An isoform of the phosphatidylinositol-transfer protein transfers sphingomyelin and is associated with the Golgi system

The expanding roles of PI4P and PI(4,5)P2 at the plasma membrane: Role of phosphatidylinositol transfer proteins

Phosphoinositides are phosphorylated derivatives of phosphatidylinositol, a phospholipid that is synthesised at the endoplasmic reticulum. The plasma membrane contains the enzymes to phosphorylate phosphatidylinositol and is therefore rich in the phosphorylated derivatives, PI4P and PI(4,5)P2. PI(4,5)P2 is a substrate for phospholipase C and during cell signaling, PI(4,5)P2 levels are reduced. Here I discuss a family of proteins, phosphatidylinositol transfer proteins (PITPs) that can restore PI(4,5)P2 levels.

Nuclear Phosphoinositides as Key Determinants of Nuclear Functions

Polyphosphoinositides (PPIns) are signalling messengers representing less than five per cent of the total phospholipid concentration within the cell. Despite their low concentration, these lipids are critical regulators of various cellular processes, including cell cycle, differentiation, gene transcription, apoptosis and motility. PPIns are generated by the phosphorylation of the inositol head group of phosphatidylinositol (PtdIns). Different pools of PPIns are found at distinct subcellular compartments, which are regulated by an array of kinases, phosphatases and phospholipases. Six of the seven PPIns species have been found in the nucleus, including the nuclear envelope, the nucleoplasm and the nucleolus. The identification and characterisation of PPIns interactor and effector proteins in the nucleus have led to increasing interest in the role of PPIns in nuclear signalling. However, the regulation and functions of PPIns in the nucleus are complex and are still being elucidated. This review summarises our current understanding of the localisation, biogenesis and physiological functions of the different PPIns species in the nucleus.

Courier service for phosphatidylinositol: PITPs deliver on demand

Phosphatidylinositol is the parent lipid for the synthesis of seven phosphorylated inositol lipids and each of them play specific roles in numerous processes including receptor-mediated signalling, actin cytoskeleton dynamics and membrane trafficking. PI synthesis is localised to the endoplasmic reticulum (ER) whilst its phosphorylated derivatives are found in other organelles where the lipid kinases also reside. Phosphorylation of PI to phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) at the plasma membrane and to phosphatidylinositol 4-phosphate (PI4P) at the Golgi are key events in lipid signalling and Golgi function respectively. Here we review a family of proteins, phosphatidylinositol transfer proteins (PITPs), that can mobilise PI from the ER to provide the substrate to the resident kinases for phosphorylation. Recent studies identify specific and overlapping functions for the three soluble PITPs (PITPα, PITPβ and PITPNC1) in phospholipase C signalling, neuronal function, membrane trafficking, viral replication and in cancer metastases.

Phosphatidylserine biosynthesis pathways in lipid homeostasis: Toward resolution of the pending central issue for decades

Enzymatic control of lipid homeostasis in the cell is a vital element in the complex organization of life. Phosphatidylserine (PS) is an essential anionic phospholipid of cell membranes, and conducts numerous roles for their structural and functional integrity. In mammalian cells, two distinct enzymes phosphatidylserine synthases-1 (PSS1) and -2 (PSS2) in the mitochondria-associated membrane (MAM) in the ER perform de novo synthesis of PS. It is based on base-exchange reactions of the preexisting dominant phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). While PSS2 specifically catalyzes the reaction "PE → PS," whether or not PSS1 is responsible for the same reaction along with the reaction "PC → PS" remains unsettled despite its fundamental impact on the major stoichiometry. We propose here that a key but the only report that appeared to have put scientists on hold for decades in answering to this issue may be viewed consistently with other available research reports; PSS1 utilizes the two dominant phospholipid classes at a similar intrinsic rate. In this review, we discuss the issue in view of the current information for the enzyme machineries, membrane structure and dynamics, intracellular network of lipid transport, and PS synthesis in health and disease. Resolution of the pending issue is thus critical in advancing our understanding of roles of the essential anionic lipid in biology, health, and disease.

Lipid Exchangers: Cellular Functions and Mechanistic Links With Phosphoinositide Metabolism

Lipids are amphiphilic molecules that self-assemble to form biological membranes. Thousands of lipid species coexist in the cell and, once combined, define organelle identity. Due to recent progress in lipidomic analysis, we now know how lipid composition is finely tuned in different subcellular regions. Along with lipid synthesis, remodeling and flip-flop, lipid transfer is one of the active processes that regulates this intracellular lipid distribution. It is mediated by Lipid Transfer Proteins (LTPs) that precisely move certain lipid species across the cytosol and between the organelles. A particular subset of LTPs from three families (Sec14, PITP, OSBP/ORP/Osh) act as lipid exchangers. A striking feature of these exchangers is that they use phosphatidylinositol or phosphoinositides (PIPs) as a lipid ligand and thereby have specific links with PIP metabolism and are thus able to both control the lipid composition of cellular membranes and their signaling capacity. As a result, they play pivotal roles in cellular processes such as vesicular trafficking and signal transduction at the plasma membrane. Recent data have shown that some PIPs are used as energy by lipid exchangers to generate lipid gradients between organelles. Here we describe the importance of lipid counter-exchange in the cell, its structural basis, and presumed links with pathologies.

Specialized ER membrane domains for lipid metabolism and transport

The endoplasmic reticulum (ER) is a highly organized organelle that performs vital functions including de novo membrane lipid synthesis and transport. Accordingly, numerous lipid biosynthesis enzymes are localized in the ER membrane. However, it is now evident that lipid metabolism is sub-compartmentalized within the ER and that lipid biosynthetic enzymes engage with lipid transfer proteins (LTPs) to rapidly shuttle newly synthesized lipids from the ER to other organelles. As such, intimate relationships between lipid metabolism and lipid transfer pathways exist within the ER network. Notably, certain LTPs enhance the activities of lipid metabolizing enzymes; likewise, lipid metabolism can ensure the specificity of LTP transfer/exchange reactions. Yet, our understanding of these mutual relationships is still emerging. Here, we highlight past and recent key findings on specialized ER membrane domains involved in efficient lipid metabolism and transport and consider unresolved issues in the field.

Nuclear Phosphoinositides-Versatile Regulators of Genome Functions

The many functions of phosphoinositides in cytosolic signaling were extensively studied; however, their activities in the cell nucleus are much less clear. In this review, we summarize data about their nuclear localization and metabolism, and review the available literature on their involvements in chromatin remodeling, gene transcription, and RNA processing. We discuss the molecular mechanisms via which nuclear phosphoinositides, in particular phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2), modulate nuclear processes. We focus on PI(4,5)P2’s role in the modulation of RNA polymerase I activity, and functions of the nuclear lipid islets—recently described nucleoplasmic PI(4,5)P2-rich compartment involved in RNA polymerase II transcription. In conclusion, the high impact of the phosphoinositide–protein complexes on nuclear organization and genome functions is only now emerging and deserves further thorough studies.

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.

Lipid transport in the lactating mammary gland

Mammalian cells depend on phospholipid (PL) and fatty acid (FA) transport to maintain membrane structure and organization, and to fuel and regulate cellular functions. In mammary glands of lactating animals, copious milk secretion, including large quantities of lipid in some species, requires adaptation and integration of PL and FA synthesis and transport processes to meet secretion demands. At present few details exist about how these processes are regulated within the mammary gland. However, recent advances in our understanding of the structural and molecular biology of membrane systems and cellular lipid trafficking provide insights into the mechanisms underlying the regulation and integration of PL and FA transport processes the lactating mammary gland. This review discusses the PL and FA transport processes required to maintain the structural integrity and organization of the mammary gland and support its secretory functions within the context of current molecular and cellular models of their regulation.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Modifier genes for mouse phosphatidylinositol transfer protein α (vibrator) that bypass juvenile lethality

Phosphatidylinositol transfer proteins (PITPs) mediate lipid signaling and membrane trafficking in eukaryotic cells. Loss-of-function mutations of the gene encoding PITPα in mice result in a range of dosage-sensitive phenotypes, including neurological dysfunction, neurodegeneration, and premature death. We have previously reported genetic suppression of a strong hypomorphic allele, vibrator, by a wild-derived variant of Nxf1, which increases the level of PITPα made from vibrator alleles and suppresses each of the neurological and survival phenotypes. Here we report discovery and genetic mapping of additional vibrator modifiers, Mvb2 and Mvb3, from a different strain background that suppresses juvenile lethality without suppressing visible phenotypes or gene expression. Genotype-specific survival analysis predicts molecular heterosis at Mvb3. These results indicate a mechanism of suppression that bypasses a quantitative requirement for PITPα function.

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.

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.

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.

Protein-sphingolipid interactions within cellular membranes

Each intracellular organelle critically depends on maintaining its specific lipid composition that in turn contributes to the biophysical properties of the membrane. With our knowledge increasing about the organization of membranes with defined microdomains of different lipid compositions, questions arise regarding the molecular mechanisms that underlie the targeting to/segregation from microdomains of a given protein. In addition to specific lipid-transmembrane segment interactions as a basis for partitioning, the presence in a given microdomain may alter the conformation of proteins and, thus, the activity and availability for regulatory modifications. However, for most proteins, the specific lipid environment of transmembrane segments as well as its relevance to protein function and overall membrane organization are largely unknown. To help fill this gap, we have synthesized a novel photoactive sphingolipid precursor that, together with a precursor for phosphoglycerolipids and with photo-cholesterol, was investigated in vivo with regard to specific protein transmembrane span-lipid interactions. As a proof of principle, we show specific labeling of the ceramide transporter with the sphingolipid probe and describe specific in vivo interactions of lipids with caveolin-1, phosphatidylinositol transfer protein beta, and the mature form of nicastrin. This novel photolabile sphingolipid probe allows the detection of protein-sphingolipid interactions within the membrane bilayer of living cells.

The pathologies associated with functional titration of phosphatidylinositol transfer protein α activity in mice

Phosphatidylinositol transfer proteins (PITPs) bind phosphatidylinositol (PtdIns) and phosphatidylcholine and play diverse roles in coordinating lipid metabolism/signaling with intracellular functions. The underlying mechanisms remain unclear. Genetic ablation of PITPalpha in mice results in neonatal lethality characterized by intestinal and hepatic steatosis, spinocerebellar neurodegeneration, and glucose homeostatic defects. We report that mice expressing a PITPalpha selectively ablated for PtdIns binding activity (Pitpalpha(T59D)), as the sole source of PITPalpha, exhibit phenotypes that recapitulate those of authentic PITPalpha nullizygotes. Analyses of mice with graded reductions in PITPalpha activity reveal proportionately graded reductions in lifespan, demonstrate that intestinal steatosis and hypoglycemia are apparent only when PITPalpha protein levels are strongly reduced (>or=90%), and correlate steatotic and glucose homeostatic defects with cerebellar inflammatory disease. Finally, reconstitution of PITPalpha expression in the small intestine substantially corrects the chylomicron retention disease and cerebellar inflammation of Pitpalpha(0/0) neonates, but does not rescue neonatal lethality in these animals. These data demonstrate that PtdIns binding is an essential functional property of PITPalpha in vivo, and suggest a causal linkage between defects in lipid transport and glucose homeostasis and cerebellar inflammatory disease. Finally, the data also demonstrate intrinsic neuronal deficits in PITPalpha-deficient mice that are independent of intestinal lipid transport defects and hypoglycemia.

Biochemical and biological functions of class I phosphatidylinositol transfer proteins

Phosphoinositides function in a diverse array of cellular activities. They include a role as substrate for lipid kinases and phospholipases to generate second messengers, regulators of the cytoskeleton, of enzymes and of ion channels, and docking sites for reversible recruitment of proteins to membranes. Mammalian phosphatidylinositol transfer proteins, PITPalpha and PITPbeta are paralogs that share 77% sequence identity and contain a hydrophobic cavity that can sequester either phosphatidylinositol or phosphatidylcholine. A string of 11 amino acid residues at the C-terminal acts as a "lid" which shields the lipid from the aqueous environment. PITPs in vitro can facilitate inter-membrane lipid transfer and this requires the movement of the "lid" to allow the lipid cargo to be released. Thus PITPs are structurally designed for delivering lipid cargo and could thus participate in cellular events that are dependent on phosphatidylinositol or derivatives of phosphatidylinositol. Phosphatidylinositol, the precursor for all phosphoinositides is synthesised at the endoplasmic reticulum and its distribution to other organelles could be facilitated by PITPs. Here we highlight recent studies that report on the three-dimensional structures of the different PITP forms and suggest how PITPs are likely to dock at the membrane surface for lipid delivery and extraction. Additionally we discuss whether PITPs are important regulators of sphingomyelin metabolism, and finally describe recent studies that link the association of PITPs with diverse functions including membrane traffic at the Golgi, neurite outgrowth, cytokinesis and stem cell growth.

RdgB proteins: Functions in lipid homeostasis and signal transduction

The RdgBs are a group of evolutionarily conserved molecules that contain a phosphatidylinositol transfer protein (PITP) domain. However in contrast to classical PITPs (PITPalpha) with whom they share the conserved PITP domain, these proteins also contain several additional sequence elements whose functional significance remains unknown. The founding member of the family DrdgB alpha (Drosophila rdgB) appears to be essential for sensory transduction and maintenance of ultra structure in photoreceptors (retinal sensory neurons). Although proposed to support the maintenance of phosphatidylinositol 4, 5 bisphosphate [PI (4, 5) P(2)] levels during G-protein coupled phospholipase C activity in these cells, the biochemical mechanism of DrdgB alpha function remains unresolved. More recently, a mammalian RdgB protein has been implicated in the maintenance of diacylglycerol (DAG) levels and secretory function at Golgi membranes. In this review we discuss existing work on the function of RdgB proteins and set out future challenges in understanding this group of lipid transfer proteins.

Tissue factor trafficking in fibroblasts: involvement of protease-activated receptor-mediated cell signaling

Tissue factor (TF) is the cellular receptor for clotting factor VIIa (FVIIa), and the formation of TF-FVIIa complexes on cell surfaces triggers the activation of the coagulation cascade and the cell signaling. Our recent studies have shown that a majority of TF resides in various intracellular compartments, predominantly in the Golgi, and that FVIIa binding to cell surface TF induces TF endocytosis and mobilizes the Golgi TF pool to translocate it to the cell surface. This present study is aimed to elucidate the mechanisms involved in TF endocytosis and its mobilization from the Golgi. Activation of protease-activated receptor 1 (PAR1) and PAR2 by specific peptide agonists and proteases, independent of FVIIa, mobilized TF from the Golgi store and increased the cell surface expression of TF. Blocking PAR2 activation, but not PAR1, with neutralizing antibodies fully attenuated the FVIIa-induced TF mobilization. Consistent with these data, silencing the PAR2 receptor, and not PAR1, abrogated the FVIIa-mediated TF mobilization. In contrast to their effect on TF mobilization, PAR1 and PAR2 activation, in the absence of FVIIa, had no effect on TF endocytosis. However, PAR2 activation is found to be critical for the FVIIa-induced TF endocytosis. Overall the data herein provide novel insights into the role of PARs in regulating cell surface TF expression.

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.

Phospholipid transfer proteins in perspective

Since their discovery and subsequent purification from mammalian tissues more than 30 years ago an impressive number of studies have been carried out to characterize and elucidate the biological functions of phosphatidylcholine transfer protein (PC-TP), phosphatidylinositol transfer protein (PI-TP) and non-specific lipid transfer protein, more commonly known as sterol carrier protein 2 (SCP-2). Here I will present information to show that these soluble, low-molecular weight proteins constitute domain structures in StArR-related lipid transfer (START) proteins (i.e. PC-TP), in retinal degeneration protein, type B (RdgB)-related PI-TPs (e.g. Dm RdgB, Nir2, Nir3) and in peroxisomal beta-oxidation enzyme-related SCP-2 (i.e. 3-oxoacyl-CoA thiolase, also denoted as SCP-X and the 80-kDa D-bifunctional protein). Further I will summarize the most recent studies pertaining to the physiological function of these soluble phospholipid transfer proteins in metazoa.

A phosphatidylinositol transfer protein α-dependent survival factor protects cultured primary neurons against serum deprivation-induced cell death

Selective neuronal loss is a prominent feature in both acute and chronic neurological disorders. Recently, a link between neurodegeneration and a deficiency in the lipid transport protein phosphatidylinositol transfer protein alpha (PI-TPalpha) has been demonstrated. In this context it may be of importance that fibroblasts overexpressing PI-TPalpha are known to produce and secrete bioactive survival factors that protect fibroblasts against UV-induced apoptosis. In the present study it was investigated whether the conditioned medium of cells overexpressing PI-TPalpha (CMalpha) has neuroprotective effects on primary neurons in culture. We show that CMalpha is capable of protecting primary, spinal cord-derived motor neurons from serum deprivation-induced cell death. Since the conditioned medium of wild-type cells was much less effective, we infer that the neuroprotective effect of CMalpha is linked (in part) to the PI-TPalpha-dependent production of arachidonic acid metabolites. The neuroprotective activity of CMalpha is partly inhibited by suramin, a broad-spectrum antagonist of G-protein coupled receptors. Western blot analysis shows that brain cortex and spinal cord express relatively high levels of PI-TPalpha, suggesting that the survival factor may be produced in neuronal tissue. We propose that the bioactive survival factor is implicated in neuronal survival. If so, PI-TPalpha could be a promising target to be evaluated in studies on the prevention and treatment of neurological disorders.

Specific and nonspecific membrane-binding determinants cooperate in targeting phosphatidylinositol transfer protein beta-isoform to the mammalian trans-Golgi network

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between lipid metabolism and specific steps in membrane trafficking through the secretory pathway in eukaryotes. Herein, we describe the cis-acting information that controls PITPbeta localization in mammalian cells. We demonstrate PITPbeta localizes predominantly to the trans-Golgi network (TGN) and that this localization is independent of the phospholipid-bound state of PITPbeta. Domain mapping analyses show the targeting information within PITPbeta consists of three short C-terminal specificity elements and a nonspecific membrane-binding element defined by a small motif consisting of adjacent tryptophan residues (the W(202)W(203) motif). Combination of the specificity elements with the W(202)W(203) motif is necessary and sufficient to generate an efficient TGN-targeting module. Finally, we demonstrate that PITPbeta association with the TGN is tolerant to a range of missense mutations at residue serine 262, we describe the TGN localization of a novel PITPbeta isoform with a naturally occurring S262Q polymorphism, and we find no other genetic or pharmacological evidence to support the concept that PITPbeta localization to the TGN is obligately regulated by conventional protein kinase C (PKC) or the Golgi-localized PKC isoforms delta or epsilon. These latter findings are at odds with a previous report that conventional PKC-mediated phosphorylation of residue Ser262 is required for PITPbeta targeting to Golgi membranes.

Structure of PITPβ in Complex with Phosphatidylcholine: Comparison of Structure and Lipid Transfer to Other PITP Isoforms,

Phosphatidylinositol transfer protein (PITP) is a ubiquitous eukaryotic protein that preferentially binds either phosphatidylinositol or phosphatidylcholine and catalyzes the exchange of these lipids between membranes. Mammalian cytosolic PITPs include the ubiquitously expressed PITPalpha and PITPbeta isoforms (269-270 residues). The crystal structure of rat PITPbeta complexed to dioleoylphosphatidylcholine was determined to 2.18 A resolution with molecular replacement using rat PITPalpha (77% sequence identify) as the phasing model. A structure comparison of the alpha and beta isoforms reveals minimal differences in protein conformation, differences in acyl conformation in the two isoforms, and remarkable conservation of solvent structure around the bound lipid. A comparison of transfer activity by human and rat PITPs, using small unilamellar vesicles with carefully controlled phospholipid composition, indicates that the beta isoforms have minimal differences in transfer preference between PtdIns and PtdCho when donor vesicles contain predominantly PtdCho. When PtdCho and PtdIns are present in equivalent concentrations in donor vesicles, PtdIns transfer occurs at approximately 3-fold the rate of PtdCho. The rat PITPbeta isoform clearly has the most diminished transfer rate of the four proteins studied. With the two rat isoforms, site-directed mutations of two locations within the lipid binding cavity that possess differing biochemical properties were characterized: I84alpha/F83beta and F225alpha/L224beta. The 225/224 locus is more critical in determining substrate specificity. Following the mutation of this locus to the other amino acid, the PtdCho transfer specific activity became PITPalpha (F225L) approximately PITPbeta and PITPbeta (L224F) approximately PITPalpha. The 225alpha/224beta locus plays a modest role in the specificity of both isoforms toward CerPCho.

Molecular mechanisms and regulation of ceramide transport

De novo biosynthesis of sphingolipids begins in the endoplasmic reticulum (ER) and continues in the Golgi apparatus and plasma membrane. A crucial step in sphingolipid biosynthesis is the transport of ceramide by vesicular and non-vesicular mechanisms from its site of synthesis in the ER to the Golgi apparatus. The recent discovery of the ceramide transport protein CERT has revealed a novel pathway for the delivery of ceramide to the Golgi apparatus for sphingomyelin (SM) synthesis. In addition to a ceramide-binding START domain, CERT has FFAT (referring to two phenylalanines [FF] in an acidic tract) and pleckstrin homology (PH) domains that recognize the ER integral membrane protein VAMP-associated protein (VAP) and Golgi-associated PtdIns 4-phosphate, respectively. Mechanisms for vectorial transport involving dual-organellar targeting and sites of deposition of ceramide in the Golgi apparatus are proposed. Similar Golgi-ER targeting motifs are also present in the oxysterol-binding protein (OSBP), which regulates ceramide transport and SM synthesis in an oxysterol-dependent manner. Consequently, this emerges as a potential mechanism for integration of sphingolipid and cholesterol metabolism. The identification of organellar targeting motifs in other related lipid-binding/transport proteins indicate that concepts learned from the study of ceramide transport can be applied to other lipid transport processes.

Lipid metabolism in phosphatidylinositol transfer protein α-deficient vibrator mice

Mice that are homozygous for the vibrator mutation express 65-85% less phosphatidylinositol transfer protein alpha (PITPalpha) than their wild type litter mates. By postnatal day 10-12 (P10-12) they exhibit signs of neurodegeneration and die prematurely by P40. In the present study, we examine the lipid content of brain, liver, and mammary glands from these animals. Lipid-mediated signal transduction is evaluated in primary fibroblast cultures. With respect to the lipid make-up of brain and liver, we report that there is a significant increase (2- to 4-fold) in the neutral lipids present in the livers of vb/vb animals when compared with wild type (+/+) litter mates. No significant changes are observed in the brains of these animals. The mammary glands of vb/vb mice are underdeveloped with respect to ductal and alveolar structures, and the fat pad is composed of predominantly brown adipose tissue rather than the white adipose tissue characteristic of age-matched wild type litter mates. No differences are observed in any aspect of lipid-mediated signal transduction.

Overexpression of phosphatidylinositol transfer protein β in NIH3T3 cells has a stimulatory effect on sphingomyelin synthesis and apoptosis

Phosphatidylinositol transfer proteins (PI-TPs) consist of two isoforms (PI-TPalpha and PI-TPbeta), which differ in phospholipid transfer properties and intracellular localization. Both PI-TP isoforms are substrates for protein kinase C and contain a minor phosphorylation site (Ser166 in PI-TPalpha; Ser165 in PI-TPbeta). Only PI-TPbeta contains a major phosphorylation site at Ser262, which must be phosphorylated for PI-TPbeta to be associated with the Golgi. The PI-TP isoforms are completely conserved between mammals. Although their function is still not clear, their importance follows from knock-out studies, showing that mice lacking PI-TPalpha die soon after birth and that embryonic stems cells lacking PI-TPbeta cannot be generated [Mol. Biol. Cell 13 (2002) 739]. We determined the levels of the PI-TP isoforms in various mouse tissues by immunoblotting. PI-TPalpha is present in all tissues investigated, with highest levels in brain (167 ng/100 microg total protein). The levels of PI-TPbeta are 50-100 times lower than those of PI-TPalpha, with relatively high levels found in liver and brain (1.2 and 1.8 ng/100 microg of total protein, respectively). In contrast to NIH3T3 cells overexpressing PI-TPalpha, cells overexpressing PI-TPbeta (SPIbeta cells) were able to maintain steady-state levels of sphingomyelin in plasma membrane under conditions where this lipid is degraded by exogenous sphingomyelinase. This process of rapid sphingomyelin replenishment is dependent on PI-TPbeta being associated with the Golgi as cells overexpressing a mutant PI-TPbeta in which the major phosphorylation site is replaced (PI-TPbeta(S262A) behave as wild-type NIH3T3 cells. Since the SPIbeta cells display a decreased growth rate (35 h as compared to 21 h for wtNIH3T3 cells), we have investigated the sensitivity of these cells towards UV-induced apoptosis. We have found that the SPIbeta cells, but not the cells overexpressing PI-TPbeta(S262A), are very sensitive. We are currently investigating whether a relationship exists between PI-TPbeta being involved in maintaining plasma membrane sphingomyelin levels and the enhanced sensitivity towards apoptosis.

Cloning and characterization of a novel variant (mM-rdgBbeta1) of mouse M-rdgBs, mammalian homologs of Drosophila retinal degeneration B gene proteins, and its mRNA localization in mouse brain in comparison with other M-rdgBs

We report the cloning, characterization and localization in the brain of a novel isoform termed mM-rdgBbeta1 (mouse type of mammalian retinal degeneration Bbeta1 protein) in comparison with the localization of three known mammalian homologs (M-rdgBbeta, M-rdgB1, M-rdgB2). mM-rdgBbeta1 cDNA contains a sequence of 119 bp as a form of insertion in the open reading frame of the known mM-rdgBbeta, and encodes a protein of 269 amino acids with a calculated molecular mass of 31.7 kDa, different from the molecular mass of 38.3 kDa of mM-rdgBbeta. It also contains a phosphatidylinositol transfer protein (PITP)-like domain similar to the known three homologs, as well as D-rdgB. The recombinant mM-rdgBbeta1 protein shows the specific binding activity to phosphatidylinositol but not to other phospholipids. This novel molecule is localized not only in the cytoplasm but also in the nucleus, different from the cytoplasmic localization of mM-rdgBbeta. In in situ hybridization analysis, the gene expression for mM-rdgBbeta1 in the brain, though weak, is rather confined to the embryonic stage, different from wider expression of mM-rdgBbeta in the gray matters of pre- and post-natal brains. Taken together, mM-rdgBbeta1 is suggested to play a role in the phosphoinositide-mediated signaling in the neural development.

The structure of phosphatidylinositol transfer protein alpha reveals sites for phospholipid binding and membrane association with major implications for its function

Elucidation of the three-dimensional structure of phosphatidylinositol transfer protein alpha (PI-TPalpha) void of phospholipid revealed a site of membrane association connected to a channel for phospholipid binding. Near the top of the channel specific binding sites for the phosphorylcholine and phosphorylinositol head groups were identified. The structure of this open form suggests a mechanism by which PI-TPalpha preferentially binds PI from a membrane interface. Modeling predicts that upon association of PI-TPalpha with the membrane the inositol moiety of bound PI is accessible from the medium. Upon release from the membrane PI-TPalpha adopts a closed structure with the phospholipid bound fully encapsulated. This structure provides new insights as to how PI-TPalpha may play a role in PI metabolism.

Current thoughts on the phosphatidylinositol transfer protein family

Monomeric transport of lipids is carried out by a class of proteins that can shield a lipid from the aqueous environment by binding the lipid in a hydrophobic cavity. One such group of proteins is the phosphatidylinositol transfer proteins (PITP) that can bind phosphatidylinositol and phosphatidylcholine and transfer them from one membrane compartment to another. PITPs are found in both unicellular and multicellular organisms but not bacteria. In mice and humans, the PITP domain responsible for lipid transfer is found in five proteins, which can be classified into two classes based on sequence. Class I PITPs comprises two family members, alpha and beta, small 35 kDa proteins with a single PITP domain which are ubiquitously expressed. Class IIA PITPs (RdgBalphaI and II) are larger proteins possessing additional domains that target the protein to membranes and are only able to bind lipids but not mediate transfer. Finally, Class IIB PITP (RdgBbeta) is similar to Class I in size (38 kDa) and is also ubiquitously expressed. Class III PITPs, exemplified by the Sec14p family, are found in yeast and plants but are unrelated in sequence and structure to Class I and Class II PITPs. In this review we discuss whether PITP proteins are passive transporters or are regulated proteins that are able to couple their transport and binding properties to specific biological functions including inositol lipid signalling and membrane turnover.

Phosphatidylinositol transfer protein beta displays minimal sphingomyelin transfer activity and is not required for biosynthesis and trafficking of sphingomyelin

Mammalian phosphatidylinositol transfer proteins (PITPs) alpha and beta, which share 77% identity, have been shown to exhibit distinct lipid-transfer activities. In addition to transferring phosphatidylinositol (PI) and phosphatidylcholine (PC), PITPbeta has been shown to transfer sphingomyelin (SM), and this has led to the suggestion that PITPbeta is important for the regulation of SM metabolism. In the present study, we have analysed the ability of human PITPbeta to transfer and regulate the metabolism of cellular SM. We report that, in vitro, the two PITP isoforms were comparable in mediating PI, PC or SM transfer. Using permeabilized HL-60 cells as the donor compartment, both PITP isoforms efficiently transferred PI and PC, and were slightly active towards SM, with the activity of PITPbeta being slightly greater. To identify which cellular lipids were selected by PITPs, PITPalpha and PITPbeta were exposed to permeabilized HL-60 cells, and subsequently repurified and analysed for their bound lipids. Both PITPs were able to select only PI and PC, but not SM. SM synthesis takes place at the Golgi, and PITPbeta was shown to localize in that compartment. To examine the role of PITPbeta in SM biosynthesis, Golgi membranes were used. Purified Golgi membranes had lost their endogenous PITPbeta, but were able to recruit PITPbeta when added exogenously. However, PITPbeta did not enhance the activities of either SM synthase or glucosylceramide synthase. Further analysis in COS-7 cells overexpressing PITPbeta showed no effects on (a) SM and glucosylceramide biosynthesis, (b) diacylglycerol or ceramide levels, (c) SM transport from the Golgi to the plasma membrane, or (d) resynthesis of SM after exogenous sphingomyelinase treatment. Altogether, these observations do not support the suggestion that PITPbeta participates in the transfer of SM, the regulation of SM biosynthesis or its intracellular trafficking.

The Golgi localization of phosphatidylinositol transfer protein beta requires the protein kinase C-dependent phosphorylation of serine 262 and is essential for maintaining plasma membrane sphingomyelin levels

Recombinant mouse phosphatidylinositol transfer protein (PI-TP)beta is a substrate for protein kinase C (PKC)-dependent phosphorylation in vitro. Based on site-directed mutagenesis and two-dimensional tryptic peptide mapping, Ser(262) was identified as the major site of phosphorylation and Ser(165) as a minor phosphorylation site. The phospholipid transfer activities of wild-type PI-TP beta and PI-TP beta(S262A) were identical, whereas PI-TP beta(S165A) was completely inactive. PKC-dependent phosphorylation of Ser(262) also had no effect on the transfer activity of PI-TP beta. To investigate the role of Ser(262) in the functioning of PI-TP beta, wtPI-TP beta and PI-TP beta(S262A) were overexpressed in NIH3T3 fibroblast cells. Two-dimensional PAGE analysis of cell lysates was used to separate PI-TP beta from its phosphorylated form. After Western blotting, wtPI-TP beta was found to be 85% phosphorylated, whereas PI-TP beta(S262A) was not phosphorylated. In the presence of the PKC inhibitor GF 109203X, the phosphorylated form of wtPI-TP beta was strongly reduced. Immunolocalization showed that wtPI-TP beta was predominantly associated with the Golgi membranes. In the presence of the PKC inhibitor, wtPI-TP beta was distributed throughout the cell similar to what was observed for PI-TP beta(S262A). In contrast to wtPI-TP beta overexpressors, cells overexpressing PI-TP beta(S262A) were unable to rapidly replenish sphingomyelin in the plasma membrane upon degradation by sphingomyelinase. This implies that PKC-dependent association with the Golgi complex is a prerequisite for PI-TP beta to express its effect on sphingomyelin metabolism.

Both isoforms of mammalian phosphatidylinositol transfer protein are capable of binding and transporting sphingomyelin

The structurally related mammalian alpha and beta isoforms of phosphatidylinositol (PtdIns) transfer protein (PITP) bind reversibly a single phospholipid molecule, preferably PtdIns or phosphatidylcholine (PtdCho), and transport that lipid between membrane surfaces. PITPbeta, but not PITPalpha, is reported extensively in the scientific literature to exhibit the additional capacity to bind and transport sphingomyelin (CerPCho). We undertook a detailed investigation of the lipid binding and transfer specificity of the soluble mammalian PITP isoforms. We employed a variety of donor and acceptor membrane lipid compositions to determine the sensitivity of recombinant rat PITPalpha and PITPbeta isoforms toward PtdIns, PtdCho, CerPCho, and phosphatidate (PtdOH). Results indicated often striking differences in protein-phospholipid and protein-membrane interactions. We demonstrated unequivocally that both isoforms were capable of binding and transferring CerPCho; we confirmed that the beta isoform was the more active. The order of transfer specific activity was similar for both isoforms: PtdIns>PtdCho>CerPCho>>PtdOH. Independently, we verified the binding of CerPCho to both isoforms by showing an increase in holoprotein isoelectric point following the exchange of protein-bound phosphatidylglycerol for membrane-associated CerPCho. We conclude that PITPalpha and PITPbeta are able to bind and transport glycero- and sphingophospholipids.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Phosphatidylinositol transfer proteins and protein kinase C make separate but non-interacting contributions to the phosphorylation state necessary for secretory competence in rat mast cells

Mast cells permeabilized by streptolysin O undergo exocytosis when stimulated with Ca(2+) and guanosine 5'-[gamma-thio]triphosphate but become progressively refractory to this stimulus if it is delayed. This run-down of responsiveness occurs over a period of 20-30 min, during which the cells leak soluble and tethered proteins. We show here that withdrawal of ATP during the process of run-down is strongly inhibitory but that as little as 25 microM ATP can extend responsiveness significantly; this effect is maximal at 50 microM. When phosphatidylinositol transfer proteins (PITPs) are provided to cells at the time of permeabilization, run-down is retarded. We conclude that in the presence of ATP they convey substrates for phosphorylation that are essential for exocytosis and thus interact with the regulatory machinery. Furthermore, we show that PITPalpha and PITPbeta have additive effects in this mechanism, suggesting that they are not functionally redundant. Alternatively, secretion from run-down cells can be inhibited by the aminoglycoside antibiotic neomycin, which is understood to bind to phosphoinositide headgroups, and by a PH (pleckstrin homology) domain polypeptide that binds phosphoinositides. The apparent displacement of neomycin by exogenous PITPs suggests that these proteins screen essential lipids. Secretion from run-down cells is also inhibited by 1-O-hexadecyl-2-O-methyl-rac-glycerol (AMG-C(16)), an inhibitor of protein kinase C. The lack of synergy between neomycin and AMG-C(16) suggests that protein kinase C independently provides a second essential component through protein phosphorylation and that there are two independent phosphorylation pathways necessary for secretion competence.

Activation of phosphatidylinositol transfer protein alpha and beta isoforms from inclusion bodies

Fully active phosphatidylinositol transfer protein (PI-TP) isoforms alpha and beta have been obtained from Escherichia coli inclusion bodies. Folding and activation of PI-TPalpha was achieved in the presence of DiC7:0-phosphatidylcholine-Triton X-114 (PtdCho-TX114) mixed micelles. Replacement of DiC7:0-PtdCho with the natural ligands of PI-TPalpha, i.e. long-chain PtdCho and phosphatidylinositol, did not stimulate activation. Efficient activation of PI-TPalpha required a low temperature (4 degrees C), the presence of dithiothreitol, and was achieved at a relatively high protein concentration (i.e. up to 500 microg ml(-1)). The inclusion bodies yielded 10 mg homogeneous PI-TPalpha per liter of E. coli culture. Conditions for full activation of PI-TPbeta were similar to those for PI-TPalpha except that long-chain PtdCho-TX114 mixed micelles and a very low protein concentration (i.e. 10 microg ml(-1)) were required. In contrast to PI-TPalpha, PI-TPbeta lost its lipid transfer activity within a few days. This inactivation could be prevented by addition of beta-alanine. In summary, despite 94% sequence similarity, PI-TPalpha and PI-TPbeta display a striking difference both in their preference for the PtdCho acyl chain length required for activation, and in their conformational stability after folding.

Reconstitution of ATP- and cytosol-dependent transport of de novo synthesized ceramide to the site of sphingomyelin synthesis in semi-intact cells

Transport of ceramide synthesized at the endoplasmic reticulum to the Golgi compartment, where sphingomyelin (SM) synthase exists, was reconstituted within semi-intact Chinese hamster ovary cells. When [(3)H]ceramide that had been produced from [(3)H]sphingosine at 15 degrees C in perforated cells was chased at 37 degrees C, [(3)H]ceramide-to-[(3)H]SM conversion occurred in a cytosol-dependent manner. In various aspects (i.e. kinetics, ATP dependence, and temperature dependence), [(3)H]ceramide-to-[(3)H]SM conversion in perforated cells was consistent with that in intact cells. The cytosol from LY-A strain, a Chinese hamster ovary cell mutant defective in endoplasmic reticulum-to-Golgi transport of ceramide, did not support [(3)H]ceramide-to-[(3)H]SM conversion in perforated wild-type cells, whereas the wild-type cytosol rescued the conversion in perforated LY-A cells. Brefeldin A-treated cells, in which the endoplasmic reticulum and the Golgi apparatus were merged, no longer required cytosol for conversion of [(3)H]ceramide to [(3)H]SM. These results indicated that the assay of [(3)H]ceramide-to-[(3)H]SM conversion in semi-intact cells is a faithful in vitro assay for the activity of cytosol-dependent transport of ceramide and that LY-A cells are defective in a cytosolic factor involved in ceramide transport. In addition, conversion of [(3)H]ceramide to [(3)H]glucosylceramide in semi-intact cells was little dependent on cytosol, suggesting that ceramide reached the site of glucosylceramide synthesis by a cytosol-independent (or less dependent) pathway.

The protein kinase C-dependent phosphorylation of serine 166 is controlled by the phospholipid species bound to the phosphatidylinositol transfer protein alpha

The charge isomers of bovine brain PI-TPalpha (i.e. PI-TPalphaI containing a phosphatidylinositol (PI) molecule and PI-TPalphaII containing a phosphatidylcholine (PC) molecule) were phosphorylated in vitro by rat brain protein kinase C (PKC) at different rates. From the double-reciprocal plot, it was estimated that the V(max) values for PI-TPalphaI and II were 2.0 and 6.0 nmol/min, respectively; the K(m) values for both charge isomers were about equal, i.e. 0.7 micrometer. Phosphorylation of charge isomers of recombinant mouse PI-TPalpha confirmed that the PC-containing isomer was the better substrate. Phosphoamino acid analysis of in vitro and in vivo (32)P-labeled PI-TPalphas showed that serine was the major site of phosphorylation. Degradation of (32)P-labeled PI-TPalpha by cyanogen bromide followed by high pressure liquid chromatography and sequence analysis yielded one (32)P-labeled peptide (amino acids 104-190). This peptide contained Ser-148, Ser-152, and the consensus PKC phosphorylation site Ser-166. Replacement of Ser-166 with an alanine residue confirmed that indeed this residue was the site of phosphorylation. This mutation completely abolished PI and PC transfer activity. This was also observed when Ser-166 was replaced with Asp, implying that this is a key amino acid residue in regulating the function of PI-TPalpha. Stimulation of NIH3T3 fibroblasts by phorbol ester or platelet-derived growth factor induced the rapid relocalization of PI-TPalpha to perinuclear Golgi structures concomitant with a 2-3-fold increase in lysophosphatidylinositol levels. This relocalization was also observed for Myc-tagged wtPI-TPalpha expressed in NIH3T3 cells. In contrast, the distribution of Myc-tagged PI-TPalpha(S166A) and Myc-tagged PI-TPalpha(S166D) were not affected by phorbol ester, suggesting that phosphorylation of Ser-166 was a prerequisite for the relocalization to the Golgi. A model is proposed in which the PKC-dependent phosphorylation of PI-TPalpha is linked to the degradation of PI.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Purification and cloning of phosphatidylinositol transfer proteins from Dictyostelium discoideum: homologues of both mammalian PITPs and Saccharomyces cerevisiae sec14p are found in the same cell

Soluble phosphatidylinositol transfer proteins (PITPs) have important roles in lipid-mediated signalling as well as in membrane traffic. Two PITPs (alpha and beta) have been cloned from mammalian cells, which are unrelated in sequence to yeast PITP (the product of the SEC14 gene). However, all three PITPs can perform interchangeably to reconstitute function in mammalian cells. We have now purified the major PITP from the cytoplasm of Dictyostelium discoideum and cloned the gene. This protein, DdPITP1, is homologous with mammalian PITPalpha and PITPbeta. We have also cloned a second gene (DdPITP2) related in sequence to DdPITP1. In addition, an independently cloned cDNA encodes a relative of the SEC14 family of yeast PITPs. DdPITP1, DdPITP2 and DdSec14 proteins were all able to mediate the transfer of PtdIns from one membrane compartment to another; they thus exhibited the hallmark of PITPs. Secondly, all three PITPs were able to rescue phospholipase C-mediated phosphoinositide hydrolysis in PITP-depleted HL60 cells, indicating that all three PITPs were capable of stimulating phosphoinositide synthesis. The identification of PITPs related to both mammalian PITPs and yeast Sec14p in a single organism will provide a unique opportunity to examine the functions of this class of protein with genetic approaches.

Rapid replenishment of sphingomyelin in the plasma membrane upon degradation by sphingomyelinase in NIH3T3 cells overexpressing the phosphatidylinositol transfer protein beta

In order to study the in vivo function of the phosphatidylinositol transfer protein beta (PI-TPbeta), mouse NIH3T3 fibroblasts were transfected with cDNA encoding mouse PI-TPbeta. Two stable cell lines were isolated (SPIbeta2 and SPIbeta8) in which the levels of PI-TPbeta were increased 16- and 11-fold respectively. The doubling time of the SPIbeta cells was about 1.7 times that of the wild-type (wt) cells. Because PI-TPbeta expresses transfer activity towards sphingomyelin (SM) in vitro, the SM metabolism of the overexpressors was investigated. By measuring the incorporation of [methyl-(3)H]choline chloride in SM and phosphatidylcholine (PtdCho), it was shown that the rate of de novo SM and PtdCho synthesis was similar in transfected and wt cells. We also determined the ability of the cells to resynthesize SM from ceramide produced in the plasma membrane by the action of bacterial sphingomyelinase (bSMase). In these experiments the cells were labelled to equilibrium (60 h) with [(3)H]choline. At relatively low bSMase concentrations (50 munits/ml), 50% of [(3)H]SM in wt NIH3T3 cells was degraded, whereas the levels of [(3)H]SM in SPIbeta cells appeared to be unaffected. Since the release of [(3)H]choline phosphate into the medium was comparable for both wt NIH3T3 and SPIbeta cells, these results strongly suggest that breakdown of SM in SPIbeta cells was masked by rapid resynthesis of SM from the ceramide formed. By increasing the bSMase concentrations to 200 munits/ml, a 50% decrease in the level of [(3)H]SM in SPIbeta cells was attained. During a recovery period of 6 h (in the absence of bSMase) the resynthesis of SM was found to be much more pronounced in these SPIbeta cells than in 50% [(3)H]SM-depleted wt NIH3T3 cells. After 6 h of recovery about 50% of the resynthesized SM in the SPIbeta cells was available for a second hydrolysis by bSMase. When monensin was present during the recovery period, the resynthesis of SM in bSMase-treated SPIbeta cells was not affected. However, under these conditions 100% of the resynthesized SM was available for hydrolysis. On the basis of these results we propose that, under conditions where ceramide is formed in the plasma membrane, PI-TPbeta plays an important role in restoring the steady-state levels of SM.