Sphingomyelin maintains the cutaneous barrier via regulation of the STAT3 pathway.

Epidermal tissues play vital roles in maintaining homeostasis and preventing the dysregulation of the cutaneous barrier. Sphingomyelin (SM), a sphingolipid synthesized by sphingomyelin synthase (SMS) 1 and 2, is involved in signal transduction via modulation of lipid-raft functions. Though the implications of SMS on inflammatory diseases have been reported, its role in dermatitis has not been clarified. In this study, we investigated the role of SM in the cutaneous barrier using a dermatitis model established by employing Sgms1 and 2 deficient mice. SM deficiency impaired the cutaneous inflammation and upregulated signal transducer and activator of transcription 3 (STAT3) phosphorylation in epithelial tissues. Furthermore, using mouse embryonic fibroblast cells, the sensitivity of STAT3 to Interleukin-6 stimulation was increased in Sgms-deficient cells. Using tofacitinib, a clinical JAK inhibitor, the study showed that SM deficiency might participate in STAT3 phosphorylation via JAK activation. Overall, these results demonstrate that SM is essential for maintaining the cutaneous barrier via the STAT3 pathway, suggesting SM could be a potential therapeutic target for dermatitis treatment.

Sphingomyelin synthase 2 loss suppresses steatosis but exacerbates fibrosis in the liver of mice fed with choline-deficient, L-amino acid-defined, high-fat diet.

Sphingomyelin synthase 2 (SMS2) regulates sphingomyelin synthesis and contributes to obesity and hepatic steatosis. Here, we investigated the effect of SMS2 deficiency on liver fibrosis in mice fed with choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD) or injected with carbon tetrachloride (CCl4), respectively. SMS2 deficiency suppressed hepatic steatosis, but exacerbated fibrosis induced by CDAHFD feeding. Sphingosine 1-phosphate (S1P), which is a key lipid mediator induces fibrosis in various organs, was increased in the liver of mice fed with CDAHFD. The increase of S1P became prominent by SMS2 deficiency. Meanwhile, SMS2 deficiency had no impact on CCl4-induced liver injury, fibrosis and S1P levels. Our findings demonstrated that SMS2 deficiency suppresses steatosis but worsens fibrosis in the liver in a specific condition with CDAHFD feeding.

Airway Resistance Caused by Sphingomyelin Synthase 2 Insufficiency in Response to Cigarette Smoke.

Sphingomyelin synthase is responsible for the production of sphingomyelin (SGM), the second most abundant phospholipid in mammalian plasma, from ceramide, a major sphingolipid. Knowledge of the effects of cigarette smoke on SGM production is limited. In the present study, we examined the effect of chronic cigarette smoke on sphingomyelin synthase (SGMS) activity and evaluated how the deficiency of Sgms2, one of the two isoforms of mammalian SGMS, impacts pulmonary function. Sgms2-knockout and wild-type control mice were exposed to cigarette smoke for 6 months, and pulmonary function testing was performed. SGMS2-dependent signaling was investigated in these mice and in human monocyte-derived macrophages of nonsmokers and human bronchial epithelial (HBE) cells isolated from healthy nonsmokers and subjects with chronic obstructive pulmonary disease (COPD). Chronic cigarette smoke reduces SGMS activity and Sgms2 gene expression in mouse lungs. Sgms2-deficient mice exhibited enhanced airway and tissue resistance after chronic cigarette smoke exposure, but had similar degrees of emphysema, compared with smoke-exposed wild-type mice. Sgms2-/- mice had greater AKT phosphorylation, peribronchial collagen deposition, and protease activity in their lungs after smoke inhalation. Similarly, we identified reduced SGMS2 expression and enhanced phosphorylation of AKT and protease production in HBE cells isolated from subjects with COPD. Selective inhibition of AKT activity or overexpression of SGMS2 reduced the production of several matrix metalloproteinases in HBE cells and monocyte-derived macrophages. Our study demonstrates that smoke-regulated Sgms2 gene expression influences key COPD features in mice, including airway resistance, AKT signaling, and protease production.

Computational study on human sphingomyelin synthase 1 (hSMS1).

Human sphingomyelin synthase 1 (hSMS1) is the last enzyme for sphingomyelin (SM) biosynthesis. It has been discovered that in different human tumor tissues the SM levels are lower compared to normal tissues and the activation of hSMS1, to restore the normal levels of SM, inhibits cell cycle proliferation of cancer cells. Since the importance of SM and other lipid metabolism genes in the malignant transformation, we decided to explore the hSMS1 mechanism of action. Enzymes capable to regulate the formation of lipids are therefore of paramount importance. Here we present a computational study on sphingomyelin synthases hSMS1. The full structure of the enzyme was obtained by means of homology and ab initio techniques. Further molecular dynamics and docking studies permitted to identify putative binding sites and to identify the key residues for binding. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Sphingomyelin synthase 2, but not sphingomyelin synthase 1, is involved in HIV-1 envelope-mediated membrane fusion.

Membrane fusion between the viral envelope and plasma membranes of target cells has previously been correlated with HIV-1 infection. Lipids in the plasma membrane, including sphingomyelin, may be crucially involved in HIV-1 infection; however, the role of lipid-metabolic enzymes in membrane fusion remains unclear. In this study, we examined the roles of sphingomyelin synthase (SMS) in HIV-1 Env-mediated membrane fusion using a cell-cell fusion assay with HIV-1 mimetics and their target cells. We employed reconstituted cells as target cells that stably express Sms1 or Sms2 in Sms-deficient cells. Fusion susceptibility was ∼5-fold higher in Sms2-expressing cells (not in Sms1-expressing cells) than in Sms-deficient cells. The enhancement of fusion susceptibility observed in Sms2-expressing cells was reversed and reduced by Sms2 knockdown. We also found that catalytically nonactive Sms2 promoted membrane fusion susceptibility. Moreover, SMS2 co-localized and was constitutively associated with the HIV receptor·co-receptor complex in the plasma membrane. In addition, HIV-1 Env treatment resulted in a transient increase in nonreceptor tyrosine kinase (Pyk2) phosphorylation in Sms2-expressing and catalytically nonactive Sms2-expressing cells. We observed that F-actin polymerization in the region of membrane fusion was more prominent in Sms2-expressing cells than Sms-deficient cells. Taken together, our research provides insight into a novel function of SMS2 which is the regulation of HIV-1 Env-mediated membrane fusion via actin rearrangement.

Cardiolipin-mediated mitochondrial dynamics and stress response in Arabidopsis.

Mitochondria are essential and dynamic organelles in eukaryotes. Cardiolipin (CL) is a key phospholipid in mitochondrial membranes, playing important roles in maintaining the functional integrity and dynamics of mitochondria in animals and yeasts. However, CL's role in plants is just beginning to be elucidated. In this study, we used Arabidopsis thaliana to examine the subcellular distribution of CL and CARDIOLIPIN SYNTHASE (CLS) and analyzed loss-of-function cls mutants for defects in mitochondrial morphogenesis and stress response. We show that CL localizes to mitochondria and is enriched at specific domains, and CLS targets to the inner membrane of mitochondria with its C terminus in the intermembrane space. Furthermore, cls mutants exhibit significantly impaired growth as well as altered structural integrity and morphogenesis of mitochondria. In contrast to animals and yeasts, in which CL's effect on mitochondrial fusion is more profound, Arabidopsis CL plays a dominant role in mitochondrial fission and exerts this function, at least in part, through stabilizing the protein complex of the major mitochondrial fission factor, DYNAMIN-RELATED PROTEIN3. CL also plays a role in plant responses to heat and extended darkness, stresses that induce programmed cell death. Our study has uncovered conserved and plant-specific aspects of CL biology in mitochondrial dynamics and the organism response to environmental stresses.

The importance of cardiolipin synthase for mitochondrial ultrastructure, respiratory function, plant development, and stress responses in Arabidopsis.

Cardiolipin (CL) is the signature phospholipid of the mitochondrial inner membrane. In animals and yeast (Saccharomyces cerevisiae), CL depletion affects the stability of respiratory supercomplexes and is thus crucial to the energy metabolism of obligate aerobes. In eukaryotes, the last step of CL synthesis is catalyzed by CARDIOLIPIN SYNTHASE (CLS), encoded by a single-copy gene. Here, we characterize a cls mutant in Arabidopsis thaliana, which is devoid of CL. In contrast to yeast cls, where development is little affected, Arabidopsis cls seedlings are slow developing under short-day conditions in vitro and die if they are transferred to long-day (LD) conditions. However, when transferred to soil under LD conditions under low light, cls plants can reach the flowering stage, but they are not fertile. The cls mitochondria display abnormal ultrastructure and reduced content of respiratory complex I/complex III supercomplexes. The marked accumulation of tricarboxylic acid cycle derivatives and amino acids demonstrates mitochondrial dysfunction. Mitochondrial and chloroplastic antioxidant transcripts are overexpressed in cls leaves, and cls protoplasts are more sensitive to programmed cell death effectors, UV light, and heat shock. Our results show that CLS is crucial for correct mitochondrial function and development in Arabidopsis under both optimal and stress conditions.

Alternative cardiolipin synthase Cls1 compensates for stalled Cls2 function in Staphylococcus aureus under conditions of acute acid stress.

Staphylococcus aureus possesses two distinct cardiolipin (CL) synthase genes, cls1 and cls2. It was previously shown that cls2 encodes a housekeeping-type CL synthase. However, the role of cls1 is elusive; a cls1 mutant was found to be equal to the wild type in terms of CL accumulation and stress tolerance. Here, we report that the physiological role of cls1 is to synthesize CL under conditions of acute low-pH stress. Below pH 2.6, the cls1 mutant (i.e. carrying Cls2 alone) could not produce CL, while the cls2 mutant (carrying Cls1) effectively accumulated CL. The cls1-dependent CL production was quick (within 5 min) and did not require de novo protein synthesis. Together with the results of phylogenetic analyses, our findings suggest that cls1 was generated through the duplication of cls2 after the divergence of the genus Staphylococcus and that the alternative CL synthase encoded by this gene confers improved survival in the face of acute acid stress.

Impact of sphingomyelin synthase 1 deficiency on sphingolipid metabolism and atherosclerosis in mice.

OBJECTIVE: Sphingomyelin synthase (SMS) catalyzes the conversion of ceramide to sphingomyelin and sits at the crossroads of sphingolipid biosynthesis. SMS has 2 isoforms: SMS1 and SMS2. Although they have the same SMS activity, they are different enzymes with distinguishable subcellular localizations and cell expression patterns. It is conceivable that these differences could yield different consequences, in terms of sphingolipid metabolism and its related atherogenesis. METHODS AND RESULTS: We created Sms1 gene knockout mice and found that Sms1 deficiency significantly decreased plasma, liver, and macrophage sphingomyelin (59%, 45%, and 54%, respectively), but only had a marginal effect on ceramide levels. Surprisingly, we found that Sms1 deficiency dramatically increased glucosylceramide and GM3 levels in plasma, liver, and macrophages (4- to 12-fold), whereas Sms2 deficiency had no such effect. We evaluated the total SMS activity in tissues and found that Sms1 deficiency causes 77% reduction in SMS activity in macrophages, indicating SMS1 is the major SMS in macrophages. Moreover, Sms1-deficient macrophages have a significantly higher glucosylceramide synthase activity. We also found that Sms1 deficiency significantly attenuated toll-like 4 receptor-mediated nuclear factor-κB and mitogen-activated protein kinase activation after lipopolysaccharide treatment. To evaluate atherogenicity, we transplanted Sms1 knockout mouse bone marrow into low-density lipoprotein receptor knockout mice (Sms1(-/-)→Ldlr(-/-)). After 3 months on a western diet, these animals showed a significant decrease of atherosclerotic lesions in the root and the entire aorta (35% and 44%, P<0.01, respectively) and macrophage content in lesions (51%, P<0.05), compared with wild-type→Ldlr(-/-) mice. CONCLUSIONS: Sms1 deficiency decreases sphingomyelin, but dramatically increases the levels of glycosphingolipids. Atherosclerosis in Sms1(-/-)→Ldlr(-/-) mice is significantly decreased.

The evolution of cardiolipin biosynthesis and maturation pathways and its implications for the evolution of eukaryotes.

BACKGROUND: Cardiolipin (CL) is an important component in mitochondrial inner and bacterial membranes. Its appearance in these two biomembranes has been considered as evidence of the endosymbiotic origin of mitochondria. But CL was reported to be synthesized through two distinct enzymes--CLS_cap and CLS_pld in eukaryotes and bacteria. Therefore, how the CL biosynthesis pathway evolved is an interesting question. RESULTS: Phylogenetic distribution investigation of CL synthase (CLS) showed: most bacteria have CLS_pld pathway, but in partial bacteria including proteobacteria and actinobacteria CLS_cap pathway has already appeared; in eukaryotes, Supergroup Opisthokonta and Archaeplastida, and Subgroup Stramenopiles, which all contain multicellular organisms, possess CLS_cap pathway, while Supergroup Amoebozoa and Excavata and Subgroup Alveolata, which all consist exclusively of unicellular eukaryotes, bear CLS_pld pathway; amitochondriate protists in any supergroups have neither. Phylogenetic analysis indicated the CLS_cap in eukaryotes have the closest relationship with those of alpha proteobacteria, while the CLS_pld in eukaryotes share a common ancestor but have no close correlation with those of any particular bacteria. CONCLUSIONS: The first eukaryote common ancestor (FECA) inherited the CLS_pld from its bacterial ancestor (e. g. the bacterial partner according to any of the hypotheses about eukaryote evolution); later, when the FECA evolved into the last eukaryote common ancestor (LECA), the endosymbiotic mitochondria (alpha proteobacteria) brought in CLS_cap, and then in some LECA individuals the CLS_cap substituted the CLS_pld, and these LECAs would evolve into the protist lineages from which multicellular eukaryotes could arise, while in the other LECAs the CLS_pld was retained and the CLS_cap was lost, and these LECAs would evolve into the protist lineages possessing CLS_pld. Besides, our work indicated CL maturation pathway arose after the emergence of eukaryotes probably through mechanisms such as duplication of other genes, and gene duplication and loss occurred frequently at different lineage levels, increasing the pathway diversity probably to fit the complicated cellular process in various cells. Our work also implies the classification putting Stramenopiles and Alveolata together to form Chromalveolata may be unreasonable; the absence of CL synthesis and maturation pathways in amitochondriate protists is most probably due to secondary loss.

Sphingomyelin synthases regulate protein trafficking and secretion.

Sphingomyelin synthases (SMS1 and 2) represent a class of enzymes that transfer a phosphocholine moiety from phosphatidylcholine onto ceramide thus producing sphingomyelin and diacylglycerol (DAG). SMS1 localizes at the Golgi while SMS2 localizes both at the Golgi and the plasma membrane. Previous studies from our laboratory showed that modulation of SMS1 and, to a lesser extent, of SMS2 affected the formation of DAG at the Golgi apparatus. As a consequence, down-regulation of SMS1 and SMS2 reduced the localization of the DAG-binding protein, protein kinase D (PKD), to the Golgi. Since PKD recruitment to the Golgi has been implicated in cellular secretion through the trans golgi network (TGN), the effect of down-regulation of SMSs on TGN-to-plasma membrane trafficking was studied. Down regulation of either SMS1 or SMS2 significantly retarded trafficking of the reporter protein vesicular stomatitis virus G protein tagged with GFP (VSVG-GFP) from the TGN to the cell surface. Inhibition of SMSs also induced tubular protrusions from the trans Golgi network reminiscent of inhibited TGN membrane fission. Since a recent study demonstrated the requirement of PKD activity for insulin secretion in beta cells, we tested the function of SMS in this model. Inhibition of SMS significantly reduced insulin secretion in rat INS-1 cells. Taken together these results provide the first direct evidence that both enzymes (SMS1 and 2) are capable of regulating TGN-mediated protein trafficking and secretion, functions that are compatible with PKD being a down-stream target for SMSs in the Golgi.

Chemopreventive effects of silymarin against 1,2-dimethylhydrazine plus dextran sodium sulfate-induced inflammation-associated carcinogenicity and genotoxicity in the colon of gpt delta rats.

Silymarin, a natural flavonoid from the seeds of milk thistle, is used for chemoprevention against various cancers in clinical settings and in experimental models. To examine the chemopreventive mechanisms of silymarin against colon cancer, we investigated suppressive effects of silymarin against carcinogenicity and genotoxicity induced by 1,2-dimethylhydrazine (DMH) plus dextran sodium sulfate (DSS) in the colon of F344 gpt delta transgenic rats. Male gpt delta rats were given a single subcutaneous injection of 40 mg/kg DMH and followed by 1.5% DSS in drinking water for a week. They were fed diets containing silymarin for 4 weeks, starting 1 week before DMH injection and samples were collected at 4, 20 and 32 weeks after the DMH treatment. Silymarin at doses of 100 and 500 p.p.m. suppressed the tumor formation in a dose-dependent manner and the reduction was statistically significant. In the mutation assays, DMH plus DSS enhanced the gpt mutant frequency (MF) in the colon, and the silymarin treatments reduced the MFs by 20%. Silymarin also reduced the genotoxicity of DMH in a dose-dependent manner in bacterial mutation assay with Salmonella typhimurium YG7108, a sensitive strain to alkylating agents, and the maximum reduction was >80%. These results suggest that silymarin is chemopreventive against DMH/DSS-induced inflammation-associated colon carcinogenesis and silymarin might act as an antigenotoxic agent, in part.

Proteinase-activated receptors, nucleotide P2Y receptors, and µ-opioid receptor-1B are under the control of the type I transmembrane proteins p23 and p24A in post-Golgi trafficking.

We recently characterized the proteinase-activated receptor (PAR)-2, a G protein-coupled receptor (GPCR), as the first cargo protein recognized by p24A. Here, we demonstrate that p24A binds to several other GPCRs, including PAR-1, the nucleotide receptors P2Y(1), P2Y(2), P2Y(4), and P2Y(11), as well as the µ-opioid receptor 1B. The acidic amino acid residues Glu and Asp at the second extracellular loop of GPCRs are essential for interaction with p24A. p23, another member of the p24 family, also interacts with GPCRs, similar to p24A. However, p23 shows a delayed dissociation from PAR-2 after activation of PAR-2, compared to the dissociation between PAR-2 and p24A. p24A and p23 arrest both P2Y(4) receptor and µ-opioid receptor 1B at the intracellular compartments, as observed for PAR-2. A comparable result was obtained when we studied primary rat astrocytes in culture. Over-expression of the N-terminal p24A fragment impairs PAR-2 resensitization in astrocytes that extends our findings to a native system. In summary, we demonstrate that p24A and p23 are specific cargo receptors of GPCRs and differentially control GPCR trafficking in the biosynthetic pathway, and thereby, p24A and p23 regulate GPCR signaling in astrocytes.

Mitochondrial dysfunction and increased reactive oxygen species impair insulin secretion in sphingomyelin synthase 1-null mice.

Sphingomyelin synthase 1 (SMS1) catalyzes the conversion of ceramide to sphingomyelin. Here, we generated and analyzed SMS1-null mice. SMS1-null mice exhibited moderate neonatal lethality, reduced body weight, and loss of fat tissues mass, suggesting that they might have metabolic abnormality. Indeed, analysis on glucose metabolism revealed that they showed severe deficiencies in insulin secretion. Isolated mutant islets exhibited severely impaired ability to release insulin, dependent on glucose stimuli. Further analysis indicated that mitochondria in mutant islet cells cannot up-regulate ATP production in response to glucose. We also observed additional mitochondrial abnormalities, such as hyperpolarized membrane potential and increased levels of reactive oxygen species (ROS) in mutant islets. Finally, when SMS1-null mice were treated with the anti-oxidant N-acetyl cysteine, we observed partial recovery of insulin secretion, indicating that ROS overproduction underlies pancreatic ß-cell dysfunction in SMS1-null mice. Altogether, our data suggest that SMS1 is important for controlling ROS generation, and that SMS1 is required for normal mitochondrial function and insulin secretion in pancreatic ß-cells.

Animal models for studying the pathophysiology of ceramide.

Bioactive sphingolipids play key roles in the regulation of several fundamental biological processes such as proliferation, apoptosis and transformation. The recent development of genetically engineered mouse (GEM) models has enabled the study of functional roles of sphingolipids in normal development and disease. In this chapter, we review the phenotypes of GEM models (knockout mice) that lack sphingolipid metabolism-related enzymes, discuss what we have learned from animal models and describe future directions of animal models in sphingolipid research.

Proteolytic processing of the gamma-subunit is associated with the failure to form GlcNAc-1-phosphotransferase complexes and mannose 6-phosphate residues on lysosomal enzymes in human macrophages.

GlcNAc-1-phosphotransferase is a Golgi-resident 540-kDa complex of three subunits, alpha(2)beta(2)gamma(2), that catalyze the first step in the formation of the mannose 6-phosphate (M6P) recognition marker on lysosomal enzymes. Anti-M6P antibody analysis shows that human primary macrophages fail to generate M6P residues. Here we have explored the sorting and intracellular targeting of cathepsin D as a model, and the expression of the GlcNAc-1-phosphotransferase complex in macrophages. Newly synthesized cathepsin D is transported to lysosomes in an M6P-independent manner in association with membranes whereas the majority is secreted. Realtime PCR analysis revealed a 3-10-fold higher GlcNAc-1-phosphotransferase subunit mRNA levels in macrophages than in fibroblasts or HeLa cells. At the protein level, the gamma-subunit but not the beta-subunit was found to be proteolytically cleaved into three fragments which form irregular 97-kDa disulfide-linked oligomers in macrophages. Size exclusion chromatography showed that the gamma-subunit fragments lost the capability to assemble with other GlcNAc-1-phosphotransferase subunits to higher molecular complexes. These findings demonstrate that proteolytic processing of the gamma-subunit represents a novel mechanism to regulate GlcNAc-1-phosphotransferase activity and the subsequent sorting of lysosomal enzymes.

Role of sphingomyelin synthase in controlling the antimicrobial activity of neutrophils against Cryptococcus neoformans.

The key host cellular pathway(s) necessary to control the infection caused by inhalation of the environmental fungal pathogen Cryptococcus neoformans are still largely unknown. Here we have identified that the sphingolipid pathway in neutrophils is required for them to exert their killing activity on the fungus. In particular, using both pharmacological and genetic approaches, we show that inhibition of sphingomyelin synthase (SMS) activity profoundly impairs the killing ability of neutrophils by preventing the extracellular release of an antifungal factor(s). We next found that inhibition of protein kinase D (PKD), which controls vesicular sorting and secretion and is regulated by diacylglycerol (DAG) produced by SMS, totally blocks the extracellular killing activity of neutrophils against C. neoformans. The expression of SMS genes, SMS activity and the levels of the lipids regulated by SMS (namely sphingomyelin (SM) and DAG) are up-regulated during neutrophil differentiation. Finally, tissue imaging of lungs infected with C. neoformans using matrix-assisted laser desorption-ionization mass spectrometry (MALDI-MS), revealed that specific SM species are associated with neutrophil infiltration at the site of the infection. This study establishes a key role for SMS in the regulation of the killing activity of neutrophils against C. neoformans through a DAG-PKD dependent mechanism, and provides, for the first time, new insights into the protective role of host sphingolipids against a fungal infection.

A novel function of the human CLS1 in phosphatidylglycerol synthesis and remodeling.

Phosphatidylglycerol (PG) is a precursor for the biosynthesis of cardiolipin and a signaling molecule required for various cellular functions. PG is subjected to remodeling subsequent to its de novo biosynthesis in mitochondria to incorporate appropriate acyl content for its biological functions and to prevent the harmful effect of lysophosphatidylglycerol (LPG) accumulation. Yet, a gene encoding a mitochondrial LPG acyltransferase has not been identified. In this report, we identified a novel function of the human cardiolipin synthase (hCLS1) in regulating PG remodeling. In addition to the reported cardiolipin synthase activity, the recombinant hCLS1 protein expressed in COS-7 cells and Sf-9 insect cells exhibited a strong acyl-CoA-dependent LPG acyltransferase activity, which was further confirmed by purified hCLS1 protein overexpressed in Sf-9 cells. The recombinant hCLS1 displayed an acyl selectivity profile in the order of in the order of C18:1>C18:2>C18:0>C16:0, which is similar to that of hCLS1 toward PGs in cardiolipin synthesis, suggesting that the PG remodeling by hCLS1 is an intrinsic property of the enzyme. In contrast, no significant acyltransferase activity was detected from the recombinant hCLS1 enzyme toward lysocardiolipin which shares a similar structure with LPG. In support of a key function of hCLS1 in PG remodeling, overexpression of hCLS1 in COS-7 cells significantly increased PG biosynthesis concurrent with elevated levels of cardiolipin without any significant effects on the biosynthesis of other phospholipids. These results demonstrate for the first time that hCLS1 catalyzes two consecutive steps in cardiolipin biosynthesis by acylating LPG to PG and then converting PG to cardiolipin.

Sfp-type 4'-phosphopantetheinyl transferase is indispensable for fungal pathogenicity.

In filamentous fungi, Sfp-type 4'-phosphopantetheinyl transferases (PPTases) activate enzymes involved in primary (alpha-aminoadipate reductase [AAR]) and secondary (polyketide synthases and nonribosomal peptide synthetases) metabolism. We cloned the PPTase gene PPT1 of the maize anthracnose fungus Colletotrichum graminicola and generated PPTase-deficient mutants (Deltappt1). Deltappt1 strains were auxotrophic for Lys, unable to synthesize siderophores, hypersensitive to reactive oxygen species, and unable to synthesize polyketides (PKs). A differential analysis of secondary metabolites produced by wild-type and Deltappt1 strains led to the identification of six novel PKs. Infection-related morphogenesis was affected in Deltappt1 strains. Rarely formed appressoria of Deltappt1 strains were nonmelanized and ruptured on intact plant. The hyphae of Deltappt1 strains colonized wounded maize (Zea mays) leaves but failed to generate necrotic anthracnose disease symptoms and were defective in asexual sporulation. To analyze the pleiotropic pathogenicity phenotype, we generated AAR-deficient mutants (Deltaaar1) and employed a melanin-deficient mutant (M1.502). Results indicated that PPT1 activates enzymes required at defined stages of infection. Melanization is required for cell wall rigidity and appressorium function, and Lys supplied by the AAR1 pathway is essential for necrotrophic development. As PPTase-deficient mutants of Magnaporthe oryzea were also nonpathogenic, we conclude that PPTases represent a novel fungal pathogenicity factor.

Kei1: a novel subunit of inositolphosphorylceramide synthase, essential for its enzyme activity and Golgi localization.

Fungal sphingolipids have inositol-phosphate head groups, which are essential for the viability of cells. These head groups are added by inositol phosphorylceramide (IPC) synthase, and AUR1 has been thought to encode this enzyme. Here, we show that an essential protein encoded by KEI1 is a novel subunit of IPC synthase of Saccharomyces cerevisiae. We find that Kei1 is localized in the medial-Golgi and that Kei1 is cleaved by Kex2, a late Golgi processing endopeptidase; therefore, it recycles between the medial- and late Golgi compartments. The growth defect of kei1-1, a temperature-sensitive mutant, is effectively suppressed by the overexpression of AUR1, and Aur1 and Kei1 proteins form a complex in vivo. The kei1-1 mutant is hypersensitive to aureobasidin A, a specific inhibitor of IPC synthesis, and the IPC synthase activity in the mutant membranes is thermolabile. A part of Aur1 is missorted to the vacuole in kei1-1 cells. We show that the amino acid substitution in kei1-1 causes release of Kei1 during immunoprecipitation of Aur1 and that Aur1 without Kei1 has hardly detectable IPC synthase activity. From these results, we conclude that Kei1 is essential for both the activity and the Golgi localization of IPC synthase.