N-terminal protein acylation confers localization to cholesterol, sphingolipid-enriched membranes but not to lipid rafts/caveolae

Multiomics analysis identifies oxidative phosphorylation as a cancer vulnerability arising from myristoylation inhibition

BACKGROUND

In humans, two ubiquitously expressed N-myristoyltransferases, NMT1 and NMT2, catalyze myristate transfer to proteins to facilitate membrane targeting and signaling. We investigated the expression of NMTs in numerous cancers and found that NMT2 levels are dysregulated by epigenetic suppression, particularly so in hematologic malignancies. This suggests that pharmacological inhibition of the remaining NMT1 could allow for the selective killing of these cells, sparing normal cells with both NMTs.

METHODS AND RESULTS

Transcriptomic analysis of 1200 NMT inhibitor (NMTI)-treated cancer cell lines revealed that NMTI sensitivity relates not only to NMT2 loss or NMT1 dependency, but also correlates with a myristoylation inhibition sensitivity signature comprising 54 genes (MISS-54) enriched in hematologic cancers as well as testis, brain, lung, ovary, and colon cancers. Because non-myristoylated proteins are degraded by a glycine-specific N-degron, differential proteomics revealed the major impact of abrogating NMT1 genetically using CRISPR/Cas9 in cancer cells was surprisingly to reduce mitochondrial respiratory complex I proteins rather than cell signaling proteins, some of which were also reduced, albeit to a lesser extent. Cancer cell treatments with the first-in-class NMTI PCLX-001 (zelenirstat), which is undergoing human phase 1/2a trials in advanced lymphoma and solid tumors, recapitulated these effects. The most downregulated myristoylated mitochondrial protein was NDUFAF4, a complex I assembly factor. Knockout of NDUFAF4 or in vitro cell treatment with zelenirstat resulted in loss of complex I, oxidative phosphorylation and respiration, which impacted metabolomes.

CONCLUSIONS

Targeting of both, oxidative phosphorylation and cell signaling partly explains the lethal effects of zelenirstat in select cancer types. While the prognostic value of the sensitivity score MISS-54 remains to be validated in patients, our findings continue to warrant the clinical development of zelenirstat as cancer treatment.

Chemical Proteomics Reveals Nε-Fatty-Acylation of Septins by Rho Inactivation Domain (RID) of the Vibrio MARTX Toxin to Alter Septin Localization and Organization

Vibrio species, the Gram-negative bacterial pathogens causing cholera and sepsis, produce multiple secreted virulence factors for infection and pathogenesis. Among these is the multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin that releases several critical effector domains with distinct functions inside eukaryotic host cells. One such effector domain, the Rho inactivation domain (RID), has been discovered to catalyze long-chain Nε-fatty-acylation on lysine residues of Rho GTPases, causing inactivation of Rho GTPases and disruption of the host actin cytoskeleton. However, whether RID modifies other host proteins to exert additional functions remains to be determined. Herein, we describe the integration of bioorthogonal chemical labeling and quantitative proteomics to globally profile the target proteins modified by RID in living cells. More than 246 proteins are identified as new RID substrates, including many involved in GTPase regulation, cytoskeletal organization, and cell division. We demonstrate that RID extensively Nε-fatty-acylates septin proteins, the fourth cytoskeletal component of mammalian cells with important roles in diverse cellular processes. While affinity purification and mass spectrometry analysis show that RID-mediated Nε-fatty-acylation does not affect septin-septin interactions, this modification increases the membrane association of septins and confers localization to detergent-resistant membrane rafts. As a result, the filamentous assembly and organization of septins are disrupted by RID-mediated Nε-fatty-acylation, further contributing to cytoskeletal and mitotic defects that phenocopy the effects of septin depletion. Overall, our work greatly expands the substrate scope and function of RID and demonstrates the role of RID-mediated Nε-fatty-acylation in manipulating septin localization and organization.

Emerging Roles of YES1 in Cancer: The Putative Target in Drug Resistance

Src family kinases (SFKs) are non-receptor tyrosine kinases that are recognized as proto-oncogenic products. Among SFKs, YES1 is frequently amplified and overexpressed in a variety of human tumors, including lung, breast, ovarian, and skin cancers. YES1 plays a pivotal role in promoting cell proliferation, survival, and invasiveness during tumor development. Recent findings indicate that YES1 expression and activation are associated with resistance to chemotherapeutic drugs and tyrosine kinase inhibitors in human malignancies. YES1 undergoes post-translational modifications, such as lipidation and nitrosylation, which can modulate its catalytic activity, subcellular localization, and binding affinity for substrate proteins. Therefore, we investigated the diverse mechanisms governing YES1 activation and its impact on critical intracellular signal transduction pathways. We emphasized the function of YES1 as a potential mechanism contributing to the anticancer drug resistance emergence.

Extracellular vesicles and co-isolated endogenous retroviruses from murine cancer cells differentially affect dendritic cells

Cells secrete extracellular vesicles (EVs) and non-vesicular extracellular (nano)particles (NVEPs or ENPs) that may play a role in intercellular communication. Tumor-derived EVs have been proposed to induce immune priming of antigen presenting cells or to be immuno-suppressive agents. We suspect that such disparate functions are due to variable compositions in EV subtypes and ENPs. We aimed to characterize the array of secreted EVs and ENPs of murine tumor cell lines. Unexpectedly, we identified virus-like particles (VLPs) from endogenous murine leukemia virus in preparations of EVs produced by many tumor cells. We established a protocol to separate small EVs from VLPs and ENPs. We compared their protein composition and analyzed their functional interaction with target dendritic cells. ENPs were poorly captured and did not affect dendritic cells. Small EVs specifically induced dendritic cell death. A mixed large/dense EV/VLP preparation was most efficient to induce dendritic cell maturation and antigen presentation. Our results call for systematic re-evaluation of the respective proportions and functions of non-viral EVs and VLPs produced by murine tumors and their contribution to tumor progression.

Rab5b-Associated Arf1 GTPase Regulates Export of N-Myristoylated Adenylate Kinase 2 From the Endoplasmic Reticulum in Plasmodium falciparum

Plasmodium falciparum extensively remodels human erythrocytes by exporting hundreds of parasite proteins. This remodeling is closely linked to the Plasmodium virulence-related functions and immune evasion. The N-terminal export signal named PEXEL (Plasmodium export element) was identified to be important for the export of proteins beyond the PVM, however, the issue of how these PEXEL-positive proteins are transported and regulated by Rab GTPases from the endoplasmic reticulum (ER) to the cell surface has remained poorly understood. Previously, we identified new aspects of the trafficking of N-myristoylated adenylate kinase 2 (PfAK2), which lacks the PEXEL motif and is regulated by the PfRab5b GTPase. Overexpression of PfRab5b suppressed the transport of PfAK2 to the parasitophorous vacuole membrane and PfAK2 was accumulated in the punctate compartment within the parasite. Here, we report the identification of PfRab5b associated proteins and dissect the pathway regulated by PfRab5b. We isolated two membrane trafficking GTPases PfArf1 and PfRab1b by coimmunoprecipitation with PfRab5b and via mass analysis. PfArf1 and PfRab1b are both colocalized with PfRab5b adjacent to the ER in the early erythrocytic stage. A super-resolution microgram of the indirect immunofluorescence assay using PfArf1 or PfRab1b- expressing parasites revealed that PfArf1 and PfRab1b are localized to different ER subdomains. We used a genetic approach to expresses an active or inactive mutant of PfArf1 that specifically inhibited the trafficking of PfAK2 to the parasitophorous vacuole membrane. While expression of PfRab1b mutants did not affect in the PfAK2 transport. In contrast, the export of the PEXEL-positive protein Rifin was decreased by the expression of the inactive mutant of PfRab1b or PfArf1. These data indicate that the transport of PfAK2 and Rifin were recognized at the different ER subdomain by the two independent GTPases: PfAK2 is sorted by PfArf1 into the pathway for the PV, and the export of Rifin might be sequentially regulated by PfArf1 and PfRab1b.

Eimeria tenella Eimeria-specific protein that interacts with apical membrane antigen 1 (EtAMA1) is involved in host cell invasion

Background

Avian coccidiosis is a widespread, economically significant disease of poultry, caused by several Eimeria species. These parasites have complex and diverse life-cycles that require invasion of their host cells. This is mediated by various proteins secreted from apical secretory organelles. Apical membrane antigen 1 (AMA1), which is released from micronemes and is conserved across all apicomplexans, plays a central role in the host cell invasion. In a previous study, some putative EtAMA1-interacting proteins of E. tenella were screened. In this study, we characterized one putative EtAMA1-interacting protein, E. tenella Eimeria -specific protein (EtEsp).

Methods

Bimolecular fluorescence complementation (BiFC) and glutathione S-transferase (GST) fusion protein pull-down (GST pull-down) were used to confirm the interaction between EtAMA1 and EtEsp in vivo and in vitro. The expression of EtEsp was analyzed in different developmental stages of E. tenella with quantitative PCR and western blotting. The secretion of EtEsp protein was tested with staurosporine when sporozoites were incubated in complete medium at 41 °C. The localization of EtEsp was analyzed with an immunofluorescence assay (IFA). An in vitro invasion inhibition assay was conducted to assess the ability of antibodies against EtEsp to inhibit cell invasion by E. tenella sporozoites.

Results

The interaction between EtAMA1 and EtEsp was confirmed with BiFC and by GST pull-down. Our results show that EtEsp is differentially expressed during distinct phases of the parasite life-cycle. IFA showed that the EtEsp protein is mainly distributed on the parasite surface, and that the expression of this protein increases during the development of the parasite in the host cells. Using staurosporine, we showed that EtEsp is a secreted protein, but not from micronemes. In inhibition tests, a polyclonal anti-rEtEsp antibody attenuated the capacity of E. tenella to invade host cells.

Conclusion

In this study, we show that EtEsp interacts with EtAMA1 and that the protein is secreted protein, but not from micronemes. The protein participates in sporozoite invasion of host cells and is maybe involved in the growth of the parasite. These data have implications for the use of EtAMA1 or EtAMA1-interacting proteins as targets in intervention strategies against avian coccidiosis.

PtdIns4P-mediated electrostatic forces influence S-acylation of peripheral proteins at the Golgi complex

Protein S-acylation is a reversible post-translational modification involving the addition of fatty acids to cysteines and is catalyzed by transmembrane protein acyltransferases (PATs) mainly expressed at the Golgi complex. In case of soluble proteins, S-acylation confers stable membrane attachment. Myristoylation or farnesylation of many soluble proteins constitutes the initial transient membrane adsorption step prior to S-acylation. However, some S-acylated soluble proteins, such as the neuronal growth-associated protein Growth-associated protein-43 (GAP-43), lack the hydrophobic modifications required for this initial membrane interaction. The signals for GAP-43 S-acylation are confined to the first 13 amino acids, including the S-acylatable cysteines 3 and 4 embedded in a hydrophobic region, followed by a cluster of basic amino acids. We found that mutation of critical basic amino acids drastically reduced membrane interaction and hence S-acylation of GAP-43. Interestingly, acute depletion of phosphatidylinositol 4-phosphate (PtdIns4P) at the Golgi complex reduced GAP-43 membrane binding, highlighting a new, pivotal role for this anionic lipid and supporting the idea that basic amino acid residues are involved in the electrostatic interactions between GAP-43 and membranes of the Golgi complex where they are S-acylated.

Characterization and expression analysis of a new small heat shock protein Hsp20.4 from Eimeria tenella

Small heat shock proteins (sHsps) are ubiquitous and diverse molecular chaperones. Found in almost all organisms, they regulate protein refolding and protect cells from stress. Until now, no sHsp has been characterized in Eimeria tenella. In this study, the novel EtsHsp20.4 gene was cloned from E. tenella by rapid amplification of cDNA ends based on a previously identified expressed sequence tag. The full-length cDNA was 1019bp in length and contained an open reading frame of 558bp that encoded a 185-amino acid polypeptide with a calculated molecular weight of 20.4 kDa. The EtsHsp20.4 protein contained a distinct HSP20/alpha-crystallin domain that is the key determinant of their function as molecular chaperones and belongs to the HSP20 protein family. EtsHsp20.4 mRNA levels were higher in sporulated oocysts than in sporozoites or second-generation merozoites by real-time quantitative PCR, the transcription of EtsHsp20.4 was barely detectable in unsporulated oocysts. Immunolocalization with EtsHsp20.4 antibody showed that EtsHsp20.4 was mainly located on the surface of sporozoites, first-generation merozoites and second-generation merozoites. Following the development of parasites in DF-1 cells, EtsHsp20.4 protein was uniformly dispersed in trophozoites, immature schizonts, and mature schizonts. Malate dehydrogenase thermal aggregation assays indicated that recombinant EtsHsp20.4 had molecular chaperone activity in vitro. These results suggested that EtsHsp20.4 might be involved in sporulation in external environments and intracellular growth of the parasite in the host.

Influence of cell confluence on the cAMP signalling pathway in vascular smooth muscle cells

The influence of cell confluence on the β-adrenoceptor (β-AR)/cAMP/phosphodiesterase (PDE) pathway was investigated in cultured rat aortic smooth muscle cells (RASMCs). Cells were plated either at low density (LD: 3·10³cells/cm²) or high density (HD: 3·10⁴cells/cm²) corresponding to non-confluent or confluent cells, respectively, on the day of experiment. β-AR-stimulated cAMP was monitored in real-time using the fluorescence resonance energy transfer (FRET)-based cAMP sensor, Epac2-camps. A brief application (15s) of the β-AR agonist isoprenaline (Iso) induced a typical transient FRET signal, reflecting cAMP production followed by its rapid degradation. The amplitude of this response, which increased with the concentration of Iso (10 or 100nM), was higher in HD than in LD cells, whatever the Iso concentration used. However, activation of adenylyl cyclase by L-858051 (100μM) induced a similar saturating response in both LD and HD cells. A β₁-AR antagonist (CGP 20712A, 100nM) reduced the Iso (100nM) response in HD but not LD cells, whereas a β₂-AR antagonist (ICI 118,551, 5nM) reduced this response in HD cells and almost abolished it in LD cells. Competitive [¹²⁵I]-ICYP binding experiments with betaxolol, a β-AR ligand, identified two binding sites in HD cells, corresponding to β₁- and β₂-ARs with a proportion of 11% and 89%, respectively, but only one binding site in LD cells, corresponding to β₂-ARs. Total cAMP-PDE activity (assessed by a radioenzymatic assay) was increased in HD cells compared to LD cells. This increase was associated with a rise in mRNA expression of five cAMP-PDEs subtypes (PDE1A, 3A, 4A, 4B and 7B) in HD cells, and a decrease in basal [cAMP]_i (assessed by an EIA assay). PDE4 inhibition with Ro-20-1724 (10μM) strongly prolonged the Iso response in LD and HD cells, whereas PDE3 inhibition with cilostamide (1μM) slightly prolonged Iso response only in LD cells. Interestingly, inhibition of PDE4 unmasked an effect of PDE3 in HD cells. Our results show that in cultured RASMCs, the β-AR/cAMP/PDE signalling pathway is substantially modulated by the cell density. In HD cells, Iso response involves both β₁- and β₂-AR stimulation and is mainly controlled by PDE4, PDE3 being recruited only after PDE4 inhibition. In LD cells, Iso response involves only β₂-AR stimulation and is controlled by PDE4 and to a lower degree by PDE3. This low density state is associated with an absence of membrane expression of the β₁-AR, a lower cAMP-PDE activity and a higher basal [cAMP]_i. This study highlights the critical role of the cellular environment in controlling the vascular β-AR signalling.

GPS-Lipid: a robust tool for the prediction of multiple lipid modification sites

As one of the most common post-translational modifications in eukaryotic cells, lipid modification is an important mechanism for the regulation of variety aspects of protein function. Over the last decades, three classes of lipid modifications have been increasingly studied. The co-regulation of these different lipid modifications is beginning to be noticed. However, due to the lack of integrated bioinformatics resources, the studies of co-regulatory mechanisms are still very limited. In this work, we developed a tool called GPS-Lipid for the prediction of four classes of lipid modifications by integrating the Particle Swarm Optimization with an aging leader and challengers (ALC-PSO) algorithm. GPS-Lipid was proven to be evidently superior to other similar tools. To facilitate the research of lipid modification, we hosted a publicly available web server at http://lipid.biocuckoo.org with not only the implementation of GPS-Lipid, but also an integrative database and visualization tool. We performed a systematic analysis of the co-regulatory mechanism between different lipid modifications with GPS-Lipid. The results demonstrated that the proximal dual-lipid modifications among palmitoylation, myristoylation and prenylation are key mechanism for regulating various protein functions. In conclusion, GPS-lipid is expected to serve as useful resource for the research on lipid modifications, especially on their co-regulation.

A membrane-delimited N-myristoylated CRMP2 peptide aptamer inhibits CaV2.2 trafficking and reverses inflammatory and postoperative pain behaviors

Targeting proteins within the N-type voltage-gated calcium channel (CaV2.2) complex has proven to be an effective strategy for developing novel pain therapeutics. We describe a novel peptide aptamer derived from the collapsin response mediator protein 2 (CRMP2), a CaV2.2-regulatory protein. Addition of a 14-carbon myristate group to the peptide (myr-tat-CBD3) tethered it to the membrane of primary sensory neurons near surface CaV2.2. Pull-down studies demonstrated that myr-tat-CBD3 peptide interfered with the CRMP2-CaV2.2 interaction. Quantitative confocal immunofluorescence revealed a pronounced reduction of CaV2.2 trafficking after myr-tat-CBD3 treatment and increased efficiency in disrupting CRMP2-CaV2.2 colocalization compared with peptide tat-CBD3. Consequently, myr-tat-CBD3 inhibited depolarization-induced calcium influx in sensory neurons. Voltage clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after myr-tat-CBD3 exposure. Current clamp electrophysiology experiments demonstrated a reduction in excitability of small-diameter dorsal root ganglion neurons after exposure to myr-tat-CBD3. Myr-tat-CBD3 was effective in significantly attenuating carrageenan-induced thermal hypersensitivity and reversing thermal hypersensitivity induced by a surgical incision of the plantar surface of the rat hind paw, a model of postoperative pain. These effects are compared with those of tat-CBD3-the nonmyristoylated tat-conjugated CRMP2 peptide as well as scrambled versions of CBD3 and CBD3-lacking control peptides. Our results demonstrate that the myristoyl tag enhances intracellular delivery and local concentration of the CRMP2 peptide aptamer near membrane-delimited calcium channels resulting in pronounced interference with the calcium channel complex, superior suppression of calcium influx, and better antinociceptive potential.

TGFβ in endosomal signaling

The transforming growth factor beta (TGFβ) signaling pathway is important for normal cell homeostasis and has critical roles in apoptosis, cell-cycle arrest, and cellular differentiation (reviewed in Massague, 2008). In the classical TGFβ pathway, the endosomal trafficking of receptors has a direct outcome on signal transduction-receptors internalized via clathrin-mediated endocytosis enter the early endosome and propagate signaling, while those internalized via membrane rafts are targeted for degradation. Recently, there have been a number of articles that have identified TGFβ receptor-binding proteins that direct receptor endocytosis and/or intracellular trafficking and affect signal output (Atfi et al., 2007; Bauge, Girard, Leclercq, Galera, & Boumediene, 2012; Bizet et al., 2011, 2012; Chen et al., 2007; Gunaratne, Benchabane, & Di Guglielmo, 2012; Hao et al., 2011; McLean, Bhattacharya, & Di Guglielmo, 2013; Zhao et al., 2012). Given the importance of TGFβ receptor trafficking to signaling outcome, this chapter will focus on strategies to isolate membrane rafts and techniques to follow the trafficking of cell-surface TGFβ receptors and provide examples of functional readouts to assess TGFβ signal transduction.

Fatty acyl chain-dependent but charge-independent association of the SH4 domain of Lck with lipid membranes

The SH4 domain of Src family of nonreceptor protein tyrosine kinases represents the extreme N-terminal 1-16 amino acid region which mediates membrane association of these proteins and facilitates their functions. The SH4 domains among Src members lack well-defined sequence consensus and vary in the net charge. However, they readily anchor to the cytoplasmic face of the plasma membrane upon fatty acid acylation. Here, we report the membrane association of differentially acylated SH4 domain of Lck kinase, which has net negative charge at physiological pH. Our results suggest that despite the net negative charge, the SH4 domain of Lck associates with membranes upon fatty acid acylation. While myristoylation at the N-terminus is sufficient for providing membrane anchorage, multiple acylation determines orientation of the peptide chain with respect to the lipid bilayer. Hence, fatty acylation serves more than just a lipid anchor. It has an important role in regulating the spatial orientation of the peptide domain with respect to the lipid bilayer, which could be important for the interaction of the other domains of these kinases with their partners.

Cyclic AMP directs inositol (1,4,5)-trisphosphate-evoked Ca2+ signalling to different intracellular Ca2+ stores

Cholesterol depletion reversibly abolishes carbachol-evoked Ca(2+) release from inositol (1,4,5)-trisphosphate (IP3)-sensitive stores, without affecting the distribution of IP3 receptors (IP3R) or endoplasmic reticulum, IP3 formation or responses to photolysis of caged IP3. Receptors that stimulate cAMP formation do not alone evoke Ca(2+) signals, but they potentiate those evoked by carbachol. We show that these potentiated signals are entirely unaffected by cholesterol depletion and that, within individual cells, different IP3-sensitive Ca(2+) stores are released by carbachol alone and by carbachol combined with receptors that stimulate cAMP formation. We suggest that muscarinic acetylcholine receptors in lipid rafts deliver IP3 at high concentration to associated IP3R, stimulating them to release Ca(2+). Muscarinic receptors outside rafts are less closely associated with IP3R and provide insufficient local IP3 to activate IP3R directly. These IP3R, probably type 2 IP3R within a discrete Ca(2+) store, are activated only when their sensitivity is increased by cAMP. Sensitization of IP3R by cAMP extends the effective range of signalling by phospholipase C, allowing muscarinic receptors that are otherwise ineffective to recruit additional IP3-sensitive Ca(2+) stores.

N-terminal palmitoylation is required for Toxoplasma gondii HSP20 inner membrane complex localization

Toxoplasma gondii is an obligate intracellular parasite and the causative agent of toxoplasmosis. Protein palmitoylation is known to play roles in signal transduction and in enhancing the hydrophobicity of proteins thus contributing to their membrane association. Global inhibition of protein palmitoylation has been shown to affect T. gondii physiology and invasion of the host cell. However, the proteins affected by this modification have been understudied. This paper shows that the small heat shock protein 20 from T. gondii (TgHSP20) is synthesized as a mature protein in the cytosol and is palmitoylated in three cysteine residues. However, its localization at the inner membrane complex (IMC) is dependent only on N-terminal palmitoylation. Absence or incomplete N-terminal palmitoylation causes TgHSP20 to partially accumulate in a membranous structure. Interestingly, TgHSP20 palmitoylation is not responsible for its interaction with the daughter cells IMCs. Together, our data describe the importance of palmitoylation in protein targeting to the IMC in T. gondii.

Identification and subcellular localization of TcHIP, a putative Golgi zDHHC palmitoyl transferase of Trypanosoma cruzi

Protein palmitoylation is a post-translational modification that contributes to determining protein localization and function. Palmitoylation has been described in trypanosomatid protozoa, but no zDHHC palmitoyl transferase has been identified in Trypanosoma cruzi, the etiological agent of Chagas disease in Latin America. In this study we identify and show the subcellular localization of TcHIP (Tc00.1047053508199.50), a putative T. cruzi zDHHC palmitoyl transferase. Analysis of the deduced protein sequence indicates that it contains ankyrin repeats (Ank and Ank2) and the zDHHC conserved domain, typical of zDHHC palmitoyl transferases. A TcHIP polyclonal antiserum obtained from mice immunized with the purified recombinant protein was used to study the presence and subcellular localization of the native enzyme. In western blots this antiserum recognized a protein of about 95 kDa, consistent with the predicted molecular mass of TcHIP (95.4 kDa), in whole extracts of T. cruzi epimastigotes, metacyclic trypomastigotes and intracellular amastigotes. Immunolocalization by confocal microscopy showed TcHIP labeling at the Golgi complex, co-localizing with the T. cruzi Golgi marker TcRab7-GFP. Transfectant T. cruzi epimastigotes containing a construct encoding TcHIP fused to proteins A and C (TcHIP/AC) were obtained. In western blotting experiments, the TcHIP polyclonal antiserum recognized both native and TcHIP/AC proteins in extracts of the transfectants. Confocal microscopy showed co-localization of native TcHIP with TcHIP/AC. These findings demonstrate the presence of a putative zDHHC palmitoyl transferase (TcHIP) containing ankyrin and zDHHC domains in different developmental forms of T. cruzi, and its association with the Golgi complex.

Monitoring lipid anchor organization in cell membranes by PIE-FCCS

This study examines the dynamic co-localization of lipid-anchored fluorescent proteins in living cells using pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS) and fluorescence lifetime analysis. Specifically, we look at the pairwise co-localization of anchors from lymphocyte cell kinase (LCK: myristoyl, palmitoyl, palmitoyl), RhoA (geranylgeranyl), and K-Ras (farnesyl) proteins in different cell types. In Jurkat cells, a density-dependent increase in cross-correlation among RhoA anchors is observed, while LCK anchors exhibit a more moderate increase and broader distribution. No correlation was detected among K-Ras anchors or between any of the different anchor types studied. Fluorescence lifetime data reveal no significant Förster resonance energy transfer in any of the data. In COS 7 cells, minimal correlation was detected among LCK or RhoA anchors. Taken together, these observations suggest that some lipid anchors take part in anchor-specific co-clustering with other existing clusters of native proteins and lipids in the membrane. Importantly, these observations do not support a simple interpretation of lipid anchor-mediated organization driven by partitioning based on binary lipid phase separation.

Palmitoylation by DHHC5/8 targets GRIP1 to dendritic endosomes to regulate AMPA-R trafficking

Palmitoylation, a key regulatory mechanism controlling protein targeting, is catalyzed by DHHC-family palmitoyl acyltransferases (PATs). Impaired PAT activity is linked to neurodevelopmental and neuropsychiatric disorders, suggesting critical roles for palmitoylation in neuronal function. However, few substrates for specific PATs are known, and functional consequences of palmitoylation events are frequently uncharacterized. Here, we identify the closely related PATs DHHC5 and DHHC8 as specific regulators of the PDZ domain protein GRIP1b. Binding, palmitoylation, and dendritic targeting of GRIP1b require a PDZ ligand unique to DHHC5/8. Palmitoylated GRIP1b is targeted to trafficking endosomes and may link endosomes to kinesin motors. Consistent with this trafficking role, GRIP1b's palmitoylation turnover rate approaches the highest of all reported proteins, and palmitoylation increases GRIP1b's ability to accelerate AMPA-R recycling. To our knowledge, these findings identify the first neuronal DHHC5/8 substrate, define novel mechanisms controlling palmitoylation specificity, and suggest further links between dysregulated palmitoylation and neuropathological conditions.

Phenotypic profiling of the human genome reveals gene products involved in plasma membrane targeting of SRC kinases

SRC proteins are non-receptor tyrosine kinases that play key roles in regulating signal transduction by a diverse set of cell surface receptors. They contain N-terminal SH4 domains that are modified by fatty acylation and are functioning as membrane anchors. Acylated SH4 domains are both necessary and sufficient to mediate specific targeting of SRC kinases to the inner leaflet of plasma membranes. Intracellular transport of SRC kinases to the plasma membrane depends on microdomains into which SRC kinases partition upon palmitoylation. In the present study, we established a live-cell imaging screening system to identify gene products involved in plasma membrane targeting of SRC kinases. Based on siRNA arrays and a human model cell line expressing two kinds of SH4 reporter molecules, we conducted a genome-wide analysis of SH4-dependent protein targeting using an automated microscopy platform. We identified and validated 54 gene products whose down-regulation causes intracellular retention of SH4 reporter molecules. To detect and quantify this phenotype, we developed a software-based image analysis tool. Among the identified gene products, we found factors involved in lipid metabolism, intracellular transport, and cellular signaling processes. Furthermore, we identified proteins that are either associated with SRC kinases or are related to various known functions of SRC kinases such as other kinases and phosphatases potentially involved in SRC-mediated signal transduction. Finally, we identified gene products whose function is less defined or entirely unknown. Our findings provide a major resource for future studies unraveling the molecular mechanisms that underlie proper targeting of SRC kinases to the inner leaflet of plasma membranes.

Functional activation of Src family kinase yes protein is essential for the enhanced malignant properties of human melanoma cells expressing ganglioside GD3

The possible roles of Src family kinases in the enhanced malignant properties of melanomas related to GD3 expression were analyzed. Among Src family kinases only Yes, not Fyn or Src, was functionally involved in the increased cell proliferation and invasion of GD3-expressing transfectant cells (GD3+). Yes was located upstream of p130Cas and paxillin and at an equivalent level to focal adhesion kinase. Yes underwent autophosphorylation even before serum treatment and showed stronger kinase activity in GD3+ cells than in GD3- cells following serum treatment. Coimmunoprecipitation experiments revealed that Yes bound to focal adhesion kinase or p130Cas more strongly in GD3+ cells than in GD3- cells. As a possible mechanism for the enhancing effects of GD3 on cellular phenotypes, it was shown that majority of Yes was localized in glycolipid-enriched microdomain/rafts in GD3+ cells even before serum treatment, whereas it was scarcely detected in glycolipid-enriched microdomain/rafts in GD3- cells. An in vitro kinase assay of Yes revealed that coexistence of GD3 with Yes in membranous environments enhances the kinase activity of GD3- cell-derived Yes toward enolase, p125, and Yes itself. Knockdown of GD3 synthase resulted in the alleviation of tumor phenotypes and reduced activation levels of Yes. Taken together, these results suggest a role of GD3 in the regulation of Src family kinases.

Post-translational myristoylation: Fat matters in cellular life and death

Myristoylation corresponds to the irreversible covalent linkage of the 14-carbon saturated fatty acid, myristic acid, to the N-terminal glycine of many eukaryotic and viral proteins. It is catalyzed by N-myristoyltransferase. Typically, the myristate moiety participates in protein subcellular localization by facilitating protein-membrane interactions as well as protein-protein interactions. Myristoylated proteins are crucial components of a wide variety of functions, which include many signalling pathways, oncogenesis or viral replication. Initially, myristoylation was described as a co-translational reaction that occurs after the removal of the initiator methionine residue. However, it is now well established that myristoylation can also occur post-translationally in apoptotic cells. Indeed, during apoptosis hundreds of proteins are cleaved by caspases and in many cases this cleavage exposes an N-terminal glycine within a cryptic myristoylation consensus sequence, which can be myristoylated. The principal objective of this review is to provide an overview on the implication of myristoylation in health and disease with a special emphasis on post-translational myristoylation. In addition, new advancements in the detection and identification of myristoylated proteins are also briefly reviewed.

The effect of membrane domains on the G protein-phospholipase Cbeta signaling pathway

The plasma membrane serves as a barrier to limit the exit and entry of components into and out of the cell, offering protection from the external environment. Communication between the cell and the external environment is mediated by multiple signaling pathways. While the plasma membrane was historically viewed as a lipid bilayer with freely diffusing proteins, the last decade has shown that the lipids and proteins in the plasma membrane are organized in a non-random manner, and that this organization can direct and modify various signaling pathways in the cell. In this review, we qualitatively discuss the ways that membrane domains can affect cell signaling. We then focus on how membrane domains can affect a specific signaling pathway--the G protein-phospholipase Cbeta pathway and show how membrane domains can play an active role in directing or redirecting G protein signals.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

The cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 harbors lipid raft association determinants

The molecular basis for localization of the human immunodeficiency virus type 1 envelope glycoprotein (Env) in detergent-resistant membranes (DRMs), also called lipid rafts, still remains unclear. The C-terminal cytoplasmic tail of gp41 contains three membrane-interacting, amphipathic alpha-helical sequences, termed lentivirus lytic peptide 2 (LLP-2), LLP-3, and LLP-1, in that order. Here we identify determinants in the cytoplasmic tail which are crucial for Env's association with Triton X-100-resistant rafts. Truncations of LLP-1 greatly reduced Env localization in lipid rafts, and the property of Gag-independent gp41 localization in rafts was conserved among different strains. Analyses of mutants containing single deletions or substitutions in LLP-1 showed that the alpha-helical structure of the LLP-1 hydrophobic face has a more-critical role in Env-raft associations than that of the hydrophilic face. With the exception of a Pro substitution for Val-833, all Pro substitution and charge-inverting mutants showed wild-type virus-like one-cycle viral infectivity, replication kinetics, and Env incorporation into the virus. The intracellular localization and cell surface expression of mutants not localized in lipid rafts, such as the TM844, TM813, 829P, and 843P mutants, were apparently normal compared to those of wild-type Env. Cytoplasmic subdomain targeting analyses revealed that the sequence spanning LLP-3 and LLP-1 could target a cytoplasmic reporter protein to DRMs. Mutations of LLP-1 that affected Env association with lipid rafts also disrupted the DRM-targeting ability of the LLP-3/LLP-1 sequence. Our results clearly demonstrate that LLP motifs located in the C-terminal cytoplasmic tail of gp41 harbor Triton X-100-resistant raft association determinants.

Reversible phosphorylation as a molecular switch to regulate plasma membrane targeting of acylated SH4 domain proteins

Acylated SH4 domains represent N-terminal targeting signals that anchor peripheral membrane proteins such as Src kinases in the inner leaflet of plasma membranes. Here we provide evidence for a novel regulatory mechanism that may control the levels of SH4 proteins being associated with plasma membranes. Using a fusion protein of the SH4 domain of Leishmania HASPB and GFP as a model system, we demonstrate that threonine 6 is a substrate for phosphorylation. Substitution of threonine 6 by glutamate (to mimic a phosphothreonine residue) resulted in a dramatic redistribution from plasma membranes to intracellular sites with a particular accumulation in a perinuclear region. As shown by both pharmacological inhibition and RNAi-mediated down-regulation of the threonine/ serine-specific phosphatases PP1 and PP2A, recycling back to the plasma membrane required dephosphorylation of threonine 6. We provide evidence that a cycle of phosphorylation and dephosphorylation may also be involved in intracellular targeting of other SH4 proteins such as the Src kinase Yes.

Ectopic expression of plasma membrane targeted subunits of the Ndc80-complex as a tool to study kinetochore biochemistry

Genomic stability depends on the normal function of the kinetochore, a multi-protein assemblage, which consists of over 80 molecules including both constitutive and transiently binding components. Information regarding the spatial-temporal assembly of kinetochore subcomplexes is often limited by technical difficulties in their isolation. To study kinetochore subcomplex formation, we targeted separately Hec1 and Spc24, two subunits of the Ndc80 kinetochore compilation, to the plasma membrane by fusing them with the amino-terminal palmitoylation and myristoylation (pm) sequence of the receptor tyrosine kinase Fyn. We found that in early mitotic cells, pm-GFP-Hec1 and pm-GFP-Spc24 fusion proteins localised to the plasma membrane and were able to recruit all subunits of the Ndc80 complex (Ndc80/Hec1, Nuf2, Spc24 and Spc25) to these foci. In interphase cells, only Hec1-Nuf2 and Spc24-Spc25 heterodimers accumulated to the plasma membrane foci. The results propose that the assembly of Ndc80 tetramer can take place outside of the kinetochore but requires co-factors that are only present in mitotic cells. These findings provide the first experimental evidence on the successful employment of the plasma membrane targeting technique in the study of kinetochore biochemistry.

Reduction of T cell-derived ghrelin enhances proinflammatory cytokine expression: implications for age-associated increases in inflammation

Ghrelin (Grln) is a peptide hormone that is predominantly produced in the stomach and stimulates appetite and induces growth hormone (GH) release. We have previously reported that ghrelin is also expressed in T cells and exerts prothymic and anti-inflammatory effects. However, the biologic relevance of T cell-derived ghrelin remains to be determined. Here, we report that acylated-bioactive ghrelin is expressed in human T cells and preferentially segregates within the lipid raft domains upon TCR ligation. The RNA interference (RNAi)-mediated down-regulation of ghrelin in primary human T cells activates IkB, and increases Th1 cytokines and IL-17 secretion. Ghrelin expression declines with increasing age in spleen and T cells and exogenous ghrelin administration in old mice reduces proinflammatory cytokines. These findings demonstrate that ghrelin functions in an autocrine and paracrine capacity to regulate proinflammatory cytokine expression in human and murine T cells and may contribute in regulating "inflamm-aging."

Cholesterol-dependent separation of the beta2-adrenergic receptor from its partners determines signaling efficacy: insight into nanoscale organization of signal transduction

Determining the role of lipid raft nanodomains in G protein-coupled receptor signaling remains fraught by the lack of assays directly monitoring rafts in native membranes. We thus combined extensive biochemical and pharmacological approaches to a nanoscale strategy based on bioluminescence resonance energy transfer (BRET) to assess the spatial and functional influence of cholesterol-rich liquid-ordered lipid nanodomains on beta2 adrenergic receptor (beta2AR) signaling. The data revealed that whereas beta2AR did not partition within liquid-ordered lipid phase, a pool of G protein and adenylyl cyclase (AC) were sequestered in these domains. Destabilization of the liquid-ordered phase by cholesterol depletion led to a lateral redistribution of Galphas and AC that favored interactions between the receptor and its signaling partners as assessed by BRET. This resulted in an increased basal and agonist-promoted beta2AR-stimulated cAMP production that was partially dampened as a result of constitutive protein kinase A-dependent phosphorylation and desensitization of the receptor. This restraining influence of nanodomains on beta2AR signaling was further substantiated by showing that liquid-ordered lipid phase stabilization using caveolin overexpression or increasing membrane cholesterol amount led to an inhibition of beta2AR-associated signaling. Given the emerging concept that clustering of receptors and effectors into signaling platforms contributes to the efficacy and selectivity of signal transduction, our results support a model whereby cholesterol-promoted liquid-ordered lipid phase-embedding Gs and AC allows their lateral separation from the receptor, thus restraining the basal activity and controlling responsiveness of beta2AR signaling machinery within larger signaling platforms.

Regulation of ghrelin structure and membrane binding by phosphorylation

The peptide hormone ghrelin requires Ser-3 acylation for receptor binding, orexigenic and anti-inflammatory effects. Functions of desacylghrelin are less well understood. In vitro kinase assays reveal that the evolutionarily conserved Ser-18 in the basic C-terminus is an excellent substrate for protein kinase C. Circular dichroism reveals that desacylghrelin is approximately 12% helical in aqueous solution and approximately 50% helical in trifluoroethanol. Ser-18-phosphorylation, Ser-18-Ala substitution, or Ser-3-acylation reduces the helical character in trifluoroethanol to approximately 24%. Both ghrelin and desacylghrelin bind to phosphatidylcholine:phosphatidylserine sucrose-loaded vesicles in a phosphatidylserine-dependent manner. Phosphoghrelin and phosphodesacylghrelin show greatly diminished phosphatidylserine-dependent binding. These results are consistent with binding of ghrelin and desacylghrelin to acidic lipids via the basic face of an amphipathic helix with Ser-18 phosphorylation disrupting both helical character and membrane binding.

Regulation of Cripto-1 signaling and biological activity by caveolin-1 in mammary epithelial cells

Human and mouse Cripto-1 (CR-1/Cr-1) proteins play an important role in mammary gland development and tumorigenesis. In this study, we examined the relationship between Cripto-1 and caveolin-1 (Cav-1), a membrane protein that acts as a tumor suppressor in the mammary gland. Cripto-1 was found to interact with Cav-1 in COS7 cells and mammary epithelial cells. Using EpH4 mouse mammary epithelial cells expressing Cr-1 (EpH4 Cr-1) or Cr-1 and Cav-1 (EpH4 Cr-1/Cav-1), we demonstrate that Cav-1 expression markedly reduced the ability of Cr-1 to enhance migration, invasion, and formation of branching structures in EpH4 Cr-1/Cav-1 cells as compared to EpH4 Cr-1 cells. Furthermore, coexpression of Cav-1 together with Cr-1 in EpH4 Cr-1/Cav-1 cells inhibited Cr-1-mediated activation of c-src and mitogen-activated protein kinase signaling pathways. Conversely, primary mammary epithelial cells isolated from Cav-1 null(-/-)/mouse mammary tumor virus-CR-1 transgenic animals showed enhanced motility and activation of mitogen-activated protein kinase and c-src as compared to Cav-1(+/-)/CR-1 mammary cells. Finally, mammary tumors derived from mouse mammary tumor virus-CR-1 mice showed a dramatic reduction of Cav-1 expression as compared to mammary tissue from normal FVB/N mice, suggesting that in vivo Cav-1 is down-regulated during the process of CR-1-mediated mammary tumorigenesis.

Rapid detection, discovery, and identification of post-translationally myristoylated proteins during apoptosis using a bio-orthogonal azidomyristate analog

Myristoylation is the attachment of the 14-carbon fatty acid myristate to the N-terminal glycine residue of proteins. Typically a co-translational modification, myristoylation of proapoptotic cysteinyl-aspartyl proteases (caspase)-cleaved Bid and PAK2 was also shown to occur post-translationally and is essential for their proper localization and proapoptotic function. Progress in the identification and characterization of myristoylated proteins has been impeded by the long exposure times required to monitor incorporation of radioactive myristate into proteins (typically 1-3 months). Consequently, we developed a nonradioactive detection methodology in which a bio-orthogonal azidomyristate analog is specifically incorporated co- or post-translationally into proteins at N-terminal glycines, chemoselectively ligated to tagged triarylphosphines and detected by Western blotting with short exposure times (seconds to minutes). This represents over a million-fold signal amplification in comparison to using radioactive labeling methods. Using rational prediction analysis to recognize putative internal myristoylation sites in caspase-cleaved proteins combined with our nonradioactive chemical detection method, we identify 5 new post-translationally myristoylatable proteins (PKC epsilon, CD-IC2, Bap31, MST3, and the catalytic subunit of glutamate cysteine ligase). We also demonstrate that 15 proteins undergo post-translational myristoylation in apoptotic Jurkat T cells. This suggests that post-translational myristoylation of caspase-cleaved proteins represents a novel mechanism widely used to regulate cell death.

Growth factor induction of Cripto-1 shedding by glycosylphosphatidylinositol-phospholipase D and enhancement of endothelial cell migration

Cripto-1 (CR-1) is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that has been shown to play an important role in embryogenesis and cellular transformation. CR-1 is reported to function as a membrane-bound co-receptor and as a soluble ligand. Although a number of studies implicate the role of CR-1 as a soluble ligand in tumor progression, it is unclear how transition from the membrane-bound to the soluble form is physiologically regulated and whether differences in biological activity exist between these forms. Here, we demonstrate that CR-1 protein is secreted from tumor cells into the conditioned medium after treatment with serum, epidermal growth factor, or lysophosphatidic acid, and this soluble form of CR-1 exhibits the ability to promote endothelial cell migration as a paracrine chemoattractant. On the other hand, membrane-bound CR-1 can stimulate endothelial cell sprouting through direct cell-cell interaction. Shedding of CR-1 occurs at the GPI-anchorage site by the activity of GPI-phospholipase D (GPI-PLD), because CR-1 shedding was suppressed by siRNA knockdown of GPI-PLD and enhanced by overexpression of GPI-PLD. These findings describe a novel molecular mechanism of CR-1 shedding, which may contribute to endothelial cell migration and possibly tumor angiogenesis.

Regulation of D1 dopamine receptor trafficking and signaling by caveolin-1

There is accumulating evidence that G protein-coupled receptor signaling is regulated by localization in lipid raft microdomains. In this report, we determined that the D1 dopamine receptor (D1R) is localized in caveolae, a subset of lipid rafts, by sucrose gradient fractionation and confocal microscopy. Through coimmunoprecipitation and bioluminescence resonance energy transfer assays, we demonstrated that this localization was mediated by an interaction between caveolin-1 and D1R in COS-7 cells and an isoform-selective interaction between D1R and caveolin-1alpha in rat brain. We determined that the D1R interaction with caveolin-1 required a putative caveolin binding motif identified in transmembrane domain 7. Agonist stimulation of D1R caused translocation of D1R into caveolin-1-enriched sucrose fractions, which was determined to be a result of D1R endocytosis through caveolae. This was found to be protein kinase A-independent and a kinetically slower process than clathrin-mediated endocytosis. Site-directed mutagenesis of the caveolin binding motif at amino acids Phe313 and Trp318 significantly attenuated caveolar endocytosis of D1R. We also found that these caveolin binding mutants had a diminished capacity to stimulate cAMP production, which was determined to be due to constitutive desensitization of these receptors. In contrast, we found that D1Rs had an enhanced ability to maximally generate cAMP in chemically induced caveolae-disrupted cells. Taken together, these data suggest that caveolae has an important role in regulating D1R turnover and signaling in brain.

Evaluation of cholesterol content and impact on antigen exposure in the outer lipid bilayer of adult schistosomes

Developing and adultSchistosoma mansoniandS. haematobiumintact worms do not bind specific antibodies, likely because of structural and biochemical modifications of the outer lipid bilayer. We have estimated the amount of cholesterol in the apical membrane of adult schistosomes via extraction with the membrane-impermeable, cholesterol-binding drug, methyl-β-cyclodextrin (MBCD), followed by filipin staining of the worms, and evaluation of the amount of cholesterol released in the medium by a commercially available, enzymatic colorimetric assay. Positive correlations between amount of released cholesterol, MBCD concentration, and worm number and age provided evidence for the sensitivity and validity of the newly developed method. Treatment with 40 mmMBCD for 2 h at 37°C led to total loss of cholesterol from the worm outer membrane, as assessed by filipin staining, and the released cholesterol values were used to estimate the amount of cholesterol per worm and per an approximate surface area unit. Additionally, total depletion of outer membrane cholesterol was associated with exposure of surface membrane antigens to specific antibody binding in 50% and 70% ofS. haematobiumandS. mansoniworms, respectively. These findings together suggest that cholesterol is an essential, but not the sole, factor in sequestration of surface membrane antigens in schistosomes.

Rapid trafficking of c-Src, a non-palmitoylated Src-family kinase, between the plasma membrane and late endosomes/lysosomes

Src-family kinases (SFKs) are co-expressed with multiple combinations of each member in a single cell and involved in various signalings. Recently, we showed by sucrose-density gradient fractionation that the subcellular distribution of c-Src is distinct from that of Lyn. However, little is known about the trafficking of c-Src in living cells. Here, we show by time-lapse monitoring combined with photobleaching techniques that c-Src, a non-palmitoylated SFK, is rapidly exchanged between the plasma membrane and intracellular organelles representing late endosomes/lysosomes possibly through its cytosolic release. Although Lyn, a palmitoylated SFK, is exocytosed to the plasma membrane via the Golgi apparatus along the secretory pathway, lack of palmitoylation directs Lyn away from the exocytotic transport to the c-Src-type trafficking between the plasma membrane and late endosomes/lysosomes. Intriguingly, c-Src and a non-palmitoylated Lyn mutant are efficiently delivered and immobilized to focal adhesions when their SH2 domains are able to mediate protein-protein interactions in place of intramolecular bindings. However, palmitoylation of Lyn inhibits its recruitment to focal adhesions. These results suggest that palmitoylation of SFKs is critical for SFK localization and trafficking and implicate that two distinct trafficking pathways for SFKs may be involved in SFKs' specific functions.

CD8 Raft Localization Is Induced by Its Assembly into CD8αβ Heterodimers, Not CD8αα Homodimers

The coreceptor CD8 is expressed as a CD8alphabeta heterodimer on major histocompatibility complex class I-restricted TCRalphabeta T cells, and as a CD8alphaalpha homodimer on subsets of memory T cells, intraepithelial lymphocytes, natural killer cells, and dendritic cells. Although the role of CD8alphaalpha is not well understood, it is increasingly clear that this protein is not a functional homologue of CD8alphabeta. On major histocompatibility complex class I-restricted T cells, CD8alphabeta is a more efficient TCR coreceptor than CD8alphaalpha. This property has for the mouse protein been attributed to the recruitment of CD8alphabeta into lipid rafts, which is dependent on CD8beta palmitoylation. Here, these divergent distributions of CD8alphabeta and CD8alphaalpha are demonstrated for the human CD8 proteins as well. However, although palmitoylation of both CD8alpha and CD8beta chains was detected, this modification did not contribute to raft localization. In contrast, arginines in the cytoplasmic domain are crucial for raft localization of CD8betabeta. Most strikingly, the assembly of a non-raft localized CD8beta chain with a non-raft localized CD8alpha chain resulted in raft-localized CD8alphabeta heterodimers. Using chimeric CD8 proteins, this property of the heterodimer was found to be determined by the assembly of CD8alpha and CD8beta extracellular regions. The presence of two CD8alpha extracellular regions, on the other hand, appears to preclude raft localization. Thus, heterodimer formation and raft association are intimately linked for CD8alphabeta. These results emphasize that lipid raft localization is a key feature of human CD8alphabeta that clearly distinguishes it from CD8alphaalpha.

Caveolin-1 interacts with the Gag precursor of murine leukaemia virus and modulates virus production

Background

Retroviral Gag determines virus assembly at the plasma membrane and the formation of virus-like particles in intracellular multivesicular bodies. Thereby, retroviruses exploit by interaction with cellular partners the cellular machineries for vesicular transport in various ways.

Results

The retroviral Gag precursor protein drives assembly of murine leukaemia viruses (MLV) at the plasma membrane (PM) and the formation of virus like particles in multivesicular bodies (MVBs). In our study we show that caveolin-1 (Cav-1), a multifunctional membrane-associated protein, co-localizes with Gag in a punctate pattern at the PM of infected NIH 3T3 cells. We provide evidence that Cav-1 interacts with the matrix protein (MA) of the Gag precursor. This interaction is mediated by a Cav-1 binding domain (CBD) within the N-terminus of MA. Interestingly, the CBD motif identified within MA is highly conserved among most other γ-retroviruses. Furthermore, Cav-1 is incorporated into MLV released from NIH 3T3 cells. Overexpression of a GFP fusion protein containing the putative CBD of the retroviral MA resulted in a considerable decrease in production of infectious retrovirus. Moreover, expression of a dominant-negative Cav-1 mutant affected retroviral titres significantly.

Conclusion

This study demonstrates that Cav-1 interacts with MLV Gag, co-localizes with Gag at the PM and affects the production of infectious virus. The results strongly suggest a role for Cav-1 in the process of virus assembly.

Specific and distinct determinants mediate membrane binding and lipid raft incorporation of HIV-1(SF2) Nef

Membrane association is believed to be a prerequisite for the biological activity of the HIV-1 pathogenicity factor Nef. Attachment to cellular membranes as well as incorporation into detergent-insoluble microdomains (lipid rafts) require the N-terminal myristoylation of Nef. However, this modification is not sufficient for sustained membrane association and a specific raft-targeting signal for Nef has not yet been identified. Using live cell confocal microscopy and membrane fractionation analyses, we found that the N-terminal anchor domain (aa 1-61) is necessary and sufficient for efficient membrane binding of Nef from HIV-1(SF2). Within this domain, highly conserved lysine and arginine residues significantly contributed to Nef's membrane association and localization. Plasma membrane localization of Nef was also governed by an additional membrane-targeting motif between residues 40 and 61. Importantly, two lysines at positions 4 and 7 were not essential for the overall membrane association but critically contributed to Nef's incorporation into lipid raft domains. Cell surface receptor downmodulation was largely unaffected by mutations of all N-terminal basic residues, while the association of Nef with Pak2 kinase activity and its ability to augment virion infectivity correlated with its lysine-mediated raft incorporation. In contrast, all basic residues were required for efficient HIV-1 replication in primary human T lymphocytes but did not contribute to the incorporation of Nef into HIV-1 virions. Together, these results unravel that Nef's membrane association is governed by a complex pattern of signature motifs that differentially contribute to individual Nef activities. The identification of a critical raft targeting determinant and the functional characterization of a membrane-bound, non-raft-associated Nef variant indicate raft incorporation as a regulatory mechanism that determines the biological activity of distinct subpopulations of Nef in HIV-infected cells.

Receptor activation alters inner surface potential during phagocytosis

The surface potential of biological membranes varies according to their lipid composition. We devised genetically encoded probes to assess surface potential in intact cells. These probes revealed marked, localized alterations in the charge of the inner surface of the plasma membrane of macrophages during the course of phagocytosis. Hydrolysis of phosphoinositides and displacement of phosphatidylserine accounted for the change in surface potential at the phagosomal cup. Signaling molecules such as K-Ras, Rac1, and c-Src that are targeted to the membrane by electrostatic interactions were rapidly released from membrane subdomains where the surface charge was altered by lipid remodeling during phagocytosis.

Palmitoylation determines the function of Vac8 at the yeast vacuole

Palmitoylation stably anchors specific proteins to membranes, but may also have a direct effect on the function of a protein. The yeast protein Vac8 is required for efficient vacuole fusion, inheritance and cytosol-to-vacuole trafficking. It is anchored to vacuoles by an N-terminal myristoylation site and three palmitoylation sites, also known as the SH4 domain. Here, we address the role of Vac8 palmitoylation and show that the position and number of substrate cysteines within the SH4 domain determine the vacuole localization of Vac8: stable vacuole binding of Vac8 requires two cysteines within the N-terminus, regardless of the combination. Importantly, our data suggest that palmitoylation adds functionality to Vac8 beyond simple localization. A mutant Vac8 protein, in which the palmitoylation sites were replaced by a stretch of basic residues, still localizes to vacuole membranes and functions in cytosol-to-vacuole transport, but can only complement the function of Vac8 in morphology and inheritance if it also contains a single cysteine within the SH4 domain. Our data suggest that palmitoylation is not a mere hydrophobic anchor required solely for localization, but influences the protein function(s).

Gamma-secretase-dependent proteolysis of CD44 promotes neoplastic transformation of rat fibroblastic cells

The metalloprotease-dependent extracellular domain cleavage of the adhesion molecule CD44 is frequently observed in human tumors and is thought to promote metastasis. This cleavage is followed by gamma-secretase-dependent release of CD44 intracellular domain (CD44-ICD), which exhibits nuclear signaling activity. Using a reversible Ret-dependent oncogenic conversion model and a restricted proteomic approach, we identified a positive correlation between the neoplastic transformation of Rat-1 cells and the expression of standard CD44. In these transformed cells, CD44 was found to undergo a sequential metalloprotease and gamma-secretase cleavage, resulting in an increase in expression of CD44-ICD. We showed that this proteolytic fragment possesses a transforming activity. In support of this role, a significant and specific reduction in Ret-induced transformation of Rat-1 cells was observed following drug-mediated inhibition of gamma-secretase. Taken together, these findings suggest that the shedding of CD44 may not only modulate metastasis but also affects earlier events in tumorigenesis through the release of CD44-ICD.

N-terminal palmitoylation within the appropriate amino acid environment conveys on NOS2 the ability to progress along the intracellular sorting pathways

We have analysed the mechanism by which palmitoylation permits the progression of nitric oxide synthase 2 (NOS2) along the ER-Golgi-TGN pathway. Introduction of an additional myristoylation site at the N-terminus of NOS2 resulted in a chimera that displayed an enhanced association with the particulate fraction and with the plasma membrane but did not display increased enzymatic activity. In the absence of palmitoylation, introduction of a surrogate myristoylation site resulted in a mutant NOS2 with only 25% activity compared with the wild-type enzyme. Hence, the novel surrogate myristoyl moiety not only failed to increase NOS2 activity when introduced in a wild-type sequence environment, but was also unable to rescue the inactive phenotype of the Cys3Ser mutant. Introduction of an additional palmitoylatable Cys at position 2 of the wild-type sequence resulted in a chimera that associated to a larger degree with membranes and displayed decreased activity. Our data indicate that palmitoylation of inducible NOS at position 3 exquisitely determines its transit along the secretory pathway following a route that cannot be mimicked by a surrogate myristoylation or by a palmitate at position 2. In addition, the exit of NOS2 from the TGN and the accumulation in the cellular plasma membrane per se did not correlate with increased ·NO synthesis.

Posttranslational myristoylation of caspase-activated p21-activated protein kinase 2 (PAK2) potentiates late apoptotic events

p21-activated protein kinase (PAK) 2 is a small GTPase-activated serine/threonine kinase regulating various cytoskeletal functions and is cleaved by caspase-3 during apoptosis. We demonstrate that the caspase-cleaved PAK2 C-terminal kinase fragment (C-t-PAK2) is posttranslationally myristoylated, although myristoylation is typically a cotranslational process. Myristoylation and an adjacent polybasic domain of C-t-PAK2 are sufficient to redirect EGFP from the cytosol to membrane ruffles and internal membranes. Membrane localization and the ability of C-t-PAK2 to induce cell death are significantly reduced when myristoylation is abolished. In addition, the proper myristoylation-dependent membrane localization of C-t-PAK2 significantly increased signaling through the stress-activated c-Jun N-terminal kinase signaling pathway, which often regulates apoptosis. Interestingly, C-t-PAK2 promoted cell death without compromising mitochondrial integrity. Posttranslational myristoylation of caspase-cleaved proteins involved in cytoskeletal dynamics (e.g., PAK2, actin, and gelsolin) might be part of a unique series of mechanisms involved in the regulation of the later events of apoptosis.

In vivo dynamics of Rac-membrane interactions

The small GTPase Rac cycles between the membrane and the cytosol as it is activated by nucleotide exchange factors (GEFs) and inactivated by GTPase-activating proteins (GAPs). Solubility in the cytosol is conferred by binding of Rac to guanine-nucleotide dissociation inhibitors (GDIs). To analyze the in vivo dynamics of Rac, we developed a photobleaching method to measure the dissociation rate constant (k(off)) of membrane-bound GFP-Rac. We find that k(off) is 0.048 s(-1) for wtRac and approximately 10-fold less (0.004 s(-1)) for G12VRac. Thus, the major route for dissociation is conversion of membrane-bound GTP-Rac to GDP-Rac; however, dissociation of GTP-Rac occurs at a detectable rate. Overexpression of the GEF Tiam1 unexpectedly decreased k(off) for wtRac, most likely by converting membrane-bound GDP-Rac back to GTP-Rac. Both overexpression and small hairpin RNA-mediated suppression of RhoGDI strongly affected the amount of membrane-bound Rac but surprisingly had only slight effects on k(off). These results indicate that RhoGDI controls Rac function mainly through effects on activation and/or membrane association.

Cholesterol depletion inhibits src family kinase-dependent calcium mobilization and apoptosis induced by rituximab crosslinking

The monoclonal antibody (mAb) rituximab produces objective clinical responses in patients with B-cell non-Hodgkin's lymphoma and antibody-based autoimmune diseases. Mechanisms mediating B-cell depletion by rituximab are not completely understood and may include direct effects of signalling via the target antigen CD20. Like most but not all CD20 mAbs, rituximab induces a sharp change in the solubility of the CD20 protein in the non-ionic detergent Triton-X-100, reflecting a dramatic increase in the innate affinity of CD20 for membrane raft signalling domains. Apoptosis induced by rituximab hypercrosslinking has been shown to require src family kinases (SFK), which are enriched in rafts. In this report we provide experimental evidence that SFK-dependent apoptotic signals induced by rituximab are raft dependent. Cholesterol depletion prevented the association of hypercrosslinked CD20 with detergent-insoluble rafts, and attenuated both calcium mobilization and apoptosis induced with rituximab. CD20 cocapped with the raft-associated transmembrane adaptor LAB/NTAL after hypercrosslinking with CD20 mAbs, regardless of their ability to induce a change in the affinity of CD20 for rafts. Taken together, the data demonstrate that CD20 hypercrosslinking via rituximab activates SFKs and downstream signalling events by clustering membrane rafts in which antibody-bound CD20 is localized in a high-affinity configuration.

Transforming growth factor-beta receptors localize to caveolae and regulate endothelial nitric oxide synthase in normal human endothelial cells

Caveolae (sphingolipid- and cholesterol-rich, 100 nm flask-shaped invaginations of the cell membrane) serve as a nexus of cell signalling. In the present study caveolin-rich lipid raft domains were extracted from HUVEC (human umbilical-vein endothelial cells) using both density gradient and immunoprecipitation techniques, and demonstrated localization of the TGF-beta (transforming growth factor-beta) receptors TbetaRI and TbetaRII to the Cav-1 (caveolin-1)-enriched raft fractions of these normal, human endothelial cells. Immunoprecipitation demonstrated an association between TbetaRI and TbetaRII, as well as an association of the TbetaRs receptors with Cav-1 and eNOS (endothelial nitric oxide synthase), suggesting a mutual co-localization to caveolae; after treatment of HUVEC with 5 ng/ml TGF-beta1 for 15 min, however, co-precipitation of eNOS with TbetaRI, TbetaRII and Cav-1 was diminished. The loss of immunoprecipitable eNOS from Cav-1-enriched fractions was accompanied by a decrease both in phosphorylation of eNOS and in enzymatic activity (conversion of arginine into citrulline). No change in the localization of eNOS to morphologically distinct caveolae could be detected by electron microscopy after treatment of HUVEC with TGF-beta1 for 20 min. The results of these investigations provide evidence that TbetaRI interacts with eNOS in the caveolae of normal, human endothelial cells and has a regulatory function on basal eNOS enzymatic activity.