Fragmentation and re-assembly of the Golgi apparatus in vitro. A requirement for phosphatidic acid and phosphatidylinositol 4,5-bisphosphate synthesis

Split but merge: Golgi fragmentation in physiological and pathological conditions

The Golgi complex is a highly dynamic and tightly regulated cellular organelle with essential roles in the processing as well as the sorting of proteins and lipids. Its structure undergoes rapid disassembly and reassembly during normal physiological processes, including cell division, migration, polarization, differentiation, and cell death. Golgi dispersal or fragmentation also occurs in pathological conditions, such as neurodegenerative diseases, infectious diseases, congenital disorders of glycosylation diseases, and cancer. In this review, current knowledge about both structural organization and morphological alterations in the Golgi in physiological and pathological conditions is summarized together with the methodologies that help to reveal its structure.

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Courier service for phosphatidylinositol: PITPs deliver on demand

Phosphatidylinositol is the parent lipid for the synthesis of seven phosphorylated inositol lipids and each of them play specific roles in numerous processes including receptor-mediated signalling, actin cytoskeleton dynamics and membrane trafficking. PI synthesis is localised to the endoplasmic reticulum (ER) whilst its phosphorylated derivatives are found in other organelles where the lipid kinases also reside. Phosphorylation of PI to phosphatidylinositol (4,5) bisphosphate (PI(4,5)P2) at the plasma membrane and to phosphatidylinositol 4-phosphate (PI4P) at the Golgi are key events in lipid signalling and Golgi function respectively. Here we review a family of proteins, phosphatidylinositol transfer proteins (PITPs), that can mobilise PI from the ER to provide the substrate to the resident kinases for phosphorylation. Recent studies identify specific and overlapping functions for the three soluble PITPs (PITPα, PITPβ and PITPNC1) in phospholipase C signalling, neuronal function, membrane trafficking, viral replication and in cancer metastases.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Phosphatidic acid in membrane rearrangements

Phosphatidic acid (PA) is the simplest cellular glycerophospholipid characterized by unique biophysical properties: a small headgroup; negative charge; and a phosphomonoester group. Upon interaction with lysine or arginine, PA charge increases from -1 to -2 and this change stabilizes protein-lipid interactions. The biochemical properties of PA also allow interactions with lipids in several subcellular compartments. Based on this feature, PA is involved in the regulation and amplification of many cellular signalling pathways and functions, as well as in membrane rearrangements. Thereby, PA can influence membrane fusion and fission through four main mechanisms: it is a substrate for enzymes producing lipids (lysophosphatidic acid and diacylglycerol) that are involved in fission or fusion; it contributes to membrane rearrangements by generating negative membrane curvature; it interacts with proteins required for membrane fusion and fission; and it activates enzymes whose products are involved in membrane rearrangements. Here, we discuss the biophysical properties of PA in the context of the above four roles of PA in membrane fusion and fission.

Giantin Is Required for Post-Alcohol Recovery of Golgi in Liver Cells

In hepatocytes and alcohol-metabolizing cultured cells, Golgi undergoes ethanol (EtOH)-induced disorganization. Perinuclear and organized Golgi is important in liver homeostasis, but how the Golgi remains intact is unknown. Work from our laboratories showed that EtOH-altered cellular function could be reversed after alcohol removal; we wanted to determine whether this recovery would apply to Golgi. We used alcohol-metabolizing HepG2 (VA-13) cells (cultured with or without EtOH for 72 h) and rat hepatocytes (control and EtOH-fed (Lieber–DeCarli diet)). For recovery, EtOH was removed and replenished with control medium (48 h for VA-13 cells) or control diet (10 days for rats). Results: EtOH-induced Golgi disassembly was associated with de-dimerization of the largest Golgi matrix protein giantin, along with impaired transport of selected hepatic proteins. After recovery from EtOH, Golgi regained their compact structure, and alterations in giantin and protein transport were restored. In VA-13 cells, when we knocked down giantin, Rab6a GTPase or non-muscle myosin IIB, minimal changes were observed in control conditions, but post-EtOH recovery was impaired. Conclusions: These data provide a link between Golgi organization and plasma membrane protein expression and identify several proteins whose expression is important to maintain Golgi structure during the recovery phase after EtOH administration.

Segregation of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate into distinct microdomains on the endosome membrane

Phosphatidylinositol 4-phosphate (PtdIns(4)P) is the immediate precursor of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂), which is located on the cytoplasmic leaflet of the plasma membrane and has been reported to possess multiple cellular functions. Although PtdIns(4)P and PtdIns(4,5)P₂ have been reported to localize to multiple intracellular compartments and to each function as regulatory molecules, their generation, regulation and functions in most intracellular compartments are not well-defined. To analyze PtdIns(4)P and PtdIns(4,5)P₂ distributions, at a nanoscale, we employed an electron microscopy technique that specifically labels PtdIns(4)P and PtdIns(4,5)P₂ on the freeze-fracture replica of intracellular biological membranes. This method minimizes the possibility of artificial perturbation, because molecules in the membrane are physically immobilized in situ. Using this technique, we found that PtdIns(4)P was localized to the cytoplasmic leaflet of Golgi apparatus and vesicular-shaped structures. The PtdIns(4,5)P₂ labeling was observed in the cytoplasmic leaflet of the mitochondrial inner membrane and vesicular-shaped structures. Double labeling of PtdIns(4)P and PtdIns(4,5)P₂ with endosome markers illustrated that PtdIns(4)P and PtdIns(4,5)P₂ were mainly localized to the late endosome/lysosome and early endosome, respectively. PtdIns(4)P and PtdIns(4,5)P₂ were colocalized in some endosomes, with the two phospholipids separated into distinct microdomains on the same endosomes. This is the first report showing, at a nanoscale, segregation of PtdIns(4)P- and PtdIns(4,5)P₂-enriched microdomains in the endosome, of likely importance for endosome functionality.

Emerging roles of PtdIns(4,5)P2--beyond the plasma membrane

Phosphoinositides are a collection of lipid messengers that regulate most subcellular processes. Amongst the seven phosphoinositide species, the roles for phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] at the plasma membrane, such as in endocytosis, exocytosis, actin polymerization and focal adhesion assembly, have been extensively studied. Recent studies have argued for the existence of PtdIns(4,5)P2 at multiple intracellular compartments, including the nucleus, endosomes, lysosomes, autolysosomes, autophagic precursor membranes, ER, mitochondria and the Golgi complex. Although the generation, regulation and functions of PtdIns(4,5)P2 are less well-defined in most other intracellular compartments, accumulating evidence demonstrates crucial roles for PtdIns(4,5)P2 in endolysosomal trafficking, endosomal recycling, as well as autophagosomal pathways, which are the focus of this Commentary. We summarize and discuss how phosphatidylinositol phosphate kinases, PtdIns(4,5)P2 and PtdIns(4,5)P2-effectors regulate these intracellular protein and membrane trafficking events.

Phospholipase D2 Modulates the Secretory Pathway in RBL-2H3 Mast Cells

Phospholipase D (PLD) hydrolyses phosphatidylcholine to produce phosphatidic acid (PA) and choline. It has two isoforms, PLD1 and PLD2, which are differentially expressed depending on the cell type. In mast cells it plays an important role in signal transduction. The aim of the present study was to clarify the role of PLD2 in the secretory pathway. RBL-2H3 cells, a mast cell line, transfected to overexpress catalytically active (PLD2CA) and inactive (PLD2CI) forms of PLD2 were used. Previous observations showed that the Golgi complex was well organized in CA cells, but was disorganized and dispersed in CI cells. Furthermore, in CI cells, the microtubule organizing center was difficult to identify and the microtubules were disorganized. These previous observations demonstrated that PLD2 is important for maintaining the morphology and organization of the Golgi complex. To further understand the role of PLD2 in secretory and vesicular trafficking, the role of PLD2 in the secretory process was investigated. Incorporation of sialic acid was used to follow the synthesis and transport of glycoconjugates in the cell lines. The modified sialic acid was subsequently detected by labeling with a fluorophore or biotin to visualize the localization of the molecule after a pulse-chase for various times. Glycoconjugate trafficking was slower in the CI cells and labeled glycans took longer to reach the plasma membrane. Furthermore, in CI cells sialic acid glycans remained at the plasma membrane for longer periods of time compared to RBL-2H3 cells. These results suggest that PLD2 activity plays an important role in regulating glycoconjugate trafficking in mast cells.

Phospholipase D is involved in the formation of Golgi associated clathrin coated vesicles in human parotid duct cells

Phospholipase D (PLD) has been implicated in many cellular functions, such as vesicle trafficking, exocytosis, differentiation, and proliferation. The aim of this study was to characterize the role of PLD in HSY cells, a human cell line originating from the intercalated duct of the parotid gland. As the function and intracellular localization of PLD varies according to cell type, initially, the intracellular localization of PLD1 and PLD2 was determined. By immunofluorescence, PLD1 and PLD2 both showed a punctate cytoplasmic distribution with extensive co-localization with TGN-46. PLD1 was also found in the nucleus, while PLD2 was associated with the plasma membrane. Treatment of cells with the primary alcohol 1-butanol inhibits the hydrolysis of phosphatidylcoline by PLD thereby suppressing phosphatidic acid (PA) production. In untreated HSY cells, there was only a slight co-localization of PLD with the clathrin coated vesicles. When HSY cells were incubated with 1-butanol the total number of clathrin coated vesicles increased, especially in the juxtanuclear region and the co-localization of PLD with the clathrin coated vesicles was augmented. Transmission electron microscopy confirmed that the number of Golgi-associated coated vesicles was greater. Treatment with 1-butanol also affected the Golgi apparatus, increasing the volume of the Golgi saccules. The decrease in PA levels after treatment with 1-butanol likewise resulted in an accumulation of enlarged lysosomes in the perinuclear region. Therefore, in HSY cells PLD appears to be involved in the formation of Golgi associated clathrin coated vesicles as well as in the structural maintenance of the Golgi apparatus.

RNAi screening reveals a large signaling network controlling the Golgi apparatus in human cells

RNAi screening and automated image analysis reveal 180 kinases and phosphatases regulating the organization of the Golgi apparatus. Most of these genes also control the expression of specific glycans, pointing to a web of interactions between signaling cascades and glycosylation at the Golgi.

Golgi organization was probed with three markers of different Golgi compartments and quantitative morphological analysis.

Knockdowns of ∼20% of all known kinases and phosphatases affected the Golgi globally or in a compartment-specific manner, and were comparable in degree to the depletion of known membrane traffic regulators such as SNAREs.

Several cell surface receptors, their cognate ligands and downstream effectors regulate Golgi organization, suggesting a large regulatory network.

Most signaling genes affected both Golgi morphology and the expression of specific glycans.

The Golgi apparatus has many important physiological functions, including sorting of secretory cargo and biosynthesis of complex glycans. These functions depend on the intricate and compartmentalized organization of the Golgi apparatus. To investigate the mechanisms that regulate Golgi architecture, we developed a quantitative morphological assay using three different Golgi compartment markers and quantitative image analysis, and performed a kinome- and phosphatome-wide RNAi screen in HeLa cells. Depletion of 159 signaling genes, nearly 20% of genes assayed, induced strong and varied perturbations in Golgi morphology. Using bioinformatics data, a large regulatory network could be constructed. Specific subnetworks are involved in phosphoinositides regulation, acto-myosin dynamics and mitogen activated protein kinase signaling. Most gene depletion also affected Golgi functions, in particular glycan biosynthesis, suggesting that signaling cascades can control glycosylation directly at the Golgi level. Our results provide a genetic overview of the signaling pathways that control the Golgi apparatus in human cells.

The lipid kinase PI4KIIIβ preserves lysosomal identity

Lipid modifications are essential in cellular sorting and trafficking inside cells. The role of phosphoinositides in trafficking between Golgi and endocytic/lysosomal compartments has been extensively explored and the kinases responsible for these lipid changes have been identified. In contrast, the mechanisms that mediate exit and recycling from lysosomes (Lys), considered for a long time as terminal compartments, are less understood. In this work, we identify a dynamic association of the lipid kinase PI4KIIIβ with Lys and unveil its regulatory function in lysosomal export and retrieval. We have found that absence of PI4KIIIβ leads to abnormal formation of tubular structures from the lysosomal surface and loss of lysosomal constituents through these tubules. We demonstrate that the kinase activity of PI4KIIIβ is necessary to prevent this unwanted lysosomal efflux under normal conditions, and to facilitate proper sorting when recycling of lysosomal material is needed, such as in the physiological context of lysosomal reformation after prolonged starvation.

Phospholipase D2: a pivotal player modulating RBL-2H3 mast cell structure

The current study examined the role of PLD2 in the maintenance of mast cell structure. Phospholipase D (PLD) catalyzes hydrolysis of phosphatidylcholine to produce choline and phosphatidic acid (PA). PLD has two isoforms, PLD1 and PLD2, which vary in expression and localization depending on the cell type. The mast cell line RBL-2H3 was transfected to overexpress catalytically active (PLD2CA) and inactive (PLD2CI) forms of PLD2. The results of this study show that PLD2CI cells have a distinct star-shaped morphology, whereas PLD2CA and RBL-2H3 cells are spindle shaped. In PLD2CI cells, the Golgi complex was also disorganized with dilated cisternae, and more Golgi-associated vesicles were present as compared with the PLD2CA and RBL-2H3 cells. Treatment with exogenous PA led to the restoration of the wild-type Golgi complex phenotype in PLD2CI cells. Conversely, treatment of RBL-2H3 and PLD2CA cells with 1% 1-Butanol led to a disruption of the Golgi complex. The distribution of acidic compartments, including secretory granules and lysosomes, was also modified in PLD2CI cells, where they concentrated in the perinuclear region. These results suggest that the PA produced by PLD2 plays an important role in regulating cell morphology in mast cells.

Integration of non-vesicular and vesicular transport processes at the Golgi complex by the PKD-CERT network

Non-vesicular transport of ceramide from endoplasmic reticulum to Golgi membranes is essential for cellular lipid homeostasis. Protein kinase D (PKD) is a serine-threonine kinase that controls vesicle fission at Golgi membranes. Here we highlight the intimate connections between non-vesicular and vesicular transport at the level of the Golgi complex, and suggest that PKD and its substrate CERT, the ceramide transfer protein, play central roles in coordinating these processes by fine-tuning the local membrane lipid composition to maintain Golgi secretory function. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

The distribution of phosphatidylinositol 4,5-bisphosphate in acinar cells of rat pancreas revealed with the freeze-fracture replica labeling method

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] is a phospholipid that has been implicated in multiple cellular activities. The distribution of PI(4,5)P₂ has been analyzed extensively using live imaging of the GFP-coupled phospholipase C-δ1 pleckstrin homology domain in cultured cell lines. However, technical difficulties have prevented the study of PI(4,5)P₂ in cells of in vivo tissues. We recently developed a method to analyze the nanoscale distribution of PI(4,5)P₂ in cultured cells by using the quick-freezing and freeze-fracture replica labeling method. In principle, this method can be applied to any cell because it does not require the expression of artificial probes. In the present study, we modified the method to study cells of in vivo tissues and applied it to pancreatic exocrine acinar cells of the rat. We found that PI(4,5)P₂ in the plasma membrane is distributed in an equivalent density in the apical and basolateral domains, but exists in a significantly higher concentration in the gap junction. The intracellular organelles did not show labeling for PI(4,5)P₂. The results are novel or different from the reported distribution patterns in cell lines and highlight the importance of studying cells differentiated in vivo.

Regulation of Golgi function via phosphoinositide lipids

Phosphoinositides play important roles in Golgi traffic and structural integrity. Specific lipid kinases and phosphatases associate with the Golgi complex and regulate the multiplicity of trafficking routes from this organelle. Work in different model systems showed that the basic elements that regulate lipid signaling at the Golgi are conserved from yeast to humans. Many of the enzymes involved in Golgi phosphoinositide metabolism are essential for viability or cause severe human disease when malfunctioning. Phosphoinositide effectors at the Golgi control both non-vesicular transfer of lipids and sorting of secretory and membrane proteins. In addition, Golgi phosphoinositides were recently implicated in the metabolic and cell growth-dependent regulation of the secretory pathway.

Pollen tube growth: a delicate equilibrium between secretory and endocytic pathways

Although pollen tube growth is a prerequisite for higher plant fertilization and seed production, the processes leading to pollen tube emission and elongation are crucial for understanding the basic mechanisms of tip growth. It was generally accepted that pollen tube elongation occurs by accumulation and fusion of Golgi-derived secretory vesicles (SVs) in the apical region, or clear zone, where they were thought to fuse with a restricted area of the apical plasma membrane (PM), defining the apical growth domain. Fusion of SVs at the tip reverses outside cell wall material and provides new segments of PM. However, electron microscopy studies have clearly shown that the PM incorporated at the tip greatly exceeds elongation and a mechanism of PM retrieval was already postulated in the mid-nineteenth century. Recent studies on endocytosis during pollen tube growth showed that different endocytic pathways occurred in distinct zones of the tube, including the apex, and led to a new hypothesis to explain vesicle accumulation at the tip; namely, that endocytic vesicles contribute substantially to V-shaped vesicle accumulation in addition to SVs and that exocytosis does not involve the entire apical domain. New insights suggested the intriguing hypothesis that modulation between exo- and endocytosis in the apex contributes to maintain PM polarity in terms of lipid/protein composition and showed distinct degradation pathways that could have different functions in the physiology of the cell. Pollen tube growth in vivo is closely regulated by interaction with style molecules. The study of endocytosis and membrane recycling in pollen tubes opens new perspectives to studying pollen tube-style interactions in vivo.

Phospholipase D in the Golgi apparatus

Phospholipase D has long been implicated in vesicle formation and vesicular transport through the secretory pathway. The Golgi apparatus has been shown to exhibit a plethora of mechanisms of vesicle formation at different stages to accommodate a wide variety of cargo. Phospholipase D has been found on the Golgi apparatus and is regulated by ADP-ribosylation factors which are themselves regulators of vesicle trafficking. Moreover, the product of phospholipase D activity, phosphatidic acid, as well as its degradation product diacylglycerol, have been implicated in vesicle fission and fusion events. Here we summarize recent advances in the understanding of the role of phospholipase D at the Golgi apparatus.

CtBP1/BARS is an activator of phospholipase D1 necessary for agonist-induced macropinocytosis

Vesicular trafficking such as macropinocytosis is a dynamic process that requires coordinated interactions between specialized proteins and lipids. A recent report suggests the involvement of CtBP1/BARS in epidermal growth factor (EGF)-induced macropinocytosis. Detailed mechanisms as to how lipid remodelling is regulated during macropinocytosis are still undefined. Here, we show that CtBP1/BARS is a physiological activator of PLD1 required in agonist-induced macropinocytosis. EGF-induced macropinocytosis was specifically blocked by 1-butanol but not by 2-butanol. In addition, stimulation of cells by serum or EGF resulted in the association of CtBP1/BARS with PLD1. Finally, CtBP1/BARS activated PLD1 in a synergistic manner with other PLD activators, including ADP-ribosylation factors as demonstrated by in vitro and intact cell systems. The present results shed light on the molecular basis of how the ‘fission protein' CtBP1/BARS controls vesicular trafficking events including macropinocytosis.

SAC1 lipid phosphatase and growth control of the secretory pathway

Phosphoinositide lipids play a dual role in cell physiology. Specific sets of these molecules are short-lived downstream mediators of growth signals, regulating cell survival and differentiation. In addition, distinct classes of phosphoinositide lipids function as constitutive mediators of membrane traffic and organelle identity. Recent work has provided the first direct evidence that phosphoinositides also play a direct role in linking protein secretion with cell growth and proliferation. This review focuses on SAC1 lipid phosphatase and how this enzyme operates in an evolutionary conserved mechanism to coordinate the secretory capacity of ER and Golgi during cell growth.

A conspicuous connection: structure defines function for the phosphatidylinositol-phosphate kinase family

The phosphatidylinositol phosphate (PIP) kinases are a unique family of enzymes that generate an assortment of lipid messengers, including the pivotal second messenger phosphatidylinositol 4,5-bisphosphate (PI4,5P2). While members of the PIP kinase family function by catalyzing a similar phosphorylation reaction, the specificity loop of each PIP kinase subfamily determines substrate preference and partially influences distinct subcellular targeting. Specific protein-protein interactions that are unique to particular isoforms or splice variants play a key role in targeting PIP kinases to appropriate subcellular compartments to facilitate the localized generation of PI4,5P2 proximal to effectors, a mechanism key for the function of PI4,5P2 as a second messenger. This review documents the discovery of the PIP kinases and their signaling products, and summarizes our current understanding of the mechanisms underlying the localized generation of PI4,5P2 by PIP kinases for the regulation of cellular events including actin cytoskeleton dynamics, vesicular trafficking, cell migration, and an assortment of nuclear events.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

Arabidopsis PLDzeta2 regulates vesicle trafficking and is required for auxin response

Phospholipase D (PLD) and its product, phosphatidic acid (PA), play key roles in cellular processes, including stress and hormonal responses, vesicle trafficking, and cytoskeletal rearrangements. We isolated and functionally characterized Arabidopsis thaliana PLDzeta2, which is expressed in various tissues and enhanced by auxin. A PLDzeta2-defective mutant, pldzeta2, and transgenic plants deficient in PLDzeta2 were less sensitive to auxin, had reduced root gravitropism, and suppressed auxin-dependent hypocotyl elongation at 29 degrees C, whereas transgenic seedlings overexpressing PLDzeta2 showed opposite phenotypes, suggesting that PLDzeta2 positively mediates auxin responses. Studies on the expression of auxin-responsive genes and observation of the beta-glucuronidase (GUS) expression in crosses between pldzeta2 and lines containing DR5-GUS indicated that PLDzeta2, or PA, stimulated auxin responses. Observations of the membrane-selective dye FM4-64 showed suppressed vesicle trafficking under PLDzeta2 deficiency or by treatment with 1-butanol, a PLD-specific inhibitor. By contrast, vesicle trafficking was enhanced by PA or PLDzeta2 overexpression. Analyses of crosses between pldzeta2 and lines containing PIN-FORMED2 (PIN2)-enhanced green fluorescent protein showed that PLDzeta2 deficiency had no effect on the localization of PIN2 but blocked the inhibition of brefeldin A on PIN2 cycling. These results suggest that PLDzeta2 and PA are required for the normal cycling of PIN2-containing vesicles as well as for function in auxin transport and distribution, and hence auxin responses.

A role for clathrin in reassembly of the Golgi apparatus

The Golgi apparatus is a highly dynamic organelle whose organization is maintained by a proteinaceous matrix, cytoskeletal components, and inositol phospholipids. In mammalian cells, disassembly of the organelle occurs reversibly at the onset of mitosis and irreversibly during apoptosis. Several pharmacological agents including nocodazole, brefeldin A (BFA), and primary alcohols (1-butanol) induce reversible fragmentation of the Golgi apparatus. To dissect the mechanism of Golgi reassembly, rat NRK and GH3 cells were treated with 1-butanol, BFA, or nocodazole. During washout of 1-butanol, clathrin, a ubiquitous coat protein implicated in vesicle traffic at the trans-Golgi network and plasma membrane, and abundant clathrin coated vesicles were recruited to the region of nascent Golgi cisternae. Knockdown of endogenous clathrin heavy chain showed that the Golgi apparatus failed to reform efficiently after BFA or 1-butanol removal. Instead, upon 1-butanol washout, it maintained a compact, tight morphology. Our results suggest that clathrin is required to reassemble fragmented Golgi elements. In addition, we show that after butanol treatment the Golgi apparatus reforms via an initial compact intermediate structure that is subsequently remodeled into the characteristic interphase lace-like morphology and that reassembly requires clathrin.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

The differential regulation of phosphatidylinositol 4-phosphate 5-kinases and phospholipase D1 by ADP-ribosylation factors 1 and 6

Phosphatidylinositol 4-phosphate 5-kinases [PtdIns4P5Ks] synthesise the majority of cellular phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and phospholipase D1 (PLD1) synthesises large amounts of phosphatidic acid (PtdOH). The activities of PtdIns4P5Ks and PLDs are thought to be coupled during cell signalling in order to support large simultaneous increases in both PtdIns(4,5)P(2) and PtdOH, since PtdOH activates PtdIns4P5Ks and PLD1 requires PtdIns(4,5)P(2) as a cofactor. However, little is known about the control of such a system. Membrane recruitment of ADP-ribosylation factors (Arfs) activates both PtdIns4P5Ks and PLDs, but it is not known if each enzyme is controlled in series by different Arfs or in parallel by a single form. We show through pull-down and vesicle sedimentation interaction assays that PtdIns4P5K activation may be facilitated by Arf-enhanced membrane association. However PtdIns4P5Ks discriminate poorly between near homogeneously myristoylated Arf1 and Arf6 although examples of all three known active isoforms (mouse alpha>beta, gamma) respond to these G-proteins. Conversely PLD1 genuinely prefers Arf1 and so the two lipid metabolising enzymes are differentially controlled. We propose that isoform selective Arf/PLD interaction and not Arf/PtdIns4P5K will be the critical trigger in the formation of distinct, optimal triples of Arf/PLDs/PtdIns4P5Ks and be the principle regulator of any coupled increases in the signalling lipids PtdIns(4,5)P(2) and PtdOH.

Signalling pathways in pollen germination and tube growth

Signalling is an integral component in the establishment and maintenance of cellular identity. In plants, tip-growing cells represent an ideal system to investigate signal transduction mechanisms, and among these, pollen tubes (PTs) are one of the favourite models. Many signalling pathways have been identified during germination and tip growth, namely, Ca(2+), calmodulin, phosphoinositides, protein kinases, cyclic AMP, and GTPases. These constitute a large and complex web of signalling networks that intersect at various levels such as the control of vesicle targeting and fusion and the physical state of the actin cytoskeleton. Here we discuss some of the most recent advances made in PT signal transduction cascades and their implications for our future research. For reasons of space, emphasis was given to signalling mechanisms that control PT reorientation, so naturally many other relevant works have not been cited.

Phospholipase D1 corrects impaired betaAPP trafficking and neurite outgrowth in familial Alzheimer's disease-linked presenilin-1 mutant neurons

Presenilins (PS1/PS2) regulate proteolysis of beta-amyloid precursor protein (betaAPP) and affect its intracellular trafficking. Here, we demonstrate that a PS1-interacting protein, phospholipase D1 (PLD1), affects intracellular trafficking of betaAPP. Overexpression of PLD1 in PS1wt cells promotes generation of betaAPP-containing vesicles from the trans-Golgi network. Conversely, inhibition of PLD1 activity by 1-butanol decreases betaAPP trafficking in both wt and PS1-deficient cells. The subcellular localization of PLD1 is altered, and PLD enzymatic activity is reduced in cells expressing familial Alzheimer's disease (FAD) PS1 mutations compared with PS1wt cells. Overexpression of wt, but not catalytically inactive, PLD1 increases budding of betaAPP-containing vesicles from the trans-Golgi network in FAD mutant cells. Surface delivery of betaAPP is also increased by PLD1 in these cells. The impaired neurite outgrowth capacity in FAD mutant neurons was corrected by introducing PLD1 into these cells. The results indicate that PLD1 may represent a therapeutic target for rescuing compromised neuronal function in AD.

Enhanced apoptosis through farnesol inhibition of phospholipase D signal transduction

Farnesol is a catabolite of the cholesterol biosynthetic pathway that preferentially causes apoptosis in tumorigenic cells. Phosphatidylcholine (PC), phosphatidic acid (PA), and diacylglycerol (DAG) were able to prevent induction of apoptosis by farnesol. Primary alcohol inhibition of PC catabolism by phospholipase D augmented farnesol-induced apoptosis. Exogenous PC was unable to prevent the increase in farnesol-induced apoptosis by primary alcohols, whereas DAG was protective. Farnesol-mediated apoptosis was prevented by transformation with a plasmid coding for the PA phosphatase LPP3, but not by an inactive LPP3 point mutant. Farnesol did not directly inhibit LPP3 PA phosphatase enzyme activity in an in vitro mixed micelle assay. We propose that farnesol inhibits the action of a DAG pool generated by phospholipase D signal transduction that normally activates an antiapoptotic/pro-proliferative target.

High-affinity interaction of the N-terminal myristoylation motif of the neuronal calcium sensor protein hippocalcin with phosphatidylinositol 4,5-bisphosphate

Many proteins are associated with intracellular membranes due to their N-terminal myristoylation. Not all myristoylated proteins have the same localization within cells, indicating that other factors must determine their membrane targeting. The NCS (neuronal calcium sensor) proteins are a family of Ca2+-binding proteins with diverse functions. Most members of the family are N-terminally myristoylated and are either constitutively membrane-bound or have a Ca2+/myristoyl switch that allows their reversible membrane association in response to Ca2+ signals. In the case of hippocalcin and NCS-1, or alternatively KChIP1 (K+ channel-interacting protein 1), their N-terminal myristoylation motifs are sufficient for targeting to distinct organelles. We have shown that an N-terminal myristoylated hippocalcin peptide is able to specifically reproduce the membrane targeting of hippocalcin/NCS-1 when introduced into permeabilized cells. The peptide binds to liposomes containing phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] with high affinity (K(d) 50 nM). Full-length hippocalcin also bound preferentially to liposomes supplemented with PtdIns(4,5)P2. Co-expression of hippocalcin-(1-14)-ECFP (enhanced cyan fluorescent protein) or NCS-1-ECFP partially displaced the expressed PH (pleckstrin homology) domain of phospholipase delta1 from the plasma membrane in live cells, indicating that they have a higher affinity for PtdIns(4,5)P2 than does this PH domain. The Golgi localization of the PH domain of FAPP1 (four-phosphate-adaptor protein 1), which binds to phosphatidylinositol 4-phosphate, was unaffected. The localization of NCS-1 and hippocalcin is likely to be determined, therefore, by their interaction with PtdIns(4,5)P2.

Modulation of endocytosis in pollen tube growth by phosphoinositides and phospholipids

In plants, tip-growing cells represent an ideal system to investigate signal transduction mechanisms, and among those, pollen tubes are one of the favourite models. Many signalling pathways have been identified during germination and tip growth, namely, Ca2+, calmodulin, phosphoinositides, cyclic AMP, and GTPases. Not surprisingly, the apical secretory machinery, essential for tip growth, seems to be an intersection point for all these pathways. Recently, the phospholipid phosphatidic acid was also suggested to actively participate in the control of endo- and exocytosis and to interfere with the correct positioning of the actin cytoskeleton. Phosphatidic acid seems to act concertedly with the phosphoinositides phosphatidylinositol 4,5-bisphosphate and D-myo-inositol 1,4,5-trisphosphate. Here we review previous data and discuss additional evidence that these three molecules have a combined action modulating both the actin cytoskeleton and the apical secretory machinery. We further discuss how these findings can be integrated into a working model for pollen tube apical secretion that contemplates the existence of a rapid endocytosis mechanism.

Microtubule actin crosslinking factor 1b: a novel plakin that localizes to the Golgi complex

MACF1 (microtubule actin crosslinking factor), also called ACF7 (actin crosslinking family 7) is a cytoskeletal linker protein that can associate with both actin filaments and microtubules. We have identified a novel alternatively spliced isoform of MACF1. We named this isoform MACF1b and renamed the original isoform MACF1a. MACF1b is identical to MACF1a, except that it has a region containing plakin (or plectin) repeats in the middle of the molecule. MACF1b is ubiquitously expressed in adult tissues with especially high levels in the lung. We studied the subcellular localization of MACF1b proteins in mammalian cell lines. In two lung cell lines, MACF1b was chiefly localized to the Golgi complex. Upon treatments that disrupt the Golgi complex, MACF1b redistributed into the cytosol, but remained co-localized with the dispersed Golgi ministacks. MACF1b proteins can be detected in the enriched Golgi fraction by western blotting. The domain of MACF1b that targets it to the Golgi was found at the N-terminal part of the region that contains the plakin repeats. Reducing the level of MACF1 proteins by small-interfering RNA resulted in the dispersal of the Golgi complex.

The role of the phosphoinositides at the Golgi complex

The phosphorylated derivatives of phosphatidylinositol (PtdIns), known as the polyphosphoinositides (PIs), represent key membrane-localized signals in the regulation of fundamental cell processes, such as membrane traffic and cytoskeleton remodelling. The reversible production of the PIs is catalyzed through the combined activities of a number of specific phosphoinositide phosphatases and kinases that can either act separately or in concert on all the possible combinations of the 3, 4, and 5 positions of the inositol ring. So far, seven distinct PI species have been identified in mammalian cells and named according to their site(s) of phosphorylation: PtdIns 3-phosphate (PI3P); PtdIns 4-phosphate (PI4P); PtdIns 5-phosphate (PI5P); PtdIns 3,4-bisphosphate (PI3,4P2); PtdIns 4,5-bisphosphate (PI4,5P2); PtdIns 3,5-bisphosphate (PI3,5P2); and PtdIns 3,4,5-trisphosphate (PI3,4,5P3). Over the last decade, accumulating evidence has indicated that the different PIs serve not only as intermediates in the synthesis of the higher phosphorylated phosphoinositides, but also as regulators of different protein targets in their own right. These regulatory actions are mediated through the direct binding of their protein targets. In this way, the PIs can control the subcellular localization and activation of their various effectors, and thus execute a variety of cellular responses. To exert these functions, the metabolism of the PIs has to be finely regulated both in time and space, and this is achieved by controlling the subcellular distribution, regulation, and activation states of the enzymes involved in their synthesis and removal (kinases and phosphatases). These exist in many different isoforms, each of which appears to have a distinctive intracellular localization and regulation. As a consequence of this subcompartimentalized PI metabolism, a sort of "PI-fingerprint" of each cell membrane compartment is generated. When combined with the targeted recruitment of their protein effectors and the different intracellular distributions of other lipids and regulatory proteins (such as small GTPases), these factors can maintain and determine the identity of the cell organelles despite the extensive membrane flux []. Here, we provide an overview of the regulation and roles of different phosphoinositide kinases and phosphatases and their lipid products at the Golgi complex.

Phosphoinositides and phosphatidic acid regulate pollen tube growth and reorientation through modulation of [Ca2+]c and membrane secretion

The maintenance of a calcium gradient and vesicle secretion in the apex of pollen tubes is essential for growth. It is shown here that phosphatidylinositol-4,5-bisphosphate (PIP2) and D-myo-inositol-1,4,5-trisphosphate (IP3), together with phosphatidic acid (PA), play a vital role in the regulation of these processes. Changes in the intracellular concentration of both PIP2 and IP3 (induced by photolysis of caged-probes), modified growth and caused reorientation of the growth axis. However, measurements of cytosolic free calcium ([Ca2+]c) and apical secretion revealed significant differences between the photo-release of PIP2 or IP3. When released in the first 50 mum of the pollen tube, PIP2 led to transient growth perturbation, [Ca2+]c increases, and inhibition of apical secretion. By contrast, a concentration of IP3 which caused a [Ca2+]c transient of similar magnitude, stimulated apical secretion and caused severe growth perturbation. Furthermore, the [Ca2+]c transient induced by IP3 was spatially different causing a pronounced elevation in the sub-apical region. These observations suggest different targets for the two phosphoinositides. One of the targets is suggested to be PA, a product of PIP2 hydrolysis via phospholipase C (PLC) or phospholipase D (PLD) activity. It was found that antagonists of PA accumulation (e.g. butan-1-ol) and inhibitors of PLC and PLD reversibly halted polarity. Reduction of PA levels caused the dissipation of the [Ca2+]c gradient and inhibited apical plasma membrane recycling. It was also found to cause abolition of the apical zonation. These data suggest that phosphoinositides and phospholipids regulate tip growth through a multiple pathway system involving regulation of [Ca2+]c levels, endo/exocytosis, and vesicular trafficking.

Accumulation of diacylglycerol in the Chlamydia inclusion vacuole: possible role in the inhibition of host cell apoptosis

Intracellular pathogens have developed strategies to survive for extended periods inside their host cells. These include avoidance of host microbicidal effectors, often by sequestration in a protected subcompartment of the host cell. In some cases, the parasites exert also an antiapoptotic effect that prolongs the life of the infected host cell. Chlamydia utilizes both strategies, but the underlying molecular mechanisms are incompletely understood. Comparatively, little is known regarding the effects that Chlamydia exerts on the metabolism and distribution of the host cell lipids. The expression of fluorescently tagged C1 domains revealed that diacylglycerol is greatly accumulated in the immediate vicinity of Chlamydia inclusion vacuoles. The concentrated diacylglycerol recruits protein kinase Cdelta (PKCdelta), a proapoptotic effector, to the immediate vicinity of the vacuole. PKCdelta normally exerts its pro-apoptotic effects at the mitochondria and in the nucleus. We speculate that Chlamydia antagonizes the pro-apoptotic effect of PKCdelta by sequestering the enzyme on the inclusion vacuole away from its conventional target sites. Accordingly, we found that the ectopic expression of a catalytic fragment of PKCdelta that cannot be recruited by the vacuole, because it lacks a functional C1 domain, overcame the anti-apoptotic effect of the bacteria. The scavenging of pro-apoptotic factors may provide a novel mechanism whereby pathogens promote their own survival by extending the life of the host cells they infect.

Protein–lipid interactions in membrane trafficking at the Golgi complex

The integrated interplay between proteins and lipids drives many key cellular processes, such as signal transduction, cytoskeleton remodelling and membrane trafficking. The last of these, membrane trafficking, has the Golgi complex as its central station. Not only does this organelle orchestrates the biosynthesis, transport and intracellular distribution of many proteins and lipids, but also its own function and structure is dictated by intimate functional and physical relationships between protein-based and lipid-based machineries. These machineries are involved in the control of the fundamental events that govern membrane traffic, such as in the budding, fission and fusion of transport intermediates, in the regulation of the shape and geometry of the Golgi membranes themselves, and, finally, in the generation of "signals" that can have local actions in the secretory system, or that may affect other cellular systems. Lipid-protein interactions rely on the abilities of certain protein domains to recognize specific lipids. These interactions are mediated, in particular, through the headgroups of the phospholipids, although a few of these protein domains are able to specifically interact with the phospholipid acyl chains. Recent evidence also indicates that some proteins and/or protein domains are more sensitive to the physical environment of the membrane bilayer (such as its curvature) than to its chemical composition.

Regulation and cellular roles of phosphoinositide 5-kinases

The membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), plays a critical role in various, apparently very different cellular processes. As precursor for second messengers generated by phospholipase C isoforms and class I phosphoinositide 3-kinases, PIP(2) is indispensable for cellular signaling by membrane receptors. In addition, PIP(2) directly affects the localization and activity of many cellular proteins via specific interaction with unique phosphoinositide-binding domains and thereby regulates actin cytoskeletal dynamics, vesicle trafficking, ion channel activity, gene expression and cell survival. The activity and subcellular localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms, which catalyze the formation of PIP(2), are actively regulated by membrane receptors, by phosphorylation and by small GTPases of the Rho and ARF families. Spatially and temporally organized regulation of PIP(2) synthesis by PIP5K enables dynamic and versatile PIP(2) signaling and represents an important link in the execution of cellular tasks by Rho and ARF GTPases.

Phospholipases D1 and D2 Coordinately Regulate Macrophage Phagocytosis

Phagocytosis is a fundamental feature of the innate immune system, required for antimicrobial defense, resolution of inflammation, and tissue remodeling. Furthermore, phagocytosis is coupled to a diverse range of cytotoxic effector mechanisms, including the respiratory burst, secretion of inflammatory mediators and Ag presentation. Phospholipase D (PLD) has been linked to the regulation of phagocytosis and subsequent effector responses, but the identity of the PLD isoform(s) involved and the molecular mechanisms of activation are unknown. We used primary human macrophages and human THP-1 promonocytes to characterize the role of PLD in phagocytosis. Macrophages, THP-1 cells, and other human myelomonocytic cells expressed both PLD1 and PLD2 proteins. Phagocytosis of complement-opsonized zymosan was associated with stimulation of the activity of both PLD1 and PLD2, as demonstrated by a novel immunoprecipitation-in vitro PLD assay. Transfection of dominant-negative PLD1 or PLD2 each inhibited the extent of phagocytosis (by 55-65%), and their combined effects were additive (reduction of 91%). PLD1 and PLD2 exhibited distinct localizations in resting macrophages and those undergoing phagocytosis, and only PLD1 localized to the phagosome membrane. The COS-7 monkey fibroblast cell line, which has been used as a heterologous system for the analysis of receptor-mediated phagocytosis, expressed PLD2 but not PLD1. These data support a model in which macrophage phagocytosis is coordinately regulated by both PLD1 and PLD2, with isoform-specific localization. Human myelomonocytic cell lines accurately model PLD-dependent signal transduction events required for phagocytosis, but the heterologous COS cell system does not.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

Phospholipase D2 localizes to the plasma membrane and regulates angiotensin II receptor endocytosis

Phospholipase D (PLD) is a key facilitator of multiple types of membrane vesicle trafficking events. Two PLD isoforms, PLD1 and PLD2, exist in mammals. Initial studies based on overexpression studies suggested that in resting cells, human PLD1 localized primarily to the Golgi and perinuclear vesicles in multiple cell types. In contrast, overexpressed mouse PLD2 was observed to localize primarily to the plasma membrane, although internalization on membrane vesicles was observed subsequent to serum stimulation. A recent report has suggested that the assignment of PLD2 to the plasma membrane is in error, because the endogenous isoform in rat secretory cells was imaged and found to be present primarily in the Golgi apparatus. We have reexamined this issue by using a monoclonal antibody specific for mouse PLD2, and find, as reported initially using overexpression studies, that endogenous mouse PLD2 is detected most readily at the plasma membrane in multiple cell types. In addition, we report that mouse, rat, and human PLD2 when overexpressed all similarly localize to the plasma membrane in cell lines from all three species. Finally, studies conducted using overexpression of wild-type active or dominant-negative isoforms of PLD2 and RNA interference-mediated targeting of PLD2 suggest that PLD2 functions at the plasma membrane to facilitate endocytosis of the angiotensin II type 1 receptor.

Membrane targeting by pleckstrin homology domains

Pleckstrin homology (PH) domains are small modular domains that occur once, or occasionally several times, in a large variety of signalling proteins. In a number of instances, PH domains act to target their host protein to the cytosolic face of cellular membranes through an ability to associate with phosphoinositides. In this review, we discuss recent advances in our understanding of PH domain function. In particular we describe the structural aspects of how PH domains have evolved to bind various phosphoinositides, how PH domains regulate phosphoinositide-mediated association to plasma and internals membranes, and finally raise the issue of PH domains in protein:protein interactions and the allosteric regulation of their host protein.

Reconstitution of phosphatidylserine transport from chemically defined donor membranes to phosphatidylserine decarboxylase 2 implicates specific lipid domains in the process

Phosphatidylserine (PtdSer) is transported from its site of synthesis in the endoplasmic reticulum to the locus of PtdSer decarboxylase 2 (Psd2p) in the Golgi/vacuole and decarboxylated to form phosphatidylethanolamine. Recent biochemical and genetic evidence has implicated the C2 domain of Psd2p and a membrane-bound form of the phosphatidylinositol binding/transfer protein, PstB2p, as essential for this transport process. We devised a reconstituted system in which chemically defined donor membranes function to transfer PtdSer to the biological acceptor membranes containing Psd2p. The transfer of PtdSer is poor when the donor membranes have a high degree of curvature but markedly enhanced when the membranes are relatively planar (> or =400-nm diameter). PtdSer transfer is also dependent upon both the bulk and the surface concentrations of the lipid, with pure PtdSer vesicles acting as the most efficient donors at all concentrations. The lipid transfer from donor membranes containing either 100% PtdSer or 50% PtdSer at a fixed concentration (e.g. 250 microM PtdSer) differs by a factor of 20. Surface dilution of PtdSer by choline, ethanolamine, glycerol, and inositol phospholipids markedly inhibits PtdSer transfer, whereas phosphatidic acid (PtdOH) stimulates the transfer. Most importantly, the transfer of PtdSer from liposomes to Psd2p fails to occur in acceptor membranes from strains lacking PstB2p or the C2 domain of Psd2p. These data support a model for PtdSer transport from planar domains highly enriched in PtdSer or in PtdSer plus PtdOH.

Phosphatidic acid produced by phospholipase D is required for tobacco pollen tube growth

Phospholipase D (PLD) and its product phosphatidic acid (PA) are involved in a number of signalling pathways regulating cell proliferation, membrane vesicle trafficking and defence responses in eukaryotic cells. Here we report that PLD and PA have a role in the process of polarised plant cell expansion as represented by pollen tube growth. Both phosphatidylinositol-4,5-bisphosphate-dependent and independent PLD activities were identified in pollen tube extracts, and activity levels during pollen tube germination and growth were measured. PLD-mediated PA production in vivo can be blocked by primary alcohols, which serve as a substrate for the transphosphatidylation reaction. Both pollen germination and tube growth are stopped in the presence 0.5% 1-butanol, whereas secondary and tertiary isomers do not show any effect. This inhibition could be overcome by addition of exogenous PA-containing liposomes. In the absence of n-butanol, addition of a micromolar concentration of PA specifically stimulates pollen germination and tube elongation. Furthermore, a recently established link between PLD and microtubule dynamics was supported by taxol-mediated partial rescue of the 1-butanol-inhibited pollen tubes. The potential signalling role for PLD-derived PA in plant cell expansion is discussed.

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

Phospholipase D2 is localized to the rims of the Golgi apparatus in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid, a molecule known to have multiple physiological roles, including release of nascent secretory vesicles from the trans-Golgi network. In mammalian cells two forms of the enzyme, PLD1 and PLD2, have been described. We recently demonstrated that PLD1 is localized to the Golgi apparatus, nuclei, and to a lesser extent, plasma membrane. Due to its low abundance, the intracellular localization of PLD2 has been characterized only indirectly through overexpression of chimeric proteins. Using antibodies specific to PLD2, together with immunofluorescence microscopy, herein we demonstrate that a significant fraction of endogenous PLD2 localized to the perinuclear Golgi region and was also distributed throughout cells in dense cytoplasmic puncta; a fraction of which colocalized with caveolin-1 and the plasma membrane. On treatment with brefeldin A, PLD2 translocated into the nucleus in a manner similar to PLD1, suggesting a potential role in nuclear signaling. Most significantly, cryoimmunogold electron microscopy demonstrated that in pituitary GH(3) cells >90% of PLD2 present in the Golgi apparatus was localized to cisternal rims and peri-Golgi vesicles exclusively. The data are consistent with a model whereby PLD2 plays a role in Golgi vesicular transport.

The regulation of phospholipase D by inositol phospholipids and small GTPases

Phospholipase D1 and D2 (PLD1, PLD2) both have PX and PH domains in their N-terminal regions with these inositol lipid binding domains playing key roles in regulating PLD activity and localisation. The activity of PLD1 is also regulated by protein kinase C and members of the Rho and Arf families of GTPases. Each of these proteins binds to unique sites; however, there appears to be little in vitro discrimination between individual family members. In agonist-stimulated cells, however, there is specificity, with, for example in RBL-2H3 cells, antigen stimulating the activation of PLD1 by association with Arf6, Rac1 and protein kinase Calpha. PLD2 appears to be less directly regulated by GTPases and rather is primarily controlled through interaction with phosphatidylinositol 4-phosphate 5-kinase that generates the activating phosphatidylinositol 4,5-bisphosphate.