Enhancement of transport-dependent decarboxylation of phosphatidylserine by S100B protein in permeabilized Chinese hamster ovary cells

Advancements in unravelling the fundamental function of the ATAD3 protein in multicellular organisms

ATPase family AAA domain containing protein 3, commonly known as ATAD3 is a versatile mitochondrial protein that is involved in a large number of pathways. ATAD3 is a transmembrane protein that spans both the inner mitochondrial membrane and outer mitochondrial membrane. It, therefore, functions as a connecting link between the mitochondrial lumen and endoplasmic reticulum facilitating their cross-talk. ATAD3 contains an N-terminal domain which is amphipathic in nature and is inserted into the membranous space of the mitochondria, while the C-terminal domain is present towards the lumen of the mitochondria and contains the ATPase domain. ATAD3 is known to be involved in mitochondrial biogenesis, cholesterol transport, hormone synthesis, apoptosis and several other pathways. It has also been implicated to be involved in cancer and many neurological disorders making it an interesting target for extensive studies. This review aims to provide an updated comprehensive account of the role of ATAD3 in the mitochondria especially in lipid transport, mitochondrial-endoplasmic reticulum interactions, cancer and inhibition of mitophagy.

Mitochondrial phospholipid transport: Role of contact sites and lipid transport proteins

One of the major constituents of mitochondrial membranes is the phospholipids, which play a key role in maintaining the structure and the functions of the mitochondria. However, mitochondria do not synthesize most of the phospholipids in situ, necessitating the presence of phospholipid import pathways. Even for the phospholipids, which are synthesized within the inner mitochondrial membrane (IMM), the phospholipid precursors must be imported from outside the mitochondria. Therefore, the mitochondria heavily rely on the phospholipid transport pathways for its proper functioning. Since, mitochondria are not part of a vesicular trafficking network, the molecular mechanisms of how mitochondria receive its phospholipids remain a relevant question. One of the major ways that hydrophobic phospholipids can cross the aqueous barrier of inter or intraorganellar spaces is by apposing membranes, thereby decreasing the distance of transport, or by being sequestered by lipid transport proteins (LTPs). Therefore, with the discovery of LTPs and membrane contact sites (MCSs), we are beginning to understand the molecular mechanisms of phospholipid transport pathways in the mitochondria. In this review, we will present a brief overview of the recent findings on the molecular architecture and the importance of the MCSs, both the intraorganellar and interorganellar contact sites, in facilitating the mitochondrial phospholipid transport. In addition, we will also discuss the role of LTPs for trafficking phospholipids through the intermembrane space (IMS) of the mitochondria. Mechanistic insights into different phospholipid transport pathways of mitochondria could be exploited to vary the composition of membrane phospholipids and gain a better understanding of their precise role in membrane homeostasis and mitochondrial bioenergetics.

ATAD3 proteins: brokers of a mitochondria-endoplasmic reticulum connection in mammalian cells

In yeast, a sequence of physical and genetic interactions termed the endoplasmic reticulum (ER)-mitochondria organizing network (ERMIONE) controls mitochondria-ER interactions and mitochondrial biogenesis. Several functions that characterize ERMIONE complexes are conserved in mammalian cells, suggesting that a similar tethering complex must exist in metazoans. Recent studies have identified a new family of nuclear-encoded ATPases associated with diverse cellular activities (AAA+-ATPase) mitochondrial membrane proteins specific to multicellular eukaryotes, called the ATPase family AAA domain-containing protein 3 (ATAD3) proteins (ATAD3A and ATAD3B). These proteins are crucial for normal mitochondrial-ER interactions and lie at the heart of processes underlying mitochondrial biogenesis. ATAD3A orthologues have been studied in flies, worms, and mammals, highlighting the widespread importance of this gene during embryonic development and in adulthood. ATAD3A is a downstream effector of target of rapamycin (TOR) signalling in Drosophila and exhibits typical features of proteins from the ERMIONE-like complex in metazoans. In humans, mutations in the ATAD3A gene represent a new link between altered mitochondrial-ER interaction and recognizable neurological syndromes. The primate-specific ATAD3B protein is a biomarker of pluripotent embryonic stem cells. Through negative regulation of ATAD3A function, ATAD3B supports mitochondrial stemness properties.

Mitochondrial cardiolipin/phospholipid trafficking: the role of membrane contact site complexes and lipid transfer proteins

Historically, cellular trafficking of lipids has received much less attention than protein trafficking, mostly because its biological importance was underestimated, involved sorting and translocation mechanisms were not known, and analytical tools were limiting. This has changed during the last decade, and we discuss here some progress made in respect to mitochondria and the trafficking of phospholipids, in particular cardiolipin. Different membrane contact site or junction complexes and putative lipid transfer proteins for intra- and intermembrane lipid translocation have been described, involving mitochondrial inner and outer membrane, and the adjacent membranes of the endoplasmic reticulum. An image emerges how cardiolipin precursors, remodeling intermediates, mature cardiolipin and its oxidation products could migrate between membranes, and how this trafficking is involved in cardiolipin biosynthesis and cell signaling events. Particular emphasis in this review is given to mitochondrial nucleoside diphosphate kinase D and mitochondrial creatine kinases, which emerge to have roles in both, membrane junction formation and lipid transfer.

MAM (mitochondria-associated membranes) in mammalian cells: lipids and beyond

One mechanism by which communication between the endoplasmic reticulum (ER) and mitochondria is achieved is by close juxtaposition between these organelles via mitochondria-associated membranes (MAM). The MAM consist of a region of the ER that is enriched in several lipid biosynthetic enzyme activities and becomes reversibly tethered to mitochondria. Specific proteins are localized, sometimes transiently, in the MAM. Several of these proteins have been implicated in tethering the MAM to mitochondria. In mammalian cells, formation of these contact sites between MAM and mitochondria appears to be required for key cellular events including the transport of calcium from the ER to mitochondria, the import of phosphatidylserine into mitochondria from the ER for decarboxylation to phosphatidylethanolamine, the formation of autophagosomes, regulation of the morphology, dynamics and functions of mitochondria, and cell survival. This review focuses on the functions proposed for MAM in mediating these events in mammalian cells. In light of the apparent involvement of MAM in multiple fundamental cellular processes, recent studies indicate that impaired contact between MAM and mitochondria might underlie the pathology of several human neurodegenerative diseases, including Alzheimer's disease. Moreover, MAM has been implicated in modulating glucose homeostasis and insulin resistance, as well as in some viral infections.

Lipid transport between the endoplasmic reticulum and mitochondria

Mitochondria are partially autonomous organelles that depend on the import of certain proteins and lipids to maintain cell survival and membrane formation. Although phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine are synthesized by mitochondrial enzymes, phosphatidylcholine, phosphatidylinositol, phosphatidylserine, and sterols need to be imported from other organelles. The origin of most lipids imported into mitochondria is the endoplasmic reticulum, which requires interaction of these two subcellular compartments. Recently, protein complexes that are involved in membrane contact between endoplasmic reticulum and mitochondria were identified, but their role in lipid transport is still unclear. In the present review, we describe components involved in lipid translocation between the endoplasmic reticulum and mitochondria and discuss functional as well as regulatory aspects that are important for lipid homeostasis.

Calcium signaling around Mitochondria Associated Membranes (MAMs)

Calcium (Ca²⁺) homeostasis is fundamental for cell metabolism, proliferation, differentiation, and cell death. Elevation in intracellular Ca²⁺ concentration is dependent either on Ca²⁺ influx from the extracellular space through the plasma membrane, or on Ca²⁺ release from intracellular Ca²⁺ stores, such as the endoplasmic/sarcoplasmic reticulum (ER/SR). Mitochondria are also major components of calcium signalling, capable of modulating both the amplitude and the spatio-temporal patterns of Ca²⁺ signals. Recent studies revealed zones of close contact between the ER and mitochondria called MAMs (Mitochondria Associated Membranes) crucial for a correct communication between the two organelles, including the selective transmission of physiological and pathological Ca²⁺ signals from the ER to mitochondria. In this review, we summarize the most up-to-date findings on the modulation of intracellular Ca²⁺ release and Ca²⁺ uptake mechanisms. We also explore the tight interplay between ER- and mitochondria-mediated Ca²⁺ signalling, covering the structural and molecular properties of the zones of close contact between these two networks.

Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles

Several recent works show structurally and functionally dynamic contacts between mitochondria, the plasma membrane, the endoplasmic reticulum, and other subcellular organelles. Many cellular processes require proper cooperation between the plasma membrane, the nucleus and subcellular vesicular/tubular networks such as mitochondria and the endoplasmic reticulum. It has been suggested that such contacts are crucial for the synthesis and intracellular transport of phospholipids as well as for intracellular Ca(2+) homeostasis, controlling fundamental processes like motility and contraction, secretion, cell growth, proliferation and apoptosis. Close contacts between smooth sub-domains of the endoplasmic reticulum and mitochondria have been shown to be required also for maintaining mitochondrial structure. The overall distance between the associating organelle membranes as quantified by electron microscopy is small enough to allow contact formation by proteins present on their surfaces, allowing and regulating their interactions. In this review we give a historical overview of studies on organelle interactions, and summarize the present knowledge and hypotheses concerning their regulation and (patho)physiological consequences.

MAM: more than just a housekeeper

The physical association between the endoplasmic reticulum (ER) and mitochondria, which is known as the mitochondria-associated ER membrane (MAM), has important roles in various cellular 'housekeeping' functions including the non-vesicular transports of phospholipids. It has recently become clear that the MAM also enables highly efficient transmission of Ca(2+) from the ER to mitochondria to stimulate oxidative metabolism and, conversely, might enable the metabolically energized mitochondria to regulate the ER Ca(2+) homeostasis. Recent studies have shed light on molecular chaperones such as calnexin, calreticulin, ERp44, ERp57, grp75 and the sigma-1 receptor at the MAM, which regulate the association between the two organelles. The MAM thus integrates signal transduction with metabolic pathways to regulate the communication and functional interactions between the ER and mitochondrion.

Regulation of redox-sensitive exofacial protein thiols in CHO cells

Thiols affect a variety of cell functions, an effect known as redox regulation, largely attributed to modification of transcription factors and intracellular signaling mechanisms. Since exofacial protein thiols are more exposed to redox-acting molecules used in cell culture and may represent sensors of the redox state of the environment, we investigated their susceptibility to redox regulation. Exofacial protein thiols were measured using cell-impermeable Ellman's reagent [5,5'-dithiobis(2-nitrobenzoic acid), DTNB]. For quantification, we also set up an ELISA assay based on the cell-impermeable biotinylated SH reagent, N-(biotinoyl)-N-(iodoacetyl) ethylendiamine (BIAM). Exposure of CHO cells to H(2)O(2) induces oxidation of surface thiols at concentrations not affecting intracellular GSH. Depletion (50%) of GSH decreases surface thiols by 88%. Surface thiols are also highly sensitive to thiol antioxidants, since exposure to 5 mM N-acetyl-L-cysteine (NAC) for 2 h augmented their expression without increasing GSH levels. Using BIAM labeling and two-dimensional gel electrophoresis, we show that this increase in surface thiols is due to the reduction of specific membrane proteins. Peptide mass fingerprinting by MALDI mass spectrometry allowed us to identify two of these proteins as Erp57 and vimentin.

Asymmetric Distribution of Phospholipids in Biomembranes

In eukaryotic cells, the biological membrane is characterized by a non-uniform distribution of membrane lipids, vertically as well as laterally. The paradigm for the vertical non-random distribution is the plasma membrane, where phosphatidylcholine (PC), sphingomyelin (SM), and glycosphingolipids are primarily located on the exoplasmic leaflet, while aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine (PE), are generally enriched in the cytoplasmic leaflet. Other minor phospholipids, such as phosphatidic acid and phosphatidylinositol (PI), are also enriched on the cytoplasmic face. Such asymmetrical distribution is related to each lipid regulating various biological events through interaction with other molecules. The clarification of the regulatory mechanism of the distribution and movement of membrane lipids is crucial to understanding the physiological roles of lipids. Here we focus on PS, which has been reported to be involved in apoptosis, blood coagulation and other biological phenomena, and summarize the present understanding of the dynamics of this phospholipid, including biosynthesis, metabolism, transport, and transbilayer movement. We also refer to diseases that have been reported to be related to phospholipid asymmetry.

Bridging gaps in phospholipid transport

Phospholipid transport between membranes is a fundamental aspect of organelle biogenesis in eukaryotes; however, little is know about this process. A significant body of data demonstrates that newly synthesized phospholipids can move between membranes by routes that are independent of the vesicular traffic that carries membrane proteins. Evidence continues to accumulate in support of a system for phospholipid transport that occurs at zones of apposition and contact between donor membranes - the source of specific phospholipids - and acceptor membranes that are unable to synthesize the necessary lipids. Recent findings identify some of the lipids and proteins that must be present on membranes for inter-organelle phospholipid transport to occur between the endoplasmic reticulum and mitochondria or Golgi. These data suggest that protein and lipid assemblies on donors and acceptors promote membrane docking and facilitate lipid movement.

Functional analysis of Chinese hamster phosphatidylserine synthase 1 through systematic alanine mutagenesis

PtdSer (phosphatidylserine) synthesis in mammalian cells occurs through the exchange of L-serine with the base moieties of phosphatidylcholine and phosphatidylethanolamine, which is catalysed by PSS (PtdSer synthase) 1 and 2 respectively. PtdSer synthesis in intact cells and an isolated membrane fraction was inhibited by exogenous PtdSer, indicating that feedback control is involved in the regulation of PtdSer biosynthesis. PSS 1 and 2 are similar in amino acid sequence, with an identity of 32%; however, due to a lack of homology with other known enzymes, their amino acid sequences do not provide information on their catalytic and regulatory mechanisms. In the present study, to identify amino acid residues crucial for the activity and/or regulation of PSS 1, we systematically introduced mutations into a Chinese hamster PSS 1 cDNA clone; namely, each of the 66 polar amino acid residues common to PSS 2 was replaced with an alanine residue. On analysis of Chinese hamster ovary cells transfected with each of the alanine mutant clones, we identified eight amino acid residues (His-172, Glu-197, Glu-200, Asn-209, Glu-212, Asp-216, Asp-221 and Asn-226) as those crucial for the enzyme reaction or the maintenance of the correct structure required for serine base-exchange activity. Among these residues, Asn-209 was suggested to be involved in the recognition and/or binding of free L-serine. We also identified six amino acid residues (Arg-95, His-97, Cys-189, Arg-262, Gln-266 and Arg-336) as those important for regulation of PSS 1. In addition, we found that the alanine mutations at Tyr-111, Asp-166, Arg-184, Arg-323, and Glu-364 affected the production and/or stability of PSS 1 in Chinese hamster ovary cells.

Phospholipid biosynthesis in mammalian cells

Identification of the genes and gene products involved in the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine has lagged behind that in many other fields because of difficulties encountered in purifying the respective proteins. Nevertheless, most of these genes have now been identified. In this review article, we have highlighted important new findings on the individual enzymes and the corresponding genes of phosphatidylcholine synthesis via its two major biosynthetic pathways: the CDP-choline pathway and the methylation pathway. We also review recent studies on phosphatidylethanolamine biosynthesis by two pathways: the CDP-ethanolamine pathway, which is active in the endoplasmic reticulum, and the phosphatidylserine decarboxylase pathway, which operates in mitochondria. Finally, the two base-exchange enzymes, phosphatidylserine synthase-1 and phosphatidylserine synthase-2, that synthesize phosphatidylserine in mammalian cells are also discussed.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Redox regulation of surface protein thiols: identification of integrin alpha-4 as a molecular target by using redox proteomics

Thiols affect a variety of cell functions, an effect known as redox regulation. We show here that treatment (1-2 h) of cells with 0.1-5 mM N-acetyl-L-cysteine (NAC) increases surface protein thiol expression in human peripheral blood mononuclear cells. This effect is not associated with changes in cellular glutathione (GSH) and is also observed with a non-GSH precursor thiol N-acetyl-D-cysteine or with GSH itself, which is not cell-permeable, suggesting a direct reducing action. NAC did not augment protein SH in the cytosol, indicating that they are already maximally reduced under normal, nonstressed, conditions. By using labeling with a non permeable, biotinylated SH reagent followed by two-dimensional gel electrophoresis and analysis by MS, we identified some of the proteins associated with the membrane that are reduced by NAC. These proteins include the following: integrin alpha-4, myosin heavy chain (nonmuscle type A), myosin light-chain alkali (nonmuscle isoform), and beta-actin. NAC pretreatment augmented integrin alpha-4-dependent fibronectin adhesion and aggregation of Jurkat cells without changing its expression by fluorescence-activated cell sorter, suggesting that reduction of surface disulfides can affect proteins function. We postulate that some of the activities of NAC or other thiol antioxidants may not only be due to free radical scavenging or increase of intracellular GSH and subsequent effects on transcription factors, but could modify the redox state of functional membrane proteins with exofacial SH critical for their activity.

Human PIG-U and yeast Cdc91p are the fifth subunit of GPI transamidase that attaches GPI-anchors to proteins

Many eukaryotic proteins are anchored to the cell surface via glycosylphosphatidylinositol (GPI), which is posttranslationally attached to the carboxyl-terminus by GPI transamidase. The mammalian GPI transamidase is a complex of at least four subunits, GPI8, GAA1, PIG-S, and PIG-T. Here, we report Chinese hamster ovary cells representing a new complementation group of GPI-anchored protein-deficient mutants, class U. The class U cells accumulated mature and immature GPI and did not have in vitro GPI transamidase activity. We cloned the gene responsible, termed PIG-U, that encoded a 435-amino-acid hydrophobic protein. The GPI transamidase complex affinity-purified from cells expressing epitope-tagged-GPI8 contained PIG-U and four other known components. Cells lacking PIG-U formed complexes of the four other components normally but had no ability to cleave the GPI attachment signal peptide. Saccharomyces cerevisiae Cdc91p, with 28% amino acid identity to PIG-U, partially restored GPI-anchored proteins on the surface of class U cells. PIG-U and Cdc91p have a functionally important short region with similarity to a region conserved in long-chain fatty acid elongases. Taken together, PIG-U and the yeast orthologue Cdc91p are the fifth component of GPI transamidase that may be involved in the recognition of either the GPI attachment signal or the lipid portion of GPI.

New perspectives on the regulation of intermembrane glycerophospholipid traffic

In eukaryotes, phosphatidylserine (PtdSer) can serve as a precursor of phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho), which are the major cellular phospholipids. PtdSer synthesis originates in the endoplasmic reticulum (ER) and its subdomain named the mitochondria-associated membrane (MAM). PtdSer is transported to the mitochondria in mammalian cells and yeast, and decarboxylated by PtdSer decarboxylase 1 (Psd1p) to form PtdEtn. A second decarboxylase, Psd2p, is also found in yeast in the Golgi-vacuole. PtdEtn produced by Psd1p and Psd2p can be transported to the ER, where it is methylated to form PtdCho. Organelle-specific metabolism of the aminoglycerophospholipids is a powerful tool for experimentally following lipid traffic that is now enabling identification of new proteins involved in the regulation of this process. Genetic and biochemical experiments demonstrate that transport of PtdSer between the MAM and mitochondria is regulated by protein ubiquitination, which affects events at both membranes. Similar analyses of PtdSer transport to the locus of Psd2p now indicate that a membrane-bound phosphatidylinositol transfer protein and the C2 domain of Psd2p are both required on the acceptor membrane for efficient transport of PtdSer. Collectively, these recent findings indicate that novel multiprotein assemblies on both donor and acceptor membranes participate in interorganelle phospholipid transport.

Molecular and cell biology of phosphatidylserine and phosphatidylethanolamine metabolism

In this review, the pathways for phosphatidylserine (PS) and phosphatidylethanolamine (PE) biosynthesis, as well as the genes and proteins involved in these pathways, are described in mammalian cells, yeast, and prokaryotes. In mammalian cells, PS is synthesized by a base-exchange reaction in which phosphatidylcholine or PE is substrate for PS synthase-1 or PS synthase-2, respectively. Isolation of Chinese hamster ovary cell mutants led to the cloning of cDNAs and genes encoding these two PS synthases. In yeast and prokaryotes PS is produced by a biosynthetic pathway completely different from that in mammals: from a reaction between CDP-diacylglycerol and serine. The major route for PE synthesis in cultured cells is from the mitochondrial decarboxylation of PS. Alternatively, PE can be synthesized in the endoplasmic reticulum (ER) from the CDP-ethanolamine pathway. Genes and/or cDNAs encoding all the enzymes in these two pathways for PE synthesis have been isolated and characterized. In mammalian cells, PS is synthesized on the ER and/or mitochondria-associated membranes (MAM). PS synthase-1 and -2 are highly enriched in MAM compared to the bulk of ER. Since MAM are a region of the ER that appears to be in close juxtaposition to the mitochondrial outer membrane, it has been proposed that MAM act as a conduit for the transfer of newly synthesized PS into mitochondria. A similar pathway appears to operate in yeast. The use of yeast mutants has led to identification of genes involved in the interorganelle transport of PS and PE in yeast, but so far none of the corresponding genes in mammalian cells has been identified. PS and PE do not act solely as structural components of membranes. Several specific functions have been ascribed to these two aminophospholipids. For example, cell-surface exposure of PS during apoptosis is thought to be the signal by which apoptotic cells are recognized and phagocytosed. Translocation of PS from the inner to outer leaflet of the plasma membrane of platelets initiates the blood-clotting cascade, and PS is an important activator of several enzymes, including protein kinase C. Recently, exposure of PE on the cell surface was identified as a regulator of cytokinesis. In addition, in Escherichia coli, PE appears to be involved in the correct folding of membrane proteins; and in Drosophila, PE regulates lipid homeostasis via the sterol response element-binding protein.

Translocation of pyrene-labeled phosphatidylserine from the plasma membrane to mitochondria diminishes systematically with molecular hydrophobicity: implications on the maintenance of high phosphatidylserine content in the inner leaflet of the plasma membrane

To study the translocation of phosphatidylserine (PS) from plasma membrane to mitochondria, dipyrene PS molecules (diPyr(n)PS; n=acyl chain length) were introduced to the plasma membrane of baby hamster kidney cells (BHK cells) using either cyclodextrin-mediated monomer transfer or fusion of cationic vesicles. Translocation of diPyr(n)PS to mitochondria was assessed based on decarboxylation by mitochondrial PS decarboxylase (PSD). It was found that the rate of translocation diminishes systematically with acyl chain length (molecular hydrophobicity) of diPyr(n)PS. Using an in vitro assay, it was shown that the spontaneous translocation rates of long-chain diPyr(n)PS species are similar to those of common natural PS species, thus supporting the biological relevance of the data. These results, and other data arguing against the involvement of vesicular traffic and lipid transfer proteins, imply that spontaneous monomeric diffusion via the cytoplasm is the main mechanism of PS movement from the plasma membrane to mitochondria. This finding could explain why a major fraction of PS synthesized by BHK cells consists of hydrophobic species: such species have little tendency to efflux from the plasma membrane to mitochondria where they would be decarboxylated. Thus, adequate molecular hydrophobicity seems to be crucial for the maintenance of high PS content in the inner leaflet of the plasma membrane.

Biochemistry and genetics of interorganelle aminoglycerophospholipid transport

The organelle specific reactions that constitute the biosynthetic pathway for aminoglycerophospholipid synthesis provide an important means for examining the biochemistry and genetics of intracellular lipid transport. Biochemical studies with intact and permeabilized cells, and isolated organelles have defined some of the essential features of lipid transport between the endoplasmic reticulum and mitochondria and Golgi/vacuole. Genetic screens have now also identified mutations and genes that are involved in aminoglycerophospholipid traffic between different membranes in mammalian cells, yeast and bacteria. Increasingly, studies focused upon intermembrane lipid movement are revealing important new information about this essential aspect of membrane biogenesis.

Translocation of phosphatidylthreonine and -serine to mitochondria diminishes exponentially with increasing molecular hydrophobicity

Some cultured cells contain significant amounts of a rarely recognized phospholipid, phosphatidylthreonine. Since phosphatidylthreonine is a structural analog of phosphatidylserine, the question rises whether it is transported to mitochondria and decarboxylated to phosphatidylisopropanolamine therein. We studied this issue with hamster kidney cell-line using a novel approach, i.e. electrospray mass-spectrometry and stable isotope-labeled precursors. Scanning for a neutral loss of 155, which is characteristic for phosphatidylisopropanolamine, indicated that this lipid is indeed present. The identity of phosphatidylisopropanolamine was supported by the following: (i) it co-chromatographed with phosphatidylethanolamine; (ii) its molecular species profile was similar to that of phosphatidylethanolamine; (iii) its head group was labeled from 13C-threonine; and (iv) its concentration increased in parallel with phosphatidylthreonine. Tests with solubilized decarboxylase and subcellular fractionation studies indicated that the low cellular content of phosphatidylisopropanolamine is due to inefficient decarboxylation, rather than poor translocation of phosphatidylthreonine to mitochondria. Importantly, the average hydrophobicity of phosphatidylisopropanolamine molecular species was significantly less than that of phosphatidylthreonine species, indicating that hydrophilic phosphatidylthreonine species translocate to mitochondria far more rapidly than hydrophobic ones. Parallel results were obtained for phosphatidylserine. These findings imply that efflux from the ER membrane could be the rate-limiting step in the phosphatidylthreonine and -serine translocation to mitochondria.