Mitochondria-localized phospholipase A2, AoPlaA, in Aspergillus oryzae displays phosphatidylethanolamine-specific activity and is involved in the maintenance of mitochondrial phospholipid composition

In mammals, cytosolic phospholipases A₂ (cPLA₂s) play important physiological roles by releasing arachidonic acid, a precursor for bioactive lipid mediators, from the biological membranes. In contrast, fungal cPLA₂-like proteins are much less characterized and their roles have remained elusive. AoPlaA is a cPLA₂-like protein in the filamentous fungus Aspergillus oryzae which, unlike mammalian cPLA₂, localizes to mitochondria. In this study, we investigated the biochemical and physiological functions of AoPlaA. Recombinant AoPlaA produced in E. coli displayed Ca²⁺-independent lipolytic activity. Mass spectrometry analysis demonstrated that AoPlaA displayed PLA₂ activity to phosphatidylethanolamine (PE), but not to other phospholipids, and generated 1-acylated lysoPE. Catalytic site mutants of AoPlaA displayed almost no or largely reduced activity to PE. Consistent with PE-specific activity of AoPlaA, AoplaA-overexpressing strain showed decreased PE content in the mitochondrial fraction. In contrast, AoplaA-disruption strain displayed increased content of cardiolipin. AoplaA-overexpressing strain, but not its counterparts overexpressing the catalytic site mutants, exhibited retarded growth at low temperature, possibly because of the impairment of the mitochondrial function caused by excess degradation of PE. These results suggest that AoPlaA is a novel PE-specific PLA₂ that plays a regulatory role in the maintenance of mitochondrial phospholipid composition.

Saccharomyces cerevisiae genome-wide mutant screen for sensitivity to 2,4-diacetylphloroglucinol, an antibiotic produced by Pseudomonas fluorescens

2,4-Diacetylphloroglucinol (2,4-DAPG), an antibiotic produced by Pseudomonas fluorescens, has broad-spectrum antibiotic activity, inhibiting organisms ranging from viruses, bacteria, and fungi to higher plants and mammalian cells. The biosynthesis and regulation of 2,4-DAPG in P. fluorescens are well described, but the mode of action against target organisms is poorly understood. As a first step to elucidate the mechanism, we screened a deletion library of Saccharomyces cerevisiae in broth and agar medium supplemented with 2,4-DAPG. We identified 231 mutants that showed increased sensitivity to 2,4-DAPG under both conditions, including 22 multidrug resistance-related mutants. Three major physiological functions correlated with an increase in sensitivity to 2,4-DAPG: membrane function, reactive oxygen regulation, and cell homeostasis. Physiological studies with wild-type yeast validated the results of the mutant screens. The chemical-genetic fitness profile of 2,4-DAPG resembled those of menthol, sodium azide, and hydrogen peroxide determined in previous high-throughput screening studies. Collectively, these findings indicate that 2,4-DAPG acts on multiple basic cellular processes.

The role of zinc in thrombosis and pulmonary embolism in the course of antiphospholipid syndrome (APS)--short review

Antiphospholipid antibodies may occur in the course of various diseases, but its presence is not necessarily associated with clinical symptoms. Zinc has multiple biological roles. For example, it stabilizes the cell's membrane and regulates its functions by influencing the synthesis of phospholipids and its distribution. The present review focuses on the possible associations between zinc and antiphospholipid antibodies and with the symptoms of antiphospholipid syndrome.

Multiple endoplasmic reticulum-to-nucleus signaling pathways coordinate phospholipid metabolism with gene expression by distinct mechanisms

In many organisms the coordinated synthesis of membrane lipids is controlled by feedback systems that regulate the transcription of target genes. However, a complete description of the transcriptional changes that accompany the remodeling of membrane phospholipids has not been reported. To identify metabolic signaling networks that coordinate phospholipid metabolism with gene expression, we profiled the sequential and temporal changes in genome-wide expression that accompany alterations in phospholipid metabolism induced by inositol supplementation in yeast. This analysis identified six distinct expression responses, which included phospholipid biosynthetic genes regulated by Opi1p, endoplasmic reticulum (ER) luminal protein folding chaperone and oxidoreductase genes regulated by the unfolded protein response pathway, lipid-remodeling genes regulated by Mga2p, as well as genes involved in ribosome biogenesis, cytosolic stress response, and purine and amino acid metabolism. We also report that the unfolded protein response pathway is rapidly inactivated by inositol supplementation and demonstrate that the response of the unfolded protein response pathway to inositol is separable from the response mediated by Opi1p. These data indicate that altering phospholipid metabolism produces signals that are relayed through numerous distinct ER-to-nucleus signaling pathways and, thereby, produce an integrated transcriptional response. We propose that these signals are generated in the ER by increased flux through the pathway of phosphatidylinositol synthesis.

Transcriptional regulation of yeast phospholipid biosynthetic genes

The last several years have been witness to significant developments in understanding transcriptional regulation of the yeast phospholipid structural genes. The response of most phospholipid structural genes to inositol is now understood on a mechanistic level. The roles of specific activators and repressors are also well established. The knowledge of specific regulatory factors that bind the promoters of phospholipid structural genes serves as a foundation for understanding the role of chromatin modification complexes. Collectively, these findings present a complex picture for transcriptional regulation of the phospholipid biosynthetic genes. The INO1 gene is an ideal example of the complexity of transcriptional control and continues to serve as a model for studying transcription in general. Furthermore, transcription of the regulatory genes is also subject to complex and essential regulation. In addition, databases resulting from a plethora of genome-wide studies have identified regulatory signals that control one of the essential phospholipid biosynthetic genes, PIS1. These databases also provide significant clues for other regulatory signals that may affect phospholipid biosynthesis. Here, we have tried to present a complete summary of the transcription factors and mechanisms that regulate the phospholipid biosynthetic genes.

Regulation of the Saccharomyces cerevisiae EKI1-encoded ethanolamine kinase by zinc depletion

Ethanolamine kinase catalyzes the committed step in the synthesis of phosphatidylethanolamine via the CDP-ethanolamine branch of the Kennedy pathway. Regulation of the EKI1-encoded ethanolamine kinase by the essential nutrient zinc was examined in Saccharomyces cerevisiae. The level of ethanolamine kinase activity increased when zinc was depleted from the growth medium. This regulation correlated with increases in the CDP-ethanolamine pathway intermediates phosphoethanolamine and CDP-ethanolamine, and an increase in the methylated derivative of phosphatidylethanolamine, phosphatidylcholine. The beta-galactosidase activity driven by the P(EKI1)-lacZ reporter gene was elevated in zinc-depleted cells, indicating that the increase in ethanolamine kinase activity was attributed to a transcriptional mechanism. The expression level of P(EKI1)-lacZ reporter gene activity in the zrt1deltazrt2delta mutant (defective in plasma membrane zinc transport) cells grown with zinc was similar to the activity expressed in wild-type cells grown without zinc. This indicated that EKI1 expression was sensitive to intracellular zinc. The zinc-mediated regulation of EKI1 expression was attenuated in the zap1delta mutant defective in the zinc-regulated transcription factor Zap1p. Direct interactions between Zap1p and putative zinc-responsive elements in the EKI1 promoter were demonstrated by electrophoretic mobility shift assays. Mutations of these elements to a nonconsensus sequence abolished Zap1p-DNA interactions. Taken together, this work demonstrated that the zinc-mediated regulation of ethanolamine kinase and the synthesis of phospholipids via the CDP-ethanolamine branch of the Kennedy pathway were controlled in part by Zap1p.

Casein kinase II phosphorylation of the yeast phospholipid synthesis transcription factor Opi1p

The transcription factor Opi1p regulates phospholipid synthesis in the yeast Saccharomyces cerevisiae by repressing the expression of several UAS(INO)-containing genes (e.g. INO1). Opi1p repressor activity is most active in inositol-supplemented cells. Regulation of Opi1p repressor activity is mediated by multiple phosphorylations catalyzed by protein kinases A and C. In this work, we showed that Opi1p was also phosphorylated by casein kinase II. Using purified maltose-binding protein-Opi1p as a substrate, casein kinase II activity was dose-and time-dependent and dependent on the concentrations of maltose-binding protein-Opi1p (Km = 25 microg/ml) and ATP (Km = 7 microM). Of three mutations (S10A, S38A, and S239A) in putative phosphorylation sites, 10 only the S10A mutation affected Opi1p phosphorylation. That Ser10 was a specific target of casein kinase II was confirmed by the loss of a phosphopeptide in the S10A mutant protein. The S10A mutation did not affect phosphorylation of Opi1p by either protein kinase A or protein kinase C. Likewise, phosphorylation of Opi1p by casein kinase II was not affected by mutations in protein kinase A (S31A and S251A) and protein (S10A) kinase C (S26A) phosphorylation sites. Expression of the OPI1 allele in an opi1Delta mutant attenuated (2-fold) the repressive effect of Opi1p on INO1 expression, and this effect was only observed when cells were grown in the absence of inositol. These data supported the conclusion that casein kinase II phosphorylation at Ser10 played a role in stimulating the repression of INO1 when Opi1p was not in its most active state (i.e. in inositol-deprived cells).