies indicating preferential coupling of isoforms to Raf and PtdIns-3-kinase pathways, we find that endogenous Ras isoforms show no specific coupling to these major Ras pathways.

High-resolution 3D quantitative analysis of caveolar ultrastructure and caveola-cytoskeleton interactions

Caveolae are characteristic invaginations of the mammalian plasma membrane (PM) implicated in lipid regulation, signal transduction and endocytosis. We have employed electron microscope tomography (ET) to quantify caveolae structure-function relationships in three-dimension (3D) at high resolution both in conventionally fixed and in fast-frozen/freeze-substituted (intact) cells as well as immunolabelled PM lawns. Our findings provide a detailed quantitative comparison of the average caveola dimensions for different cell types including tissue endothelial cells and cultured 3T3-L1 adipocytes. These studies revealed the presence of a spiked caveolar coat and a wide caveolar neck open to the extracellular milieu that is sensitive to conventional fixation; the neck region appeared to form a specialized microdomain with associated cytoplasmic material. In endothelial cells in situ in pancreatic islets of Langerhans, the diaphragm spanning the caveolar opening was clearly resolved by ET, and the involuted 3D topology of the cell surface mapped to measure the contribution of caveolar membranes to local increases in the surface area of the PM. The complexity of connections among caveolae and to the actin cytoskeleton and microtubules suggests that individual caveolae may be interconnected through a complex filamentous network to form a single functional unit.

Galectin-1 is a novel structural component and a major regulator of h-ras nanoclusters

The organization of Ras proteins into nanoclusters on the inner plasma membrane is essential for Ras signal transduction, but the mechanisms that drive nanoclustering are unknown. Here we show that epidermal growth factor receptor activation stimulates the formation of H-Ras.GTP-Galectin-1 (Gal-1) complexes on the plasma membrane that are then assembled into transient nanoclusters. Gal-1 is therefore an integral structural component of the H-Ras-signaling nanocluster. Increasing Gal-1 levels increases the stability of H-Ras nanoclusters, leading to enhanced effector recruitment and signal output. Elements in the H-Ras C-terminal hypervariable region and an activated G-domain are required for H-Ras-Gal-1 interaction. Palmitoylation is not required for H-Ras-Gal-1 complex formation, but is required to anchor H-Ras-Gal-1 complexes to the plasma membrane. Our data suggest a mechanism for H-Ras nanoclustering that involves a dual role for Gal-1 as a critical scaffolding protein and a molecular chaperone that contributes to H-Ras trafficking by returning depalmitoylated H-Ras to the Golgi complex for repalmitoylation.

Caveolin regulates endocytosis of the muscle repair protein, dysferlin

Dysferlin and Caveolin-3 are plasma membrane proteins associated with muscular dystrophy. Patients with mutations in the CAV3 gene show dysferlin mislocalization in muscle cells. By utilizing caveolin-null cells, expression of caveolin mutants, and different mutants of dysferlin, we have dissected the site of action of caveolin with respect to dysferlin trafficking pathways. We now show that Caveolin-1 or -3 can facilitate exit of a dysferlin mutant that accumulates in the Golgi complex of Cav1(-/-) cells. In contrast, wild type dysferlin reaches the plasma membrane but is rapidly endocytosed in Cav1(-/-) cells. We demonstrate that the primary effect of caveolin is to cause surface retention of dysferlin. Caveolin-1 or Caveolin-3, but not specific caveolin mutants, inhibit endocytosis of dysferlin through a clathrin-independent pathway colocalizing with internalized glycosylphosphatidylinositol-anchored proteins. Our results provide new insights into the role of this endocytic pathway in surface remodeling of specific surface components. In addition, they highlight a novel mechanism of action of caveolins relevant to the pathogenic mechanisms underlying caveolin-associated disease.

A novel switch region regulates H-ras membrane orientation and signal output

The plasma membrane nanoscale distribution of H-ras is regulated by guanine nucleotide binding. To explore the structural basis of H-ras membrane organization, we combined molecular dynamic simulations and medium-throughput FRET measurements on live cells. We extracted a set of FRET values, termed a FRET vector, to describe the lateral segregation and orientation of H-ras with respect to a large set of nanodomain markers. We show that mutation of basic residues in helix alpha4 or the hypervariable region (HVR) selectively alter the FRET vectors of GTP- or GDP-loaded H-ras, demonstrating a critical role for these residues in stabilizing GTP- or GDP-H-ras interactions with the plasma membrane. By a similar analysis, we find that the beta2-beta3 loop and helix alpha5 are involved in a novel conformational switch that operates through helix alpha4 and the HVR to reorient the H-ras G-domain with respect to the plasma membrane. Perturbation of these switch elements enhances MAPK activation by stabilizing GTP-H-ras in a more productive signalling conformation. The results illustrate how the plasma membrane spatially constrains signalling conformations by acting as a semi-neutral interaction partner.

Altered localization of H-Ras in caveolin-1-null cells is palmitoylation-independent

Caveolin-1 is a palmitoylated protein involved in the formation of plasma membrane subdomains termed caveolae, intracellular cholesterol transport, and assembly and regulation of signaling molecules in caveolae. Caveolin-1 interacts via a consensus binding motif with several signaling proteins, including H-Ras. Ras oncogene products function as molecular switches in several signal transduction pathways regulating cell growth and differentiation. Post-translational modifications, including palmitoylation, are critical for the membrane targeting and function of H-Ras. Subcellular localization regulates the signaling pathways engaged by H-Ras activation. We show here that H-Ras is localized at the plasma membrane in caveolin-1-expressing cells but not in caveolin-1-deficient cells. Since palmitoylation is required for trafficking of H-Ras from the endomembrane system to the plasma membrane, we tested whether the altered localization of H-Ras in caveolin-1-null cells is due to decreased H-Ras palmitoylation. Although the palmitoylation profiles of cultured embryo fibroblasts isolated from wild type and caveolin-1 gene-disrupted mice differed, suggesting that caveolin-1, or caveolae, play a role in the palmitate incorporation of a subset of palmitoylated proteins, the palmitoylation of H-Ras was not decreased in caveolin-1-null cells. We conclude that the altered localization of H-Ras in caveolin-1-deficient cells is palmitoylation-independent. This article shows two important new mechanisms by which loss of caveolin-1 expression may perturb intracellular signaling, namely the mislocalization of signaling proteins and alterations in protein palmitoylation.

Spatial control of Rho (Rac-Rop) signaling in tip-growing plant cells

Spatially restricted signaling by Rho GTPases is essential for the polarization of eukaryotic cells, which is required for the morphogenesis, mobility and division of single cells, and for the development of multicellular organisms. Rac-Rop GTPases, which constitute a plant-specific Rho GTPase subfamily, accumulate at the apical plasma membrane of pollen tubes and root hairs, where they control rapid polar cell expansion by a process known as tip growth. Here, recent insights into the spatial control of Rac-Rop-dependent signaling in tip-growing plant cells by regulatory proteins (i.e. Rho GTPase-activating proteins, Rho guanine nucleotide dissociation inhibitors, Rho guanine nucleotide-exchange factors and phosphoinositide-specific phospholipase C) and lipids [phosphatidylinositol (4,5)-bisphosphate and diacyl glycerol] are summarized. A model is presented, which integrates the current knowledge concerning the molecular mechanisms that maintain the polarization of Rho signaling in plant cells.

Membranes: a meeting point for lipids, proteins and therapies

Membranes constitute a meeting point for lipids and proteins. Not only do they define the entity of cells and cytosolic organelles but they also display a wide variety of important functions previously ascribed to the activity of proteins alone. Indeed, lipids have commonly been considered a mere support for the transient or permanent association of membrane proteins, while acting as a selective cell/organelle barrier. However, mounting evidence demonstrates that lipids themselves regulate the location and activity of many membrane proteins, as well as defining membrane microdomains that serve as spatio-temporal platforms for interacting signalling proteins. Membrane lipids are crucial in the fission and fusion of lipid bilayers and they also act as sensors to control environmental or physiological conditions. Lipids and lipid structures participate directly as messengers or regulators of signal transduction. Moreover, their alteration has been associated with the development of numerous diseases. Proteins can interact with membranes through lipid co-/post-translational modifications, and electrostatic and hydrophobic interactions, van der Waals forces and hydrogen bonding are all involved in the associations among membrane proteins and lipids. The present study reviews these interactions from the molecular and biomedical point of view, and the effects of their modulation on the physiological activity of cells, the aetiology of human diseases and the design of clinical drugs. In fact, the influence of lipids on protein function is reflected in the possibility to use these molecular species as targets for therapies against cancer, obesity, neurodegenerative disorders, cardiovascular pathologies and other diseases, using a new approach called membrane-lipid therapy.

Docosahexaenoic acid alters the size and distribution of cell surface microdomains

We recently generated nutritional data suggesting that chemoprotective dietary n-3 polyunsaturated fatty acids (n-3 PUFA) are capable of displacing acylated proteins from lipid raft microdomains in vivo [D.W. Ma, J. Seo, L.A. Davidson, E.S. Callaway, Y.Y. Fan, J.R. Lupton, R.S. Chapkin, n-3 PUFA alter caveolae lipid composition and resident protein localization in mouse colon, FASEB J. 18 (2004) 1040-1042; Y.Y. Fan, L.H. Ly, R. Barhoumi, D.N. McMurray, R.S. Chapkin, Dietary docosahexaenoic acid suppresses T cell protein kinase Ctheta lipid raft recruitment and IL-2 recruitment, J. Immunol. 173 (2004) 6151-6160]. A primary source of very long chain n-3 PUFA in the diet is derived from fish enriched with docosahexaenoic acid (DHA, 22:6n-3). In this study, we sought to determine the effect of DHA on cell surface microdomain organization in situ. Using immuno-gold electron microscopy of plasma membrane sheets coupled with spatial point analysis of validated microdomain markers, morphologically featureless microdomains were visualized in HeLa cells at high resolution. Clustering of probes within cholesterol-dependent (GFP-tH) versus cholesterol-independent (GFP-tK) nanoclusters was differentially sensitive to n-3 PUFA treatment of cells. Univariate K-function analysis of GFP-tH (5 nm gold) revealed a significant increase in clustering (p<0.05) by pre-treatment with DHA and linoleic acid (LA, 18:2(Delta9,12)) compared to control fatty acids; whereas LA significantly (p<0.05) reduced GFP-tK clustering. These novel data suggest that the plasma membrane organization of inner leaflets is fundamentally altered by PUFA-enrichment. We speculate that our findings may help define a new paradigm to better understand the complexity of n-3 PUFA modulation of signaling networks.

A palmitoylation cycle dynamically regulates partitioning of the GABA-synthesizing enzyme GAD65 between ER-Golgi and post-Golgi membranes

GAD65, the smaller isoform of the enzyme glutamic acid decarboxylase, synthesizes GABA for fine-tuning of inhibitory neurotransmission. GAD65 is synthesized as a soluble hydrophilic protein but undergoes a hydrophobic post-translational modification and becomes anchored to the cytosolic face of Golgi membranes. A second hydrophobic modification, palmitoylation of Cys30 and Cys45 in GAD65, is not required for the initial membrane anchoring but is crucial for post-Golgi trafficking of the protein to presynaptic clusters. The mechanism by which palmitoylation directs targeting of GAD65 through and out of the Golgi complex is unknown. Here, we show that prior to palmitoylation, GAD65 anchors to both ER and Golgi membranes. Palmitoylation, however, clears GAD65 from the ER-Golgi, targets it to the trans-Golgi network and then to a post-Golgi vesicular pathway. FRAP analyses of trafficking of GAD65-GFP reveal a rapid and a slow pool of protein replenishing the Golgi complex. The rapid pool represents non-palmitoylated hydrophobic GAD65-GFP, which exchanges rapidly between the cytosol and ER/Golgi membranes. The slow pool represents palmitoylation-competent GAD65-GFP, which replenishes the Golgi complex via a non-vesicular pathway and at a rate consistent with a depalmitoylation step. We propose that a depalmitoylation-repalmitoylation cycle serves to cycle GAD65 between Golgi and post-Golgi membranes and dynamically control levels of enzyme directed to the synapse.

Palmitoylation and localisation of RAS isoforms are modulated by the hypervariable linker domain

RAS isoforms have been proposed to exhibit differing biological outputs due to differences in their relative occupancy of cellular organelles and signalling microdomains. The membrane binding and targeting motifs of RAS are encoded by the C-terminal hypervariable region (HVR), and the precise localisation depends upon interactions between the HVR and the host membrane. Classic studies revealed that all RAS proteins rely on farnesylation and either palmitoylation or a polybasic stretch for stable binding to membranes. We now show that, for N-RAS and Ki-RAS4A, mono-palmitoylation and farnesylation are not sufficient for specifying stable cell-surface localisation. A third motif that is present within the linker domain of all palmitoylated RAS HVRs is necessary for stabilising localisation to the plasma membrane. This motif comprises acidic residues that stabilise palmitoylation and basic amino acids that are likely to interact electrostatically with acidic phospholipids enriched at the cell surface. Importantly, altered localisation is achieved without changes in palmitoylation status. Our data provide a mechanism for distinct HVR membrane interactions controlling subcellular distribution. In the context of the full-length RAS proteins, this is likely to be of crucial importance for controlling signalling output and engagement with different pools of effectors.

Rasl11b knock down in zebrafish suppresses one-eyed-pinhead mutant phenotype

The EGF-CFC factor Oep/Cripto1/Frl1 has been implicated in embryogenesis and several human cancers. During vertebrate development, Oep/Cripto1/Frl1 has been shown to act as an essential coreceptor in the TGFβ/Nodal pathway, which is crucial for germ layer formation. Although studies in cell cultures suggest that Oep/Cripto1/Frl1 is also implicated in other pathways, in vivo it is solely regarded as a Nodal coreceptor. We have found that Rasl11b, a small GTPase belonging to a Ras subfamily of putative tumor suppressor genes, modulates Oep function in zebrafish independently of the Nodal pathway. rasl11b down regulation partially rescues endodermal and prechordal plate defects of zygotic oep^−/− mutants (Zoep). Rasl11b inhibitory action was only observed in oep-deficient backgrounds, suggesting that normal oep expression prevents Rasl11b function. Surprisingly, rasl11b down regulation does not rescue mesendodermal defects in other Nodal pathway mutants, nor does it influence the phosphorylation state of the downstream effector Smad2. Thus, Rasl11b modifies the effect of Oep on mesendoderm development independently of the main known Oep output: the Nodal signaling pathway. This data suggests a new branch of Oep signaling that has implications for germ layer development, as well as for studies of Oep/Frl1/Cripto1 dysfunction, such as that found in tumors.

Caveolae as organizers of pharmacologically relevant signal transduction molecules

Caveolae, a subset of membrane (lipid) rafts, are flask-like invaginations of the plasma membrane that contain caveolin proteins, which serve as organizing centers for cellular signal transduction. Caveolins (-1, -2, and -3) have cytoplasmic N and C termini, palmitolylation sites, and a scaffolding domain that facilitates interaction and organization of signaling molecules so as to help provide coordinated and efficient signal transduction. Such signaling components include upstream entities (e.g., G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and steroid hormone receptors) and downstream components (e.g., heterotrimeric and low-molecular-weight G proteins, effector enzymes, and ion channels). Diseases associated with aberrant signaling may result in altered localization or expression of signaling proteins in caveolae. Caveolin-knockout mice have numerous abnormalities, some of which may reflect the impact of total body knockout throughout the life span. This review provides a general overview of caveolins and caveolae, signaling molecules that localize to caveolae, the role of caveolae/caveolin in cardiac and pulmonary pathophysiology, pharmacologic implications of caveolar localization of signaling molecules, and the possibility that caveolae might serve as a therapeutic target.

The Ras inhibitor farnesylthiosalicylic acid (Salirasib) disrupts the spatiotemporal localization of active Ras: a potential treatment for cancer

Chronic activation of Ras proteins by mutational activation or by growth factor stimulation is a common occurrence in many human cancers and was shown to induce and be required for tumor growth. Even if additional genetic defects are present, "correction" of the Ras defect has been shown to reverse Ras-dependent tumorigenesis. One way to block Ras protein activity is by interfering with their spatiotemporal localization in cellular membranes or in membrane microdomains, a prerequisite for Ras signaling and biological activity. Detailed reports describe the use of this method in studies employing farnesylthiosalicylic acid (FTS, Salirasib), a Ras farnesylcysteine mimetic, which selectively disrupts the association of chronically active Ras proteins with the plasma membrane. FTS competes with Ras for binding to Ras-escort proteins, which possess putative farnesyl-binding domains and interact only with the activated form of Ras proteins, thereby promoting Ras nanoclusterization in the plasma membrane and robust signals. This chapter presents three-dimensional time-lapse images that track the FTS-induced inhibition of membrane-activated Ras in live cells on a real-time scale. It also describes a mechanistic model that explains FTS selectivity toward activated Ras. Selective blocking of activated Ras proteins results in the inhibition of Ras transformation in vitro and in animal models, with no accompanying toxicity. Phase I clinical trials have demonstrated a safe profile for oral FTS, with minimal side effects and promising activity in hematological malignancies. Salirasib is currently undergoing trials in patients with pancreatic cancer and with nonsmall cell lung cancer, with or without identified K-Ras mutations. The findings might indicate whether with the disruption of the spatiotemporal localization of oncogenic Ras proteins and the targeting of prenyl-binding domains by anticancer drugs is worth developing as a means of cancer treatment.

G-protein-coupled receptor-signaling components in membrane raft and caveolae microdomains

The efficiency of signal transduction in cells derives in part from subcellular, in particular plasma membrane, microdomains that organize signaling molecules and signaling complexes. Two related plasma membrane domains that compartmentalize G-protein coupled receptor (GPCR) signaling complexes are lipid (membrane) rafts, domains that are enriched in certain lipids, including cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that are enriched in the protein caveolin. This review focuses on the properties of lipid rafts and caveolae, the mechanisms by which they localize signaling molecules and the identity of GPCR signaling components that are organized in these domains.

Conformational plasticity and navigation of signaling proteins in antigen-activated B lymphocytes

Over the past two decades our view of the B cell antigen receptor (BCR) has fundamentally changed. Being initially regarded as a mute antibody orphan of the B cell surface, the BCR turned out to be a complex multimolecular machine monitoring almost all stages of B cell development, selection, and activation through a plethora of ubiquitously and cell-type-specific effector proteins. A comprehensive understanding of the many BCR signaling facets is still out but a few common biochemical principles outlined in this review operate at the level of receptor activation and orchestrate specific wiring of intracellular transducer cascades. First, initiation and processing of antigen-induced signal transduction relies on transient conformational changes in the signaling proteins to trigger their physical interaction with downstream elements. Second, this dynamic assembly of signalosomes occurs at distinct subcellular locations, most prominently the plasma membrane, which requires dynamic relocalization of one or more of the engaged molecules. For both, precise complex formation and efficient subcellular targeting, B cell signaling components are equipped with a variety of protein interaction domains. Here we provide an overview on how these simple rules are applied by a limited number of transmembrane and cytosolic proteins to convert BCR ligation into Ca(2+) mobilization and Ras activation in an adjustable manner.

Dissecting lipid raft facilitated cell signaling pathways in cancer

Cancer is one of the most devastating disorders in our lives. Higher rate of proliferation than death of cells is one of the essential factors for development of cancer. The dynamicity of cell membrane plays some vital roles in cell survival and cell death, including protection, endocytosis, signaling, and increases in mechanical stability during cell division, as well as decrease of shear forces during separation of two cells after division, and cell separation from tissues for cancer metastasis. Within the membrane, there are specialized domains, known as lipid rafts. A raft can coordinate various signaling pathways. Recent data on the proteomics of lipid rafts/caveolae have highlighted the enigmatic role of various signaling proteins in cancer development. Analysis of these data of raft proteome from various tumors, cancer tissues, and cell lines cultured without and with therapeutic agents, as well as from model rafts revealed that there may be two subsets of raft assemblage in cell membrane. One subset of raft is enriched with cholesterol-sphingomyeline-ganglioside-cav-1/Src/EGFR (hereafter, "chol-raft") that is involved in normal cell signaling, and when dysregulated promotes cell transformation and tumor progression; another subset of raft is enriched with ceramide-sphingomyeline-ganglioside-FAS/Ezrin (hereafter, "cer-raft") that generally promotes apoptosis. In view of this, and to focus insight into the cancer cell physiology caused by the lipid rafts mediated signals and their receptors, and the downstream transmitters, either proliferative (for example, EGF and EGFR) or death-inducing (for example, FASL and FAS), and the precise roles of some therapeutic drugs and endogenous acid sphingomylenase in this scenario in in situ transformation of "chol-raft" into "cer-raft" are summarized and discussed in this contribution.

Partial functional overlap of the three ras genes in mouse embryonic development

In mammals, three ras genes, H-ras, N-ras and K-ras, encode homologous but distinct 21-kDa Ras proteins. We examined the in vivo functional relationship of the three ras genes in mouse embryonic development by investigating the phenotypes of mice deficient in one or multiple ras genes. H-ras-/- mice and N-ras-/- mice as well as a substantial proportion of H-ras-/-/N-ras-/- mice expressing only the K-ras gene were viable, while K-ras-/- mice were embryonically lethal, as have been reported previously. N-ras-/-/K-ras+/- mice died neonatally, while H-ras-/-/K-ras-/- embryos died much earlier than K-ras homozygous mutant fetuses. To further investigate the functional relationship of the ras genes in embryonic development, we introduced a human H-ras transgene into single or multiple ras mutant mice and found that the transgene rescued mice, including triple ras mutants, from embryonic lethality in association with correction of thin ventricular walls of the heart in null K-ras mutant mice. In situ hybridization revealed that the expression of the H-ras transgene on embryonic day E13.5 and E15.5 was more intense in major organs, including the heart, than those of endogenous ras genes. We therefore conclude that the functions of the ras genes are partially overlapping in mouse embryonic development.

Lipid raft specific knockdown of SRC family kinase activity inhibits cell adhesion and cell cycle progression of breast cancer cells

Src family kinase (SFK) is known to control various cell functions, but the significance of the location of its activation was largely unknown. We herein revealed that SFK activation occurs in lipid rafts. Based on this finding, we have developed a lipid raft-targeted SFK inhibitory fusion protein (LRT-SIFP) that inhibits the SFK activity in lipid rafts. LRT-SIFP has a peptide inhibitor of SFK and a lipid raft-targeting sequence in which two cysteine residues are palmitoylated for clustering in lipid rafts. LRT-SIFP was found to inhibit cell adhesion and cell cycle progression of human breast cancer cell lines MCF-7 and MDA-MB231. On the other hand, the cell functions of MCF-7 cells were found to be not affected with a previously developed peptide inhibitor of SFK that lacks the lipid raft-targeting sequence. In addition, when we replaced the targeting sequence of LRT-SIFP with the consensus sequence for geranylgeranylation to make LRT-SIFP unable to cluster in lipid rafts, this mutated LRT-SIFP did not show any effect on the above cell functions of MCF-7 cells. Furthermore, in contrast to the breast cancer cell lines, LRT-SIFP did not show any inhibitory effect on cell adhesion and cell cycle progression of human normal cell line HEK293. The present lipid raft-specific knockdown of SFK activity would potentially be useful for selective cancer therapy to prevent tumorigenesis and metastasis of breast cancer cells.

H-ras protein in a bilayer: interaction and structure perturbation

Ras GTPases become functionally active when anchored to membranes by inserting their lipid modified side chains. Their role in cell division, development, and cancer has made them targets of extensive research efforts, yet the mechanism of membrane insertion and the structure of the resulting complex remain elusive. Recently, the structure of the full-length H-ras protein in a DMPC bilayer has been computationally characterized. Here, the atomic interactions between the H-ras membrane anchor and the DMPC bilayer are investigated in detail. We find that the palmitoylated cysteines and Met182 have dual contributions to membrane affinity: hydrogen bonding by their amides and van der Waals interaction by their hydrophobic side chains. The polar side chains help maintain the orientation of the anchor. Although the overall structure of the bilayer is similar to that of a pure DMPC, there are localized perturbations. These perturbations depend on the insertion depth and backbone localization of the anchor, which in turn is modulated by the catalytic domain and the linker. The pattern of anchor amide-DMPC phosphate/carbonyl hydrogen bonds and the flexibility of Palm184 are important in discriminating between different modes of ras-DMPC interactions. The results provide structural arguments in support of the proposed participation of ras in the organization of membrane nanoclusters.

Ras proteins: paradigms for compartmentalised and isoform-specific signalling

Ras GTPases mediate a wide variety of cellular processes by converting a multitude of extracellular stimuli into specific biological responses including proliferation, differentiation and survival. In mammalian cells, three ras genes encode four Ras isoforms (H-Ras, K-Ras4A, K-Ras4B and N-Ras) that are highly homologous but functionally distinct. Differences between the isoforms, including their post-translational modifications and intracellular sorting, mean that Ras has emerged as an important model system of compartmentalised signalling and membrane biology. Ras isoforms in different subcellular locations are proposed to recruit distinct upstream and downstream accessory proteins and activate multiple signalling pathways. Here, we summarise data relating to isoform-specific signalling, its role in disease and the mechanisms promoting compartmentalised signalling. Further understanding of this field will reveal the role of Ras signalling in development, cellular homeostasis and cancer and may suggest new therapeutic approaches.

Ras nanoclusters: molecular structure and assembly

H-, N- and K-ras4B are lipid-anchored, peripheral membrane guanine nucleotide binding proteins. Recent work has shown that Ras proteins are laterally segregated into non-overlapping, dynamic domains of the plasma membrane called nanoclusters. This lateral segregation is important to specify Ras interactions with membrane-associated proteins, effectors and scaffolding proteins and is critical for Ras signal transduction. Here we review biological, in vitro and structural data that provide insight into the molecular basis of how palmitoylated Ras proteins are anchored to the plasma membrane. We explore possible mechanisms for how the interactions of H-ras with a lipid bilayer may drive nanocluster formation.

Lipid rafts and caveolae in signaling by growth factor receptors

Lipid rafts and caveolae are microdomains of the plasma membrane enriched in sphingolipids and cholesterol, and hence are less fluid than the remainder of the membrane. Caveolae have an invaginated structure, while lipid rafts are flat regions of the membrane. The two types of microdomains have different protein compositions (growth factor receptors and their downstream molecules) suggesting that lipid rafts and caveolae have a role in the regulation of signaling by these receptors. The purpose of this review is to discuss this model, and the implications that it might have regarding a potential role for lipid rafts and caveolae in human cancer. Particular attention will be paid to the epidermal growth factor receptor, for which the largest amount of information is available. It has been proposed that caveolins act as tumor suppressors. The role of lipid rafts is less clear, but they seem to be capable of acting as 'signaling platforms', in which signal initiation and propagation can occur efficiently.

Lipid mediators in membrane rafts are important determinants of human health and disease

The new field of membrane rafts has provided fresh insight and a novel framework in which to understand the interaction, relation, and organization of lipids and proteins within cell membranes. This review will examine our current understanding of membrane rafts and their role in human health. In addition, the effect of various lipids, including dietary lipids, on membrane raft structure and function will be discussed. Membrane rafts are found in all cells and are characterized by their high concentration of cholesterol, sphingolipids, and saturated fatty acids. These lipids impart lateral segregation of membrane proteins, thus facilitating the spatial organization and regulation of membrane proteins involved in many cellular processes, such as cell proliferation, apoptosis, and cell signaling. Therefore, membrane rafts are shedding new light on the origins of metabolic disturbances and diseases such as cancer, insulin resistance, inflammation, cardiovascular disease, and Alzheimer's disease, which will be further discussed in this review.

Interactions of Ras proteins with the plasma membrane and their roles in signaling

The complex dynamic structure of the plasma membrane plays critical roles in cellular signaling; interactions with the membrane lipid milieu, spatial segregation within and between cellular membranes and/or targeting to specific membrane-associated scaffolds are intimately involved in many signal transduction pathways. In this review, we focus on the membrane interactions of Ras proteins. These small GTPases play central roles in the regulation of cell growth and proliferation, and their excessive activation is commonly encountered in human tumors. Ras proteins associate with the membrane continuously via C-terminal lipidation and additional interactions in both their inactive and active forms; this association, as well as the targeting of specific Ras isoforms to plasma membrane microdomains and to intracellular organelles, have recently been implicated in Ras signaling and oncogenic potential. We discuss biochemical and biophysical evidence for the roles of specific domains of Ras proteins in mediating their association with the plasma membrane, and consider the potential effects of lateral segregation and interactions with membrane-associated protein assemblies on the signaling outcomes.

A FRET map of membrane anchors suggests distinct microdomains of heterotrimeric G proteins

The standard model of heterotrimeric G protein signaling postulates a dissociation of Gα and Gβγ subunits after activation. We hypothesized that the different combination of lipid-modifications on Gα and Gαβγ subunits directs them into different microdomains. By characterizing rapidly and at high sensitivity 38 fluorescence resonance energy transfer (FRET) pairs of heterotrimeric-G-protein constructs, we defined their microdomains in relation to each other, free from the constraints of the raft/non-raft dualism. We estimated that in a cell ∼30% of these membrane-anchored proteins are mostly clustered in 3400-16,200 copies of 30-nm microdomains. We found that the membrane anchors of Gα and Gαβγ subunits of both the Gi/o and Gq family co-cluster differently with microdomain markers. Moreover, anchors of the Gαi/o and Gαq subunits co-clustered only weakly, whereas constructs that contained the anchors of the corresponding heterotrimers co-clustered considerably, suggesting the existence of at least three types of microdomain. Finally, FRET experiments with full-length heterotrimeric G proteins confirmed that the inactive, heterotrimerized Gα subunit is in microdomains shared by heterotrimers from different subclasses, from where it displaces upon activation into a membrane-anchor- and subclass-specific microdomain.

B- and C-RAF display essential differences in their binding to Ras: the isotype-specific N terminus of B-RAF facilitates Ras binding

Recruitment of RAF kinases to the plasma membrane was initially proposed to be mediated by Ras proteins via interaction with the RAF Ras binding domain (RBD). Data reporting that RAF kinases possess high affinities for particular membrane lipids support a new model in which Ras-RAF interactions may be spatially restricted to the plane of the membrane. Although the coupling features of Ras binding to the isolated RAF RBD were investigated in great detail, little is known about the interactions of the processed Ras with the functional and full-length RAF kinases. Here we present a quantitative analysis of the binding properties of farnesylated and nonfarnesylated H-Ras to both full-length B- and C-RAF in the presence and absence of lipid environment. Although isolated RBD fragments associate with high affinity to both farnesylated and nonfarnesylated H-Ras, the full-length RAF kinases revealed fundamental differences with respect to Ras binding. In contrast to C-RAF that requires farnesylated H-Ras, cytosolic B-RAF associates effectively and with significantly higher affinity with both farnesylated and nonfarnesylated H-Ras. To investigate the potential farnesyl binding site(s) we prepared several N-terminal fragments of C-RAF and found that in the presence of cysteine-rich domain only the farnesylated form of H-Ras binds with high association rates. The extreme N terminus of B-RAF turned out to be responsible for the facilitation of lipid independent Ras binding to B-RAF, since truncation of this region resulted in a protein that changed its kinase properties and resembles C-RAF. In vivo studies using PC12 and COS7 cells support in vitro results. Co-localization measurements using labeled Ras and RAF documented essential differences between B- and C-RAF with respect to association with Ras. Taken together, these data suggest that the activation of B-RAF, in contrast to C-RAF, may take place both at the plasma membrane and in the cytosolic environment.

Dynamic recruitment of phospholipase C gamma at transiently immobilized GPI-anchored receptor clusters induces IP3-Ca2+ signaling: single-molecule tracking study 2

Clusters of CD59, a glycosylphosphatidylinositol-anchored receptor (GPI-AR), with physiological sizes of approximately six CD59 molecules, recruit Gαi2 and Lyn via protein–protein and raft interactions. Lyn is activated probably by the Gαi2 binding in the same CD59 cluster, inducing the CD59 cluster's binding to F-actin, resulting in its immobilization, termed stimulation-induced temporary arrest of lateral diffusion (STALL; with a 0.57-s lifetime, occurring approximately every 2 s). Simultaneous single-molecule tracking of GFP-PLCγ2 and CD59 clusters revealed that PLCγ2 molecules are transiently (median = 0.25 s) recruited from the cytoplasm exclusively at the CD59 clusters undergoing STALL, producing the IP₃–Ca²⁺ signal. Therefore, we propose that the CD59 cluster in STALL may be a key, albeit transient, platform for transducing the extracellular GPI-AR signal to the intracellular IP₃–Ca²⁺ signal, via PLCγ2 recruitment. The prolonged, analogue, bulk IP₃–Ca²⁺ signal, which lasts for more than several minutes, is likely generated by the sum of the short-lived, digital-like IP₃ bursts, each created by the transient recruitment of PLCγ2 molecules to STALLed CD59.

Lipidated ras and rab peptides and proteins--synthesis, structure, and function

Chemical biology can be defined as the study of biological phenomena from a chemical approach. Based on the analysis of relevant biological phenomena and their structural foundation, unsolved problems are identified and tackled through a combination of chemistry and biology. Thus, new synthetic methods and strategies are developed and employed for the construction of compounds that are used to investigate biological procedures. Solid-phase synthesis has emerged as the preferred method for the synthesis of lipidated peptides, which can be chemoselectively ligated to proteins of the Ras superfamily. The generated peptides and proteins have solved biological questions in the field of the Ras-superfamily GTPases that are not amendable to chemical or biological techniques alone.

Nerve growth factor stimulates the concentration of TrkA within lipid rafts and extracellular signal-regulated kinase activation through c-Cbl-associated protein

Nerve growth factor (NGF) acts through its receptor, TrkA, to elicit the neuronal differentiation of PC12 cells through the action of extracellular signal-regulated kinase 1 (ERK1) and ERK2. Upon NGF binding, TrkA translocates and concentrates in cholesterol-rich membrane microdomains or lipid rafts, facilitating formation of receptor-associated signaling complexes, activation of downstream signaling pathways, and internalization into endosomes. We have investigated the mechanisms responsible for the localization of TrkA within lipid rafts and its ability to activate ERK1 and ERK2. We report that NGF treatment results in the translocation of activated forms of TrkA to lipid rafts, and this localization is important for efficient activation of the ERKs. TrkA is recruited and retained within lipid rafts through its association with flotillin, an intrinsic constituent of these membrane microdomains, via the adapter protein, c-Cbl associated protein (CAP). Mutant forms of CAP that lack protein interaction domains block TrkA localization to lipid rafts and attenuate ERK activation. Importantly, suppression of endogenous CAP expression inhibited NGF-stimulated neurite outgrowth from primary dorsal root ganglion neurons. These data provide a mechanism for the lipid raft localization of TrkA and establish the importance of the CAP adaptor protein for NGF activation of the ERKs and neuronal differentiation.

Hyperactive Ras in developmental disorders and cancer

Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.

Overexpression of caveolin-1 in a human melanoma cell line results in dispersion of ganglioside GD3 from lipid rafts and alteration of leading edges, leading to attenuation of malignant properties

Caveolin-1 is a component of lipid rafts, and is considered to be a tumor suppressor molecule. However, the mechanisms by which caveolin-1 functions in cancer cells are not well understood. We generated caveolin-1 transfectant cells (Cav-1(+) cells) using a human melanoma cell line (SK-MEL-28) and investigated the effects of caveolin-1 overexpression on the GD3-mediated malignant properties of melanomas. Cav-1(+) cells had decreased cell growth and motility, and reduced phosphorylation levels of p130Cas and paxillin relative to controls. In floatation analysis, although GD3 was mainly localized in glycolipid-enriched microdomain (GEM)/rafts in control cells, it was dispersed from GEM/rafts in Cav-1(+) cells. Correspondingly, GD3 in Cav-1(+) cells stained uniformly throughout the membrane, whereas control cells showed partial staining of the membrane, probably at the leading edge. p130Cas and paxillin were stained in the leading edges and colocalized with GD3 in the control cells. In contrast, these molecules were diffusely stained and no definite leading edges were detected in Cav-1(+) cells. These results suggest that caveolin-1 regulates GD3-mediated malignant signals by altering GD3 distribution and leading edge formation. These results reveal one of the mechanisms by which caveolin-1 curtails the malignant properties of tumor cells.

A novel negative regulatory function of the phosphoprotein associated with glycosphingolipid-enriched microdomains: blocking Ras activation

In primary human T cells, anergy induction results in enhanced p59Fyn activity. Because Fyn is the kinase primarily responsible for the phosphorylation of PAG (the phosphoprotein associated with glycosphingolipid-enriched microdomains), which negatively regulates Src-kinase activity by recruiting Csk (the C-terminal Src kinase) to the membrane, we investigated whether anergy induction also affects PAG. Analysis of anergic T cells revealed that PAG is hyperphosphorylated at the Csk binding site, leading to enhanced Csk recruitment and inhibitory tyrosine phosphorylation within Fyn. This together with enhanced phosphorylation of a tyrosine within the SH2 domain of Fyn leads to the formation of a hyperactive conformation, thus explaining the enhanced Fyn kinase activity. In addition, we have also identified the formation of a multiprotein complex containing PAG, Fyn, Sam68, and RasGAP in stimulated T cells. We demonstrate that PAG-Fyn overexpression is sufficient to suppress Ras activation in Jurkat T cells and show that this activity is independent of Csk binding. Thus, in addition to negatively regulating Src family kinases by recruiting Csk, PAG also negatively regulates Ras by recruiting RasGAP to the membrane. Finally, by knocking down PAG, we demonstrate both enhanced Src kinase activity and Ras activation, thereby establishing PAG as an important negative regulator of T-cell activation.