Phospholipid transfer proteins revisited

Discovering the Radiation Biomarkers in the Plasma of Total-Body Irradiated Leukemia Patients

The increased risk of acute large-scale radiological exposure for the world's population underlines the need for optimal radiation biomarkers. Ionizing radiation triggers a complex response by the genome, proteome, and metabolome, all of which have been reported as suitable indicators of radiation-induced damage in vivo. This study analyzed peripheral blood samples from total-body irradiation (TBI) leukemia patients through mass spectrometry (MS) to identify and quantify differentially regulated proteins in plasma before and after irradiation. In brief, samples were taken from 16 leukemic patients prior to and 24 h after TBI (2 × 2.0 Gy), processed with Tandem Mass Tag isobaric labelling kit (TMTpro-16-plex), and analyzed by MS. In parallel, label-free relative quantification was performed with a RP-nanoLC-ESI-MS/MS system in a Q-Exactive mass spectrometer. Protein identification was done in Proteome Discoverer v.2.2 platform (Thermo). Data is available via ProteomeXchange with identifier PXD043516. Using two different methods, we acquired two datasets of up-regulated (ratio ≥ 1.2) or down-regulated (ratio ≤ 0.83) plasmatic proteins 24 h after irradiation, identifying 356 and 346 proteins in the TMT-16plex and 285 and 308 label-free analyses, respectively (P ≤ 0.05). Combining the two datasets yielded 15 candidates with significant relation to gamma-radiation exposure. The majority of these proteins were associated with the inflammatory response and lipid metabolism. Subsequently, from these, five proteins showed the strongest potential as radiation biomarkers in humans (C-reactive protein, Alpha amylase 1A, Mannose-binding protein C, Phospholipid transfer protein, and Complement C5). These candidate biomarkers might have implications for practical biological dosimetry.

Integrated regulation of the phosphatidylinositol cycle and phosphoinositide-driven lipid transport at ER-PM contact sites

Among the structural phospholipids that form the bulk of eukaryotic cell membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the common precursor for low-abundance regulatory lipids, collectively referred to as polyphosphoinositides (PPIn). The metabolic turnover of PPIn species has received immense attention because of the essential functions of these lipids as universal regulators of membrane biology and their dysregulation in numerous human pathologies. The diverse functions of PPIn lipids occur, in part, by orchestrating the spatial organization and conformational dynamics of peripheral or integral membrane proteins within defined subcellular compartments. The emerging role of stable contact sites between adjacent membranes as specialized platforms for the coordinate control of ion exchange, cytoskeletal dynamics, and lipid transport has also revealed important new roles for PPIn species. In this review, we highlight the importance of membrane contact sites formed between the endoplasmic reticulum (ER) and plasma membrane (PM) for the integrated regulation of PPIn metabolism within the PM. Special emphasis will be placed on non-vesicular lipid transport during control of the PtdIns biosynthetic cycle as well as toward balancing the turnover of the signaling PPIn species that define PM identity.

Weighted gene coexpression network analysis reveals hub genes involved in cholangiocarcinoma progression and prognosis

AIM

Cholangiocarcinoma (CCA) is a highly malignant tumor found in the bile duct epithelial cells, and the second most common primary tumor of the liver. However, the pivotal roles of molecular biomarkers in oncogenesis of CCA are unclear. Therefore, we aim to explore the underlying mechanisms of progression and screen for novel prognostic biomarkers and treatment targets.

METHOD

The data of mRNA sequencing and clinical information of CCA patients in The Cancer Genome Atlas was analyzed by weighted gene coexpression network analysis (WGCNA). Modules and clinical traits were constructed according to Pearson's correlation analysis, and Gene Ontology and pathway enrichment analysis were applied. Hub genes of these modules were screened by intramodule analysis; Cytoscape with Search Tool for the Retrieval of Interacting Genes was utilized to visualize protein-protein interaction of these modules; hub genes of these modules were validated afterwards. Furthermore, the significance of these genes was confirmed by survival analysis.

RESULTS

Genes MRPS18A, CST1, and SCP2 were identified as candidate genes in the module, which was associated with clinical traits including pathological stage, histological grade, and liver function and which also affected overall survival of CCA patients. Nineteen hub genes were analyzed together and were associated with progression and prognosis of CCA. Survival analyses found that several of the multiple genes could serve as biomarkers to stratify CCA patients into low- and high-risk groups.

CONCLUSION

These candidate genes could be involved in progression of CCA, which could serve as novel prognostic markers and treatment targets. Moreover, most of them were first reported in CCA and deserve further research.

Involvement of OpsLTP1 from Opuntia streptacantha in abiotic stress adaptation and lipid metabolism

Plant lipid transfer proteins (LTPs) exhibit the ability to transfer lipids between membranes in vitro, and have been implicated in diverse physiological processes associated to plant growth, reproduction, development, biotic and abiotic stress responses. However, their mode of action is not yet fully understood. To explore the functions of the OpsLTP1 gene encoding a LTP from cactus pear Opuntia streptacantha Lem., we generated transgenic Arabidopsis thaliana (L.) Heynh. plants to overexpress OpsLTP1 and contrasted our results with the loss-of-function mutant ltp3 from A. thaliana under abiotic stress conditions. The ltp3 mutant seeds showed impaired germination under salt and osmotic treatments, in contrast to OpsLTP1 overexpressing lines that displayed significant increases in germination rate. Moreover, stress recovery assays showed that ltp3 mutant seedlings were more sensitive to salt and osmotic treatments than wild-type plants suggesting that AtLTP3 is required for stress-induced responses, while the OpsLTP1 overexpressing line showed no significant differences. In addition, OpsLTP1 overexpressing and ltp3 mutant seeds stored lower amount of total lipids compared with wild-type seeds, showing changes primarily on 16C and 18C fatty acids. However, ltp3 mutant also lead changes in lipid profile and no over concrete lipids which may suggest a compensatory activation of other LTPs. Interestingly, linoleic acid (18:2ω6) was consistently increased in neutral, galactoglycerolipids and phosphoglycerolipids of OpsLTP1 overexpressing line indicating a role of OpsLTP1 in the modulation of lipid composition in A. thaliana.

Identification of Peroxisomal Protein Complexes with PTS Receptors, Pex5 and Pex7, in Mammalian Cells

Pex5 and Pex7 are cytosolic receptors for peroxisome targeting signal type-1 (PTS1) and type-2 (PTS2), respectively, and play a pivotal role in import of peroxisomal matrix proteins. Recent advance in mass spectrometry analysis has facilitated comprehensive analysis of protein-protein interaction network by a combination with immunoprecipitation or biochemical purification. In this chapter, we introduce several findings obtained by these methods applied to mammalian cells. Exploring Pex5-binding partners in mammalian cells revealed core components comprising the import machinery complex of matrix proteins and a number of PTS1-type cargo proteins. Biochemical purification of the Pex5-export stimulating factor from rat liver cytosol fraction identified Awp1, providing further insight into molecular mechanisms of the export step of mono-ubiquitinated Pex5. Identification of DDB1 (damage-specific DNA-binding protein 1), a component of CRL4 (Cullin4A-RING ubiquitin ligase) E3 complex, as a Pex7-interacting protein revealed that quality control of Pex7 by CRL4A is important for PTS2 protein import by preventing the accumulation of dysfunctional Pex7. Furthermore, analysis of binding partners of an intraperoxisomal processing enzyme, trypsin-domain containing 1 (Tysnd1), showed a protein network regulating peroxisomal fatty acid β-oxidation.

Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

Constitutive cyclic GMP accumulation in Arabidopsis thaliana compromises systemic acquired resistance induced by an avirulent pathogen by modulating local signals

The infection of Arabidopsis thaliana plants with avirulent pathogens causes the accumulation of cGMP with a biphasic profile downstream of nitric oxide signalling. However, plant enzymes that modulate cGMP levels have yet to be identified, so we generated transgenic A. thaliana plants expressing the rat soluble guanylate cyclase (GC) to increase genetically the level of cGMP and to study the function of cGMP in plant defence responses. Once confirmed that cGMP levels were higher in the GC transgenic lines than in wild-type controls, the GC transgenic plants were then challenged with bacterial pathogens and their defence responses were characterized. Although local resistance was similar in the GC transgenic and wild-type lines, differences in the redox state suggested potential cross-talk between cGMP and the glutathione redox system. Furthermore, large-scale transcriptomic and proteomic analysis highlighted the significant modulation of both gene expression and protein abundance at the infection site, inhibiting the establishment of systemic acquired resistance. Our data indicate that cGMP plays a key role in local responses controlling the induction of systemic acquired resistance in plants challenged with avirulent pathogens.

Bifunctional Sphingosine for Cell-Based Analysis of Protein-Sphingolipid Interactions

Sphingolipids are essential structural components of cellular membranes and are crucial regulators of cellular processes. While current high-throughput approaches allow for the systematic mapping of interactions of soluble proteins with their lipid-binding partners, photo-cross-linking is the only technique that enables for the proteome-wide mapping of integral membrane proteins with their direct lipid environment. Here, we report the synthesis of a photoactivatable and clickable analog of sphingosine (pacSph). When administered to sphingosine-1-phosphate lyase deficient cells, pacSph allows its metabolic fate and the subcellular flux of de novo synthesized sphingolipids to be followed in a time-resolved manner. The chemoproteomic profiling yielded over 180 novel sphingolipid-binding proteins, of which we validated a number, demonstrating the unique value of this technique as a discovery tool. This work provides an important resource for the understanding of the global cellular interplay between sphingolipids and their interacting proteins.

Overexpression of full-length cholesteryl ester transfer protein in SW872 cells reduces lipid accumulation

Cells produce two cholesteryl ester transfer protein (CETP) isoforms, full-length and a shorter variant produced by alternative splicing. Blocking synthesis of both isoforms disrupts lipid metabolism and storage. To further define the role of CETP in cellular lipid metabolism, we stably overexpressed full-length CETP in SW872 cells. These CETP(+) cells had several-fold higher intracellular CETP and accumulated 50% less TG due to a 26% decrease in TG synthesis and 2.5-fold higher TG turnover rate. Reduced TG synthesis was due to decreased fatty acid uptake and impaired conversion of diglyceride to TG even though diacylglycerol acyltransferase activity was normal. Sterol-regulatory element binding protein 1 mRNA levels were normal, and although PPARγ expression was reduced, the expression of several of its target genes including adipocyte triglyceride lipase, FASN, and APOE was normal. CETP(+) cells contained smaller lipid droplets, consistent with their higher levels of perilipin protein family (PLIN) 3 compared with PLIN1 and PLIN2. Intracellular CETP was mostly associated with the endoplasmic reticulum, although CETP near lipid droplets poorly colocalized with this membrane. A small pool of CETP resided in the cytoplasm, and a subfraction coisolated with lipid droplets. These data show that overexpression of full-length CETP disrupts lipid homeostasis resulting in the formation of smaller, more metabolically active lipid droplets.

Nonvesicular lipid transfer from the endoplasmic reticulum

The transport of lipids from their synthesis site at the endoplasmic reticulum (ER) to different target membranes could be mediated by both vesicular and nonvesicular transport mechanisms. Nonvesicular lipid transport appears to be the major transport route of certain lipid species, and could be mediated by either spontaneous lipid transport or by lipid-transfer proteins (LTPs). Although nonvesicular lipid transport has been extensively studied for more than four decades, its underlying mechanism, advantage and regulation, have not been fully explored. In particular, the function of LTPs and their involvement in intracellular lipid movement remain largely controversial. In this article, we describe the pathways by which lipids are synthesized at the ER and delivered to different cellular membranes, and discuss the role of LTPs in lipid transport both in vitro and in intact cells.

The phosphatidylinositol transfer protein RdgBβ binds 14-3-3 via its unstructured C-terminus, whereas its lipid-binding domain interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein)

PITPs [PI (phosphatidylinositol) transfer proteins] bind and transfer PI between intracellular membranes and participate in many cellular processes including signalling, lipid metabolism and membrane traffic. The largely uncharacterized PITP RdgBβ (PITPNC1; retinal degeneration type B β), contains a long C-terminal disordered region following its defining N-terminal PITP domain. In the present study we report that the C-terminus contains two tandem phosphorylated binding sites (Ser274 and Ser299) for 14-3-3. The C-terminus also contains PEST sequences which are shielded by 14-3-3 binding. Like many proteins containing PEST sequences, the levels of RdgBβ are regulated by proteolysis. RdgBβ is degraded with a half-life of 4 h following ubiquitination via the proteasome. A mutant RdgBβ which is unable to bind 14-3-3 is degraded even faster with a half-life of 2 h. In vitro, RdgBβ is 100-fold less active than PITPα for PI transfer, and RdgBβ proteins (wild-type and a mutant that cannot bind 14-3-3) expressed in COS-7 cells or endogenous proteins from heart cytosol do not exhibit transfer activity. When cells are treated with PMA, the PITP domain of RdgBβ interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein; also known as AGTRAP) causing membrane recruitment. We suggest that RdgBβ executes its function following recruitment to membranes via its PITP domain and the C-terminal end of the protein could regulate entry to the hydrophobic cavity.

Viruses exploiting peroxisomes

Viruses that are of great importance for global public health, including HIV, influenza and rotavirus, appear to exploit a remarkable organelle, the peroxisome, during intracellular replication in human cells. Peroxisomes are sites of lipid biosynthesis and catabolism, reactive oxygen metabolism, and other metabolic pathways. Viral proteins are targeted to peroxisomes (the spike protein of rotavirus) or interact with peroxisomal proteins (HIV's Nef and influenza's NS1) or use the peroxisomal membrane for RNA replication. The Nef interaction correlates strongly with the crucial Nef function of CD4 downregulation. Viral exploitation of peroxisomal lipid metabolism appears likely. Mostly, functional significance and mechanisms remain to be elucidated. Recently, peroxisomes were discovered to play a crucial role in the innate immune response by signaling the presence of intracellular virus, leading to the first rapid antiviral response. This review unearths, interprets and connects old data, in the hopes of stimulating new and promising research.

Overexpression of Sterol Carrier Protein 2 in patients with hereditary cholesterol gallstones

BACKGROUND

Lithogenic bile is the major cause of cholesterol gallstone, but its pathogenesis is not well understood. The hypersecretion of biliary cholesterol is believed to be an important cause of lithogenic bile. Sterol Carrier Protein 2 (SCP2) participates in cholesterol trafficking and lipid metabolism in hepatocytes and may play a key role in cholesterol gallstone formation.

METHODS

21 cholesterol gallstone genealogies were studied to investigate the expression of SCP2 gene in liver tissue of hereditary and non-hereditary cholesterol gallstone patients as well as non-gallstone patients. The mRNA expression of liver SCP2 in 28 hereditary patients, 30 non-hereditary cholesterol gallstone patients and 32 non-gallstone patients was measured by Reverse Transcription Polymerase Chain Reaction (RT-PCR). The protein expression of liver SCP2 was also detected in all the patients by Western blotting. At the same time, the bile was also analyzed with biochemical techniques and the Cholesterol Saturation Index (CSI) was calculated.

RESULTS

The mRNA and protein expression of SCP2 was significantly increased in cholesterol gallstone patients compared to those of non-gallstone patients. Moreover, SCP2 was expressed at higher levels in hereditary cholesterol gallstone patients than that of non-hereditary cholesterol gallstone patients. There was significant difference observed in CSI between cholesterol gallstone patients and non-gallstone patients, but not in CSI between hereditary and non-hereditary cholesterol gallstone patients.

CONCLUSIONS

SCP2 was overexpressed in hereditary cholesterol gallstone patients compared to non-hereditary cholesterol gallstone patients. This finding indicated that SCP2 might be one of the genetic factors contributing to cholesterol gallstone formation, which was always accompanied by the increase of bile lithogenicity.

Zebrafish class 1 phosphatidylinositol transfer proteins: PITPbeta and double cone cell outer segment integrity in retina

Phosphatidylinositol transfer proteins (PITPs) in yeast co-ordinate lipid metabolism with the activities of specific membrane trafficking pathways. The structurally unrelated metazoan PITPs (mPITPs), on the other hand, are an under-investigated class of proteins. It remains unclear what biological activities mPITPs discharge, and the mechanisms by which these proteins function are also not understood. The soluble class 1 mPITPs include the PITPalpha and PITPbeta isoforms. Of these, the beta-isoforms are particularly poorly characterized. Herein, we report the use of zebrafish as a model vertebrate for the study of class 1 mPITP biological function. Zebrafish express PITPalpha and PITPbeta-isoforms (Pitpna and Pitpnb, respectively) and a novel PITPbeta-like isoform (Pitpng). Pitpnb expression is particularly robust in double cone cells of the zebrafish retina. Morpholino-mediated protein knockdown experiments demonstrate Pitpnb activity is primarily required for biogenesis/maintenance of the double cone photoreceptor cell outer segments in the developing retina. By contrast, Pitpna activity is essential for successful navigation of early developmental programs. This study reports the initial description of the zebrafish class 1 mPITP family, and the first analysis of PITPbeta function in a vertebrate.

Kinetics of PIP2 metabolism and KCNQ2/3 channel regulation studied with a voltage-sensitive phosphatase in living cells

The signaling phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP(2)) is synthesized in two steps from phosphatidylinositol by lipid kinases. It then interacts with KCNQ channels and with pleckstrin homology (PH) domains among many other physiological protein targets. We measured and developed a quantitative description of these metabolic and protein interaction steps by perturbing the PIP(2) pool with a voltage-sensitive phosphatase (VSP). VSP can remove the 5-phosphate of PIP(2) with a time constant of tau <300 ms and fully inhibits KCNQ currents in a similar time. PIP(2) was then resynthesized from phosphatidylinositol 4-phosphate (PIP) quickly, tau = 11 s. In contrast, resynthesis of PIP(2) after activation of phospholipase C by muscarinic receptors took approximately 130 s. These kinetic experiments showed that (1) PIP(2) activation of KCNQ channels obeys a cooperative square law, (2) the PIP(2) residence time on channels is <10 ms and the exchange time on PH domains is similarly fast, and (3) the step synthesizing PIP(2) by PIP 5-kinase is fast and limited primarily by a step(s) that replenishes the pool of plasma membrane PI(4)P. We extend the kinetic model for signaling from M(1) muscarinic receptors, presented in our companion paper in this issue (Falkenburger et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.200910344), with this new information on PIP(2) synthesis and KCNQ interaction.

The Diverse Biological Functions of Phosphatidylinositol Transfer Proteins in Eukaryotes

Phosphatidylinositol/phosphatidylcholine transfer proteins (PITPs) remain largely functionally uncharacterized, despite the fact that they are highly conserved and are found in all eukaryotic cells thus far examined by biochemical or sequence analysis approaches. The available data indicate a role for PITPs in regulating specific interfaces between lipid-signaling and cellular function. In this regard, a role for PITPs in controlling specific membrane trafficking events is emerging as a common functional theme. However, the mechanisms by which PITPs regulate lipid-signaling and membrane-trafficking functions remain unresolved. Specific PITP dysfunctions are now linked to neurodegenerative and intestinal malabsorption diseases in mammals, to stress response and developmental regulation in higher plants, and to previously uncharacterized pathways for regulating membrane trafficking in yeast and higher eukaryotes, making it clear that PITPs are integral parts of a highly conserved signal transduction strategy in eukaryotes. Herein, we review recent progress in deciphering the biological functions of PITPs, and discuss some of the open questions that remain.

Dynamics of lipid transfer by phosphatidylinositol transfer proteins in cells

Of many lipid transfer proteins identified, all have been implicated in essential cellular processes, but the activity of none has been demonstrated in intact cells. Among these, phosphatidylinositol transfer proteins (PITP) are of particular interest as they can bind to and transfer phosphatidylinositol (PtdIns) – the precursor of important signalling molecules, phosphoinositides – and because they have essential functions in neuronal development (PITPα) and cytokinesis (PITPβ). Structural analysis indicates that, in the cytosol, PITPs are in a ‘closed’ conformation completely shielding the lipid within them. But during lipid exchange at the membrane, they must transiently ‘open’. To study PITP dynamics in intact cells, we chemically targeted their C95 residue that, although non-essential for lipid transfer, is buried within the phospholipid-binding cavity, and so, its chemical modification prevents PtdIns binding because of steric hindrance. This treatment resulted in entrapment of open conformation PITPs at the membrane and inactivation of the cytosolic pool of PITPs within few minutes. PITP isoforms were differentially inactivated with the dynamics of PITPβ faster than PITPα. We identify two tryptophan residues essential for membrane docking of PITPs.

Alternative metabolic fates of phosphatidylinositol produced by phosphatidylinositol synthase isoforms in Arabidopsis thaliana

PtdIns is an important precursor for inositol-containing lipids, including polyphosphoinositides, which have multiple essential functions in eukaryotic cells. It was previously proposed that different regulatory functions of inositol-containing lipids may be performed by independent lipid pools; however, it remains unclear how such subcellular pools are established and maintained. In the present paper, a previously uncharacterized Arabidopsis gene product with similarity to the known Arabidopsis PIS (PtdIns synthase), PIS1, is shown to be an active enzyme, PIS2, capable of producing PtdIns in vitro. PIS1 and PIS2 diverged slightly in substrate preferences for CDP-DAG [cytidinediphospho-DAG (diacylglycerol)] species differing in fatty acid composition, PIS2 preferring unsaturated substrates in vitro. Transient expression of fluorescently tagged PIS1 or PIS2 in onion epidermal cells indicates localization of both enzymes in the ER (endoplasmic reticulum) and, possibly, Golgi, as was reported previously for fungal and mammalian homologues. Constitutive ectopic overexpression of PIS1 or PIS2 in Arabidopsis plants resulted in elevated levels of PtdIns in leaves. PIS2-overexpressors additionally exhibited significantly elevated levels of PtdIns(4)P and PtdIns(4,5)P(2), whereas polyphosphoinositides were not elevated in plants overexpressing PIS1. In contrast, PIS1-overexpressors contained significantly elevated levels of DAG and PtdEtn (phosphatidylethanolamine), an effect not observed in plants overexpressing PIS2. Biochemical analysis of transgenic plants with regards to fatty acids associated with relevant lipids indicates that lipids increasing with PIS1 overexpression were enriched in saturated or monounsaturated fatty acids, whereas lipids increasing with PIS2 overexpression, including polyphosphoinositides, contained more unsaturated fatty acids. The results indicate that PtdIns populations originating from different PIS isoforms may enter alternative routes of metabolic conversion, possibly based on specificity and immediate metabolic context of the biosynthetic enzymes.

Translocation as a means of disseminating lipid hydroperoxide-induced oxidative damage and effector action

Lipid hydroperoxides (LOOHs) generated in cells and lipoproteins under oxidative pressure may induce waves of damaging chain lipid peroxidation near their sites of origin if O2 is readily available and antioxidant capacity is overwhelmed. However, recent studies have demonstrated that chain induction is not necessarily limited to a nascent LOOH's immediate surroundings but can extend to other cell membranes or lipoproteins by means of LOOH translocation through the aqueous phase. Mobilization and translocation can also extend the range of LOOHs as redox signaling molecules and in this sense they could act like the small, readily diffusible inorganic analogue H2O2, which has been studied much more extensively in this regard. In this article, basic mechanisms of free-radical- and singlet-oxygen-mediated LOOH formation and one-electron and two-electron LOOH reduction pathways and their biological consequences are reviewed. The first studies to document spontaneous and protein-assisted LOOH transfer in model systems and cells are described. Finally, LOOH translocation is discussed in the context of cytotoxicity vs detoxification and expanded effector action, i.e., redox signaling activity.

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.

Phosphatidylethanolamine critically supports internalization of cell-penetrating protein C inhibitor

Although their contribution remains unclear, lipids may facilitate noncanonical routes of protein internalization into cells such as those used by cell-penetrating proteins. We show that protein C inhibitor (PCI), a serine protease inhibitor (serpin), rapidly transverses the plasma membrane, which persists at low temperatures and enables its nuclear targeting in vitro and in vivo. Cell membrane translocation of PCI necessarily requires phosphatidylethanolamine (PE). In parallel, PCI acts as a lipid transferase for PE. The internalized serpin promotes phagocytosis of bacteria, thus suggesting a function in host defense. Membrane insertion of PCI depends on the conical shape of PE and is associated with the formation of restricted aqueous compartments within the membrane. Gain- and loss-of-function mutations indicate that the transmembrane passage of PCI requires a branched cavity between its helices H and D, which, according to docking studies, precisely accommodates PE. Our findings show that its specific shape enables cell surface PE to drive plasma membrane translocation of cell-penetrating PCI.

The anti-apoptotic MAP kinase pathway is inhibited in NIH3T3 fibroblasts with increased expression of phosphatidylinositol transfer protein β

Mouse NIH3T3 fibroblast cells overexpressing phosphatidylinositol transfer protein beta (PI-TPbeta, SPIbeta cells) demonstrate a low rate of proliferation and a high sensitivity towards UV-induced apoptosis when compared with wtNIH3T3 cells. In contrast, SPIbetaS262A cells overexpressing a mutant PI-TPbeta that lacks the protein kinase C-dependent phosphorylation site Ser-262, demonstrate a phenotype comparable with wtNIH3T3 cells. This suggests that the phosphorylation of Ser-262 in PI-TPbeta is involved in the regulation of apoptosis. Conditioned medium (CM) from wtNIH3T3 cells contains bioactive factors, presumably arachidonic acid metabolites [H. Bunte, et al., 2006; M. Schenning, et al., 2004] that are able to protect SPIbeta cells against UV-induced apoptosis. CM from SPIbeta cells lacks this protective activity. However, after heat denaturation CM from SPIbeta cells regains a protective activity comparable with that of wtNIH3T3 cells. This indicates that CM from SPIbeta cells contains an antagonistic factor interfering with the anti-apoptotic activity present. SPIbetaS262A cells do not produce the antagonist suggesting that phosphorylation of Ser-262 is required. Moreover, in line with the apparent lack of anti-apoptotic activity, CM from SPIbeta cells does not induce the expression of COX-2 or the activation of p42/p44 MAP kinase in SPIbeta cells. In contrast, CM from wtNIH3T3 and SPIbetaS262A cells or heat-treated CM from SPIbeta cells does induce these anti-apoptotic markers. Since we have previously shown that some of the arachidonic acid metabolites present in CM from wtNIH3T3 cells are prostaglandin (PG) E(2) and PGF(2alpha), we investigated the effect of these PGs on cell survival. Although PGE(2) and PGF(2alpha) were found to protect wtNIH3T3 and SPIbetaS262A cells against UV-induced apoptosis, these PGs failed to rescue SPIbeta cells. The fact that the concentrations of PGE(2) and PGF(2alpha) in the CM from SPIbeta cells and wtNIH3T3 cells were found to be comparable suggests that the failure of these PGs to protect SPIbeta cells could render these cells more apoptosis sensitive. Concomitantly, upon incubation with PGE(2) and PGF(2alpha), an increased expression of COX-2 and activation of p42/p44 MAP kinase were observed in wtNIH3T3 and SPIbetaS262A cells but not in SPIbeta cells. Hence, it appears that specific mechanisms of cell survival are impaired in SPIbeta cells.

A conspicuous connection: structure defines function for the phosphatidylinositol-phosphate kinase family

The phosphatidylinositol phosphate (PIP) kinases are a unique family of enzymes that generate an assortment of lipid messengers, including the pivotal second messenger phosphatidylinositol 4,5-bisphosphate (PI4,5P2). While members of the PIP kinase family function by catalyzing a similar phosphorylation reaction, the specificity loop of each PIP kinase subfamily determines substrate preference and partially influences distinct subcellular targeting. Specific protein-protein interactions that are unique to particular isoforms or splice variants play a key role in targeting PIP kinases to appropriate subcellular compartments to facilitate the localized generation of PI4,5P2 proximal to effectors, a mechanism key for the function of PI4,5P2 as a second messenger. This review documents the discovery of the PIP kinases and their signaling products, and summarizes our current understanding of the mechanisms underlying the localized generation of PI4,5P2 by PIP kinases for the regulation of cellular events including actin cytoskeleton dynamics, vesicular trafficking, cell migration, and an assortment of nuclear events.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.

Glycolipid transfer proteins

Glycolipid transfer proteins (GLTPs) are small (24 kDa), soluble, ubiquitous proteins characterized by their ability to accelerate the intermembrane transfer of glycolipids in vitro. GLTP specificity encompasses both sphingoid- and glycerol-based glycolipids, but with a strict requirement that the initial sugar residue be beta-linked to the hydrophobic lipid backbone. The 3D architecture of GLTP reveals liganded structures with unique lipid-binding modes. The biochemical properties of GLTP action at the membrane surface have been studied rather comprehensively, but the biological role of GLTP remains enigmatic. What is clear is that GLTP differs distinctly from other known glycolipid-binding proteins, such as nonspecific lipid transfer proteins, lysosomal sphingolipid activator proteins, lectins, lung surfactant proteins as well as other lipid-binding/transfer proteins. Based on the unique conformational architecture that targets GLTP to membranes and enables glycolipid binding, GLTP is now considered the prototypical and founding member of a new protein superfamily in eukaryotes.

Phosphatidylinositol transfer proteins and cellular nanoreactors for lipid signaling

Membrane lipids function as structural molecules, reservoirs for second messengers, membrane platforms that scaffold protein assembly and regulators of enzymes and ion channels. Such diverse lipid functions contribute substantially to cellular mechanisms for fine-tuning membrane-signaling events. Meaningful coordination of these events requires exquisite spatial and temporal control of lipid metabolism and organization, and reliable mechanisms for specifically coupling these parameters to dedicated physiological processes. Recent studies suggest such integration is linked to the action of phosphatidylinositol transfer proteins that operate at the interface of the metabolism, trafficking and organization of specific lipids.

Differential expression of a C-terminal splice variant of phosphatidylinositol transfer protein beta lacking the constitutive-phosphorylated Ser262 that localizes to the Golgi compartment

Mammalian PITPbeta (phosphatidylinositol transfer protein beta) is a 272-amino-acid polypeptide capable of transferring PtdIns, PtdCho and SM (sphingomyelin) between membrane bilayers. It has been reported that Ser262 present in the C-terminus of PITPbeta is constitutively phosphorylated and determines Golgi localization. We provide evidence for the expression of an sp (splice) variant of PITPbeta (PITPbeta-sp2) where the C-terminal 15 amino acids of PITPbeta-sp1 are replaced by an alternative C-terminus of 16 amino acids. PITPbeta-sp1 is the product of the first 11 exons, whereas PITPbeta-sp2 is a product of the first 10 exons followed by the twelfth exon--exon 11 being 'skipped'. Both splice variants are capable of PtdIns and PtdCho transfer, with PITPbeta-sp2 being unable to transport SM. PITPbeta is ubiquitously expressed, with the highest amounts of PITPbeta found in HL60 cells and in rat liver; HL60 cells express only PITPbeta-sp1, whereas rat liver expresses both sp variants in similar amounts. In both cell types, PITPbeta-sp1 is constitutively phosphorylated and both the PtdIns and PtdCho forms of PITPbeta-sp1 are present. In contrast, PITPbeta-sp2 lacks the constitutively phosphorylated Ser262 (replaced with glutamine). Nonetheless, both PITPbeta variants localize to the Golgi and, moreover, dephosphorylation of Ser262 of PITPbeta-sp1 does not affect its Golgi localization. The presence of PITPbeta sp variants adds an extra level of proteome complexity and, in rat liver, the single gene for PITPbeta gives rise to seven distinct protein species that can be resolved on the basis of their charge differences.

Lipid homoeostasis and Golgi secretory function

The unique lipid composition of the Golgi membranes is critical for maintaining their structural and functional identity, and is regulated by local lipid metabolism, a variety of lipid-binding, -modifying, -sensing and -transfer proteins, and by selective lipid sorting mechanisms. A growing body of evidence suggests that certain lipids, such as phosphoinositides and diacylglycerol, regulate Golgi-mediated transport events. However, their exact role in this process, and the underlying mechanisms that maintain their critical levels in specific membrane domains of the Golgi apparatus, remain poorly understood. Nevertheless, recent advances have revealed key regulators of lipid homoeostasis in the Golgi complex and have demonstrated their role in Golgi secretory function.

Phosphatidylinositol Transfer Protein Expression Altered by Aging and Parkinson Disease

1. Phosphatidylinositol transfer proteins (PI-TP) are responsible for the transport of phosphatidylinositol (PI) and other phospholipids from endoplasmic reticulum to the other membranes and indirectly for lipid mediated signaling. Till now little is known about PI-TPs in brain aging and neurodegeneration. The aim of this study was to investigate expression of PI-TP in the brain during aging and in animal's model of Parkinson disease (PD) induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Moreover, in vitro, effect of 1-methyl-4-phenyl-pyridine cation (MPP(+)) on PI-TP, tyrosine hydroxylase (TH) protein level, and viability of cells was investigated. 2. Wistar rats 4, 24, and 36 months old and C57/BL mice and rat pheochromocytoma (PC12) cell line were used for the studies. Mice C57/BL received three injections of MPTP in saline at 2 h intervals in a total dose of 40 mg/kg and then after 3, 7, and 14 days they were used for the investigation. PC12 cells were treated with increasing concentration (50-300 microM) of MPP(+) for 24 h at 37 degrees C. The level of PI-TP(alpha and beta) and TH were determined using Western Blot analysis. 3. Our data indicated that PI-TP(alpha and beta) level decreased in brain of 36 months old rat by 20% comparing to the control value (4 months old). In animal's model of PD, PI-TP(alpha and beta) level was significantly lower by 85, 69, 64% in striatum at 3, 7, and 14 days after MPTP injection, respectively, compared to the control value. MPP(+) decreased PI-TP(alpha and beta), TH expression, and viability of PC12 cells in a dose-dependent manner. H(2)O(2), menadione, and NO donor significantly decreased the PI-TP level and viability of PC12 cells. 4. Our results indicate the lower protein expression of PI-TP(alpha and beta) in aged brain and in PD and suggest that oxidative stress may be responsible for the alteration of PI-TP.

Mutations in the gene encoding peroxisomal sterol carrier protein X (SCPx) cause leukencephalopathy with dystonia and motor neuropathy

In this report, we describe the first known patient with a deficiency of sterol carrier protein X (SCPx), a peroxisomal enzyme with thiolase activity, which is required for the breakdown of branched-chain fatty acids. The patient presented with torticollis and dystonic head tremor as well as slight cerebellar signs with intention tremor, nystagmus, hyposmia, and azoospermia. Magnetic resonance imaging showed leukencephalopathy and involvement of the thalamus and pons. Metabolite analyses of plasma revealed an accumulation of the branched-chain fatty acid pristanic acid, and abnormal bile alcohol glucuronides were excreted in urine. In cultured skin fibroblasts, the thiolytic activity of SCPx was deficient, and no SCPx protein could be detected by western blotting. Mutation analysis revealed a homozygous 1-nucleotide insertion, 545_546insA, leading to a frameshift and premature stop codon (I184fsX7).

Sterol carrier protein-2 expression alters sphingolipid metabolism in transfected mouse L-cell fibroblasts

The influence of sterol carrier protein-2 (SCP-2) on the cellular metabolism of sphingolipids was examined in control mouse L-cells and stably transfected clones expressing the protein SCP-2. Three approaches were used to examine for differences; (1) compositional analysis of endogenous sphingolipid classes, (2) metabolism of NBD-ceramide, and (3) live cell labelling via endocytic uptake of BODIPY-sphingomyelin. SCP-2 over expression significantly altered the endogenous levels of both neutral and acidic sphingolipid classes. Among the neutral sphingolipids, expression of SCP-2 induced a 1.7-fold increase in the level of lactosylceramide (LacCer, p < 0.05) and a similar fold decrease in the level of the higher-order neutral glycosylceramides (p < 0.05). Among the acidic sphingolipids, SCP-2 resulted in a 5.2-fold decrease in the endogenous plasma membrane level of ganglioside GM1 (p < 0.03). Incubation of both control and transfected cell lines with NBD-ceramide resulted in the rapid establishment of a steady-state distribution of NBD-labelled sphingomyelin (NBD-SM) and glucosylceramide (NBD-GlcCer). In the SCP-2 expressing clones the conversion of NBD-Cer to NBD-GlcCer was 30% lower during incubation periods between 5 and 30 min (p < 0.025). Inspection of the cells by fluorescence microscopy after incubation with BODIPY labelled sphingomyelin (BODIPY-SM) revealed similar punctuated patterns with no distinguishable differences between the cell types. These results imply that SCP-2 plays a role in modulating enzymatic steps involved in metabolism of sphingolipid homeostasis.

Nucleolar localization of phosphatidylinositol 4-kinase PI4K230 in various mammalian cells

BACKGROUND

Previous immunohistochemical investigations could not detect PI4K230, an isoform of mammalian phosphatidylinositol 4-kinases (also called type III alpha), in the nucleus and nucleolus of cells in spite of its predicted nuclear localization signals.

METHODS

Immunofluorescent detection of PI4K230 and other PI4K isoforms was performed on formaldehyde (PFA) or ethanol fixed cells and rat brain cryosections. Costaining with nucleolin and the effect of siRNA, Triton X-100, DNase, and RNase treatments were also tested to determine the localization of PI4K230.

RESULTS

PI4K230 gives a prominent signal in the nucleolus of ethanol fixed rat brain cryosections and of several cell types in addition to its presence in the nucleus and cytoplasm. The PI4K230 immunoreactivity of the nucleolus is masked in PFA fixed cells, but it can be restored by treatment of PFA fixed cells with hot wet citrate buffer or by washing the cryosections with PBS prior to PFA fixation. Nucleolar PI4K230 occurs in a Triton X-100 resistant complex. Treatment of COS-7 cells with siRNA targeting PI4K230 and permeabilized B50 cells with DNase or RNase results in the loss of PI4K230 signal from the nucleolus.

CONCLUSION

These experiments suggest the participation of PI4K230 in a DNase and RNase sensitive complex with a unique localization and function in the nucleolus.

Phospholipase-C sensitive GPI-anchored proteins of goat sperm: possible role in sperm protection

The role of glycosylphosphatidylinositol (GPI)-anchored sperm proteins in reproduction has been investigated. SDS-polyacrylamide gels (PAGE) analysis of goat sperm (Capra indica) indicated that several GPI-anchored proteins were released by phosphatidylinositol-specific phospholipase-C (PI-PLC) treatment. The distribution of this category of PI-PLC-sensitive GPI-anchored proteins on the surface of sperm was examined by indirect immunofluorescence. The fluorescence microscopic study clearly demonstrated that the PI-PLC-sensitive GPI-anchored proteins are confined predominantly to the head region of goat sperm. Further experiments were conducted on intact and PI-PLC treated sperm in order to decipher the function of GPI proteins. Co-incubation of sperm with peritoneal macrophages led to the enhanced phagocytosis of PI-PLC treated sperm by macrophages compared with the untreated intact sperm. Transmission electron micrographs of the macrophages acquired from the phagocytosis assay are provided to corroborate the same. From the results obtained it is inferred that one or more of the PI-PLC-sensitive GPI-anchored proteins on the sperm surface could act as protection factor(s) that shield the sperm from macrophages.

Point mutational analysis of the liganding site in human glycolipid transfer protein. Functionality of the complex

Mammalian glycolipid transfer proteins (GLTPs) facilitate the selective transfer of glycolipids between lipid vesicles in vitro. Recent structural determinations of the apo- and glycolipid-liganded forms of human GLTP have provided the first insights into the molecular architecture of the protein and its glycolipid binding site (Malinina, L., Malakhova, M. L., Brown, R. E., and Patel, D. J. (2004) Nature 430, 1048-1053). In the present study, we have evaluated the functional consequences of point mutation of the glycolipid liganding site of human GLTP within the context of a carrier-based mechanism of glycolipid intermembrane transfer. Different approaches were developed to rapidly and efficiently assess the uptake and release of glycolipid by GLTP. They included the use of glass-immobilized, glycolipid films to load GLTP with glycolipid and separation of GLTP/glycolipid complexes from vesicles containing glycolipid (galactosylceramide or lactosylceramide) or from monosialoganglioside dispersions by employing nickel-nitrilotriacetic acid-based affinity or gel filtration strategies. Point mutants of the sugar headgroup recognition center (Trp-96, Asp-48, Asn-52) and of the ceramide-accommodating hydrophobic tunnel (Phe-148, Phe-183, Leu-136) were analyzed for their ability to acquire and release glycolipid ligand. Two manifestations of point mutation within the liganding site were apparent: (i) impaired formation of the GLTP/glycolipid complex; (ii) impaired acquisition and release of bound glycolipid by GLTP. The results are consistent with a carrier-based mode of GLTP action to accomplish the intermembrane transfer of glycolipid. Also noteworthy was the inefficient release of glycolipid by wtGLTP into phosphatidylcholine acceptor vesicles, raising the possibility of a function other than intermembrane glycolipid transfer in vivo.

Gene expression changes related to growth and differentiation in the fetal and juvenile reproductive system of the female rat: evaluation of microarray results

Microarrays make it possible to evaluate the responses of a major fraction of the genome in response to physiological perturbation or exogenous insult. This represents a huge advance in our ability to detect changes in gene expression that may be responsible for physiological or toxicological responses. Our laboratory is interested in the effects of estrogens on female reproductive system development and function. We have evaluated the changes in gene expression in response to estrogens in the female reproductive tract of rats during embryo/fetal development and in the juvenile rat (which is capable of mounting a uterotrophic response). The results of these experiments indicate that a number of genes (dozens to hundreds) are changed in a reproducible, dose-related manner in response to estrogens. These results have been published elsewhere; the purpose of this review is to evaluate, based on information from the literature, the potential role of selected genes on processes of cell proliferation and differentiation, and to suggest plausible relationships among these genes in eliciting responses at the tissue or organ level. We also discuss the utility of gene-expression experiments in elucidating the shape of the dose-response curve at low doses. In particular, we show that the dose-response for gene expression in the juvenile rat uterus is monotonic down to levels a few orders of magnitude below the NOEL for a uterotrophic response, suggesting that gene expression (and by inference higher order responses) do not follow patterns that are unpredictable based on response at higher dosages.

Characterization of lymphoblast mitochondria from patients with Barth syndrome

Barth syndrome (BTHS) is a multisystem disorder of individuals who carry mutations in tafazzin, a putative phospholipid acyltransferase. We investigated the hypothesis that BTHS is caused by specific impairment of the mitochondrial lipid metabolism. The fatty acid composition of all major mitochondrial phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cardiolipin (CL), changed in lymphoblasts from BTHS patients. These changes were most extensive in CL and least extensive in PE. The complementary nature of the fatty acid alterations in CL and PC suggested that fatty acid transfer between these two lipids was inhibited in BTHS. Fluorescence staining and electron microscopy showed abnormal proliferation of mitochondria in BTHS lymphoblasts. The mitochondrial membrane potential, monitored with the fluorescence probe JC-1, was reduced in BTHS lymphoblasts. However, mitochondrial ATP formation of permeabilized lymphoblasts remained unaffected in BTHS. The data suggest that phospholipid abnormalities of BTHS mitochondria led to partial uncoupling of oxidative phosphorylation and that lymphoblasts compensated for this deficiency by expanding the mitochondrial compartment.

Multiple lipid transport pathways to the plasma membrane in yeast

The plasma membrane of the yeast Saccharomyces cerevisiae is devoid of lipid-synthesizing enzymes, but contains all classes of bilayer-forming lipids. As the lipid composition of the plasma membrane does not match any of the intracellular membranes, specific trafficking of lipids from internal membranes, especially the endoplasmic reticulum and the Golgi, to the cell periphery is required. Although the secretory pathway is an obvious route to translocate glycerophospholipids, sphingolipids and sterols to the plasma membrane, experimental evidence for the role of this pathway in lipid transport is rare. Addressing this issue in a systematic way, we labeled temperature-sensitive secretory yeast mutants (sec mutants) with appropriate lipid precursors, isolated the plasma membranes at high purity and quantified labeled lipids of this compartment. Shifting sec mutants to the restrictive temperature reduced transport of both proteins and lipids to the plasma membrane, indicating that the latter compounds are also trafficked to the cell periphery through the protein secretory pathway. However, efficient sec blocks did not abrogate protein and lipid transport, suggesting that parallel pathway(s) for the translocation of membrane components to the plasma membrane of yeast must exist.

Short-range intracellular trafficking of small molecules across endoplasmic reticulum junctions

Intracellular trafficking is not mediated exclusively by vesicles. Additional, non-vesicular mechanisms transport material, in particular small molecules such as lipids and Ca(2+) ions, from one organelle to another. This transport occurs at narrow cytoplasmic gaps called membrane contact sites (MCSs), at which two organelles come into close apposition. Despite the conservation of these structures throughout evolution, little is known about this transport, largely because of a lack of knowledge of almost all molecular components of MCSs. Recently, this situation has started to change because the structural proteins that bridge an MCS are now known in a single case, and proteins implicated in lipid trafficking have been localized to MCSs. In the light of these advances, I hypothesize that the endoplasmic reticulum has a central role in the trafficking of lipids and ions by forming a network of MCSs with most other intracellular organelles.

Maintenance of the diacylglycerol level in the Golgi apparatus by the Nir2 protein is critical for Golgi secretory function

The level of diacylglycerol (DAG) in the Golgi apparatus is crucial for protein transport to the plasma membrane. Studies in budding yeast indicate that Sec14p, a phosphatidylinositol (PI)-transfer protein, is involved in regulating DAG homeostasis in the Golgi complex. Here, we show that Nir2, a peripheral Golgi protein containing a PI-transfer domain, is essential for maintaining the structural and functional integrity of the Golgi apparatus in mammalian cells. Depletion of Nir2 by RNAi leads to substantial inhibition of protein transport from the trans-Golgi network to the plasma membrane, and causes a reduction in the DAG level in the Golgi apparatus. Remarkably, inactivation of cytidine [corrected] 5'-diphosphate (CDP)-choline pathway for phosphatidylcholine biosynthesis restores both effects. These results indicate that Nir2 is involved in maintaining a critical DAG pool in the Golgi apparatus by regulating its consumption via the CDP-choline pathway, demonstrating the interface between secretion from the Golgi and lipid homeostasis.

Effects of D‐Myo‐Inositol 1‐Phosphate on the Transfer Function of Phosphatidylinositol Transfer Protein α

The lipid metabolite D-myo-inositol-1-phosphate is shown to increase the phospholipid transfer activity of phosphatidylinositol transfer protein alpha from liposomal and liver microsomal membranes. Dose-response curves indicated substantial enhancements of transfer in the low mM range that upon normalization were independent of membrane composition or the identity of the transferred phospholipid. The unnormalized effect is potentiated by anionic membrane surface charge and substantial membrane phosphatidylethanolamine content consistent with alterations of the protein's membrane binding affinity and alterations of surface electrostatic interactions as contributing factors.