The multiple roles of PtdIns(4)P -- not just the precursor of PtdIns(4,5)P2

PI4P and Rab inputs collaborate in myosin-V-dependent transport of secretory compartments in yeast

Cell polarity involves transport of specific membranes and macromolecules at the right time to the right place. In budding yeast, secretory vesicles are transported by the myosin-V Myo2p to sites of cell growth. We show that phosphatidylinositol 4-phosphate (PI4P) is present in late secretory compartments and is critical for their association with, and transport by, Myo2p. Further, the trans-Golgi network Rab Ypt31/32p and secretory vesicle Rab Sec4p each bind directly, but distinctly, to Myo2p, and these interactions are also required for secretory compartment transport. Enhancing the interaction of Myo2p with PI4P bypasses the requirement for interaction with Ypt31/32p and Sec4p. Together with additional genetic data, the results indicate that Rab proteins and PI4P collaborate in the association of secretory compartments with Myo2p. Thus, we show that a coincidence detection mechanism coordinates inputs from PI4P and the appropriate Rab for secretory compartment transport.

The Golgi apparatus: an organelle with multiple complex functions

Remarkable advances have been made during the last few decades in defining the organizational principles of the secretory pathway. The Golgi complex in particular has attracted special attention due to its central position in the pathway, as well as for its fascinating and complex structure. Analytical studies of this organelle have produced significant advances in our understanding of its function, although some aspects still seem to elude our comprehension. In more recent years a level of complexity surrounding this organelle has emerged with the discovery that the Golgi complex is involved in cellular processes other than the 'classical' trafficking and biosynthetic pathways. The resulting picture is that the Golgi complex can be considered as a cellular headquarters where cargo sorting/processing, basic metabolism, signalling and cell-fate decisional processes converge.

Cog2 null mutant CHO cells show defective sphingomyelin synthesis

The COG (conserved oligomeric Golgi complex) is a Golgi-associated tethering complex involved in retrograde trafficking of multiple Golgi enzymes. COG deficiencies lead to misorganization of the Golgi, defective trafficking of glycosylation enzymes, and abnormal N-, O- and ceramide-linked oligosaccharides. Here, we show that in Cog2 null mutant ldlC cells, the content of sphingomyelin (SM) is reduced to ∼25% of WT cells. Sphingomyelin synthase (SMS) activity is essentially normal in ldlC cells, but in contrast with the typical Golgi localization in WT cells, in ldlC cells, transfected SMS1 localizes to vesicular structures scattered throughout the cytoplasm, which show almost no signal of co-transfected ceramide transfer protein (CERT). Cog2 transfection restores SM formation and the typical SMS1 Golgi localization phenotype. Adding exogenous N-6-[(7-nitrobenzo-2-oxa-1,3-diazol-4-yl)amino]hexanoyl-4-d-erythro-sphingosine (C(6)-NBD-ceramide) to ldlC cell cultures results in normal SM formation. Endogenous ceramide levels were 3-fold higher in ldlC cells than in WT cells, indicating that Golgi misorganization caused by Cog2 deficiency affects the delivery of ceramide to sites of SM synthesis by SMS1. Considering the importance of SM as a structural component of membranes, this finding is also worth of consideration in relation to a possible contribution to the clinical phenotype of patients suffering congenital disorders of glycosylation type II.

Mammalian diseases of phosphatidylinositol transfer proteins and their homologs

Inositol and phosphoinositide signaling pathways represent major regulatory systems in eukaryotes. The physiological importance of these pathways is amply demonstrated by the variety of diseases that involve derangements in individual steps in inositide and phosphoinositide production and degradation. These diseases include numerous cancers, lipodystrophies and neurological syndromes. Phosphatidylinositol transfer proteins (PITPs) are emerging as fascinating regulators of phosphoinositide metabolism. Recent advances identify PITPs (and PITP-like proteins) to be coincidence detectors, which spatially and temporally coordinate the activities of diverse aspects of the cellular lipid metabolome with phosphoinositide signaling. These insights are providing new ideas regarding mechanisms of inherited mammalian diseases associated with derangements in the activities of PITPs and PITP-like proteins.

One lipid, multiple functions: how various pools of PI(4,5)P(2) are created in the plasma membrane

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] is a minor lipid of the inner leaflet of the plasma membrane that controls the activity of numerous proteins and serves as a source of second messengers. This multifunctionality of PI(4,5)P(2) relies on mechanisms ensuring transient appearance of PI(4,5)P(2) clusters in the plasma membrane. One such mechanism involves phosphorylation of PI(4)P to PI(4,5)P(2) by the type I phosphatidylinositol-4-phosphate 5-kinases (PIP5KI) at discrete membrane locations coupled with PI(4)P delivery/synthesis at the plasma membrane. Simultaneously, both PI(4)P and PI(4,5)P(2) participate in anchoring PIP5KI at the plasma membrane via electrostatic bonds. PIP5KI isoforms are also selectively recruited and activated at the plasma membrane by Rac1, talin, or AP-2 to generate PI(4,5)P(2) in ruffles and lamellipodia, focal contacts, and clathrin-coated pits. In addition, PI(4,5)P(2) can accumulate at sphingolipid/cholesterol-based rafts following activation of distinct membrane receptors or be sequestered in a reversible manner due to electrostatic constrains posed by proteins like MARCKS.

Requirement for Golgi-localized PI(4)P in fusion of COPII vesicles with Golgi compartments

The role of specific membrane lipids in ER-Golgi transport is unclear. Using cell-free assays that measure stages in ER-Golgi transport, a variety of enzyme inhibitors, lipid-modifying enzymes, and lipid ligands were screened. The results indicate that PI(4)P is required for SNARE-dependent fusion of COPII vesicles with the Golgi complex.

The role of specific membrane lipids in transport between endoplasmic reticulum (ER) and Golgi compartments is poorly understood. Using cell-free assays that measure stages in ER-to-Golgi transport, we screened a variety of enzyme inhibitors, lipid-modifying enzymes, and lipid ligands to investigate requirements in yeast. The pleckstrin homology (PH) domain of human Fapp1, which binds phosphatidylinositol-4-phosphate (PI(4)P) specifically, was a strong and specific inhibitor of anterograde transport. Analysis of wild type and mutant PH domain proteins in addition to recombinant versions of the Sac1p phosphoinositide-phosphatase indicated that PI(4)P was required on Golgi membranes for fusion with coat protein complex II (COPII) vesicles. PI(4)P inhibition did not prevent vesicle tethering but significantly reduced formation of soluble n-ethylmaleimide sensitive factor adaptor protein receptor (SNARE) complexes between vesicle and Golgi SNARE proteins. Moreover, semi-intact cell membranes containing elevated levels of the ER-Golgi SNARE proteins and Sly1p were less sensitive to PI(4)P inhibitors. Finally, in vivo analyses of a pik1 mutant strain showed that inhibition of PI(4)P synthesis blocked anterograde transport from the ER to early Golgi compartments. Together, the data presented here indicate that PI(4)P is required for the SNARE-dependent fusion stage of COPII vesicles with the Golgi complex.

ZSTK474, a novel phosphatidylinositol 3-kinase inhibitor identified using the JFCR39 drug discovery system

JFCR39 is an informatic anticancer drug discovery system that utilizes a panel of 39 human cancer cells coupled with a drug-activity database. This system not only provides disease-oriented information but can also predict the mechanism of action of a given antitumor agent. Development of a phosphatidylinositol 3-kinase (PI3K) inhibitor as an anticancer drug candidate has attracted a great deal of attention from both academia and industry because PI3K is known to be closely involved in carcinogenesis. ZSTK474 was identified as a PI3K inhibitor using JFCR39 system in combination with COMPARE analysis program. These findings were based on the similar fingerprint (growth inhibition profiles for JFCR39 human cancer cell line panel) with that of a classical PI3K inhibitor LY294002. Biochemical experiments confirmed ZSTK474 to be a potent pan-class I PI3K inhibitor, with high selectivity over other classes of PI3K and protein kinases. We previously reported the in vitro and in vivo antitumor efficacy of ZSTK474, together with the G(0)/G(1) arrest and antiangiogenic activity. Here, we review the JFCR39 system and summarize recent studies on PI3K biology and the development of PI3K inhibitors before discussing ZSTK474 in some detail.

Phosphoinositides: lipid regulators of membrane proteins

Phosphoinositides are a family of minority acidic phospholipids in cell membranes. Their principal role is instructional: they interact with proteins. Each cellular membrane compartment uses a characteristic species of phosphoinositide. This signature phosphoinositide attracts a specific complement of functionally important, loosely attached peripheral proteins to that membrane. For example, the phosphatidylinositol 4,5-bisphosphate (PIP(2)) of the plasma membrane attracts phospholipase C, protein kinase C, proteins involved in membrane budding and fusion, proteins regulating the actin cytoskeleton, and others. Phosphoinositides also regulate the activity level of the integral membrane proteins. Many ion channels of the plasma membrane need the plasma-membrane-specific PIP(2) to function. Their activity decreases when the abundance of this lipid falls, as for example after activation of phospholipase C. This behaviour is illustrated by the suppression of KCNQ K(+) channel current by activation of M(1) muscarinic receptors; KCNQ channels require PIP(2) for their activity. In summary, phosphoinositides contribute to the selection of peripheral proteins for each membrane and regulate the activity of the integral proteins.

Viral reorganization of the secretory pathway generates distinct organelles for RNA replication

Many RNA viruses remodel intracellular membranes to generate specialized sites for RNA replication. How membranes are remodeled and what properties make them conducive for replication are unknown. Here we show how RNA viruses can manipulate multiple components of the cellular secretory pathway to generate organelles specialized for replication that are distinct in protein and lipid composition from the host cell. Specific viral proteins modulate effector recruitment by Arf1 GTPase and its guanine nucleotide exchange factor GBF1, promoting preferential recruitment of phosphatidylinositol-4-kinase IIIbeta (PI4KIIIbeta) to membranes over coat proteins, yielding uncoated phosphatidylinositol-4-phosphate (PI4P) lipid-enriched organelles. The PI4P-rich lipid microenvironment is essential for both enteroviral and flaviviral RNA replication; PI4KIIIbeta inhibition interferes with this process; and enteroviral RNA polymerases specifically bind PI4P. These findings reveal how RNA viruses can selectively exploit specific elements of the host to form specialized organelles where cellular phosphoinositide lipids are key to regulating viral RNA replication.

Phosphoinositide signalling in cancer: beyond PI3K and PTEN

There are numerous studies that suggest multiple links between the cellular phosphoinositide system and cancer. As key roles in cancer have been established for PI3K and PTEN - enzymes that regulate the levels of phosphatidylinositol-3,4,5-trisphosphate - compounds targeting this pathway are entering the clinic at a rapid pace. Several other phosphoinositide-modifying enzymes, including phosphoinositide kinases, phosphatases and phospholipase C enzymes, have been implicated in the generation and progression of tumours. Studies of these enzymes are providing new insights into the mechanisms and the extent of their involvement in cancer, highlighting new potential targets for therapeutic intervention.

Signaling from the Golgi: mechanisms and models for Golgi phosphoprotein 3-mediated oncogenesis

Golgi phosphoprotein 3 (GOLPH3; also known as GPP34/GMx33/MIDAS) represents an exciting new class of oncoproteins involved in vesicular trafficking. Encoded by a gene residing on human chromosome 5p13, which is frequently amplified in multiple solid tumor types, GOLPH3 was initially discovered as a phosphorylated protein localized to the Golgi apparatus. Recent functional, cell biological, and biochemical analyses show that GOLPH3 can function as an oncoprotein to promote cell transformation and tumor growth by enhancing activity of the mammalian target of rapamycin, a serine/threonine protein kinase known to regulate cell growth, proliferation, and survival. Although its precise mode of action in cancer remains to be elucidated, the fact that GOLPH3 has been implicated in protein trafficking, receptor recycling, and glycosylation points to potential links of these cellular processes to tumorigenesis. Understanding how these processes may be deregulated and contribute to cancer pathogenesis and drug response will uncover new avenues for therapeutic intervention.

Distinct plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in secretagogue-stimulated beta-cells

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P(2) concentration, little is known about its precursor, PtdIns(4)P, in particular whether there are stimulus-induced alterations independent of those of PtdIns(4,5)P(2). We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P(2) dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P(2) induced by phospholipase C (PLC)-activating receptor agonists or stimulatory glucose concentrations was paralleled by increased PtdIns(4)P levels. In addition, glucose-stimulated cells regularly showed anti-synchronous oscillations of the two lipids. Whereas glucose-induced PtdIns(4)P elevation required voltage-gated Ca(2+) entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca(2+) independent, but sensitive to protein kinase C (PKC) inhibition and mimicked by phorbol ester stimulation. We conclude that glucose and PLC-activating receptor stimuli trigger Ca(2+)- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P(2). These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P(2) pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Inhibition profiles of phosphatidylinositol 3-kinase inhibitors against PI3K superfamily and human cancer cell line panel JFCR39

As accumulating evidences suggest close involvement of phosphatidylinositol 3-kinase (PI3K) in various diseases particularly cancer, considerable competition occurs in development of PI3K inhibitors. Consequently, novel PI3K inhibitors such as ZSTK474, GDC-0941 and NVP-BEZ235 have been developed. Even though all these inhibitors were reported to inhibit class I PI3K but not dozens of protein kinases, whether they have different molecular targets remained unknown. To investigate such molecular target specificity, we have determined the inhibitory effects of these novel inhibitors together with classical PI3K inhibitor LY294002 on PI3K superfamily (including classes I, II, and III PI3Ks, PI4K and PI3K-related kinases) by using several novel non-radioactive biochemical assays. As a result, ZSTK474 and GDC-0941 indicated highly similar inhibition profiles for PI3K superfamily, with class I PI3K specificity much higher than NVP-BEZ235 and LY294002. We further investigated their growth inhibition effects on JFCR39, a human cancer cell line panel which we established for molecular target identification, and analysed their cell growth inhibition profiles (fingerprints) by using COMPARE analysis programme. Interestingly, we found ZSTK474 exhibited a highly similar fingerprint with GDC-0941 (r=0.863), more similar than with that of either NVP-BEZ235 or LY294002, suggesting that ZSTK474 shares more in molecular targets with GDC-0941 than with either of the other two PI3K inhibitors, consistent with the biochemical assay result. The biological implication of the difference in molecular target specificity of these PI3K inhibitors is under investigation.

An electrostatic switch displaces phosphatidylinositol phosphate kinases from the membrane during phagocytosis

PIP5K is held at the membrane of forming phagosomes by a conserved, positively charged patch. During particle engulfment, the surface charge of the phagosome decreases, releasing PIP5K and enabling phagocytosis to proceed.

Plasmalemmal phosphatidylinositol (PI) 4,5-bisphosphate (PI4,5P₂) synthesized by PI 4-phosphate (PI4P) 5-kinase (PIP5K) is key to the polymerization of actin that drives chemotaxis and phagocytosis. We investigated the means whereby PIP5K is targeted to the membrane and its fate during phagosome formation. Homology modeling revealed that all PIP5K isoforms feature a positively charged face. Together with the substrate-binding loop, this polycationic surface is proposed to constitute a coincidence detector that targets PIP5Ks to the plasmalemma. Accordingly, manipulation of the surface charge displaced PIP5Ks from the plasma membrane. During particle engulfment, PIP5Ks detached from forming phagosomes as the surface charge at these sites decreased. Precluding the change in surface charge caused the PIP5Ks to remain associated with the phagosomal cup. Chemically induced retention of PIP5K-γ prevented the disappearance of PI4,5P₂ and aborted phagosome formation. We conclude that a bistable electrostatic switch mechanism regulates the association/dissociation of PIP5Ks from the membrane during phagocytosis and likely other processes.

Histidine-rich glycoprotein is a novel plasma pattern recognition molecule that recruits IgG to facilitate necrotic cell clearance via FcgammaRI on phagocytes

Under normal physiologic conditions, necrotic cells resulting from tissue injury are rapidly removed from the circulation and tissues by phagocytes, thus preventing the exposure of intracellular antigenic and immunostimulatory molecules that can aid the development of autoimmune disease. Histidine-rich glycoprotein (HRG), a relatively abundant plasma glycoprotein, has a multidomain structure that can interact with many ligands including components of the fibrinolytic and immune systems. Recently, it has been reported that HRG can bind strongly to cytoplasmic ligand(s) exposed in necrotic cells to enhance clearance by phagocytes. Here we describe the molecular mechanisms underpinning this process. A complex consisting of both HRG and immunoglobulin G (IgG) was found as necessary to aid necrotic cell uptake by monocytes, predominantly via an FcgammaRI-dependent mechanism. The findings in this study also show that HRG can potentially interact with anionic phospholipids exposed in necrotic cells. Furthermore, the enhanced phagocytosis of necrotic cells induced by HRG-IgG complexes triggers phagocytes to release proinflammatory cytokines such as interleukin-8 and tumor necrosis factor. Thus, HRG has the unique property of complexing with IgG and facilitating a proinflammatory innate immune response to promote the clearance of necrotic cells.

Intracellular trafficking of ceramide by ceramide transfer protein

The transport and sorting of lipids are fundamental to membrane biogenesis. In the synthesis of sphingolipids in mammalian cells, ceramide is newly produced at the endoplasmic reticulum (ER), and transported from the ER to the trans Golgi regions, where it is converted to sphingomyelin. Ceramide transfer protein (CERT) mediates the ER-to-Golgi trafficking of ceramide. It has been suggested that CERT extracts ceramide from the ER and carries it to the Golgi apparatus in a non-vesicular manner and that efficient CERT-mediated trafficking of ceramide occurs at membrane contact sites between the ER and the Golgi apparatus.

PtdIns4P recognition by Vps74/GOLPH3 links PtdIns 4-kinase signaling to retrograde Golgi trafficking

Targeting and retention of resident integral membrane proteins of the Golgi apparatus underly the function of the Golgi in glycoprotein and glycolipid processing and sorting. In yeast, steady-state Golgi localization of multiple mannosyltransferases requires recognition of their cytosolic domains by the peripheral Golgi membrane protein Vps74, an orthologue of human GOLPH3/GPP34/GMx33/MIDAS (mitochondrial DNA absence sensitive factor). We show that targeting of Vps74 and GOLPH3 to the Golgi apparatus requires ongoing synthesis of phosphatidylinositol (PtdIns) 4-phosphate (PtdIns4P) by the Pik1 PtdIns 4-kinase and that modulation of the levels and cellular location of PtdIns4P leads to mislocalization of these proteins. Vps74 and GOLPH3 bind specifically to PtdIns4P, and a sulfate ion in a crystal structure of GOLPH3 indicates a possible phosphoinositide-binding site that is conserved in Vps74. Alterations in this site abolish phosphoinositide binding in vitro and Vps74 function in vivo. These results implicate Pik1 signaling in retention of Golgi-resident proteins via Vps74 and show that GOLPH3 family proteins are effectors of Golgi PtdIns 4-kinases.

Changing phosphoinositides "on the fly": how trafficking vesicles avoid an identity crisis

Joining an antagonistic phosphoinositide (PtdInsP) kinase and phosphatase into a single protein complex may regulate rapid and local PtdInsP changes. This may be important for processes such as membrane fission that require a specific PtdInsP and that are innately local and rapid. Such a complex could couple vesicle formation, with erasing of the identity of the donor organelle from the vesicle prior to its fusion with target organelles, thus preventing organelle identity intermixing. Coordinating signals are postulated to switch the relative activities of the kinase and phosphatase in a spatio-temporal manner that matches membrane fission events. The discovery of two such complexes supports this hypothesis. One regulates the interconversion of phosphatidylinositol and PtdIns(3)P by joining the Vps34 PtdIns 3-kinase and the myotubularin 3-phosphatases. The other regulates the interconversion between PtdIns(3)P and PtdIns(3,5)P(2) through the Fab1/PIKfyve kinase and the Fig4/mFig4 phosphatase. These lipids are essential components of the endosomal identity code.

The Sec14 superfamily and mechanisms for crosstalk between lipid metabolism and lipid signaling

Lipid signaling pathways define central mechanisms for cellular regulation. Productive lipid signaling requires an orchestrated coupling between lipid metabolism, lipid organization and the action of protein machines that execute appropriate downstream reactions. Using membrane trafficking control as primary context, we explore the idea that the Sec14-protein superfamily defines a set of modules engineered for the sensing of specific aspects of lipid metabolism and subsequent transduction of 'sensing' information to a phosphoinositide-driven 'execution phase'. In this manner, the Sec14 superfamily connects diverse territories of the lipid metabolome with phosphoinositide signaling in a productive 'crosstalk' between these two systems. Mechanisms of crosstalk, by which non-enzymatic proteins integrate metabolic cues with the action of interfacial enzymes, represent unappreciated regulatory themes in lipid signaling.

Phosphoinositides, exocytosis and polarity in yeast: all about actin?

Cell polarity is necessary for cell division, morphogenesis and motility in eukaryotes, and is determined by dynamic control of the cytoskeleton and secretory pathway to promote directional growth. In yeast, three essential and tightly-regulated processes orchestrate polarization and facilitate bud growth. These processes include phosphoinositide (PI) signaling, Rho GTPase regulation of the actin cytoskeleton, and exocytosis. As yet, the interplay between these different processes is unclear, and two main models (Spatial Landmark and Allosteric Local Activation) have been proposed for Rho GTPase control of polarization in yeast. Here, we summarize the inter-relationship between these growth processes and present a more unified model, the Exocytic Signal model, which proposes that exocytosis and actin regulation are fully integrated events mediated by PI signaling.

Immunocytochemical techniques reveal multiple, distinct cellular pools of PtdIns4P and PtdIns(4,5)P(2)

PtdIns4P is the major precursor for the synthesis of the multifunctional plasma membrane lipid, PtdIns(4,5)P₂. Yet PtdIns4P also functions as a regulatory lipid in its own right, particularly at the Golgi apparatus. In the present study we define specific conditions that enable preservation of several organellar membranes for the immunocytochemical detection of PtdIns4P. We report distinct pools of this lipid in both Golgi and plasma membranes, which are synthesized by different PI4K (phosphatidylinositol 4-kinase) activities, and also the presence of PtdIns4P in cytoplasmic vesicles, which are not readily identifiable as PI4K containing trafficking intermediates. In addition, we present evidence that the majority of PtdIns4P resides in the plasma membrane, where it is metabolically distinct from the steady-state plasma membrane pool of PtdIns(4,5)P₂.

Arf GAP2 is positively regulated by coatomer and cargo

Arf GAP2 is one of four Arf GAPs that function in the Golgi apparatus. We characterized the kinetics of Arf GAP2 and its regulation. Purified Arf GAP2 had little activity compared to purified Arf GAP1. Of the potential regulators we examined, coatomer had the greatest effect, stimulating activity one to two orders of magnitude. The effect was biphasic, with half-maximal activation observed at 50 nM coatomer and activation peaking at approximately 150 nM coatomer. Activation by coatomer was greater for Arf GAP2 than has been reported for Arf GAP1. The effects of phosphoinositides and changes in vesicle curvature on GAP activity were small compared to coatomer; however, both increased coatomer-dependent activity. Peptides from p24 cargo proteins increased Arf GAP2 activity by an additional 2- to 4-fold. The effect of cargo peptide was dependent on coatomer. Overexpressing the cargo protein p25 decreased cellular Arf1*GTP levels. The differential sensitivity of Arf GAP1 and Arf GAP2 to coatomer could coordinate their activities. Based on the common regulatory features of Arf GAP1 and 2, we propose a mechanism for cargo selection in which GTP hydrolysis triggered by cargo binding to the coat protein is coupled to coat polymerization.

Function and regulation of phospholipid signalling in plants

As an important metabolic pathway, phosphatidylinositol metabolism generates both constitutive and signalling molecules that are crucial for plant growth and development. Recent studies using genetic and molecular approaches reveal the important roles of phospholipid molecules and signalling in multiple processes of higher plants, including root growth, pollen and vascular development, hormone effects and cell responses to environmental stimuli plants. The present review summarizes the current progress in our understanding of the functional mechanism of phospholipid signalling, with an emphasis on the regulation of Ins(1,4,5)P3-Ca2+ oscillation, the second messenger molecule phosphatidic acid and the cytoskeleton.

CERT-mediated trafficking of ceramide

The transport and sorting of lipids from the sites of their synthesis to their appropriate destinations are fundamental for membrane biogenesis. In the synthesis of sphingolipids in mammalian cells, ceramide is newly produced at the endoplasmic reticulum (ER), and transported from the ER to the trans Golgi regions, where it is converted to sphingomyelin. CERT mediates the ER-to-Golgi trafficking of ceramide. CERT contains several functional domains and motifs including i) a START domain capable of catalyzing inter-membrane transfer of ceramide, ii) a pleckstrin homology domain, which serves to target the Golgi apparatus, iii) a FFAT motif which interacts with the ER-resident membrane protein VAP, and iv) a serine-repeat motif, of which hyperphosphorylation down-regulates CERT activity. It has been suggested that CERT extracts ceramide from the ER and carries it to the Golgi apparatus in a non-vesicular manner and that efficient CERT-mediated trafficking of ceramide occurs at membrane contact sites between the ER and the Golgi apparatus.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.