Post-translational processing of the phosphatidylserine decarboxylase gene product in Chinese hamster ovary cells

Maturation of the malarial phosphatidylserine decarboxylase is mediated by high affinity binding to anionic phospholipids

Decarboxylation of phosphatidylserine (PS) to form phosphatidylethanolamine by PS decarboxylases (PSDs) is an essential process in most eukaryotes. Processing of a malarial PSD proenzyme into its active alpha and beta subunits is by an autoendoproteolytic mechanism regulated by anionic phospholipids, with PS serving as an activator and phosphatidylglycerol (PG), phosphatidylinositol, and phosphatidic acid acting as inhibitors. The biophysical mechanism underlying this regulation remains unknown. We used solid phase lipid binding, liposome-binding assays, and surface plasmon resonance to examine the binding specificity of a processing-deficient Plasmodium PSD (PkPSDS308A) mutant enzyme and demonstrated that the PSD proenzyme binds strongly to PS and PG but not to phosphatidylethanolamine and phosphatidylcholine. The equilibrium dissociation constants (Kd) of PkPSD with PS and PG were 80.4 nM and 66.4 nM, respectively. The interaction of PSD with PS is inhibited by calcium, suggesting that the binding mechanism involves ionic interactions. In vitro processing of WT PkPSD proenzyme was also inhibited by calcium, consistent with the conclusion that PS binding to PkPSD through ionic interactions is required for the proenzyme processing. Peptide mapping identified polybasic amino acid motifs in the proenzyme responsible for binding to PS. Altogether, the data demonstrate that malarial PSD maturation is regulated through a strong physical association between PkPSD proenzyme and anionic lipids. Inhibition of the specific interaction between the proenzyme and the lipids can provide a novel mechanism to disrupt PSD enzyme activity, which has been suggested as a target for antimicrobials, and anticancer therapies.

Eugene P. Kennedy's Legacy: Defining Bacterial Phospholipid Pathways and Function

In the 1950's and 1960's Eugene P. Kennedy laid out the blueprint for phospholipid biosynthesis in somatic cells and Escherichia coli, which have been coined the Kennedy Pathways for phospholipid biosynthesis. His research group continued to make seminal contributions in the area of phospholipids until his retirement in the early 1990's. During these years he mentored many young scientists that continued to build on his early discoveries and who also mentored additional scientists that continue to make important contributions in areas related to phospholipids and membrane biogenesis. This review will focus on the initial E. coli Kennedy Pathways and how his early contributions have laid the foundation for our current understanding of bacterial phospholipid genetics, biochemistry and function as carried on by his scientific progeny and others who have been inspired to study microbial phospholipids.

Impaired phosphatidylethanolamine metabolism activates a reversible stress response that detects and resolves mutant mitochondrial precursors

Phosphatidylethanolamine (PE) made in mitochondria has long been recognized as an important precursor for phosphatidylcholine production that occurs in the endoplasmic reticulum (ER). Recently, the strict mitochondrial localization of the enzyme that makes PE in the mitochondrion, phosphatidylserine decarboxylase 1 (Psd1), was questioned. Since a dual localization of Psd1 to the ER would have far-reaching implications, we initiated our study to independently re-assess the subcellular distribution of Psd1. Our results support the unavoidable conclusion that the vast majority, if not all, of functional Psd1 resides in the mitochondrion. Through our efforts, we discovered that mutant forms of Psd1 that impair a self-processing step needed for it to become functional are dually localized to the ER when expressed in a PE-limiting environment. We conclude that severely impaired cellular PE metabolism provokes an ER-assisted adaptive response that is capable of identifying and resolving nonfunctional mitochondrial precursors.

The mitochondrial protein PGAM5 suppresses energy consumption in brown adipocytes by repressing expression of uncoupling protein 1

Accumulating evidence suggests that brown adipose tissue (BAT) is a potential therapeutic target for managing obesity and related diseases. PGAM family member 5, mitochondrial serine/threonine protein phosphatase (PGAM5), is a protein phosphatase that resides in the mitochondria and regulates many biological processes, including cell death, mitophagy, and immune responses. Because BAT is a mitochondria-rich tissue, we have hypothesized that PGAM5 has a physiological function in BAT. We previously reported that PGAM5-knockout (KO) mice are resistant to severe metabolic stress. Importantly, lipid accumulation is suppressed in PGAM5-KO BAT, even under unstressed conditions, raising the possibility that PGAM5 deficiency stimulates lipid consumption. However, the mechanism underlying this observation is undetermined. Here, using an array of biochemical approaches, including quantitative RT-PCR, immunoblotting, and oxygen consumption assays, we show that PGAM5 negatively regulates energy expenditure in brown adipocytes. We found that PGAM5-KO brown adipocytes have an enhanced oxygen consumption rate and increased expression of uncoupling protein 1 (UCP1), a protein that increases energy consumption in the mitochondria. Mechanistically, we found that PGAM5 phosphatase activity and intramembrane cleavage are required for suppression of UCP1 activity. Furthermore, utilizing a genome-wide siRNA screen in HeLa cells to search for regulators of PGAM5 cleavage, we identified a set of candidate genes, including phosphatidylserine decarboxylase (PISD), which catalyzes the formation of phosphatidylethanolamine at the mitochondrial membrane. Taken together, these results indicate that PGAM5 suppresses mitochondrial energy expenditure by down-regulating UCP1 expression in brown adipocytes and that its phosphatase activity and intramembrane cleavage are required for UCP1 suppression.

Multitiered and Cooperative Surveillance of Mitochondrial Phosphatidylserine Decarboxylase 1

Phosphatidylserine decarboxylase 1 (Psd1p), an ancient enzyme that converts phosphatidylserine to phosphatidylethanolamine in the inner mitochondrial membrane, must undergo an autocatalytic self-processing event to gain activity. Autocatalysis severs the protein into a large membrane-anchored β subunit that noncovalently associates with the small α subunit on the intermembrane space side of the inner membrane. Here, we determined that a temperature sensitive (ts) PSD1 allele is autocatalytically impaired and that its fidelity is closely monitored throughout its life cycle by multiple mitochondrial quality control proteases. Interestingly, the proteases involved in resolving misfolded Psd1^ts vary depending on its autocatalytic status. Specifically, the degradation of a Psd1^ts precursor unable to undergo autocatalysis requires the unprecedented cooperative and sequential actions of two inner membrane proteases, Oma1p and Yme1p. In contrast, upon heat exposure postautocatalysis, Psd1^ts β subunits accumulate in protein aggregates that are resolved by Yme1p acting alone, while the released α subunit is degraded in parallel by an unidentified protease. Importantly, the stability of endogenous Psd1p is also influenced by Yme1p. We conclude that Psd1p, the key enzyme required for the mitochondrial pathway of phosphatidylethanolamine production, is closely monitored at several levels and by multiple mitochondrial quality control mechanisms present in the intermembrane space.

Involvement of a putative substrate binding site in the biogenesis and assembly of phosphatidylserine decarboxylase 1 from Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the mitochondrial phosphatidylserine decarboxylase 1 (Psd1p) produces the largest amount of cellular phosphatidylethanolamine (PE). Psd1p is synthesized as a larger precursor on cytosolic ribosomes and then imported into mitochondria in a three-step processing event leading to the formation of an α-subunit and a β-subunit. The α-subunit harbors a highly conserved motif, which was proposed to be involved in phosphatidylserine (PS) binding. Here, we present a molecular analysis of this consensus motif for the function of Psd1p by using Psd1p variants bearing either deletions or point mutations in this region. Our data show that mutations in this motif affect processing and stability of Psd1p, and consequently the enzyme's activity. Thus, we conclude that this consensus motif is essential for structural integrity and processing of Psd1p.

Phosphatidylserine synthase 2 and phosphatidylserine decarboxylase are essential for aminophospholipid synthesis in Trypanosoma brucei

Phosphatidylethanolamine (PE) and phosphatidylserine (PS) are ubiquitously expressed and metabolically interconnected glycerophospholipids in eukaryotes and prokaryotes. In Trypanosoma brucei, PE synthesis has been shown to occur mainly via the Kennedy pathway, one of the three routes leading to PE synthesis in eukaryotes, while PS synthesis has not been studied experimentally. We now reveal the importance of T. brucei PS synthase 2 (TbPSS2) and T. brucei PS decarboxylase (TbPSD), two key enzymes involved in aminophospholipid synthesis, for trypanosome viability. By using tetracycline-inducible down-regulation of gene expression and in vivo and in vitro metabolic labeling, we found that TbPSS2 (i) is necessary for normal growth of procyclic trypanosomes, (ii) localizes to the endoplasmic reticulum and (iii) represents the unique route for PS formation in T. brucei. In addition, we identified TbPSD as type I PS decarboxylase in the mitochondrion and found that it is processed proteolytically at a WGSS cleavage site into a heterodimer. Down-regulation of TbPSD expression affected mitochondrial integrity in both procyclic and bloodstream form trypanosomes, decreased ATP production via oxidative phosphorylation in procyclic form and affected parasite growth.

Cell biology, physiology and enzymology of phosphatidylserine decarboxylase

Phosphatidylethanolamine is one of the most abundant phospholipids whose major amounts are formed by phosphatidylserine decarboxylases (PSD). Here we provide a comprehensive description of different types of PSDs in the different kingdoms of life. In eukaryotes, type I PSDs are mitochondrial enzymes, whereas other PSDs are localized to other cellular compartments. We describe the role of mitochondrial Psd1 proteins, their function, enzymology, biogenesis, assembly into mitochondria and their contribution to phospholipid homeostasis in much detail. We also discuss briefly the cellular physiology and the enzymology of Psd2. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

Phosphatidylserine decarboxylase 1 autocatalysis and function does not require a mitochondrial-specific factor

Phosphatidylethanolamine (PE) is a major cellular phospholipid that can be made by four separate pathways, one of which resides in the mitochondrion. The mitochondrial enzyme that generates PE is phosphatidylserine decarboxylase 1 (Psd1p). The pool of PE produced by Psd1p, which cannot be compensated for by the other cellular PE metabolic pathways, is important for numerous mitochondrial functions, including oxidative phosphorylation and mitochondrial dynamics and morphology, and is essential for murine development. To become catalytically active, Psd1p undergoes an autocatalytic processing step involving a conserved LGST motif that separates the enzyme into α and β subunits that remain non-covalently attached and are anchored to the inner membrane by virtue of the membrane-embedded β subunit. It was speculated that Psd1p autocatalysis requires a mitochondrial-specific factor and that for Psd1p to function in vivo, it had to be embedded with the correct topology in the mitochondrial inner membrane. However, the identity of the mitochondrial factor required for Psd1p autocatalysis has not been identified. With the goal of defining molecular requirements for Psd1p autocatalysis, we demonstrate that: 1) despite the conservation of the LGST motif from bacteria to humans, only the serine residue is absolutely required for Psd1p autocatalysis and function; 2) yeast Psd1p does not require its substrate phosphatidylserine for autocatalysis; and 3) contrary to a prior report, yeast Psd1p autocatalysis does not require mitochondrial-specific phospholipids, proteins, or co-factors, because Psd1p re-directed to the secretory pathway undergoes autocatalysis normally and is fully functional in vivo.

From Protease to Decarboxylase: THE MOLECULAR METAMORPHOSIS OF PHOSPHATIDYLSERINE DECARBOXYLASE

Phosphatidylserine decarboxylase (PSDs) play a central role in the synthesis of phosphatidylethanolamine in numerous species of prokaryotes and eukaryotes. PSDs are unusual decarboxylase containing a pyruvoyl prosthetic group within the active site. The covalently attached pyruvoyl moiety is formed in a concerted reaction when the PSD proenzyme undergoes an endoproteolytic cleavage into a large β-subunit, and a smaller α-subunit, which harbors the prosthetic group at its N terminus. The mechanism of PSD proenzyme cleavage has long been unclear. Using a coupled in vitro transcription/translation system with the soluble Plasmodium knowlesi enzyme (PkPSD), we demonstrate that the post-translational processing is inhibited by the serine protease inhibitor, phenylmethylsulfonyl fluoride. Comparison of PSD sequences across multiple phyla reveals a uniquely conserved aspartic acid within an FFXRX6RX12PXD motif, two uniquely conserved histidine residues within a PXXYHXXHXP motif, and a uniquely conserved serine residue within a GS(S/T) motif, suggesting that PSDs belong to the D-H-S serine protease family. The function of the conserved D-H-S residues was probed using site-directed mutagenesis of PkPSD. The results from these mutagenesis experiments reveal that Asp-139, His-198, and Ser-308 are all essential for endoproteolytic processing of PkPSD, which occurs in cis. In addition, within the GS(S/T) motif found in all PSDs, the Gly-307 residue is also essential, but the Ser/Thr-309 is non-essential. These results define the mechanism whereby PSDs begin their biochemical existence as proteases that execute one autoendoproteolytic cleavage reaction to give rise to a mature PSD harboring a pyruvoyl prosthetic group.

Phosphatidylethanolamine synthesis in the parasite mitochondrion is required for efficient growth but dispensable for survival of Toxoplasma gondii

Toxoplasma gondii is a highly prevalent obligate intracellular parasite of the phylum Apicomplexa, which also includes other parasites of clinical and/or veterinary importance, such as Plasmodium, Cryptosporidium, and Eimeria. Acute infection by Toxoplasma is hallmarked by rapid proliferation in its host cells and requires a significant synthesis of parasite membranes. Phosphatidylethanolamine (PtdEtn) is the second major phospholipid class in T. gondii. Here, we reveal that PtdEtn is produced in the parasite mitochondrion and parasitophorous vacuole by decarboxylation of phosphatidylserine (PtdSer) and in the endoplasmic reticulum by fusion of CDP-ethanolamine and diacylglycerol. PtdEtn in the mitochondrion is synthesized by a phosphatidylserine decarboxylase (TgPSD1mt) of the type I class. TgPSD1mt harbors a targeting peptide at its N terminus that is required for the mitochondrial localization but not for the catalytic activity. Ablation of TgPSD1mt expression caused up to 45% growth impairment in the parasite mutant. The PtdEtn content of the mutant was unaffected, however, suggesting the presence of compensatory mechanisms. Indeed, metabolic labeling revealed an increased usage of ethanolamine for PtdEtn synthesis by the mutant. Likewise, depletion of nutrients exacerbated the growth defect (∼56%), which was partially restored by ethanolamine. Besides, the survival and residual growth of the TgPSD1mt mutant in the nutrient-depleted medium also indicated additional routes of PtdEtn biogenesis, such as acquisition of host-derived lipid. Collectively, the work demonstrates a metabolic cooperativity between the parasite organelles, which ensures a sustained lipid synthesis, survival and growth of T. gondii in varying nutritional milieus.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

Processing and topology of the yeast mitochondrial phosphatidylserine decarboxylase 1

The inner mitochondrial membrane plays a crucial role in cellular lipid homeostasis through biosynthesis of the non-bilayer-forming lipids phosphatidylethanolamine and cardiolipin. In the yeast Saccharomyces cerevisiae, the majority of cellular phosphatidylethanolamine is synthesized by the mitochondrial phosphatidylserine decarboxylase 1 (Psd1). The biogenesis of Psd1 involves several processing steps. It was speculated that the Psd1 precursor is sorted into the inner membrane and is subsequently released into the intermembrane space by proteolytic removal of a hydrophobic sorting signal. However, components involved in the maturation of the Psd1 precursor have not been identified. We show that processing of Psd1 involves the action of the mitochondrial processing peptidase and Oct1 and an autocatalytic cleavage at a highly conserved LGST motif yielding the α- and β-subunit of the enzyme. The Psd1 β-subunit (Psd1β) forms the membrane anchor, which binds the intermembrane space-localized α-subunit (Psd1α). Deletion of a transmembrane segment in the β-subunit results in mislocalization of Psd1 and reduced enzymatic activity. Surprisingly, autocatalytic cleavage does not depend on proper localization to the inner mitochondrial membrane. In summary, membrane integration of Psd1 is crucial for its functionality and for maintenance of mitochondrial lipid homeostasis.

Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are metabolically related membrane aminophospholipids. In mammalian cells, PS is required for targeting and function of several intracellular signaling proteins. Moreover, PS is asymmetrically distributed in the plasma membrane. Although PS is highly enriched in the cytoplasmic leaflet of plasma membranes, PS exposure on the cell surface initiates blood clotting and removal of apoptotic cells. PS is synthesized in mammalian cells by two distinct PS synthases that exchange serine for choline or ethanolamine in phosphatidylcholine (PC) or PE, respectively. Targeted disruption of each PS synthase individually in mice demonstrated that neither enzyme is required for viability whereas elimination of both synthases was embryonic lethal. Thus, mammalian cells require a threshold amount of PS. PE is synthesized in mammalian cells by four different pathways, the quantitatively most important of which are the CDP-ethanolamine pathway that produces PE in the ER, and PS decarboxylation that occurs in mitochondria. PS is made in ER membranes and is imported into mitochondria for decarboxylation to PE via a domain of the ER [mitochondria-associated membranes (MAM)] that transiently associates with mitochondria. Elimination of PS decarboxylase in mice caused mitochondrial defects and embryonic lethality. Global elimination of the CDP-ethanolamine pathway was also incompatible with mouse survival. Thus, PE made by each of these pathways has independent and necessary functions. In mammals PE is a substrate for methylation to PC in the liver, a substrate for anandamide synthesis, and supplies ethanolamine for glycosylphosphatidylinositol anchors of cell-surface signaling proteins. Thus, PS and PE participate in many previously unanticipated facets of mammalian cell biology. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Regulation of the CgPdr1 transcription factor from the pathogen Candida glabrata

Candida glabrata is an opportunistic human pathogen that is increasingly associated with candidemia, owing in part to the intrinsic and acquired high tolerance the organism exhibits for the important clinical antifungal drug fluconazole. This elevated fluconazole resistance often develops through gain-of-function mutations in the zinc cluster-containing transcriptional regulator C. glabrata Pdr1 (CgPdr1). CgPdr1 induces the expression of an ATP-binding cassette (ABC) transporter-encoding gene, CgCDR1. Saccharomyces cerevisiae has two CgPdr1 homologues called ScPdr1 and ScPdr3. These factors control the expression of an ABC transporter-encoding gene called ScPDR5, which encodes a homologue of CgCDR1. Loss of the mitochondrial genome (ρ(0) cell) or overexpression of the mitochondrial enzyme ScPsd1 induces ScPDR5 expression in a strictly ScPdr3-dependent fashion. ScPdr3 requires the presence of a transcriptional Mediator subunit called Gal11 (Med15) to fully induce ScPDR5 transcription in response to ρ(0) signaling. ScPdr1 does not respond to either ρ(0) signals or ScPsd1 overproduction. In this study, we employed transcriptional fusions between CgPdr1 target promoters, like CgCDR1, to demonstrate that CgPdr1 stimulates gene expression via binding to elements called pleiotropic drug response elements (PDREs). Deletion mapping and electrophoretic mobility shift assays demonstrated that a single PDRE in the CgCDR1 promoter was capable of supporting ρ(0)-induced gene expression. Removal of one of the two ScGal11 homologues from C. glabrata caused a major defect in drug-induced expression of CgCDR1 but had a quantitatively minor effect on ρ(0)-stimulated transcription. These data demonstrate that CgPdr1 appears to combine features of ScPdr1 and ScPdr3 to produce a transcription factor with chimeric regulatory properties.

Compartment-specific synthesis of phosphatidylethanolamine is required for normal heavy metal resistance

The enzyme Psd2 catalyzes endosomal synthesis of the phospholipid PE. While this pool of PE represents a minority of total cellular PE, function of Psd2 is required for normal activity of the vacuolar ABC transporter Ycf1. Psd2 controls vacuolar PE levels by acting at the level of the endosome.

Control of lipid composition of membranes is crucial to ensure normal cellular functions. Saccharomyces cerevisiae has two different phosphatidylserine decarboxylase enzymes (Psd1 and Psd2) that catalyze formation of phosphatidylethanolamine. The mitochondrial Psd1 provides roughly 70% of the phosphatidylethanolamine (PE) biosynthesis in the cell with Psd2 carrying out the remainder. Here, we demonstrate that loss of Psd2 causes cells to acquire sensitivity to cadmium even though Psd1 remains intact. This cadmium sensitivity results from loss of normal activity of a vacuolar ATP-binding cassette transporter protein called Ycf1. Measurement of phospholipid levels indicates that loss of Psd2 causes a specific reduction in vacuolar membrane PE levels, whereas total PE levels are not significantly affected. The presence of a phosphatidylinositol transfer protein called Pdr17 is required for Psd2 function and normal cadmium tolerance. We demonstrate that Pdr17 and Psd2 form a complex in vivo that seems essential for maintenance of vacuolar PE levels. Finally, we refine the localization of Psd2 to the endosome arguing that this enzyme controls vacuolar membrane phospholipid content by regulating phospholipids in compartments that will eventually give rise to the vacuole. Disturbance of this regulation of intracellular phospholipid balance leads to selective loss of membrane protein function in the vacuole.

Molecular electroporation and the transduction of oligoarginines

Certain short polycations, such as TAT and polyarginine, rapidly pass through the plasma membranes of mammalian cells by an unknown mechanism called transduction as well as by endocytosis and macropinocytosis. These cell-penetrating peptides (CPPs) promise to be medically useful when fused to biologically active peptides. I offer a simple model in which one or more CPPs and the phosphatidylserines of the inner leaflet form a kind of capacitor with a voltage in excess of about 200 mV, high enough to create a molecular electropore. The model is consistent with an empirical upper limit on the cargo peptide of 40-60 amino acids and with experimental data on how the transduction of a polyarginine-fluorophore into mouse C(2)C(12) myoblasts depends on the number of arginines in the CPP and on the CPP concentration. The model makes three testable predictions.

Vibrio cholerae proteome-wide screen for immunostimulatory proteins identifies phosphatidylserine decarboxylase as a novel Toll-like receptor 4 agonist

Recognition of conserved bacterial components provides immediate and efficient immune responses and plays a critical role in triggering antigen-specific adaptive immunity. To date, most microbial components that are detected by host innate immune system are non-proteinaceous structural components. In order to identify novel bacterial immunostimulatory proteins, we developed a new high-throughput approach called “EPSIA”, Expressed Protein Screen for Immune Activators. Out of 3,882 Vibrio cholerae proteins, we identified phosphatidylserine decarboxylase (PSD) as a conserved bacterial protein capable of activating host innate immunity. PSD in concentrations as low as 100 ng/ml stimulated RAW264.7 murine macrophage cells and primary peritoneal macrophage cells to secrete TNFα and IL-6, respectively. PSD-induced proinflammatory response was dependent on the presence of MyD88, a known adaptor molecule for innate immune response. An enzymatically inactive PSD mutant and heat-inactivated PSD induced ∼40% and ∼15% of IL-6 production compared to that by native PSD, respectively. This suggests that PSD induces the production of IL-6, in part, via its enzymatic activity. Subsequent receptor screening determined TLR4 as a receptor mediating the PSD-induced proinflammatory response. Moreover, no detectable IL-6 was produced in TLR4-deficient mouse macrophages by PSD. PSD also exhibited a strong adjuvant activity against a co-administered antigen, BSA. Anti-BSA response was decreased in TLR4-deficient mice immunized with BSA in combination with PSD, further proving the role of TLR4 in PSD signaling in vivo. Taken together, these results provide evidence for the identification of V. cholerae PSD as a novel TLR4 agonist and further demonstrate the potential application of PSD as a vaccine adjuvant.

Innate immune responses are the first line of defense and involve the early recognition of pathogenic microorganisms. Furthermore, these early innate responses can help shape and influence the development of more specific adaptive immune responses. One way that innate immunity is triggered is by activation of TLRs, or Toll-like Receptors. TLRs recognize a wide spectrum of microbes by binding to pathogen-associated molecular patterns (PAMPs), which are conserved microbial products. Here, we have used a high-throughput method to understand more about how a pathogen can trigger early innate immune responses and also how these early responses to infection can influence the adaptive, more specific, immune response. This technique can also be utilized for adjuvant discovery which is important in vaccine development since different adjuvants can induce or enhance different kinds of immune responses to a particular antigen. Using this method, we identified a novel bacterial protein that activates a TLR and further characterized its role as an adjuvant. Identifying the TLRs, their ligands, and the signal transduction events that they initiate has provided insight into our understanding of how the immune response to infection begins, and how these factors also collectively influence the adaptive immune response.

Phosphatidylserine decarboxylases, key enzymes of lipid metabolism

Phosphatidylserine decarboxylases (PSDs) (E.C. 4.1.1.65) are enzymes which catalyze the formation of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). This enzymatic activity has been identified in both prokaryotic and eukaryotic organisms. PSDs occur as two types of proteins depending on their localization and the sequence of a conserved motif. Type I PSDs include enzymes of eukaryotic mitochondria and bacterial origin which contain the amino acid sequence LGST as a characteristic motif. Type II PSDs are found in the endomembrane system of eukaryotes and contain a typical GGST motif. These characteristic motifs are considered as autocatalytic cleavage sites where proenzymes are split into alpha- and beta-subunits. The S-residue set free by this cleavage serves as an attachment site of a pyruvoyl group which is required for the activity of the enzymes. Moreover, PSDs harbor characteristic binding sites for the substrate PtdSer. Substrate supply to eukaryotic PSDs requires lipid transport because PtdSer synthesis and decarboxylation are spatially separated. Targeting of PSDs to their proper locations requires additional intramolecular domains. Mitochondrially localized type I PSDs are directed to the inner mitochondrial membrane by N-terminal targeting sequences. Type II PSDs also contain sequences in their N-terminal extensions which might be required for subcellular targeting. Lack of PSDs causes various defects in different cell types. The physiological relevance of these findings and the central role of PSDs in lipid metabolism will be discussed in this review.

Evidence for the bifunctional nature of mitochondrial phosphatidylserine decarboxylase: role in Pdr3-dependent retrograde regulation of PDR5 expression

Multidrug resistance in the yeast Saccharomyces cerevisiae is sensitive to the mitochondrial genome status of cells. Cells that lose their organellar genome ([rho(0)] cells) dramatically induce transcription of multiple or pleiotropic drug resistance genes via increased expression of a zinc cluster-containing transcription factor designated Pdr3. A major Pdr3 target gene is the ATP-binding cassette transporter-encoding gene PDR5. Pdr5 has been demonstrated to act as a phospholipid floppase catalyzing the net outward movement of phosphatidylethanolamine (PE). Since the mitochondrially localized Psd1 enzyme provides a major route of PE biosynthesis, we evaluated the potential linkage between Psd1 function and PDR5 regulation. Overproduction of Psd1 in wild-type ([rho(+)]) cells was found to induce PDR5 transcription and drug resistance in a Pdr3-dependent manner. Loss of the PSD1 gene from [rho(0)] cells prevented the normal activation of PDR5 expression. Surprisingly, expression of a catalytically inactive form of Psd1 still supported PDR5 transcriptional activation, suggesting that PE levels were not the signal triggering PDR5 induction. Expression of green fluorescent protein fusions mapped the region required to induce PDR5 expression to the noncatalytic amino-terminal portion of Psd1. Psd1 is a novel bifunctional protein required both for PE biosynthesis and regulation of multidrug resistance.

The phosphatidylethanolamine level of yeast mitochondria is affected by the mitochondrial components Oxa1p and Yme1p

The majority of phosphatidylethanolamine, an essential component of yeast mitochondria, is synthesized by phosphatidylserine decarboxylase 1 (Psd1p), a component of the inner mitochondrial membrane. Here, we report that deletion of OXA1 encoding an inner mitochondrial membrane protein translocase markedly affects the mitochondrial phosphatidylethanolamine level. In an oxa1Delta mutant, cellular and mitochondrial levels of phosphatidylethanolamine were lowered similar to a mutant with PSD1 deleted, and the rate of phosphatidylethanolamine synthesis by decarboxylation of phosphatidylserine in vivo and in vitro was decreased. This was due to a lower PSD1 transcription rate in the oxa1Delta mutant compared with wild-type and compromised assembly of Psd1p into the inner mitochondrial membrane. Lack of Mba1p, another component involved in the assembly of mitochondrial proteins into the inner mitochondrial membrane, did not affect the amount of phosphatidylethanolamine or the assembly of Psd1p. Deletion of the inner membrane protease Yme1p enhanced Psd1p stability suggesting that Yme1p contributed substantially to the proteolytic turnover of Psd1p in wild-type. In summary, our results demonstrate a link between the mitochondrial protein import machinery, assembly and stability of Psd1p, and phosphatidylethanolamine homeostasis in yeast mitochondria.

HOXBES2: a novel epididymal HOXB2 homeoprotein and its domain-specific association with spermatozoa

The sperm from the testis acquires complete fertilizing ability and forward progressive motility following its transit through the epididymis. Acquisition of these characteristics results from the modification of the sperm proteome following interactions with epididymal secretions. In our attempts to identify epididymis-specific sperm plasma membrane proteins, a partial 2.83-kb clone was identified by immunoscreening a monkey epididymal cDNA library with an agglutinating monoclonal antibody raised against washed human spermatozoa. The sequence of the 2.83-kb clone exhibited homology to the region between 1 and 1097 bp of the homeobox gene, Hoxb2. This sequence was found to be species conserved, as revealed by RT-PCR analysis. To obtain a full-length clone of the sequence, 5' RACE-PCR (rapid amplification of cDNA ends PCR) was carried out using rat epididymal RNA as the template. It resulted in a full-length 1.657-kb cDNA encoding a 32.9-kDa putative protein. The protein designated HOXBES2 exhibited homology to the conserved 61-amino acid homeodomain region of the HOXB2 homeoprotein. However, characteristic differences were noted in its amino and carboxyl termini compared with HOXB2. A putative 30-kDa protein was detected in the tissue extracts from adult rat epididymis and caudal spermatozoa, and a 37-kDa protein was detected in the rat embryo when probed with a polyclonal antibody against HOXB2 protein. Multiple tissue Western blot and immunohistochemical analysis further indicated its expression in the cytoplasm of the principal and basal epithelial cells, with maximal expression in the distal epididymal segments. Northern blot analysis detected a single approximately 2.5-kb transcript from the adult epididymis. Indirect immunofluorescence localized the protein to the acrosome, midpiece, and equatorial segments of rat caudal and ejaculated human and monkey spermatozoa, respectively. In conclusion, we have identified and characterized a novel epididymal homeoprotein different from HOXB2 protein and hereafter referred to as HOXBES2, (HOXB2 homeodomain containing epididymis-specific sperm protein) with a probable role in fertilization.

Characterization of a non-mitochondrial type I phosphatidylserine decarboxylase in Plasmodium falciparum

In search of key enzymes in Plasmodium phospholipid metabolism, we demonstrate the presence of a parasite-encoded phosphatidylserine decarboxylase (PSD) in the membrane fraction of Plasmodium falciparum-infected erythrocytes. PSD cDNA, encoding phosphatidylserine decarboxylase (PfPSD), was cloned by screening a directional cDNA library derived from the trophozoite erythrocytic stage. The corresponding PfPSD gene is located on chromosome 9 of P. falciparum, contains one intron of 938 nucleotides and is transcribed into a 3.7 kb mRNA. PfPSD cDNA encodes a putative protein of 362 amino acids, with a predicted molecular mass of 42.6 kDa, which clearly belongs to the type I PSD family. Only a 35 kDa polypeptide was detected in the parasite using a specific rabbit antiserum. PfPSD has a 314VGSS317 sequence near its carboxyl-terminus that is related to the Escherichia coli, yeast and human LGST motif, which is the site of proenzyme processing. PSD enzyme was expressed in E. coli with a KM of 63 +/- 19 microM and a VMAX of 680 +/- 49 nmol of phosphatidylethanolamine formed h-1 mg-1 protein. Site-directed mutagenesis of the VGSS active site demonstrated that the PfPSD proenzyme was processed into two non-identical subunits (alpha and beta) and revealed the crucial role played by each residue in enzyme processing and activity. Using indirect immunofluorescence, PfPSD labelling was co-localized with an endoplasmic reticulum marker, but not with a mitochondrial vital dye. This P. falciparum PSD is the first type I PSD identified in the endoplasmic reticulum compartment.

Mitochondrial phosphatidylserine decarboxylase from higher plants. Functional complementation in yeast, localization in plants, and overexpression in Arabidopsis

Plants are known to synthesize ethanolamine (Etn) moieties by decarboxylation of free serine (Ser), but there is also some evidence for phosphatidyl-Ser (Ptd-Ser) decarboxylation. Database searches identified diverse plant cDNAs and an Arabidopsis gene encoding 50-kD proteins homologous to yeast (Saccharomyces cerevisiae) and mammalian mitochondrial Ptd-Ser decarboxylases (PSDs). Like the latter, the plant proteins have putative mitochondrial targeting and inner membrane sorting sequences and contain near the C terminus a Glycine-Serine-Threonine motif corresponding to the site of proteolysis and catalytic pyruvoyl residue formation. A truncated tomato (Lycopersicon esculentum) cDNA lacking the targeting sequence and a chimeric construct in which the targeting and sorting sequences were replaced by those from yeast PSD1 both complemented the Etn requirement of a yeast psd1 psd2 mutant, and PSD activity was detected in the mitochondria of the complemented cells. Immunoblot analysis of potato (Solanum tuberosum) mitochondria demonstrated that PSD is located in mitochondrial membranes, and mRNA analysis in Arabidopsis showed that the mitochondrial PSD gene is expressed at low levels throughout the plant. An Arabidopsis knockup mutant grew normally but had 6- to 13-fold more mitochondrial PSD mRNA and 9-fold more mitochondrial PSD activity. Total membrane PSD activity was, however, unchanged in the mutant, showing mitochondrial activity to be a minor part of the total. These results establish that plants can synthesize Etn moieties via a phospholipid pathway and have both mitochondrial and extramitochondrial PSDs. They also indicate that mitochondrial PSD is an important housekeeping enzyme whose expression is strongly regulated at the transcriptional level.

The C2 domain of phosphatidylserine decarboxylase 2 is not required for catalysis but is essential for in vivo function

Phosphatidylserine decarboxylase 2 (Psd2p) is currently being used to study lipid trafficking processes in intact and permeabilized yeast cells. The Psd2p contains a C2 homology domain and a putative Golgi retention/localization (GR) domain. C2 domains play important functions in membrane binding and docking reactions involving phospholipids and proteins. We constructed a C2 domain deletion variant (C2Delta) and a GR deletion variant (GRDelta) of Psd2p and examined their effects on in vivo function and catalysis. Immunoblotting confirmed that the predicted immature and mature forms of Psd2(C2Delta)p, Psd2(GRDelta)p, and wild type Psd2p were produced in vivo and that the proteins localized normally. Enzymology revealed that the Psd2(C2Delta)p and Psd2(GRDelta)p were catalytically active and could readily be expressed at levels 10-fold higher than endogenous Psd2p. Both Psd2p and Psd2(GRDelta)p expression complemented the growth defect of psd1Deltapsd2Delta strains and resulted in normal aminoglycerophospholipid metabolism. In contrast, the Psd2(C2Delta)p failed to complement psd1Deltapsd2Delta strains, and [(3)H]serine labeling revealed a severe defect in the formation of PtdEtn in both intact and permeabilized cells, indicative of disruption of lipid trafficking. These findings identify an essential, non-catalytic function of the C2 domain of Psd2p and raise the possibility that it plays a direct role in membrane docking and/or PtdSer transport to the enzyme.

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.

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

Phosphatidylethanolamine synthesis through the phosphatidylserine (PtdSer) decarboxylation pathway requires PtdSer transport from the endoplasmic reticulum or mitochondrial-associated membrane to the mitochondrial inner membrane in mammalian cells. The transport-dependent PtdSer decarboxylation in permeabilized Chinese hamster ovary (CHO) cells was enhanced by cytosolic factors from bovine brain. A cytosolic protein factor exhibiting this enhancing activity was purified, and its amino acid sequence was partially determined. The sequence was identical to part of the amino acid sequence of an EF-hand type calcium-binding protein, S100B. A His(6)-tagged recombinant CHO S100B protein was able to remarkably enhance the transport-dependent PtdSer decarboxylation in permeabilized CHO cells. Under the standard assay conditions for PtdSer decarboxylase, the recombinant S100B protein did not stimulate PtdSer decarboxylase activity and exhibited no PtdSer decarboxylase activity. These results implicated the S100B protein in the transport of PtdSer to the mitochondrial inner membrane.

Regulation of mammalian cell membrane biosynthesis

This review explores current information on the interrelationship between phospholipid biochemistry and cell biology. Phosphatidylcholine is the most abundant phospholipid and it biosynthesis has been studied extensively. The choline cytidylyltransferase regulates phosphatidylcholine production, and recent advances in our understanding of the mechanisms that govern cytidylyltransferase include the discovery of multiple isoforms and a more complete understanding of the lipid regulation of enzyme activity. Similarities between phosphatidylcholine formation and the phosphatidylethanolamine and phosphatidylinositol biosynthetic pathways are discussed, together with current insight into control mechanisms. Membrane phospholipid doubling during cell cycle progression is a function of periodic biosynthesis and degradation. Membrane homeostasis is maintained by a phospholipase A-mediated degradation of excess phospholipid, whereas insufficient phosphatidylcholine triggers apoptosis in cells.

Interorganelle transport of aminoglycerophospholipids

The aminoglycerophospholipids of eukaryotic cells, phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho), can be synthesized by multiple pathways. The PtdSer pathway encompasses the synthesis of PtdSer, its decarboxylation to PtdEtn and subsequent methylation reactions to form PtdCho. The Kennedy pathways consist of the synthesis of PtdEtn and PtdCho from Etn and Cho precursors via CDP-Etn and CDP-Cho intermediates. The reactions along the PtdSer pathway are spatially segregated with PtdSer synthesis occurring in the endoplasmic reticulum or mitochondria-associated membrane (MAM), PtdEtn formation occurring in the mitochondria and Golgi/vacuole compartments and PtdCho formation occurring in the endoplasmic reticulum or MAM. The organelle-specific metabolism of the different lipids in the PtdSer pathway has provided a convenient biochemical means for defining events in the interorganelle transport of the aminoglycerophospholipids in intact cells, isolated organelles and permeabilized cells. Studies with both mammalian cells and yeast demonstrate many significant similarities in lipid transport processes between the two systems. Genetic experiments in yeast now provide the tools to create new strains with mutations along the PtdSer pathway that can be conditionally rescued by the Kennedy pathway reactions. The genetic studies in yeast indicate that it is now possible to begin to define genes that participate in the interorganelle transport of the aminoglycerophospholipids.

Isolation of a Chinese hamster ovary cell mutant defective in intramitochondrial transport of phosphatidylserine

A CHO-K1 cell mutant with a specific decrease in cellular phosphatidylethanolamine (PE) level was isolated as a variant resistant to Ro09-0198, a PE-directed antibiotic peptide. The mutant was defective in the phosphatidylserine (PS) decarboxylation pathway for PE formation, in which PS produced in the endoplasmic reticulum is transported to mitochondria and then decarboxylated by an inner mitochondrial membrane enzyme, PS decarboxylase. Neither PS formation nor PS decarboxylase activity was reduced in the mutant, implying that the mutant is defective in some step of PS transport. The transport processes of phospholipids between the outer and inner mitochondrial membrane were analyzed by use of isolated mitochondria and two fluorescence-labeled phospholipid analogs, 1-palmitoyl-2-[N-[6(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino]caproyl]-PS (C6-NBD-PS) and C6-NBD-phosphatidylcholine (C6-NBD-PC). On incubation with the CHO-K1 mitochondria, C6-NBD-PS was readily decarboxylated to C6-NBD-PE, suggesting that the PS analog was partitioned into the outer leaflet of mitochondria and then translocated to the inner mitochondrial membrane. The rate of decarboxylation of C6-NBD-PS in the mutant mitochondria was reduced to approximately 40% of that in the CHO-K1 mitochondria. The quantity of phospholipid analogs translocated from the outer leaflet of mitochondria into inner mitochondrial membranes was further examined by selective extraction of the analogs from the outer leaflet of mitochondria. In the mutant mitochondria, the translocation of C6-NBD-PS was significantly reduced, whereas the translocation of C6-NBD-PC was not affected. These results indicate that the mutant is defective in PS transport between the outer and inner mitochondrial membrane and provide genetic evidence for the existence of a specific mechanism for intramitochondrial transport of PS.

Phosphatidylserine decarboxylase

Phosphatidylserine decarboxylase (PSD) is an important enzyme in the synthesis of phosphatidylethanolamine in both prokaryotes and eukaryotes. The cloned bacterial gene encodes an integral membrane protein that is first made as a proenzyme, and subsequently proteolyzed to an alpha subunit, containing a pyruvoyl prosthetic group, and a beta subunit. Two types of decarboxylases are found in yeast, PSD1 and PSD2, that localize to the inner mitochondrial membrane and the Golgi/vacuole membrane, respectively. The mammalian enzyme is also found in the inner mitochondrial membrane. The yeast genes and mammalian cDNA have been cloned and sequenced. The yeast genes contain 5' sequences associated with regulation of expression by inositol and choline. The yeast PSD1 and the mammalian PSD both contain an LGST amino acid motif that identifies the site of proteolysis and pyruvoyl prosthetic group attachment in the bacterial enzyme. The yeast PSD1 and mammalian PSD also have mitochondrial targeting and inner membrane sorting sequences. Processing intermediates have been defined in the mammalian enzyme that correspond to the sequential removal of the mitochondrial targeting and inner membrane sorting sequence, followed by formation of the alpha and beta subunits. In contrast, the PSD2 enzyme contains a putative Golgi localization/retention sequence and a C2 homology domain, in addition to predicted alpha and beta subunits. The transport requirements for substrate access to the PSD enzymes have provided important information about lipid trafficking, and the availability of yeast mutants is likely to provide important new genetic selections in the future.

Mammalian cell mutants of membrane phospholipid biogenesis

Cultured mammalian cell mutants defective in the biosynthesis of membrane phospholipids, although limited in number, are increasing our understanding of the molecular mechanisms underlying the biogenesis and the biological significance of membrane phospholipids in higher eukaryotes. This review summarizes the progress in the isolation and characterization of such mutants, focusing on those isolated from cultured Chinese hamster ovary (CHO) cells.