Multiple roles for phosphatidylinositol 4-kinase in biosynthetic transport in polarized Madin-Darby canine kidney cells

Membrane Lipids in Epithelial Polarity: Sorting out the PIPs

The development of cell polarity in epithelia, is critical for tissue morphogenesis and vectorial transport between the environment and the underlying tissue. Epithelial polarity is defined by the development of distinct plasma membrane domains: the apical membrane interfacing with the exterior lumen compartment, and the basolateral membrane directly contacting the underlying tissue. The de novo generation of polarity is a tightly regulated process, both spatially and temporally, involving changes in the distribution of plasma membrane lipids, localization of apical and basolateral membrane proteins, and vesicular trafficking. Historically, the process of epithelial polarity has been primarily described in relation to the localization and function of protein 'polarity complexes.' However, a critical and foundational role is emerging for plasma membrane lipids, and in particular phosphoinositide species. Here, we broadly review the evidence for a primary role for membrane lipids in the generation of epithelial polarity and highlight key areas requiring further research. We discuss the complex interchange that exists between lipid species and briefly examine how major membrane lipid constituents are generated and intersect with vesicular trafficking to be preferentially localized to different membrane domains with a focus on some of the key protein-enzyme complexes involved in these processes.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Accumulation of PtdIns(4)P at the Golgi mediated by reversible oxidation of the PtdIns(4)P phosphatase Sac1 by H2O2

Phosphatidylinositol 4-phosphate [PtdIns(4)P] plays a key role in the biogenesis of transport vesicles at the Golgi complex by recruiting coat proteins and their accessory factors. The PtdIns(4)P content of the Golgi is determined by the concerted action of PtdIns 4-kinase (PI4K) and PtdIns(4)P phosphatase enzymes. Sac1 (suppressor of actin 1) is the major PtdIns(4)P phosphatase and is localized to the Golgi and endoplasmic reticulum. The targeting of both PI4Ks and Sac1 to the Golgi membrane is extensively regulated, as is the catalytic activity of PI4Ks at the Golgi. However, regulation of the catalytic activity of Sac1 has been largely unexplored. Here we show that Sac1undergoes reversible inactivation in mammalian cells when its catalytic Cys³⁸⁹ residue is oxidized by exogenous H₂O₂ to form an intramolecular disulfide with Cys³⁹². The oxidative inactivation of Sac1 results in the accumulation of PtdIns(4)P at the Golgi, with this effect also being supported by the H₂O₂-induced activation of p38 mitogen-activated protein kinase (MAPK), which was previously shown to promote the translocation of Sac1 from the Golgi to the endoplasmic reticulum. The increase in Golgi PtdIns(4)P due to Sac1 inactivation, however, is faster than that due to Sac1 translocation. Exposure of cells to H₂O₂ also increased membrane protein trafficking from the Golgi to the plasma membrane as well as protein secretion.

A map of the subcellular distribution of phosphoinositides in the erythrocytic cycle of the malaria parasite Plasmodium falciparum

Despite representing a small percentage of the cellular lipids of eukaryotic cells, phosphoinositides (PIPs) are critical in various processes such as intracellular trafficking and signal transduction. Central to their various functions is the differential distribution of PIP species to specific membrane compartments through the actions of kinases, phosphatases and lipases. Despite their importance in the malaria parasite lifecycle, the subcellular distribution of most PIP species in this organism is still unknown. We here localise several species of PIPs throughout the erythrocytic cycle of Plasmodium falciparum. We show that PI3P is mostly found at the apicoplast and the membrane of the food vacuole, that PI4P associates with the Golgi apparatus and the plasma membrane and that PI(4,5)P2, in addition to being detected at the plasma membrane, labels some cavity-like spherical structures. Finally, we show that the elusive PI5P localises to the plasma membrane, the nucleus and potentially to the transitional endoplasmic reticulum (ER). Our map of the subcellular distribution of PIP species in P. falciparum will be a useful tool to shed light on the dynamics of these lipids in this deadly parasite.

Components of the CtBP1/BARS-dependent fission machinery

The brefeldin A ADP-ribosylated substrate, a member of the C-terminal-binding protein family that is referred to as CtBP1/BARS, is a dual-function protein that acts as a transcriptional co-repressor in the nucleus and as an inducer of membrane fission in the cytoplasm. In this review, we first discuss the mechanisms that enable CtBP1/BARS to shift between the nuclear transcriptional co-repressor and the cytosolic fission-inducing activities. Then, we focus on the role of CtBP1/BARS in membrane fission. CtBP1/BARS controls several fission events including macropinocytosis, fluid-phase endocytosis, COPI-coated vesicle formation, basolaterally directed post-Golgi carrier formation, and Golgi partitioning in mitosis. We report on recent advances in our understanding of the CtBP1/BARS membrane fission machineries that operate at the trans-side and at the cis-side of the Golgi complex. Specifically, we discuss how these machineries are assembled and regulated, and how they operate in the formation of the basolaterally directed post-Golgi carriers.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Mammalian phosphatidylinositol 4-kinases as modulators of membrane trafficking and lipid signaling networks

The four mammalian phosphatidylinositol 4-kinases modulate inter-organelle lipid trafficking, phosphoinositide signalling and intracellular vesicle trafficking. In addition to catalytic domains required for the synthesis of PI4P, the phosphatidylinositol 4-kinases also contain isoform-specific structural motifs that mediate interactions with proteins such as AP-3 and the E3 ubiquitin ligase Itch, and such structural differences determine isoform-specific roles in membrane trafficking. Moreover, different permutations of phosphatidylinositol 4-kinase isozymes may be required for a single cellular function such as occurs during distinct stages of GPCR signalling and in Golgi to lysosome trafficking. Phosphatidylinositol 4-kinases have recently been implicated in human disease. Emerging paradigms include increased phosphatidylinositol 4-kinase expression in some cancers, impaired functioning associated with neurological pathologies, the subversion of PI4P trafficking functions in bacterial infection and the activation of lipid kinase activity in viral disease. We discuss how the diverse and sometimes overlapping functions of the phosphatidylinositol 4-kinases present challenges for the design of isoform-specific inhibitors in a therapeutic context.

Roles of phosphoinositides and phosphoinositides kinases in hepatitis C virus RNA replication

Phosphoinositides (PIs) play an essential role in mediating key signaling pathways on biological membranes. Hepatitis C virus (HCV) replicates its RNA genome by establishing a viral replication complex (RC) on host cell membranes. Recently, an increasing body of literature reported that not only PIs themselves but also several PIs-specific kinases are required for efficient replication of HCV RNA genome. Especially, PI 4-kinases type III alpha, beta as well as their enzymatic products including phosphatidylinositol 4-phosphate (PI(4)P) and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are consistently identified to be host factors essential for HCV replication. In this article, the current state of our knowledge of PIs and PIs-specific kinases together with their roles in modulating HCV replication is reviewed. The effects of various PIs-specific kinases inhibitors on HCV replication are also highlighted, proposing them as promising candidate targets to which a new class of anti-HCV therapeutics can be envisaged.

A 14-3-3γ dimer-based scaffold bridges CtBP1-S/BARS to PI(4)KIIIβ to regulate post-Golgi carrier formation

Large pleiomorphic carriers leave the Golgi complex for the plasma membrane by en bloc extrusion of specialized tubular domains, which then undergo fission. Several components of the underlying molecular machinery have been identified, including those involved in the budding/initiation of tubular carrier precursors (for example, the phosphoinositide kinase PI(4)KIIIβ, the GTPase ARF, and FAPP2), and in the fission of these precursors (for example, PKD, CtBP1-S/BARS). However, how these proteins interact to bring about carrier formation is poorly understood. Here, we describe a protein complex that mediates carrier formation and contains budding and fission molecules, as well as other molecules, such as the adaptor protein 14-3-3γ. Specifically, we show that 14-3-3γ dimers bridge CtBP1-S/BARS with PI(4)KIIIβ, and that the resulting complex is stabilized by phosphorylation by PKD and PAK. Disrupting the association of these proteins inhibits the fission of elongating carrier precursors, indicating that this complex couples the carrier budding and fission processes.

The phosphatidylinositol 4-kinases: don't call it a comeback

Phosphatidylinositol 4-phosphate (PtdIns4P) is a quantitatively minor membrane phospholipid which is the precursor of PtdIns(4,5)P (2) in the classical agonist-regulated phospholipase C signalling pathway. However, PtdIns4P also governs the recruitment and function of numerous trafficking molecules, principally in the Golgi complex. The majority of phosphoinositides (PIs) phosphorylated at the D4 position of the inositol headgroup are derived from PtdIns4P and play roles in a diverse array of fundamental cellular processes including secretion, cell migration, apoptosis and mitogenesis; therefore, PtdIns4P biosynthesis can be regarded as key point of regulation in many PI-dependent processes.Two structurally distinct sequence families, the type II and type III PtdIns 4-kinases, are responsible for PtdIns4P synthesis in eukaryotic organisms. These important proteins are differentially expressed, localised and regulated by distinct mechanisms, indicating that the enzymes perform non-redundant roles in trafficking and signalling. In recent years, major advances have been made in our understanding of PtdIns4K biology and here we summarise current knowledge of PtdIns4K structure, function and regulation.

The lipid kinase PI4KIIIβ and the eEF1A2 oncogene co-operate to disrupt three-dimensional in vitro acinar morphogenesis

The study of in vitro morphogenesis is fundamental to understanding neoplasia since the dysregulation of morphogenic pathways that create multi-cellular organisms is a common hallmark of oncogenesis. The in vitro culture of human breast epithelial cells on reconstituted basement membranes recapitulate some features of in vivo breast development, including the formation of a three-dimensional structure termed an acinus. Importantly, the capacity to disrupt in vitro acinar morphogenesis is a common property of human breast oncogenes. In this report, we find that phosphatidylinositol 4-kinase IIIβ (PI4KIIIβ), a lipid kinase that phosphorylates phosphatidylinositol (PI) to PI(4)P, disrupts in vitro mammary acinar formation. The PI4KIIIβ protein localizes to the basal surface of acini created by human MCF10A cells and ectopic expression of PI4KIIIβ induces multi-acinar devlopment. Furthermore, expression of an oncogenic PI4KIIIβ activator, eEF1A2 (eukaryotic elongation factor 1 alpha 2), phenocopies the PI4KIIIβ multi-acinar phenotype. Ectopic expression of PI4KIIIβ or eEF1A2 alters the localization of PI(4)P and PI(4,5)P(2) within acini, indicating the importance of these lipids in acinar development. Our work shows that PI4KIIIβ, eEF1A2 and PI(4)P likely play an important role in mammary neoplasia and acinar development.

Acute manipulation of Golgi phosphoinositides to assess their importance in cellular trafficking and signaling

Phosphoinositides are essential lipid regulators of trafficking and signaling pathways of all eukaryotic cells. Phosphatidylinositol 4-phosphate (PtdIns4P) is an intermediate in the synthesis of several important phosphoinositide species but also serves as a regulatory molecule in its own right. Phosphatidylinositol 4-kinases are most abundant in the Golgi but are also found in the plasma membrane and in endocytic compartments. To investigate the role of Golgi PtdIns4P in orchestrating trafficking events, we used a unique drug-inducible molecular approach to rapidly deplete PtdIns4P from Golgi membranes by a recruitable Sac1 phosphatase enzyme. The utility of the system was shown by the rapid loss of Golgi localization of PH domains known to bind PtdIns4P after Sac1 recruitment to the Golgi. Acute PtdIns4P depletion prevented the exit of cargo from the Golgi destined to both the plasma membrane and the late endosomes and led to the loss of some but not all clathrin adaptors from the Golgi membrane. Rapid PtdIns4P depletion in the Golgi also impaired but did not eliminate the replenishment of the plasma membrane PtdIns(4,5)P(2) during phospholipase C activation revealing a hitherto unrecognized contribution of Golgi PtdIns4P to this process. This unique approach will allow further studies on the role of phosphoinositides in endocytic compartments that have evaded detection using the conventional long-term manipulations of inositide kinase and phosphatase activities.

The PX-RICS-14-3-3zeta/theta complex couples N-cadherin-beta-catenin with dynein-dynactin to mediate its export from the endoplasmic reticulum

We have recently shown that beta-catenin-facilitated export of cadherins from the endoplasmic reticulum requires PX-RICS, a beta-catenin-interacting GTPase-activating protein for Cdc42. Here we show that PX-RICS interacts with isoforms of 14-3-3 and couples the N-cadherin-beta-catenin complex to the microtubule-based molecular motor dynein-dynactin. Similar to knockdown of PX-RICS, knockdown of either 14-3-3zeta or - resulted in the disappearance of N-cadherin and beta-catenin from the cell-cell boundaries. Furthermore, we found that PX-RICS and 14-3-3zeta/ are present in a large multiprotein complex that contains dynein-dynactin components as well as N-cadherin and beta-catenin. Both RNAi- and dynamitin-mediated inhibition of dynein-dynactin function also led to the absence of N-cadherin and beta-catenin at the cell-cell contact sites. Our results suggest that the PX-RICS-14-3-3zeta/ complex links the N-cadherin-beta-catenin cargo with the dynein-dynactin motor and thereby mediates its endoplasmic reticulum export.

OCRL1 function in renal epithelial membrane traffic

The X-linked disorder Lowe syndrome arises from mutations in OCRL1, a lipid phosphatase that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP(2)). Most patients with Lowe syndrome develop proteinuria very early in life. PIP(2) dynamics are known to modulate numerous steps in membrane trafficking, and it has been proposed that OCRL1 activity regulates the biogenesis or trafficking of the multiligand receptor megalin. To examine this possibility, we investigated the effects of siRNA-mediated OCRL1 knockdown on biosynthetic and postendocytic membrane traffic in canine and human renal epithelial cells. Cells depleted of OCRL1 did not have significantly elevated levels of cellular PIP(2) but displayed an increase in actin comets, as previously observed in cultured cells derived from Lowe patients. Using assays to independently quantitate the endocytic trafficking of megalin and of megalin ligands, we could observe no defect in the trafficking or function of megalin upon OCRL1 knockdown. Moreover, apical delivery of a newly synthesized marker protein was unaffected. OCRL1 knockdown did result in a significant increase in secretion of the lysosomal hydrolase cathepsin D, consistent with a role for OCRL1 in membrane trafficking between the trans-Golgi network and endosomes. Together, our studies suggest that OCRL1 does not directly modulate endocytosis or postendocytic membrane traffic and that the renal manifestations observed in Lowe syndrome patients are downstream consequences of the loss of OCRL1 function.

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.

Kinases required in hepatitis C virus entry and replication highlighted by small interference RNA screening

The entry pathway of the hepatitis C virus (HCV), a major human pathogen, into the cell is incompletely defined. To better characterize this viral life cycle stage, we screened a small interfering RNA library dedicated to the membrane trafficking and remodeling with the infection model of Huh-7.5.1 cells by HCV pseudoparticles (HCVpp). Results showed that the down-regulation of different factors implied in clathrin-mediated endocytosis (CME) inhibits HCVpp cell infection. In addition, knockdown of the phosphatidylinositol 4-kinase type III-alpha (PI4KIIIalpha) prevented infection by HCVpp or by cell-culture grown JFH-1-based HCV. Moreover, the replication activity of an HCV replicon was also affected by the PI4KIIIalpha knockdown. Additional investigations on the different members of the PI4K family revealed that the presence of PI4KIIIbeta in the host cells influenced their susceptibility to HCVpp infection and their capacity to sustain the HCV replication. The PI4KIII involvement during the HCV life cycle seemed to occur by other ways than the control of the CME or of the membranous expression of HCV receptors. Finally, our library screening completed data on the CME-dependant entry route of HCV and identified 2 kinases, PI4KIIIalpha and beta, as relevant potential therapeutic targets.

Proteomic and functional analysis of NCS-1 binding proteins reveals novel signaling pathways required for inner ear development in zebrafish

BACKGROUND

The semicircular canals, a subdivision of the vestibular system of the vertebrate inner ear, function as sensors of angular acceleration. Little is currently known, however, regarding the underlying molecular mechanisms that govern the development of this intricate structure. Zebrafish represent a particularly tractable model system for the study of inner ear development. This is because the ear can be easily visualized during early embryogenesis, and both forward and reverse genetic techniques are available that can be applied to the discovery of novel genes that contribute to proper ear development. We have previously shown that in zebrafish, the calcium sensing molecule neuronal calcium sensor-1 (NCS-1) is required for semicircular canal formation. The function of NCS-1 in regulating semicircular canal formation has not yet been elucidated.

RESULTS

We initiated a multistep functional proteomic strategy to identify neuronal calcium sensor-1 (NCS-1) binding partners (NBPs) that contribute to inner ear development in zebrafish. By performing a Y2H screen in combination with literature and database searches, we identified 10 human NBPs. BLAST searches of the zebrafish EST and genomic databases allowed us to clone zebrafish orthologs of each of the human NBPs. By investigating the expression profiles of zebrafish NBP mRNAs, we identified seven that were expressed in the developing inner ear and overlapped with the ncs-1a expression profile. GST pulldown experiments confirmed that selected NBPs interacted with NCS-1, while morpholino-mediated knockdown experiments demonstrated an essential role for arf1, pi4kbeta, dan, and pink1 in semicircular canal formation.

CONCLUSION

Based on their functional profiles, the hypothesis is presented that Ncs-1a/Pi4kbeta/Arf1 form a signaling pathway that regulates secretion of molecular components, including Dan and Bmp4, that are required for development of the vestibular apparatus. A second set of NBPs, consisting of Pink1, Hint2, and Slc25a25, are destined for localization in mitochondria. Our findings reveal a novel signalling pathway involved in development of the semicircular canal system, and suggest a previously unrecognized role for NCS-1 in mitochondrial function via its association with several mitochondrial proteins.

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.

PX-RICS mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex

Cadherins mediate Ca2+-dependent cell-cell adhesion. Efficient export of cadherins from the endoplasmic reticulum (ER) is known to require complex formation with beta-catenin. However, the molecular mechanisms underlying this requirement remain elusive. Here we show that PX-RICS, a beta-catenin-interacting GTPase-activating protein (GAP) for Cdc42, mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex. Knockdown of PX-RICS expression induced the accumulation of the N-cadherin/beta-catenin complex in the ER and ER exit site, resulting in a decrease in cell-cell adhesion. PX-RICS was also required for ER-to-Golgi transport of the fibroblast growth factor-receptor 4 (FGFR4) associated with N-cadherin. PX-RICS-mediated ER-to-Golgi transport was dependent on its interaction with beta-catenin, phosphatidylinositol-4-phosphate (PI4P), Cdc42, and its novel binding partner gamma-aminobutyric acid type A receptor-associated protein (GABARAP). These results suggest that PX-RICS ensures the efficient entry of the N-cadherin/beta-catenin complex into the secretory pathway, and thereby regulates the amount of N-cadherin available for cell adhesion and FGFR4-mediated signaling.

FAPP2 is involved in the transport of apical cargo in polarized MDCK cells

Phosphatidylinositol-4-phosphate (PI(4)P) is the main phosphoinositide in the Golgi complex and has been reported to play a pleiotropic role in transport of cargo from the trans-Golgi network to the plasma membrane (PM) in polarized Madin–Darby canine kidney (MDCK) cells. Overexpression of the chimeric fluorescent protein encoding the pleckstrin homology domain, which is specific for PI(4)P, inhibited both apical and basolateral transport pathways. The transport of apical cargo from the Golgi was shown to be specifically decreased by adenovirus-mediated RNA interference directed against PI(4)P adaptor protein (FAPP) 2. FAPP1 depletion had no effect on transport. On the other hand, FAPP2 was not involved in the Golgi-to-PM transport of cargo that was targeted to the basolateral membrane domain. Thus, we conclude that FAPP2 plays a specific role in apical transport in MDCK cells.

Phosphatidylinositol 4-phosphate 5-kinase reduces cell surface expression of the epithelial sodium channel (ENaC) in cultured collecting duct cells

Ubiquitination of ENaC subunits has been shown to negatively regulate the cell surface expression of ENaC channels. We have previously demonstrated that epsin links ubiquitinated ENaC to clathrin adaptors for clathrin-mediated endocytosis. Epsin is thought to directly modify the curvature of membranes upon binding to phosphatidylinositol 4,5-bisphosphate (PIP2) where it recruits clathrin and stimulates lattice assembly. Murine phosphatidylinositol 4-phosphate 5-kinase alpha (PI5KIalpha) has been shown to enhance endocytosis in a PIP2-dependent manner. We tested the hypothesis that PI5KIalpha-mediated PIP2 production would negatively regulate ENaC current by enhancing epsin-mediated endocytosis of the channel. Expression of PI5KIalpha decreased ENaC currents in Xenopus oocytes by 80%, entirely because of a decrease in cell surface ENaC levels. Catalytically inactive mutants of PI5Kalpha had no effect on ENaC activity. Expression of the PIP2 binding region of epsin increased ENaC current in oocytes, an effect completely reversed by co-expression of PI5KIalpha. Overexpression of epsin reduced amiloride-sensitive current in CCD cells. Overexpression of PI5KIalpha enhanced membrane PIP2 levels and reduced apical surface expression of ENaC in CCD cells, down-regulating amiloride-sensitive current. Knockdown of PI5KIalpha with isoform-specific siRNA resulted in a 4-fold enhancement of ENaC activity. PI5KIalpha localized exclusively to the apical plasma membrane domain when overexpressed in mouse CCD cells, consistent for a role in regulating PIP2 production at the apical plasma membrane. We conclude that membrane turnover events regulating ENaC surface expression and activity in oocytes and CCD cells can be regulated by PI5KIalpha.

Microarray analysis reveals potential mechanisms of BRMS1-mediated metastasis suppression

We used Affymetrix microarrays to compare gene expression profiles of the metastatic parental breast cancer cell line MDA-MB-435 (435) and the non-metastatic daughter cell line created by the stable expression of the BReast cancer Metastasis Suppressor 1 (BRMS1) gene in 435 cells, MDA-MB-435-BRMS1 (435/BRMS1). Analysis of microarray data provided insight into some of the potential mechanisms by which BRMS1 inhibits tumor formation at secondary sites. Furthermore, due to the importance of the microenvironment, we also examined gene expression under different growth conditions (i.e., plus or minus serum). Expression of 565 genes was significantly (adjusted P-value <0.05) altered regardless of in vitro growth conditions. BRMS1 expression significantly increased multiple major histocompatability complex (MHC) genes and significantly decreased expression of several genes associated with protein localization and secretion. The pattern of gene expression associated with BRMS1 expression suggests that metastasis suppression may be mediated by enhanced immune recognition, altered transport, and/or secretion of metastasis-associated proteins.

Phosphatidylinositol 4-kinase III-beta is required for Golgi maintenance and cytokinesis in Trypanosoma brucei

The parasitic protozoan Trypanosoma brucei contains two type III phosphatidylinositol 4-kinases (alpha and beta). We have cloned the gene encoding the T. brucei type III phosphatidylinositol 4-kinase beta (TbPI4KIII-beta), expressed the protein in COS-7 cells, and confirmed that the protein catalyzes the phosphorylation of phosphatidylinositol. Depletion of TbPI4KIII-beta in procyclic T. brucei by RNA interference (RNAi) resulted in inhibition of cell growth and a distorted cellular morphology. RNAi cells had a distorted Golgi apparatus, and lysosomal and flagellar pocket proteins were mislocalized. Ultrastructural analysis revealed the internal accumulation of a heterogeneous population of vesicles, abnormal positioning of organelles, and a loss of cell polarity. Scanning electron microcopy revealed a twisted phenotype, and dividing cells often exhibited a detached daughter flagellum and lacked a cleavage furrow. Cell cycle analysis confirmed that cells depleted of TbPI4KIII-beta have a postmitotic cytokinesis block that occurs after a single round of mitosis, suggestive of a specific cell cycle block. In summary, TbPI4KIII-beta is an essential protein in procyclic T. brucei, required for maintenance of Golgi structure, protein trafficking, normal cellular shape, and cytokinesis.

Avl9p, a member of a novel protein superfamily, functions in the late secretory pathway

The branching of exocytic transport routes in both yeast and mammalian cells has complicated studies of the late secretory pathway, and the mechanisms involved in exocytic cargo sorting and exit from the Golgi and endosomes are not well understood. Because cargo can be sorted away from a blocked route and secreted by an alternate route, mutants defective in only one route do not exhibit a strong secretory phenotype and are therefore difficult to isolate. In a genetic screen designed to isolate such mutants, we identified a novel conserved protein, Avl9p, the absence of which conferred lethality in a vps1Delta apl2Delta strain background (lacking a dynamin and an adaptor-protein complex 1 subunit). Depletion of Avl9p in this strain resulted in secretory defects as well as accumulation of Golgi-like membranes. The triple mutant also had a depolarized actin cytoskeleton and defects in polarized secretion. Overexpression of Avl9p in wild-type cells resulted in vesicle accumulation and a post-Golgi defect in secretion. Phylogenetic analysis indicated evolutionary relationships between Avl9p and regulators of membrane traffic and actin function.

Phosphatidylinositol 4-Phosphate Is Required for Translation Initiation in Saccharomyces cerevisiae

The small natural product wortmannin inhibits protein synthesis by modulating several phosphatidylinositol (PI) metabolic pathways. A primary target of wortmannin in yeast is the plasma membrane-associated PI 4-kinase (PI4K) Stt4p, which is required for actin cytoskeleton organization. Here we show that wortmannin treatment or inactivation of Stt4p, but not disorganization of the actin cytoskeleton per se, leads to a rapid attenuation of translation initiation. Interestingly, inactivation of Pik1p, a wortmannin-insensitive, functionally distinct PI4K, implicated in the regulation of Golgi functions and secretion, also results in severe translation initiation defects with a marked increase of the phosphorylation of the translation initiation factor eIF2alpha. Because wortmannin largely phenocopies the effects of rapamycin (e.g. it triggers nuclear accumulation of Gln3p), it likely also inhibits the PI kinase-related, target of rapamycin (TOR) kinases. Importantly, however, neither inactivation of Stt4p nor Pik1p significantly affects TOR-controlled readouts other than translation initiation, indicating that these PI4Ks do not simply function upstream of TOR. Together, our results reveal the existence of a novel translation initiation control mechanism in yeast that is tightly coupled to the synthesis of distinct PI4P pools.

Binding of elongation factor eEF1A2 to phosphatidylinositol 4-kinase beta stimulates lipid kinase activity and phosphatidylinositol 4-phosphate generation

Eukaryotic protein translation elongation factor 1 alpha 2 (eEF1A2) is an oncogene that transforms mammalian cell lines and increases their tumorigenicity in nude mice. Increased expression of eEF1A2 occurs during the development of breast, ovarian, and lung cancer. Here, we report that eEF1A2 directly binds to and activates phosphatidylinositol 4-kinase III beta (PI4KIIIbeta), an enzyme that converts phosphatidylinositol to phosphatidylinositol 4-phosphate. Purified recombinant eEF1A2 increases PI4KIIIbeta lipid kinase activity in vitro, and expression of eEF1A2 in rat and human cells is sufficient to increase overall cellular phosphatidylinositol 4-kinase activity and intracellular phosphatidylinositol 4-phosphate abundance. siRNA-mediated reduction in eEF1A2 expression concomitantly reduces phosphatidylinositol 4-kinase activity. This identifies a physical and functional relationship between eEF1A2 and PI4KIIIbeta.

Phosphatidylinositol 4-kinases: old enzymes with emerging functions

Phosphoinositides account for only a tiny fraction of cellular phospholipids but are extremely important in the regulation of the recruitment and activity of many signaling proteins in cellular membranes. Phosphatidylinositol (PtdIns) 4-kinases generate PtdIns 4-phosphate, the precursor of important regulatory phosphoinositides but also an emerging regulatory molecule in its own right. The four mammalian PtdIns 4-kinases regulate a diverse array of signaling events, as well as vesicular trafficking and lipid transport, but the mechanisms by which their lipid product PtdIns 4-phosphate controls these processes is only beginning to unfold.

Lipid-binding hSH3 domains in immune cell adapter proteins

SH3 domains represent versatile scaffolds within eukaryotic cells by targeting proline-rich sequences within intracellular proteins. More recently, binding of SH3 domains to unusual peptide motifs, folded proteins or lipids has been reported. Here we show that the newly defined hSH3 domains of immune cell adapter proteins bind lipid membranes with distinct affinities. The interaction of the hSH3 domains of adhesion and degranulation promoting adapter protein (ADAP) and PRAM-1 (Promyelocytic-Retinoic acid receptor alpha target gene encoding an Adaptor Molecule-1), with phosphatidylcholine-containing liposomes is observed upon incorporation of phosphatidylserine (PS) or phosphoinositides (PIs) into the membrane bilayer. Mechanistically we show that stable association of the N-terminal, amphipathic helix with the beta-sheet scaffold favours lipid binding and that the interaction with PI(4,5)P(2)-containing liposomes is consistent with a single-site, non-cooperative binding mechanism. Functional investigations indicate that deletion of both amphipathic helices of the hSH3 domains reduces the ability of ADAP to enhance adhesion and migration in stimulated T cells.

Phosphatidylinositol 5-kinase stimulates apical biosynthetic delivery via an Arp2/3-dependent mechanism

The mechanisms by which polarized epithelial cells target distinct carriers enriched in newly synthesized proteins to the apical or basolateral membrane remain largely unknown. Here we investigated the effect of phosphatidylinositol metabolism and modulation of the actin cytoskeleton, two regulatory mechanisms that have individually been suggested to function in biosynthetic traffic, on polarized traffic in Madin-Darby canine kidney cells. Overexpression of phosphatidylinositol 5-kinase (PI5K) increased actin comet frequency in Madin-Darby canine kidney cells and concomitantly stimulated trans-Golgi network (TGN) to apical membrane delivery of the raft-associated protein influenza hemagglutinin (HA), but did not affect delivery of a non-raft-associated apical protein or a basolateral marker. Modulation of actin comet formation by pharmacologic means, by overexpression of the TGN-localized inositol polyphosphate 5-phosphatase Ocrl, or by blockade of Arp2/3 function had parallel effects on the rate of apical delivery of HA. Moreover, HA released from a TGN block was colocalized in transport carriers in association with PI5K and actin comets. Inhibition of Arp2/3 function in combination with microtubule depolymerization led to a virtual block in HA delivery, suggesting synergistic coordination of these cytoskeletal assemblies in membrane transport. Our results suggest a previously unidentified role for actin comet-mediated propulsion in the biosynthetic delivery of a subset of apical proteins.

PKCeta is required for beta1gamma2/beta3gamma2- and PKD-mediated transport to the cell surface and the organization of the Golgi apparatus

Protein kinase D (PKD) binds to a pool of diacylglycerol (DAG) in the TGN and undergoes a process of activation that involves heterotrimeric GTP-binding protein subunits βγ to regulate membrane fission. This fission reaction is used to generate transport carriers at the TGN that are en route to the cell surface. We now report that PKD is activated specifically by G protein subunit β1γ2 and β3γ2 via the Golgi apparatus–associated PKCη. Compromising the kinase activity of PKCη-inhibited protein transport from TGN to the cell surface. Expression of constitutively activated PKCη caused Golgi fragmentation, which was inhibited by a kinase inactive form of PKD. Our findings reveal that βγ, PKCη, and PKD act in series to generate transport carriers from the TGN and their overactivation results in complete vesiculation of the Golgi apparatus.

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.

Neuronal calcium sensor-1 facilitates neuronal exocytosis through phosphatidylinositol 4-kinase

This work tested the theory that neuronal calcium sensor-1 (NCS-1) has effects on neurotransmitter release beyond its actions on membrane channels. We used nerve-ending preparations where membrane channels are bypassed through membrane permeabilization made by mechanical disruption or streptolysin-O. Nerve ending NCS-1 and phosphatidylinositol 4-kinase (PI4K) are largely or entirely particulate, so their concentrations in nerve endings remain constant after breaching the membrane. Exogenous, myristoylated NCS-1 stimulated nerve ending phosphatidylinositol 4-phosphate [PI(4)P] synthesis, but non-myristoylated-NCS-1 did not. The N-terminal peptide of NCS-1 interfered with PI(4)P synthesis, and with spontaneous and Ca(2+)-evoked release of both [(3)H]-norepinephrine (NA) and [(14)C]-glutamate (glu) in a concentration-dependent manner. An antibody raised against the N-terminal of NCS-1 inhibited perforated nerve ending PI(4)P synthesis, but the C-terminal antibody had no effects. Antibodies against the N- and C-termini of NCS-1 caused significant increases in mini/spontaneous/stimulation-independent release of [(3)H]-NA from perforated nerve endings, but had no effect on [(14)C]-glu release. These results support the idea that NCS-1 facilitates nerve ending neurotransmitter release and phosphoinositide production via PI4K and localizes these effects to the N-terminal of NCS-1. Combined with previous work on the regulation of channels by NCS-1, the data are consistent with the hypothesis that a NCS-1-PI4K (NP, neuropotentiator) complex may serve as an essential linker between lipid and protein metabolism to regulate membrane traffic and co-ordinate it with ion fluxes and plasticity in the nerve ending.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

Distinct Golgi Populations of Phosphatidylinositol 4-Phosphate Regulated by Phosphatidylinositol 4-Kinases

Phosphatidylinositol 4-phosphate (PI4P) regulates biosynthetic membrane traffic at multiple steps and differentially affects the surface delivery of apically and basolaterally destined proteins in polarized cells. Two phosphatidylinositol 4-kinases (PI4Ks) have been localized to the Golgi complex in mammalian cells, type III PI4Kbeta (PI4KIIIbeta) and type II PI4Kalpha (PI4KIIalpha). Here we report that PI4KIIIbeta and PI4KIIalpha localize to discrete subcompartments of the Golgi complex in Madin-Darby canine kidney (MDCK) cells. PI4KIIIbeta was enriched in early Golgi compartments, whereas PI4KIIalpha colocalized with markers of the trans-Golgi network (TGN). To understand the temporal and spatial control of PI4P generation across the Golgi complex, we quantitated the steady state distribution of a fluorescent PI4P-binding domain relative to cis/medial Golgi and TGN markers in transiently transfected MDCK cells. The density of the signal from this PI4P reporter was roughly 2-fold greater in the early Golgi compartments compared with that of the TGN. Furthermore, this ratio could be modulated in vivo by overexpression of catalytically inactive PI4KIIIbeta and PI4KIIalpha or in vitro by the PI4KIIIbeta inhibitor wortmannin. Our data suggest that both PI4KIIIbeta and PI4KIIalpha contribute to the compartmental regulation of PI4P synthesis within the Golgi complex. We discuss our results with respect to the kinetic effects of modulating PI4K activity on polarized biosynthetic traffic in MDCK cells.

Interaction of neuronal calcium sensor-1 and ADP-ribosylation factor 1 allows bidirectional control of phosphatidylinositol 4-kinase beta and trans-Golgi network-plasma membrane traffic

We have identified a novel Ca(2+)-dependent interaction between neuronal calcium sensor-1 (NCS-1) and the GTPase ARF1. Both of these proteins are localized to the Golgi complex, and both regulate phosphatidylinositol 4-kinase IIIbeta (PI(4)Kbeta). Spatial and temporal control of phosphatidylinositol 4-phosphate levels through activation of PI(4)Kbeta is important for the recruitment of trafficking complexes to the trans-Golgi network (TGN) and vesicular traffic from this organelle. The NCS-1-ARF1 interaction and its specificity have been demonstrated through in vitro binding assays, in vitro enzyme assay, and through functional cellular assays. We show that NCS-1 can exert bidirectional effects to activate PI(4)Kbeta on its own or inhibit the activation by ARF1. NCS-1 was shown to modulate the effects of expression of ARF mutants that disrupt Golgi morphology and to recruit GDP-loaded ARF to the Golgi complex in a Ca(2+)-dependent manner. We demonstrate antagonist effects of NCS-1 and ARF on constitutive and regulated exocytosis. The NCS-1-ARF1 interaction provides evidence for functional cross-talk between Ca(2+)-dependent and ARF-dependent pathways in TGN to plasma membrane traffic.

ADP-ribosylation factor 1-independent protein sorting and export from the trans-Golgi network

Polarized epithelial cells efficiently sort newly synthesized apical and basolateral proteins into distinct transport carriers that emerge from the trans-Golgi network (TGN), and this sorting is recapitulated in nonpolarized cells. While the targeting signals of basolaterally destined proteins are generally cytoplasmically disposed, apical sorting signals are not typically accessible to the cytosol, and the transport machinery required for segregation and export of apical cargo remains largely unknown. Here we investigated the molecular requirements for TGN export of the apical marker influenza hemagglutinin (HA) in HeLa cells using an in vitro reconstitution assay. HA was released from the TGN in intact membrane-bound compartments, and export was dependent on addition of an ATP-regenerating system and exogenous cytosol. HA release was inhibited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as well as under conditions known to negatively regulate apical transport in vivo, including expression of the acid-activated proton channel influenza M2. Interestingly, release of HA was unaffected by depletion of ADP-ribosylation factor 1, a small GTPase that has been implicated in the recruitment of all known adaptors and coat proteins to the Golgi complex. Furthermore, regulation of HA release by GTPgammaS or M2 expression was unaffected by cytosolic depletion of ADP-ribosylation factor 1, suggesting that HA sorting remains functionally intact in the absence of the small GTPase. These data suggest that TGN sorting and export of influenza HA does not require classical adaptors involved in the formation of other classes of exocytic carriers and thus appears to proceed via a novel mechanism.

The role of the Nir/rdgB protein family in membrane trafficking and cytoskeleton remodeling

The Nir/rdgB family of proteins has been identified in a variety of eukaryotic organisms, ranging from worms to mammals. The Drosophila retinal degeneration B (rdgB), a protein that is required for photoreceptor cell viability and light response, was the first to be identified. It consists an amino-terminal phosphatidylinositol (PI)-transfer domain and was proposed to play an essential role in photoreceptor membrane renewal and biogenesis. The other Nir/rdgB family members are functionally and structurally related to the Drosophila homolog and are implicated in regulation of lipid trafficking, metabolism, and signaling. Recent advances have revealed that Nir/rdgB proteins are also involved in regulation of cytoskeletal elements. Thus, these family members exert a broad spectrum of cellular functions and are involved in multiple cellular processes. The physiological functions of these closely related proteins are described in this review.

FAPPs control Golgi-to-cell-surface membrane traffic by binding to ARF and PtdIns(4)P

The molecular mechanisms underlying the formation of carriers trafficking from the Golgi complex to the cell surface are still ill-defined; nevertheless, the involvement of a lipid-based machinery is well established. This includes phosphatidylinositol 4-phosphate (PtdIns(4)P), the precursor for phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). In yeast, PtdIns(4)P exerts a direct role, however, its mechanism of action and its targets in mammalian cells remain uncharacterized. We have identified two effectors of PtdIns(4)P, the four-phosphate-adaptor protein 1 and 2 (FAPP1 and FAPP2). Both proteins localize to the trans-Golgi network (TGN) on nascent carriers, and interact with PtdIns(4)P and the small GTPase ADP-ribosylation factor (ARF) through their plekstrin homology (PH) domain. Displacement or knockdown of FAPPs inhibits cargo transfer to the plasma membrane. Moreover, overexpression of FAPP-PH impairs carrier fission. Therefore, FAPPs are essential components of a PtdIns(4)P- and ARF-regulated machinery that controls generation of constitutive post-Golgi carriers.

Phosphatidylinositol 4-kinasebeta is critical for functional association of rab11 with the Golgi complex

Phosphatidylinositol 4-kinasebeta (PI4Kbeta) plays an essential role in maintaining the structural integrity of the Golgi complex. In a search for PI4Kbeta-interacting proteins, we found that PI4Kbeta specifically interacts with the GTP-bound form of the small GTPase rab11. The PI4Kbeta-rab11 interaction is of functional significance because inhibition of rab11 binding to PI4Kbeta abolished the localization of rab11 to the Golgi complex and significantly inhibited transport of vesicular stomatitis virus G protein from the Golgi complex to the plasma membrane. We propose that a novel function of PI4Kbeta is to act as a docking protein for rab11 in the Golgi complex, which is important for biosynthetic membrane transport from the Golgi complex to the plasma membrane.

Neuronal Calcium Sensor-1 and Phosphatidylinositol 4-Kinase β Regulate IgE Receptor-Triggered Exocytosis in Cultured Mast Cells

We examined the possible occurrence and function of neuronal Ca(2+) sensor 1 (NCS-1/frequenin) in the mast cell line rat basophilic leukemia, RBL-2H3. This protein has been implicated in the control of neurosecretion from dense core granules in neuronal cells as well as in the control of constitutive secretory pathways in both yeast and mammalian cells. We show that RBL-2H3 cells, secretory cells of the immune system, endogenously express the 22-kDa NCS-1 protein as well as an immune-related 50-kDa protein. Both proteins associate in vivo with phosphatidylinositol 4-kinase beta (PI4Kbeta) and colocalize with the enzyme in the Golgi region. We show further that overexpression of NCS-1 in RBL-2H3 cells stimulates the catalytic activity of PI4Kbeta, increases IgE receptor (FcepsilonRI)-triggered hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), and stimulates FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Conversely, expression of a kinase-dead mutant of PI4Kbeta reduces PI4Kbeta activity, decreases FcepsilonRI-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis, and blocks FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Our results indicate that PI(4)P, produced by the Golgi-localized PI4Kbeta, is the rate-limiting factor in the synthesis of the pool of PI(4,5)P(2) that serves as substrate for the generation of lipid-derived second messengers in FcepsilonRI-triggered cells. We conclude that NCS-1 is involved in the control of regulated exocytosis in nonneural cells, where it contributes to stimulus-secretion coupling by interacting with PI4Kbeta and positive regulation of its activity.

Isolation and functional characterization of the C-terminus of rice phosphatidylinositol 4-kinase in vitro

A partial rice (Oryza sativa L.) cDNA clone, OsPI4K1c, was isolated through screening of a cDNA library constructed from tillering materials. OsPI4K1c encoded a peptide of 608 amino acids with a calculated molecular mass of 68.4 kDa. The OsPI4K1c peptide shared high homology and possessed the highly conserved domains present in most isolated cloned PI4-kinases, i.e. a lipid kinase unique (LKU) domain and a catalytic (CAT) domain. A region with similarity to pleckstrin homology (PH) domain was present in OsPI4K1c as well. Further comparison with genomic sequences in databases revealed that OsPI4K1c is located at the 3'-end of a putative rice PI 4-kinase coding gene OsPI4K1, and its coding region corresponded to the C-terminal half of OsPI4K1 protein. Twelve exons (49-562 bp in size) and 11 introns (77-974 bp in size) were identified in OsPI4K1c. The recombinant protein expressed in Escherichia coli phosphorylates phosphatidylinositol at the D4 position of the inositol ring. OsPI4K1 transcript levels were detected in a low but constitutive manner in shoot, stem, leaf, spike and root tissues and did not change upon treatment with different hormones, calcium and jasmonic acid (JA). However, treatment with salicylic acid (SA) elevated the mRNA level of the OsPI4K1 gene, which suggested the involvement of OsPI4K1 in wounding responses.

The Saccharomyces cerevisiae LSB6 gene encodes phosphatidylinositol 4-kinase activity

The LSB6 gene product was identified from the Saccharomyces Genome Data Base (locus YJL100W) as a putative member of a novel type II phosphatidylinositol (PI) 4-kinase family. Cell extracts lacking the LSB6 gene had a reduced level of PI 4-kinase activity. In addition, multicopy plasmids containing the LSB6 gene directed the overexpression of PI 4-kinase activity in cell extracts of wild-type cells, in an lsb6Delta mutant, in a pik1(ts) stt4(ts) double mutant, and in an pik1(ts) stt4(ts) lsb6Delta triple mutant. The heterologous expression of the S. cerevisiae LSB6 gene in Escherichia coli resulted in the expression of a protein that possessed PI 4-kinase activity. Although the lsb6Delta mutant did not exhibit a growth phenotype and failed to exhibit a defect in phosphoinositide synthesis in vivo, the overexpression of the LSB6 gene could partially suppress the lethal phenotype of an stt4Delta mutant defective in the type III STT4-encoded PI 4-kinase indicating that Lsb6p functions as a PI 4-kinase in vivo. Lsb6p was localized to the membrane fraction of the cell, and when overexpressed, GFP-tagged Lsb6p was observed on both the plasma membrane and the vacuole membrane. The enzymological properties (pH optimum, dependence on magnesium or manganese as a cofactor, the dependence of activity on Triton X-100, the dependence on the PI surface concentration, and temperature sensitivity) of the LSB6-encoded enzyme were very similar to the membrane-associated 55-kDa PI 4-kinase previously purified from S. cerevisiae.

Phosphoinositides and the golgi complex

Phosphoinositides act as precursors of second messengers and membrane ligands for protein modules. Specific lipid kinases and phosphatases are located and differentially regulated in cell organelles, generating a non-uniform distribution of phosphoinositides. Although it is not clear whether and how the phosphoinositide pools are integrated, it is certain that they locally control fundamental processes, including membrane trafficking. This applies to the Golgi complex, where a direct, central role of the phosphatidylinositol 4,5-bisphosphate precursor phosphatidylinositol 4-phosphate has recently been reported.