Yeast Pah1p phosphatidate phosphatase is regulated by proteasome-mediated degradation

Insights into phosphatidic acid phosphatase and its potential role as a therapeutic target

Phosphatidic acid phosphatase, a conserved eukaryotic enzyme that catalyzes the Mg2+-dependent dephosphorylation of phosphatidic acid to produce diacylglycerol, has emerged as a vital regulator of lipid homeostasis. By controlling the balance of phosphatidic acid and diacylglycerol, the enzyme governs the use of the lipids for synthesis of the storage lipid triacylglycerol and the membrane phospholipids needed for cell growth. The mutational, biochemical, and cellular analyses of yeast phosphatidic acid phosphatase have provided insights into the structural determinants of enzyme function with the understanding of its regulation by phosphorylation and dephosphorylation. The key role that the enzyme plays in triacylglycerol synthesis indicates it may be a potential drug target to ameliorate obesity in humans. The enzyme activity, which is critical to the growth and virulence of pathogenic fungi, is a proposed target for therapeutic development to ameliorate fungal infections.

Architecture and function of yeast phosphatidate phosphatase Pah1 domains/regions

Phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, provides a direct precursor for the synthesis of the storage lipid triacylglycerol and the membrane phospholipids phosphatidylcholine and phosphatidylethanolamine. The enzyme controlling the key phospholipid PA also plays a crucial role in diverse aspects of lipid metabolism and cell physiology. PA phosphatase is a peripheral membrane enzyme that is composed of multiple domains/regions required for its catalytic function and subcellular localization. In this review, we discuss the domains/regions of PA phosphatase from the yeast Saccharomyces cerevisiae with reference to the homologous enzyme from mammalian cells.

The CTR hydrophobic residues of Nem1 catalytic subunit are required to form a protein phosphatase complex with Spo7 to activate yeast Pah1 PA phosphatase

The Nem1-Spo7 phosphatase complex plays a key role in lipid metabolism as an activator of Pah1 phosphatidate phosphatase, which produces diacylglycerol for the synthesis of triacylglycerol and membrane phospholipids. For dephosphorylation of Pah1, the Nem1 catalytic subunit requires Spo7 for the recruitment of the protein substrate and interacts with the regulatory subunit through its conserved region (residues 251-446). In this work, we found that the Nem1 C-terminal region (CTR) (residues 414-436), which flanks the haloacid dehalogenase-like catalytic domain (residues 251-413), contains the conserved hydrophobic residues (L414, L415, L417, L418, L421, V430, L434, and L436) that are necessary for the complex formation with Spo7. AlphaFold predicts that some CTR residues of Nem1 interact with Spo7 conserved regions, whereas some residues interact with the haloacid dehalogenase-like domain. By site-directed mutagenesis, Nem1 variants were constructed to lack (Δ(414-446)) or substitute alanines (8A) and arginines (8R) for the hydrophobic residues. When co-expressed with Spo7, the CTR variants of Nem1 did not form a complex with Spo7. In addition, the Nem1 variants were incapable of catalyzing the dephosphorylation of Pah1 in the presence of Spo7. Moreover, the Nem1 variants expressed in nem1Δ cells did not complement the phenotypes characteristic of a defect in the Nem1-Spo7/Pah1 phosphatase cascade function (e.g., lipid synthesis, lipid droplet formation, and phospholipid biosynthetic gene expression). These findings support that Nem1 interacts with Spo7 through its CTR hydrophobic residues to form a phosphatase complex for catalytic activity and physiological functions.

Protein kinase Hsl1 phosphorylates Pah1 to inhibit phosphatidate phosphatase activity and regulate lipid synthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Pah1 phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, plays a key role in utilizing PA for the synthesis of the neutral lipid triacylglycerol and thereby controlling the PA-derived membrane phospholipids. The enzyme function is controlled by its subcellular location as regulated by phosphorylation and dephosphorylation. Pah1 is initially inactivated in the cytosol through phosphorylation by multiple protein kinases and then activated via its recruitment and dephosphorylation by the protein phosphatase Nem1-Spo7 at the nuclear/endoplasmic reticulum membrane where the PA phosphatase reaction occurs. Many of the protein kinases that phosphorylate Pah1 have yet to be characterized with the identification of the target residues. Here, we established Pah1 as a bona fide substrate of septin-associated Hsl1, a protein kinase involved in mitotic morphogenesis checkpoint signaling. The Hsl1 activity on Pah1 was dependent on reaction time and the amounts of protein kinase, Pah1, and ATP. The Hsl1 phosphorylation of Pah1 occurred on Ser-748 and Ser-773, and the phosphorylated protein exhibited a 5-fold reduction in PA phosphatase catalytic efficiency. Analysis of cells expressing the S748A and S773A mutant forms of Pah1 indicated that Hsl1-mediated phosphorylation of Pah1 promotes membrane phospholipid synthesis at the expense of triacylglycerol, and ensures the dependence of Pah1 function on the Nem1-Spo7 protein phosphatase. This work advances the understanding of how Hsl1 facilitates membrane phospholipid synthesis through the phosphorylation-mediated regulation of Pah1.

Catalytic core function of yeast Pah1 phosphatidate phosphatase reveals structural insight into its membrane localization and activity control

The PAH1-encoded phosphatidate (PA) phosphatase is a major source of diacylglycerol for the production of the storage lipid triacylglycerol and a key regulator for the de novo phospholipid synthesis in Saccharomyces cerevisiae. The catalytic function of Pah1 depends on its membrane localization which is mediated through its phosphorylation by multiple protein kinases and dephosphorylation by the Nem1-Spo7 protein phosphatase complex. The full-length Pah1 is composed of a catalytic core (N-LIP and HAD-like domains, amphipathic helix, and the WRDPLVDID domain) and non-catalytic regulatory sequences (intrinsically disordered regions, RP domain, and acidic tail) for phosphorylation and interaction with Nem1-Spo7. How the catalytic core regulates Pah1 localization and cellular function is not clear. In this work, we analyzed a variant of Pah1 (i.e., Pah1-CC (catalytic core)) that is composed only of the catalytic core. Pah1-CC expressed on a low-copy plasmid complemented the pah1Δ mutant phenotypes (e.g., nuclear/ER membrane expansion, reduced levels of triacylglycerol, and lipid droplet formation) without requiring Nem1-Spo7. The cellular function of Pah1-CC was supported by its PA phosphatase activity mostly associated with the membrane fraction. Although functional, Pah1-CC was distinct from Pah1 in the protein and enzymological properties, which include overexpression toxicity, association with heat shock proteins, and significant reduction of the Vmax value. These findings on the Pah1 catalytic core enhance the understanding of its structural requirements for membrane localization and activity control.

The Saccharomyces cerevisiae Spo7 basic tail is required for Nem1-Spo7/Pah1 phosphatase cascade function in lipid synthesis

The Saccharomyces cerevisiae Nem1-Spo7 protein phosphatase complex dephosphorylates and thereby activates Pah1 at the nuclear/endoplasmic reticulum membrane. Pah1, a phosphatidate phosphatase catalyzing the dephosphorylation of phosphatidate to produce diacylglycerol, is one of the most highly regulated enzymes in lipid metabolism. The diacylglycerol produced in the lipid phosphatase reaction is utilized for the synthesis of triacylglycerol that is stored in lipid droplets. Disruptions of the Nem1-Spo7/Pah1 phosphatase cascade cause a plethora of physiological defects. Spo7, the regulatory subunit of the Nem1-Spo7 complex, is required for the Nem1 catalytic function and interacts with the acidic tail of Pah1. Spo7 contains three conserved homology regions (CR1-3) that are important for the interaction with Nem1, but its region for the interaction with Pah1 is unknown. Here, by deletion and site-specific mutational analyses of Spo7, we revealed that the C-terminal basic tail (residues 240-259) containing five arginine and two lysine residues is important for the Nem1-Spo7 complex-mediated dephosphorylation of Pah1 and its cellular function (triacylglycerol synthesis, lipid droplet formation, maintenance of nuclear/endoplasmic reticulum membrane morphology, and cell growth at elevated temperatures). The glutaraldehyde cross-linking analysis of synthetic peptides indicated that the Spo7 basic tail interacts with the Pah1 acidic tail. This work advances our understanding of the Spo7 function and the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis.

Impairment of RPN4, a transcription factor, induces ER stress and lipid abnormality in Saccharomyces cerevisiae

Accumulation of misfolded/unfolded proteins in the endoplasmic reticulum (ER) induces ER stress. The transcription factor RPN4 {"Regulatory Particle Non-ATPase"} regulates protein homeostasis by degrading proteins that elude proper folding or assembly via the proteasomal degradation pathway. Here, we studied the lipid alterations exerted by Saccharomyces cerevisiae to mitigate (ER) stress during adaptive responses in rpn4∆ cells. The loss of RPN4-induced ER stress increased phospholipid synthesis, leading to altered membrane structures and accumulation of neutral lipids, causing an increase in lipid droplets (LDs). There was a significant upregulation of genes involved in neutral lipid and membrane lipid synthesis in rpn4∆ cells. Overexpression of RPN4 restored the defects caused by rpn4∆ cells. Thus, our study provides new insight that RPN4 impacts lipid homeostasis.

Phosphatidate phosphatase Pah1 contains a novel RP domain that regulates its phosphorylation and function in yeast lipid synthesis

The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, is one of the most highly regulated enzymes in lipid metabolism. The enzyme controls whether cells utilize PA to produce membrane phospholipids or the major storage lipid triacylglycerol. PA levels, which are regulated by the enzyme reaction, also control the expression of UASINO-containing phospholipid synthesis genes via the Henry (Opi1/Ino2-Ino4) regulatory circuit. Pah1 function is largely controlled by its cellular location, which is mediated by phosphorylation and dephosphorylation. Multiple phosphorylations sequester Pah1 in the cytosol and protect it from 20S proteasome-mediated degradation. The endoplasmic reticulum-associated Nem1-Spo7 phosphatase complex recruits and dephosphorylates Pah1 allowing the enzyme to associate with and dephosphorylate its membrane-bound substrate PA. Pah1 contains domains/regions that include the N-LIP and haloacid dehalogenase-like catalytic domains, N-terminal amphipathic helix for membrane binding, C-terminal acidic tail for Nem1-Spo7 interaction, and a conserved tryptophan within the WRDPLVDID domain required for enzyme function. Through bioinformatics, molecular genetics, and biochemical approaches, we identified a novel RP (regulation of phosphorylation) domain that regulates the phosphorylation state of Pah1. We showed that the ΔRP mutation results in a 57% reduction in the endogenous phosphorylation of the enzyme (primarily at Ser-511, Ser-602, and Ser-773/Ser-774), an increase in membrane association and PA phosphatase activity, but reduced cellular abundance. This work not only identifies a novel regulatory domain within Pah1 but emphasizes the importance of the phosphorylation-based regulation of Pah1 abundance, location, and function in yeast lipid synthesis.

The Phosphatase Cascade Nem1/Spo7-Pah1 Regulates Fungal Development, Lipid Homeostasis, and Virulence in Botryosphaeria dothidea

Protein phosphatase complex Nem1/Spo7 plays crucial roles in the regulation of various biological processes in eukaryotes. However, its biological functions in phytopathogenic fungi are not well understood. In this study, genome-wide transcriptional profiling analysis revealed that Nem1 was significantly upregulated during the infection process of Botryosphaeria dothidea, and we identified and characterized the phosphatase complex Nem1/Spo7 and its substrate Pah1 (a phosphatidic acid phosphatase) in B. dothidea. Nem1/Spo7 physically interacted with and dephosphorylated Pah1 to promote triacylglycerol (TAG) and subsequent lipid droplet (LD) synthesis. Moreover, the Nem1/Spo7-dependently dephosphorylated Pah1 functioned as a transcriptional repressor of the key nuclear membrane biosynthesis genes to regulate nuclear membrane morphology. In addition, phenotypic analyses showed that the phosphatase cascade Nem1/Spo7-Pah1 was involved in regulating mycelial growth, asexual development, stress responses, and virulence of B. dothidea. IMPORTANCE Botryosphaeria canker and fruit rot caused by the fungus Botryosphaeria dothidea is one of the most destructive diseases of apple worldwide. Our data indicated that the phosphatase cascade Nem1/Spo7-Pah1 plays important roles in the regulation of fungal growth, development, lipid homeostasis, environmental stress responses, and virulence in B. dothidea. The findings will contribute to the in-depth and comprehensive understanding of Nem1/Spo7-Pah1 in fungi and the development of target-based fungicides for disease management.

Conserved regions of the regulatory subunit Spo7 are required for Nem1-Spo7/Pah1 phosphatase cascade function in yeast lipid synthesis

In the yeast Saccharomyces cerevisiae, the Nem1-Spo7 complex is a protein phosphatase that activates Pah1 phosphatidate (PA) phosphatase at the nuclear/endoplasmic reticulum membrane for the synthesis of triacylglycerol. The Nem1-Spo7/Pah1 phosphatase cascade largely controls whether PA is partitioned into the storage lipid triacylglycerol or into membrane phospholipids. The regulated synthesis of the lipids is crucial for diverse physiological processes during cell growth. Spo7 in the protein phosphatase complex is required as a regulatory subunit for the Nem1 catalytic subunit to dephosphorylate Pah1. The regulatory subunit contains three conserved homology regions (CR1, CR2, and CR3). Previous work showed that the hydrophobicity of LLI (residues 54-56) within CR1 is important for Spo7 function in the Nem1-Spo7/Pah1 phosphatase cascade. In this work, by deletion and site-specific mutational analyses, we revealed that CR2 and CR3 are also required for Spo7 function. Mutations in any one of the conserved regions were sufficient to disrupt the function of the Nem1-Spo7 complex. We determined that the uncharged hydrophilicity of STN (residues 141-143) within CR2 was required for Nem1-Spo7 complex formation. Additionally, the hydrophobicity of LL (residues 217 and 219) within CR3 was important for Spo7 stability, which indirectly affected complex formation. Finally, we showed the loss of Spo7 CR2 or CR3 function by the phenotypes (e.g., reduced amounts of triacylglycerol and lipid droplets, temperature sensitivity) that are attributed to defects in membrane translocation and dephosphorylation of Pah1 by the Nem1-Spo7 complex. These findings advance knowledge of the Nem1-Spo7 complex and its role in lipid synthesis regulation.

Glycogen synthase kinase homolog Rim11 regulates lipid synthesis through the phosphorylation of Pah1 phosphatidate phosphatase in yeast

Pah1 phosphatidate (PA) phosphatase plays a major role in triacylglycerol synthesis in Saccharomyces cerevisiae by producing its precursor diacylglycerol and concurrently regulates de novo phospholipid synthesis by consuming its precursor PA. The function of Pah1 requires its membrane localization, which is controlled by its phosphorylation state. Pah1 is dephosphorylated by the Nem1-Spo7 protein phosphatase, whereas its phosphorylation occurs by multiple known and unknown protein kinases. In this work, we show that Rim11, a yeast homolog of mammalian glycogen synthase kinase-3β, is a protein kinase that phosphorylates Pah1 on serine (Ser12, Ser602, and Ser818) and threonine (Thr163, Thr164, Thr522) residues. Enzymological characterization of Rim11 showed that its Km for Pah1 (0.4 μM) is similar to those of other Pah1-phosphorylating protein kinases, but its Km for ATP (30 μM) is significantly higher than those of these same kinases. Furthermore, we demonstrate Rim11 phosphorylation of Pah1 does not require substrate prephosphorylation but was increased ∼2-fold upon its prephosphorylation by the Pho85-Pho80 protein kinase. In addition, we show Rim11-phosphorylated Pah1 was a substrate for dephosphorylation by Nem1-Spo7. Finally, we demonstrate the Rim11 phosphorylation of Pah1 exerted an inhibitory effect on its PA phosphatase activity by reduction of its catalytic efficiency. Mutational analysis of the major phosphorylation sites (Thr163, Thr164, and Ser602) indicated that Rim11-mediated phosphorylation at these sites was required to ensure Nem1-Spo7-dependent localization of the enzyme to the membrane. Overall, these findings advance our understanding of the phosphorylation-mediated regulation of Pah1 function in lipid synthesis.

Phosphorylation-mediated regulation of the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis

The PAH1-encoded phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to produce diacylglycerol, controls the divergence of phosphatidate into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the nuclear/endoplasmic reticulum membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase complex. The phosphorylation of Pah1 by protein kinases, which include casein kinases I and II, Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C, controls its cellular location, catalytic activity, and susceptibility to proteasomal degradation. Nem1 (catalytic subunit) and Spo7 (regulatory subunit), which form a protein phosphatase complex catalyzing the dephosphorylation of Pah1 for its activation, are phosphorylated by protein kinases A and C. In this review, we discuss the functions and interrelationships of the protein kinases in the control of the Nem1-Spo7/Pah1 phosphatase cascade and lipid synthesis.

Mutant phosphatidate phosphatase Pah1-W637A exhibits altered phosphorylation, membrane association, and enzyme function in yeast

The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the dephosphorylation of PA to produce diacylglycerol, controls the bifurcation of PA into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase. We show that the conserved Trp-637 residue of Pah1, located in the intrinsically disordered region, is required for normal synthesis of membrane phospholipids, sterols, triacylglycerol, and the formation of lipid droplets. Analysis of mutant Pah1-W637A showed that the tryptophan residue is involved in the phosphorylation-mediated/dephosphorylation-mediated membrane association of the enzyme and its catalytic activity. The endogenous phosphorylation of Pah1-W637A was increased at the sites of the N-terminal region but was decreased at the sites of the C-terminal region. The altered phosphorylation correlated with an increase in its membrane association. In addition, membrane-associated PA phosphatase activity in vitro was elevated in cells expressing Pah1-W637A as a result of the increased membrane association of the mutant enzyme. However, the inherent catalytic function of Pah1 was not affected by the W637A mutation. Prediction of Pah1 structure by AlphaFold shows that Trp-637 and the catalytic residues Asp-398 and Asp-400 in the haloacid dehalogenase-like domain almost lie in the same plane, suggesting that these residues are important to properly position the enzyme for substrate recognition at the membrane surface. These findings underscore the importance of Trp-637 in Pah1 regulation by phosphorylation, membrane association of the enzyme, and its function in lipid synthesis.

NLIP and HAD-like Domains of Pah1 and Lipin 1 Phosphatidate Phosphatases Are Essential for Their Catalytic Activities

Saccharomyces cerevisiae Pah1 phosphatidate phosphatase (PAP) catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol, controlling phospholipids and triacylglycerol metabolisms. Pah1 and human Lipin 1 are intrinsically disordered proteins with 56% and 43% unfolded regions, respectively. Truncation analysis of the conserved and non-conserved regions showed that N- and C-conserved regions are essential for the catalytic activity of Pah1. PAP activities can be detected in the conserved N-terminal Lipin (NLIP) domain and C-terminal Lipin (CLIP)/haloacid dehalogenase (HAD)-like domain of Pah1 and Lipin 1, suggesting that the evolutionarily conserved domains are essential for the catalytic activity. The removal of disordered hydrophilic regions drastically reduced the protein solubility of Pah1. Thioredoxin is an efficient fusion protein for production of soluble NLIP–HAD recombinant proteins in Escherichia coli.

Target of Rapamycin Complex 1 (TORC1), Protein Kinase A (PKA) and Cytosolic pH Regulate a Transcriptional Circuit for Lipid Droplet Formation

Lipid droplets (LDs) are ubiquitous organelles that fulfill essential roles in response to metabolic cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). However, our understanding of signaling networks, especially transcriptional mechanisms, regulating membrane biogenesis is very limited. Here, we show that the nutrient-sensing Target of Rapamycin Complex 1 (TORC1) regulates LD formation at a transcriptional level, by targeting DGA1 expression, in a Sit4-, Mks1-, and Sfp1-dependent manner. We show that cytosolic pH (pHc), co-regulated by the plasma membrane H+-ATPase Pma1 and the vacuolar ATPase (V-ATPase), acts as a second messenger, upstream of protein kinase A (PKA), to adjust the localization and activity of the major transcription factor repressor Opi1, which in turn controls the metabolic switch between phospholipid metabolism and lipid storage. Together, this work delineates hitherto unknown molecular mechanisms that couple nutrient availability and pHc to LD formation through a transcriptional circuit regulated by major signaling transduction pathways.

The yeast FIT2 homologs are necessary to maintain cellular proteostasis and membrane lipid homeostasis

Lipid droplets (LDs) are implicated in conditions of lipid and protein dysregulation. The fat storage-inducing transmembrane (FIT; also known as FITM) family induces LD formation. Here, we establish a model system to study the role of the Saccharomyces cerevisiae FIT homologues (ScFIT), SCS3 and YFT2, in the proteostasis and stress response pathways. While LD biogenesis and basal endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) remain unaltered in ScFIT mutants, SCS3 was found to be essential for proper stress-induced UPR activation and for viability in the absence of the sole yeast UPR transducer IRE1 Owing to not having a functional UPR, cells with mutated SCS3 exhibited an accumulation of triacylglycerol within the ER along with aberrant LD morphology, suggesting that there is a UPR-dependent compensatory mechanism that acts to mitigate lack of SCS3 Additionally, SCS3 was necessary to maintain phospholipid homeostasis. Strikingly, global protein ubiquitylation and the turnover of both ER and cytoplasmic misfolded proteins is impaired in ScFITΔ cells, while a screen for interacting partners of Scs3 identifies components of the proteostatic machinery as putative targets. Together, our data support a model where ScFITs play an important role in lipid metabolism and proteostasis beyond their defined roles in LD biogenesis.This article has an associated First Person interview with the first author of the paper.

The Spo7 sequence LLI is required for Nem1-Spo7/Pah1 phosphatase cascade function in yeast lipid metabolism

The Nem1-Spo7 complex in the yeast Saccharomyces cerevisiae is a protein phosphatase that catalyzes the dephosphory-lation of Pah1 phosphatidate phosphatase, required for its translocation to the nuclear/endoplasmic reticulum membrane. The Nem1-Spo7/Pah1 phosphatase cascade plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. In this work, we examined Spo7, a regulatory subunit required for Nem1 catalytic function, to identify residues that govern formation of the Nem1-Spo7 complex. By deletion analysis of Spo7, we identified a hydrophobic Leu-Leu-Ile (LLI) sequence comprising residues 54-56 as being required for the protein to complement the temperature-sensitive phenotype of an spo7Δ mutant strain. Mutational analysis of the LLI sequence with alanine and arginine substitutions showed that its overall hydrophobicity is crucial for the formation of the Nem1-Spo7 complex as well as for the Nem1 catalytic function on its substrate, Pah1, in vivo Consistent with the role of the Nem1-Spo7 complex in activating the function of Pah1, we found that the mutational effects of the Spo7 LLI sequence were on the Nem1-Spo7/Pah1 axis that controls lipid synthesis and related cellular processes (e.g. triacylglycerol/phospholipid synthesis, lipid droplet formation, nuclear/endoplasmic reticulum membrane morphology, vacuole fusion, and growth on glycerol medium). These findings advance the understanding of Nem1-Spo7 complex formation and its role in the phosphatase cascade that regulates the function of Pah1 phosphatidate phosphatase.

Yeast phosphatidic acid phosphatase Pah1 hops and scoots along the membrane phospholipid bilayer

PA phosphatase, encoded by PAH1 in the yeast Saccharomyces cerevisiae, catalyzes the Mg²⁺-dependent dephosphorylation of PA, producing DAG at the nuclear/ER membrane. This enzyme plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. As an interfacial enzyme, PA phosphatase interacts with the membrane surface, binds its substrate, and catalyzes its reaction. The Triton X-100/PA-mixed micellar system has been utilized to examine the activity and regulation of yeast PA phosphatase. This system, however, does not resemble the in vivo environment of the membrane phospholipid bilayer. We developed an assay system that mimics the nuclear/ER membrane to assess PA phosphatase activity. PA was incorporated into unilamellar phospholipid vesicles (liposomes) composed of the major nuclear/ER membrane phospholipids, PC, PE, PI, and PS. We optimized this system to support enzyme-liposome interactions and to afford activity that is greater than that obtained with the aforementioned detergent system. Activity was regulated by phospholipid composition, whereas the enzyme's interaction with liposomes was insensitive to composition. Greater activity was attained with large (≥100 nm) versus small (50 nm) vesicles. The fatty-acyl moiety of PA had no effect on this activity. PA phosphatase activity was dependent on the bulk (hopping mode) and surface (scooting mode) concentrations of PA, suggesting a mechanism by which the enzyme operates along the nuclear/ER membrane in vivo.

Phosphatidate-mediated regulation of lipid synthesis at the nuclear/endoplasmic reticulum membrane

In yeast and higher eukaryotes, phospholipids and triacylglycerol are derived from phosphatidate at the nuclear/endoplasmic reticulum membrane. In de novo biosynthetic pathways, phosphatidate is channeled into membrane phospholipids via its conversion to CDP-diacylglycerol. Its dephosphorylation to diacylglycerol is required for the synthesis of triacylglycerol as well as for the synthesis of phosphatidylcholine and phosphatidylethanolamine via the Kennedy pathway. In addition to the role of phosphatidate as a precursor, it is a regulatory molecule in the transcriptional control of phospholipid synthesis genes via the Henry regulatory circuit. Pah1 phosphatidate phosphatase and Dgk1 diacylglycerol kinase are key players that function counteractively in the control of the phosphatidate level at the nuclear/endoplasmic reticulum membrane. Loss of Pah1 phosphatidate phosphatase activity not only affects triacylglycerol synthesis but also disturbs the balance of the phosphatidate level, resulting in the alteration of lipid synthesis and related cellular defects. The pah1Δ phenotypes requiring Dgk1 diacylglycerol kinase exemplify the importance of the phosphatidate level in the misregulation of cellular processes. The catalytic function of Pah1 requires its translocation from the cytoplasm to the nuclear/endoplasmic reticulum membrane, which is regulated through its phosphorylation in the cytoplasm by multiple protein kinases as well as through its dephosphorylation by the membrane-associated Nem1-Spo7 protein phosphatase complex. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

The Yarrowia lipolytica PAH1 homologue contributes but is not required for triacylglycerol biosynthesis during growth on glucose

The PAH1-encoded phosphatidate phosphatase (PAP) catalyzes the Mg²⁺ -dependent dephosphorylation of phosphatidate to produce diacylglycerol, which can be acylated to form triacylglycerol (TAG). In the model oleaginous yeast Yarrowia lipolytica, TAG is the major lipid produced, and its biosynthesis requires a continuous supply of diacylglycerol, which can be provided by the PAP reaction. However, the regulation of Pah1 has not been studied in detail in Y. lipolytica, and thus its contribution to the biosynthesis of TAG in this yeast is not well understood. In this work, we examined the regulation of the PAH1-mediated PAP activity and Pah1 abundance and localization in cells growing on glucose. We found that Pah1 abundance and localization were regulated in a growth-dependent manner, yet the loss of Pah1 did not have a major effect on PAP activity. We also examined the effects of the Y. lipolytica pah1Δ mutation on cell physiology and lipid biosynthesis. The lack of Pah1 in the pah1Δ mutant resulted in a moderate decrease in TAG levels and an increase in phospholipid levels. These results showed that Pah1 contributed to TAG biosynthesis in Y. lipolytica but also suggested the presence of other activities in the pah1Δ mutant that compensate for the loss of Pah1. Also, the levels of linoleic acid were elevated in pah1Δ cells with a concomitant decrease in the oleic acid levels suggesting that the pah1Δ mutation affected the biosynthesis of fatty acids.

Yck1 casein kinase I regulates the activity and phosphorylation of Pah1 phosphatidate phosphatase from Saccharomyces cerevisiae

The PAH1-encoded phosphatidate phosphatase in Saccharomyces cerevisiae plays a major role in triacylglycerol synthesis and the control of phospholipid synthesis. For its catalytic function on the nuclear/endoplasmic reticulum membrane, Pah1 translocates to the membrane through its phosphorylation/dephosphorylation. Pah1 phosphorylation on multiple serine/threonine residues is complex and catalyzed by diverse protein kinases. In this work, we demonstrate that Pah1 is phosphorylated by the YCK1-encoded casein kinase I (CKI), regulating Pah1 catalytic activity and phosphorylation. Phosphoamino acid analysis coupled with phosphopeptide mapping of the CKI-phosphorylated Pah1 indicated that it is phosphorylated mainly on multiple serine residues. Using site-directed mutagenesis and phosphorylation analysis of Pah1, we identified eight serine residues (i.e. Ser-114, Ser-475, Ser-511, Ser-602, Ser-677, Ser-705, Ser-748, and Ser-774) as the target sites of CKI. Of these residues, Ser-475 and Ser-511 were specific for CKI, whereas the others were shared by casein kinase II (Ser-705), Cdc28-cyclin B (Ser-602), Pho85-Pho80 (Ser-114, Ser-602, and Ser-748), protein kinase A (Ser-667 and Ser-774), and protein kinase C (Ser-677). CKI-mediated phosphorylation of Pah1 stimulated both its phosphatidate phosphatase activity and its subsequent phosphorylation by casein kinase II. However, the CKI-mediated phosphorylation of Pah1 strongly inhibited its subsequent phosphorylation by Pho85-Pho80, protein kinase A, and protein kinase C. In a reciprocal analysis, Pah1 phosphorylation by Pho85-Pho80 inhibited subsequent phosphorylation by CKI. CKI-mediated Pah1 phosphorylation was also inhibited by a peptide containing the Pah1 residues 506-517, including the kinase-specific Ser-511 residue. These findings advance our understanding of how Pah1 catalytic activity and phosphorylation are regulated by multiple protein kinases.

Protein kinase C mediates the phosphorylation of the Nem1-Spo7 protein phosphatase complex in yeast

The Nem1-Spo7 complex in the yeast Saccharomyces cerevisiae is a protein phosphatase required for the nuclear/endoplasmic reticulum membrane localization of Pah1, a phosphatidate phosphatase that produces diacylglycerol for triacylglycerol synthesis at the expense of phospholipid synthesis. In a previous study, we showed that the protein phosphatase is subject to phosphorylation by protein kinase A (PKA). Here, we demonstrate that Nem1-Spo7 is regulated through its phosphorylation by protein kinase C (PKC), which plays multiple roles, including the regulation of lipid synthesis and cell wall integrity. Phosphorylation analyses of Nem1-Spo7 and its synthetic peptides indicate that both subunits of the complex are bona fide PKC substrates. Site-directed mutagenesis of NEM1 and SPO7, coupled with phosphopeptide mapping and immunoblotting with a phosphoserine-specific PKC substrate antibody, revealed that Ser-201 in Nem1 and Ser-22/Ser-28 in Spo7 are major PKC target sites of phosphorylation. Activity analysis of mutant Nem1-Spo7 complexes indicates that the PKC phosphorylation of Nem1 exerts a stimulatory effect, but the phosphorylation of Spo7 has no effect. Lipid-labeling analysis of cells expressing the phosphorylation-deficient alleles of NEM1 and SPO7 indicates that the stimulation of the Nem1-Spo7 activity has the effect of increasing triacylglycerol synthesis. Prephosphorylation of Nem1-Spo7 by PKC inhibits the PKA phosphorylation of Nem1, whereas prephosphorylation of the phosphatase complex by PKA inhibits the PKC phosphorylation of Spo7. Collectively, this work advances the understanding of the Nem1-Spo7 regulation by phosphorylation and its impact on lipid synthesis.

Lipid droplet biogenesis regulated by the FgNem1/Spo7-FgPah1 phosphatase cascade plays critical roles in fungal development and virulence in Fusarium graminearum

Lipid droplets (LDs) control lipid metabolism in eukaryotic cells in general. However, the biogenesis regulation and biological functions of LDs are largely unknown in pathogenic fungi. Rapamycin treatment results in a significant increase of LD biogenesis in Fusarium graminearum. Molecular mechanisms of the target of rapamycin (TOR) pathway in regulating LD biogenesis and the functions of LD in virulence of F. graminearum were investigated in depth by combining genetic, cytological and phenotypic strategies. TOR in Fusarium graminearum (FgTOR) inhibition by rapamycin induces LD biogenesis through the FgPpg1/Sit4 signaling branch. FgPpg1 promotes phosphorylation of protein phosphatase FgNem1 by the protein kinase FgCak1. The phosphorylated FgNem1 dephosphorylates the phosphatidate phosphatase FgPah1. Dephosphorylated FgPah1 is active and stimulates LD biogenesis. Moreover, deletion of FgNem1/Spo7 or FgPah1 leads to serious defects in vegetative growth, sexual development and virulence. The results of this study provide novel insights into the regulatory mechanism and biological functions of the LDs in the devastating pathogenic fungus F. graminearum.

A conserved tryptophan within the WRDPLVDID domain of yeast Pah1 phosphatidate phosphatase is required for its in vivo function in lipid metabolism

PAH1-encoded phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to produce diacylglycerol at the endoplasmic reticulum membrane, plays a major role in controlling the utilization of phosphatidate for the synthesis of triacylglycerol or membrane phospholipids. The conserved N-LIP and haloacid dehalogenase-like domains of Pah1 are required for phosphatidate phosphatase activity and the in vivo function of the enzyme. Its non-conserved regions, which are located between the conserved domains and at the C terminus, contain sites for phosphorylation by multiple protein kinases. Truncation analyses of the non-conserved regions showed that they are not essential for the catalytic activity of Pah1 and its physiological functions (e.g. triacylglycerol synthesis). This analysis also revealed that the C-terminal region contains a previously unrecognized WRDPLVDID domain (residues 637-645) that is conserved in yeast, mice, and humans. The deletion of this domain had no effect on the catalytic activity of Pah1 but caused the loss of its in vivo function. Site-specific mutational analyses of the conserved residues within WRDPLVDID indicated that Trp-637 plays a crucial role in Pah1 function. This work also demonstrated that the catalytic activity of Pah1 is required but is not sufficient for its in vivo functions.

Lipin proteins and glycerolipid metabolism: Roles at the ER membrane and beyond

The regulation of glycerolipid biosynthesis is critical for homeostasis of cellular lipid stores and membranes. Here we review the role of lipin phosphatidic acid phosphatase enzymes in glycerolipid synthesis. Lipin proteins are unique among glycerolipid biosynthetic enzymes in their ability to transit among cellular membranes, rather than remain membrane tethered. We focus on the mechanisms that underlie lipin protein interactions with membranes and the versatile roles of lipins in several organelles, including the endoplasmic reticulum, mitochondria, endolysosomes, lipid droplets, and nucleus. We also review the corresponding physiological roles of lipins, which have been uncovered by the study of genetic lipin deficiencies. We propose that the growing body of knowledge concerning the biochemical and cellular activities of lipin proteins will be valuable for understanding the physiological functions of lipin proteins in health and disease. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Phosphatidate phosphatase regulates membrane phospholipid synthesis via phosphatidylserine synthase

The yeast Saccharomyces cerevisiae serves as a model eukaryote to elucidate the regulation of lipid metabolism. In exponentially growing yeast, a diverse set of membrane lipids are synthesized from the precursor phosphatidate via the liponucleotide intermediate CDP-diacylglycerol. As cells exhaust nutrients and progress into the stationary phase, phosphatidate is channeled via diacylglycerol to the synthesis of triacylglycerol. The CHO1-encoded phosphatidylserine synthase, which catalyzes the committed step in membrane phospholipid synthesis via CDP-diacylglycerol, and the PAH1-encoded phosphatidate phosphatase, which catalyzes the committed step in triacylglycerol synthesis are regulated throughout cell growth by genetic and biochemical mechanisms to control the balanced synthesis of membrane phospholipids and triacylglycerol. The loss of phosphatidate phosphatase activity (e.g., pah1Δ mutation) increases the level of phosphatidate and its conversion to membrane phospholipids by inducing Cho1 expression and phosphatidylserine synthase activity. The regulation of the CHO1 expression is mediated through the inositol-sensitive upstream activation sequence (UAS_INO), a cis-acting element for the phosphatidate-controlled Henry (Ino2-Ino4/Opi1) regulatory circuit. Consequently, phosphatidate phosphatase activity regulates phospholipid synthesis through the transcriptional regulation of the phosphatidylserine synthase enzyme.

Pah1p negatively regulates the expression of V-ATPase genes as well as vacuolar acidification

In yeast, PAH1 plays an important role in cell homeostasis and lipid biosynthesis. PAH1 encodes for the PA phosphatase, Pah1p, which is responsible for de novo TAG and phospholipid synthesis. It has been suggested that the lack of Pah1p causes irregular vacuolar morphology and dysfunctional V-ATPase pump activity. However, the molecular connection between Pah1p and V-ATPase activity has remained unclear. Through real-time PCR, we have shown that PAH1 is maximally induced at the stationary stage in the presence of inositol. We also found that vacuoles were less fragmented when PAH1 is maximally expressed. Subsequently, we observed that vacuoles from pah1Δ cells were more acidic than those in WT cells. Furthermore, V-ATPase genes were upregulated in the absence of Pah1p. These results suggest that Pah1p plays an important role in vacuolar activity by negatively regulating the expression of V-ATPase genes. As such, we provide evidence to show the role of Pah1p in vacuolar acidification and fragmentation.

Phosphorylation of lipid metabolic enzymes by yeast protein kinase C requires phosphatidylserine and diacylglycerol

Protein kinase C in Saccharomyces cerevisiae, i.e., Pkc1, is an enzyme that plays an important role in signal transduction and the regulation of lipid metabolic enzymes. Pkc1 is structurally similar to its counterparts in higher eukaryotes, but its requirement of phosphatidylserine (PS) and diacylglycerol (DAG) for catalytic activity has been unclear. In this work, we examined the role of these lipids in Pkc1 activity with protein and peptide substrates. In agreement with previous findings, yeast Pkc1 did not require PS and DAG for its activity on the peptide substrates derived from lipid metabolic proteins such as Pah1 [phosphatidate (PA) phosphatase], Nem1 (PA phosphatase phosphatase), and Spo7 (protein phosphatase regulatory subunit). However, the lipids were required for Pkc1 activity on the protein substrates Pah1, Nem1, and Spo7. Compared with DAG, PS had a greater effect on Pkc1 activity, and its dose-dependent interaction with the protein kinase was shown by the liposome binding assay. The Pkc1-mediated degradation of Pah1 was attenuated in the cho1Δ mutant, which is deficient in PS synthase, supporting the notion that the phospholipid regulates Pkc1 activity in vivo.

Phosphorylation of Dgk1 Diacylglycerol Kinase by Casein Kinase II Regulates Phosphatidic Acid Production in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, Dgk1 diacylglycerol (DAG) kinase catalyzes the CTP-dependent phosphorylation of DAG to form phosphatidic acid (PA). The enzyme in conjunction with Pah1 PA phosphatase controls the levels of PA and DAG for the synthesis of triacylglycerol and membrane phospholipids, the growth of the nuclear/endoplasmic reticulum membrane, and the formation of lipid droplets. Little is known about how DAG kinase activity is regulated by posttranslational modification. In this work, we examined the phosphorylation of Dgk1 DAG kinase by casein kinase II (CKII). When phosphate groups were globally reduced using nonspecific alkaline phosphatase, Triton X-100-solubilized membranes from DGK1-overexpressing cells showed a 7.7-fold reduction in DAG kinase activity; the reduced enzyme activity could be increased 5.5-fold by treatment with CKII. Dgk1(1-77) expressed heterologously in Escherichia coli was phosphorylated by CKII on a serine residue, and its phosphorylation was dependent on time as well as on the concentrations of CKII, ATP, and Dgk1(1-77). We used site-specific mutagenesis, coupled with phosphorylation analysis and phosphopeptide mapping, to identify Ser-45 and Ser-46 of Dgk1 as the CKII target sites, with Ser-46 being the major phosphorylation site. In vivo, the S46A and S45A/S46A mutations of Dgk1 abolished the stationary phase-dependent stimulation of DAG kinase activity. In addition, the phosphorylation-deficient mutations decreased Dgk1 function in PA production and in eliciting pah1Δ phenotypes, such as the expansion of the nuclear/endoplasmic reticulum membrane, reduced lipid droplet formation, and temperature sensitivity. This work demonstrates that the CKII-mediated phosphorylation of Dgk1 regulates its function in the production of PA.

Energy Storage in Yeast: Regulation and Competition with Ethanol Production

Mechanisms that may regulate the storage of energy as triacylglycerol in Saccharomyces cerevisiae were examined. First, the kinetics of Dga1p, which mediates the majority of diacylglycerol esterification, the lone committed step in triacylglycerol synthesis, was measured in vitro. With an apparent K _m of 17.0 μM, Dga1p has higher affinity for oleoyl-CoA than the only S. cerevisiae acyltransferase previously kinetically characterized, Lpt1p. Lpt1p is a 1-acylglycerol-3-phosphate O-acyltransferase that produces phosphatidate, a precursor to diacylglycerol. Therefore, limiting triacylglycerol synthesis to situations of elevated acyl-CoA concentration is unlikely. However, Dga1p's apparent V _max of 5.8 nmol/min/mg was 20 times lower than Lpt1p's. This supports Dga1p being rate limiting for TAG synthesis. Dga1p activity was not activated or inhibited when seven different molecules (e.g., ATP) which reflect cellular energy status were provided at physiological concentrations. Thus, allosteric regulation was not found. Coordination between triacylglycerol and glycogen synthesis was also tested. Yeast genetically deficient in triacylglycerol synthesis did not store more energy in glycogen and vice versa. Lastly, we tested whether genetically limiting energy storage in triacylglycerol, glycogen, steryl esters, or combinations of these will increase ethanol production efficiency. In nutrient-rich media containing 5 % glucose, solely limiting glycogen synthesis had the greatest affect, increasing ethanol production efficiency by 12 %. Since limiting glycogen synthesis only had a modest effect on growth in media containing 10 % ethanol, such genetic manipulation may improve commercial ethanol production.

Lipid synthesis and membrane contact sites: a crossroads for cellular physiology

Membrane contact sites (MCSs) are regions of close apposition between different organelles that contribute to the functional integration of compartmentalized cellular processes. In recent years, we have gained insight into the molecular architecture of several contact sites, as well as into the regulatory mechanisms that underlie their roles in cell physiology. We provide an overview of two selected topics where lipid metabolism intersects with MCSs and organelle dynamics. First, the role of phosphatidic acid phosphatase, Pah1, the yeast homolog of metazoan lipin, toward the synthesis of triacylglycerol is outlined in connection with the seipin complex, Fld1/Ldb16, and lipid droplet formation. Second, we recapitulate the different contact sites connecting mitochondria and the endomembrane system and emphasize their contribution to phospholipid synthesis and their coordinated regulation. A comprehensive view is emerging where the multiplicity of contact sites connecting different cellular compartments together with lipid transfer proteins functioning at more than one MCS allow for functional redundancy and cross-regulation.

Lipid biosynthesis in yeasts: A comparison of the lipid biosynthetic pathway between the model nonoleaginous yeast Saccharomyces cerevisiae and the model oleaginous yeast Yarrowia lipolytica

Lipid biosynthesis and its regulation have been studied mostly in the nonoleaginous yeast Saccharomyces cerevisiae that serves as a model for eukaryotic cells. On the other hand, the yeast Yarrowia lipolytica has been put forward as a model for oleaginous microorganisms because its genetics is known and tools for its genetic manipulation are becoming increasingly available. A comparison of the lipid biosynthetic pathways that function in these two microorganisms shows many similarities in key biosynthetic and regulatory steps. An example is the enzyme phosphatidic acid phosphatase that controls the synthesis of triacylglycerol (TAG) in both yeasts. Controlling the TAG synthesis is crucial for metabolic engineering efforts that aim to increase the production of microbial lipids (i.e. single cell oils) because TAG comprises the final product of these processes. At the same time the comparison reveals fundamental differences (e.g. in the generation of acetyl-CoA for lipid biosynthesis) stemming from the oleaginous nature of Y. lipolytica. These differences warranty more studies in Y. lipolytica where the biochemistry and molecular biology of oleaginicity can be further explored.

Phosphorylation of Yeast Pah1 Phosphatidate Phosphatase by Casein Kinase II Regulates Its Function in Lipid Metabolism

Pah1 phosphatidate phosphatase in Saccharomyces cerevisiae catalyzes the penultimate step in the synthesis of triacylglycerol (i.e. the production of diacylglycerol by dephosphorylation of phosphatidate). The enzyme playing a major role in lipid metabolism is subject to phosphorylation (e.g. by Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C) and dephosphorylation (e.g. by Nem1-Spo7) that regulate its cellular location, catalytic activity, and stability/degradation. In this work, we show that Pah1 is a substrate for casein kinase II (CKII); its phosphorylation was time- and dose-dependent and was dependent on the concentrations of Pah1 (Km = 0.23 μm) and ATP (Km = 5.5 μm). By mass spectrometry, truncation analysis, site-directed mutagenesis, phosphopeptide mapping, and phosphoamino acid analysis, we identified that >90% of its phosphorylation occurs on Thr-170, Ser-250, Ser-313, Ser-705, Ser-814, and Ser-818. The CKII-phosphorylated Pah1 was a substrate for the Nem1-Spo7 protein phosphatase and was degraded by the 20S proteasome. The prephosphorylation of Pah1 by protein kinase A or protein kinase C reduced its subsequent phosphorylation by CKII. The prephosphorylation of Pah1 by CKII reduced its subsequent phosphorylation by protein kinase A but not by protein kinase C. The expression of Pah1 with combined mutations of S705D and 7A, which mimic its phosphorylation by CKII and lack of phosphorylation by Pho85-Pho80, caused an increase in triacylglycerol content and lipid droplet number in cells expressing the Nem1-Spo7 phosphatase complex.

Lipid partitioning at the nuclear envelope controls membrane biogenesis

Partitioning of lipid precursors between membranes and storage is crucial for cell growth, and its disruption underlies pathologies such as cancer, obesity, and type 2 diabetes. However, the mechanisms and signals that regulate this process are largely unknown. In yeast, lipid precursors are mainly used for phospholipid synthesis in nutrient-rich conditions in order to sustain rapid proliferation but are redirected to triacylglycerol (TAG) stored in lipid droplets during starvation. Here we investigate how cells reprogram lipid metabolism in the endoplasmic reticulum. We show that the conserved phosphatidate (PA) phosphatase Pah1, which generates diacylglycerol from PA, targets a nuclear membrane subdomain that is in contact with growing lipid droplets and mediates TAG synthesis. We find that cytosol acidification activates the master regulator of Pah1, the Nem1-Spo7 complex, thus linking Pah1 activity to cellular metabolic status. In the absence of TAG storage capacity, Pah1 still binds the nuclear membrane, but lipid precursors are redirected toward phospholipids, resulting in nuclear deformation and a proliferation of endoplasmic reticulum membrane. We propose that, in response to growth signals, activation of Pah1 at the nuclear envelope acts as a switch to control the balance between membrane biogenesis and lipid storage.

Altered Lipid Synthesis by Lack of Yeast Pah1 Phosphatidate Phosphatase Reduces Chronological Life Span

In Saccharomyces cerevisiae, Pah1 phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol, plays a crucial role in the synthesis of the storage lipid triacylglycerol. This evolutionarily conserved enzyme also plays a negative regulatory role in controlling de novo membrane phospholipid synthesis through its consumption of phosphatidate. We found that the pah1Δ mutant was defective in the utilization of non-fermentable carbon sources but not in oxidative phosphorylation; the mutant did not exhibit major changes in oxygen consumption rate, mitochondrial membrane potential, F1F0-ATP synthase activity, or gross mitochondrial morphology. The pah1Δ mutant contained an almost normal complement of major mitochondrial phospholipids with some alterations in molecular species. Although oxidative phosphorylation was not compromised in the pah1Δ mutant, the cellular levels of ATP in quiescent cells were reduced by 2-fold, inversely correlating with a 4-fold increase in membrane phospholipids. In addition, the quiescent pah1Δ mutant cells had 3-fold higher levels of mitochondrial superoxide and cellular lipid hydroperoxides, had reduced activities of superoxide dismutase 2 and catalase, and were hypersensitive to hydrogen peroxide. Consequently, the pah1Δ mutant had a shortened chronological life span. In addition, the loss of Tsa1 thioredoxin peroxidase caused a synthetic growth defect with the pah1Δ mutation. The shortened chronological life span of the pah1Δ mutant along with its growth defect on non-fermentable carbon sources and hypersensitivity to hydrogen peroxide was suppressed by the loss of Dgk1 diacylglycerol kinase, indicating that the underpinning of pah1Δ mutant defects was the excess synthesis of membrane phospholipids.

Rhabdomyolysis-Associated Mutations in Human LPIN1 Lead to Loss of Phosphatidic Acid Phosphohydrolase Activity

Rhabdomyolysis is an acute syndrome due to extensive injury of skeletal muscle. Recurrent rhabdomyolysis is often caused by inborn errors in intermediary metabolism, and recent work has suggested that mutations in the human gene encoding lipin 1 (LPIN1) may be a common cause of recurrent rhabdomyolysis in children. Lipin 1 dephosphorylates phosphatidic acid to form diacylglycerol (phosphatidic acid phosphohydrolase; PAP) and acts as a transcriptional regulatory protein to control metabolic gene expression. Herein, a 3-year-old boy with severe recurrent rhabdomyolysis was determined to be a compound heterozygote for a novel c.1904T>C (p.Leu635Pro) substitution and a previously reported genomic deletion of exons 18-19 (E766-S838_del) in LPIN1. Western blotting with patient muscle biopsy lysates demonstrated a marked reduction in lipin 1 protein, while immunohistochemical staining for lipin 1 showed abnormal subcellular localization. We cloned cDNAs to express recombinant lipin 1 proteins harboring pathogenic mutations and showed that the E766-S838_del allele was not expressed at the RNA or protein level. Lipin 1 p.Leu635Pro was expressed, but the protein was less stable, was aggregated in the cytosol, and was targeted for proteosomal degradation. Another pathogenic single amino acid substitution, lipin 1 p.Arg725His, was well expressed and retained its transcriptional regulatory function. However, both p.Leu635Pro and p.Arg725His proteins were found to be deficient in PAP activity. Kinetic analyses demonstrated a loss of catalysis rather than diminished substrate binding. These data suggest that loss of lipin 1-mediated PAP activity may be involved in the pathogenesis of rhabdomyolysis in lipin 1 deficiency.

Phosphorylation regulates the ubiquitin-independent degradation of yeast Pah1 phosphatidate phosphatase by the 20S proteasome

Saccharomyces cerevisiae Pah1 phosphatidate phosphatase, which catalyzes the conversion of phosphatidate to diacylglycerol for triacylglycerol synthesis and simultaneously controls phosphatidate levels for phospholipid synthesis, is subject to the proteasome-mediated degradation in the stationary phase of growth. In this study, we examined the mechanism for its degradation using purified Pah1 and isolated proteasomes. Pah1 expressed in S. cerevisiae or Escherichia coli was not degraded by the 26S proteasome, but by its catalytic 20S core particle, indicating that its degradation is ubiquitin-independent. The degradation of Pah1 by the 20S proteasome was dependent on time and proteasome concentration at the pH optimum of 7.0. The 20S proteasomal degradation was conserved for human lipin 1 phosphatidate phosphatase. The degradation analysis using Pah1 truncations and its fusion with GFP indicated that proteolysis initiates at the N- and C-terminal unfolded regions. The folded region of Pah1, in particular the haloacid dehalogenase-like domain containing the DIDGT catalytic sequence, was resistant to the proteasomal degradation. The structural change of Pah1, as reflected by electrophoretic mobility shift, occurs through its phosphorylation by Pho85-Pho80, and the phosphorylation sites are located within its N- and C-terminal unfolded regions. Phosphorylation of Pah1 by Pho85-Pho80 inhibited its degradation, extending its half-life by ∼2-fold. The dephosphorylation of endogenously phosphorylated Pah1 by the Nem1-Spo7 protein phosphatase, which is highly specific for the sites phosphorylated by Pho85-Pho80, stimulated the 20S proteasomal degradation and reduced its half-life by 2.6-fold. These results indicate that the proteolysis of Pah1 by the 20S proteasome is controlled by its phosphorylation state.

Yeast Nem1-Spo7 protein phosphatase activity on Pah1 phosphatidate phosphatase is specific for the Pho85-Pho80 protein kinase phosphorylation sites

Pah1 is the phosphatidate phosphatase in the yeast Saccharomyces cerevisiae that produces diacylglycerol for triacylglycerol synthesis and concurrently controls the levels of phosphatidate used for phospholipid synthesis. Phosphorylation and dephosphorylation of Pah1 regulate its subcellular location and phosphatidate phosphatase activity. Compared with its phosphorylation by multiple protein kinases, Pah1 is dephosphorylated by a protein phosphatase complex consisting of Nem1 (catalytic subunit) and Spo7 (regulatory subunit). In this work, we characterized the Nem1-Spo7 phosphatase complex for its enzymological, kinetic, and regulatory properties with phosphorylated Pah1. The dephosphorylation of Pah1 by Nem1-Spo7 phosphatase resulted in the stimulation (6-fold) of phosphatidate phosphatase activity. For Pah1 phosphorylated by the Pho85-Pho80 kinase complex, maximum Nem1-Spo7 phosphatase activity required Mg(2+) ions (8 mm) and Triton X-100 (0.25 mm) at pH 5.0. The energy of activation for the reaction was 8.4 kcal/mol, and the enzyme was thermally labile at temperatures above 40 °C. The enzyme activity was inhibited by sodium vanadate, sodium fluoride, N-ethylmaleimide, and phenylglyoxal but was not significantly affected by lipids or nucleotides. Nem1-Spo7 phosphatase activity was dependent on the concentrations of Pah1 phosphorylated by Pho85-Pho80, Cdc28-cyclin B, PKA, and PKC with kcat and Km values of 0.29 s(-1) and 81 nm, 0.11 s(-1) and 127 nm, 0.10 s(-1) and 46 nm, and 0.02 s(-1) and 38 nm, respectively. Its specificity constant (kcat/Km) for Pah1 phosphorylated by Pho85-Pho80 was 1.6-, 4-, and 6-fold higher, respectively, than that phosphorylated by PKA, Cdc28-cyclin B, and PKC.

TORC1 regulates Pah1 phosphatidate phosphatase activity via the Nem1/Spo7 protein phosphatase complex

The evolutionarily conserved target of rapamycin complex 1 (TORC1) controls growth-related processes such as protein, nucleotide, and lipid metabolism in response to growth hormones, energy/ATP levels, and amino acids. Its deregulation is associated with cancer, type 2 diabetes, and obesity. Among other substrates, mammalian TORC1 directly phosphorylates and inhibits the phosphatidate phosphatase lipin-1, a central enzyme in lipid metabolism that provides diacylglycerol for the synthesis of membrane phospholipids and/or triacylglycerol as neutral lipid reserve. Here, we show that yeast TORC1 inhibits the function of the respective lipin, Pah1, to prevent the accumulation of triacylglycerol. Surprisingly, TORC1 regulates Pah1 in part indirectly by controlling the phosphorylation status of Nem1 within the Pah1-activating, heterodimeric Nem1-Spo7 protein phosphatase module. Our results delineate a hitherto unknown TORC1 effector branch that controls lipin function in yeast, which, given the recent discovery of Nem1-Spo7 orthologous proteins in humans, may be conserved.

Lipin-1 regulates autophagy clearance and intersects with statin drug effects in skeletal muscle

LPIN1 encodes lipin-1, a phosphatidic acid phosphatase (PAP) enzyme that catalyzes the dephosphorylation of phosphatidic acid to form diacylglycerol. Homozygous LPIN1 gene mutations cause severe rhabdomyolysis, and heterozygous LPIN1 missense mutations may promote statin-induced myopathy. We demonstrate that lipin-1-related myopathy in the mouse is associated with a blockade in autophagic flux and accumulation of aberrant mitochondria. Lipin-1 PAP activity is required for maturation of autolysosomes, through its activation of the protein kinase D (PKD)-Vps34 phosphatidylinositol 3-kinase signaling cascade. Statin treatment also reduces PKD activation and autophagic flux, which are compounded by diminished mammalian target of rapamycin (mTOR) abundance in lipin-1-haploinsufficent and -deficient muscle. Lipin-1 restoration in skeletal muscle prevents myonecrosis and statin toxicity in vivo, and activated PKD rescues autophagic flux in lipin-1-deficient cells. Our findings identify lipin-1 PAP activity as a component of the macroautophagy pathway and define the basis for lipin-1-related myopathies.

Cross-talk phosphorylations by protein kinase C and Pho85p-Pho80p protein kinase regulate Pah1p phosphatidate phosphatase abundance in Saccharomyces cerevisiae

Yeast Pah1p is the phosphatidate phosphatase that catalyzes the penultimate step in triacylglycerol synthesis and plays a role in the transcriptional regulation of phospholipid synthesis genes. The enzyme is multiply phosphorylated, some of which is mediated by Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A. Here, we showed that Pah1p is a bona fide substrate of protein kinase C; the phosphorylation reaction was time- and dose-dependent and dependent on the concentrations of ATP (Km = 4.5 μm) and Pah1p (Km = 0.75 μm). The stoichiometry of the reaction was 0.8 mol of phosphate/mol of Pah1p. By combining mass spectrometry, truncation analysis, site-directed mutagenesis, and phosphopeptide mapping, we identified Ser-677, Ser-769, Ser-773, and Ser-788 as major sites of phosphorylation. Analysis of Pah1p phosphorylations by different protein kinases showed that prephosphorylation with protein kinase C reduces its subsequent phosphorylation with protein kinase A and vice versa. Prephosphorylation with Pho85p-Pho80p had an inhibitory effect on its subsequent phosphorylation with protein kinase C; however, prephosphorylation with protein kinase C had no effect on the subsequent phosphorylation with Pho85p-Pho80p. Unlike its phosphorylations by Pho85p-Pho80p and protein kinase A, which cause a significant reduction in phosphatidate phosphatase activity, the phosphorylation of Pah1p by protein kinase C had a small stimulatory effect on the enzyme activity. Analysis of phosphorylation-deficient forms of Pah1p indicated that protein kinase C does not have a major effect on its location or its function in triacylglycerol synthesis, but instead, the phosphorylation favors loss of Pah1p abundance when it is not phosphorylated with Pho85p-Pho80p.