The human erythrocyte contains two forms of phosphatidylinositol-4-phosphate 5-kinase which are differentially active toward membranes

Functional molecular evolution of a GTP sensing kinase: PI5P4Kβ

Over 4 billion years of evolution, multiple mutations, including nucleotide substitutions, gene and genome duplications and recombination, have established de novo genes that translate into proteins with novel properties essential for high-order cellular functions. However, molecular processes through which a protein evolutionarily acquires a novel function are mostly speculative. Recently, we have provided evidence for a potential evolutionary mechanism underlying how, in mammalian cells, phosphatidylinositol 5-phosphate 4-kinase β (PI5P4Kβ) evolved into a GTP sensor from ATP-utilizing kinase. Mechanistically, PI5P4Kβ has acquired the guanine efficient association (GEA) motif by mutating its nucleotide base recognition sequence, enabling the evolutionary transition from an ATP-dependent kinase to a distinct GTP/ATP dual kinase with its KM for GTP falling into physiological GTP concentrations-the genesis of GTP sensing activity. Importantly, the GTP sensing activity of PI5P4Kβ is critical for the manifestation of cellular metabolism and tumourigenic activity in the multicellular organism. The combination of structural, biochemical and biophysical analyses used in our study provides a novel framework for analysing how a protein can evolutionarily acquire a novel activity, which potentially introduces a critical function to the cell.

Hit Identification Driven by Combining Artificial Intelligence and Computational Chemistry Methods: A PI5P4K-β Case Study

Computer-aided drug design (CADD), especially artificial intelligence-driven drug design (AIDD), is increasingly used in drug discovery. In this paper, a novel and efficient workflow for hit identification was developed within the ID4Inno drug discovery platform, featuring innovative artificial intelligence, high-accuracy computational chemistry, and high-performance cloud computing. The workflow was validated by discovering a few potent hit compounds (best IC50 is ∼0.80 μM) against PI5P4K-β, a novel anti-cancer target. Furthermore, by applying the tools implemented in ID4Inno, we managed to optimize these hit compounds and finally obtained five hit series with different scaffolds, all of which showed high activity against PI5P4K-β. These results demonstrate the effectiveness of ID4inno in driving hit identification based on artificial intelligence, computational chemistry, and cloud computing.

Nuclear Phosphoinositides as Key Determinants of Nuclear Functions

Polyphosphoinositides (PPIns) are signalling messengers representing less than five per cent of the total phospholipid concentration within the cell. Despite their low concentration, these lipids are critical regulators of various cellular processes, including cell cycle, differentiation, gene transcription, apoptosis and motility. PPIns are generated by the phosphorylation of the inositol head group of phosphatidylinositol (PtdIns). Different pools of PPIns are found at distinct subcellular compartments, which are regulated by an array of kinases, phosphatases and phospholipases. Six of the seven PPIns species have been found in the nucleus, including the nuclear envelope, the nucleoplasm and the nucleolus. The identification and characterisation of PPIns interactor and effector proteins in the nucleus have led to increasing interest in the role of PPIns in nuclear signalling. However, the regulation and functions of PPIns in the nucleus are complex and are still being elucidated. This review summarises our current understanding of the localisation, biogenesis and physiological functions of the different PPIns species in the nucleus.

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

The GTP responsiveness of PI5P4Kβ evolved from a compromised trade-off between activity and specificity

Unlike most kinases, phosphatidylinositol 5-phosphate 4-kinase β (PI5P4Kβ) utilizes GTP as a physiological phosphate donor and regulates cell growth under stress (i.e., GTP-dependent stress resilience). However, the genesis and evolution of its GTP responsiveness remain unknown. Here, we reveal that PI5P4Kβ has acquired GTP preference by generating a short dual-nucleotide-recognizing motif called the guanine efficient association (GEA) motif. Comparison of nucleobase recognition with 660 kinases and 128 G proteins has uncovered that most kinases and PI5P4Kβ use their main-chain atoms for adenine recognition, while the side-chain atoms are required for guanine recognition. Mutational analysis of the GEA motif revealed that the acquisition of GTP reactivity is accompanied by an extended activity toward inosine triphosphate (ITP) and xanthosine triphosphate (XTP). Along with the evolutionary analysis data that point to strong negative selection of the GEA motif, these results suggest that the GTP responsiveness of PI5P4Kβ has evolved from a compromised trade-off between activity and specificity, underpinning the development of the GTP-dependent stress resilience.

Crucial Players for Inter-Organelle Communication: PI5P4Ks and Their Lipid Product PI-4,5-P2 Come to the Surface

While organelles are individual compartments with specialized functions, it is becoming clear that organellar communication is essential for maintaining cellular homeostasis. This cooperation is carried out by various interactions taking place on the membranes of organelles. The membranes themselves contain a multitude of proteins and lipids that mediate these connections and one such class of molecules facilitating these relations are the phospholipids. There are several phospholipids, but the focus of this perspective is on a minor group called the phosphoinositides and specifically, phosphatidylinositol 4,5-bisphosphate (PI-4,5-P₂). This phosphoinositide, on intracellular membranes, is largely generated by the non-canonical Type II PIPKs, namely, Phosphotidylinositol-5-phosphate-4-kinases (PI5P4Ks). These evolutionarily conserved enzymes are emerging as key stress response players in cells. Further, PI5P4Ks have been shown to modulate pathways by regulating organelle crosstalk, revealing roles in preserving metabolic homeostasis. Here we will attempt to summarize the functions of the PI5P4Ks and their product PI-4,5-P₂ in facilitating inter-organelle communication and how they impact cellular health as well as their relevance to human diseases.

Expanding role of PI5P4Ks in cancer: A promising druggable target

Cancer cells are challenged by a myriad of microenvironmental stresses, and it is their ability to efficiently adapt to the constantly changing nutrient, energy, oxidative, and/or immune landscape that allows them to survive and proliferate. Such adaptations, however, result in distinct vulnerabilities that are attractive therapeutic targets. Phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks) are a family of druggable stress-regulated phosphoinositide kinases that become conditionally essential as a metabolic adaptation, paving the way to targeting cancer cell dependencies. Further, PI5P4Ks have a synthetic lethal interaction with the tumor suppressor p53, the loss of which is one of the most prevalent genetic drivers of malignant transformation. PI5P4K's emergence as a crucial axis in the expanding landscape of phosphoinositide signaling in cancer has already stimulated the development of specific inhibitors. Thus, a better understanding of the biology of the PI5P4Ks will allow for targeted and effective therapeutic interventions. Here, we attempt to summarize the mounting roles of the PI5P4Ks in cancer, including evidence that targeting them is a therapeutic vulnerability and promising next-in-line treatment for multiple cancer subtypes.

Pharmacological inhibition of PI5P4Kα/β disrupts cell energy metabolism and selectively kills p53-null tumor cells

Most human cancer cells harbor loss-of-function mutations in the p53 tumor suppressor gene. Genetic experiments have shown that phosphatidylinositol 5-phosphate 4-kinase α and β (PI5P4Kα and PI5P4Kβ) are essential for the development of late-onset tumors in mice with germline p53 deletion, but the mechanism underlying this acquired dependence remains unclear. PI5P4K has been previously implicated in metabolic regulation. Here, we show that inhibition of PI5P4Kα/β kinase activity by a potent and selective small-molecule probe disrupts cell energy homeostasis, causing AMPK activation and mTORC1 inhibition in a variety of cell types. Feedback through the S6K/insulin receptor substrate (IRS) loop contributes to insulin hypersensitivity and enhanced PI3K signaling in terminally differentiated myotubes. Most significantly, the energy stress induced by PI5P4Kαβ inhibition is selectively toxic toward p53-null tumor cells. The chemical probe, and the structural basis for its exquisite specificity, provide a promising platform for further development, which may lead to a novel class of diabetes and cancer drugs.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Lipid kinases PIP5K7 and PIP5K9 are required for polyamine-triggered K+ efflux in Arabidopsis roots

Polyamines, such as putrescine, spermidine and spermine (Spm), are low-molecular-weight polycationic molecules present in all living organisms. Despite their implication in plant cellular processes, little is known about their molecular mode of action. Here, we demonstrate that polyamines trigger a rapid increase in the regulatory membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ), and that this increase is required for polyamine effects on K⁺ efflux in Arabidopsis roots. Using in vivo ³² P_i -labelling of Arabidopsis seedlings, low physiological (μm) concentrations of Spm were found to promote a rapid PIP₂ increase in roots that was time- and dose-dependent. Confocal imaging of a genetically encoded PIP₂ biosensor revealed that this increase was triggered at the plasma membrane. Differential ³² P_i -labelling suggested that the increase in PIP₂ was generated through activation of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) activity rather than inhibition of a phospholipase C or PIP₂ 5-phosphatase activity. Systematic analysis of transfer DNA insertion mutants identified PIP5K7 and PIP5K9 as the main candidates involved in the Spm-induced PIP₂ response. Using non-invasive microelectrode ion flux estimation, we discovered that the Spm-triggered K⁺ efflux response was strongly reduced in pip5k7 pip5k9 seedlings. Together, our results provide biochemical and genetic evidence for a physiological role of PIP₂ in polyamine-mediated signalling controlling K⁺ flux in plants.

PIP4K2B: Coupling GTP Sensing to PtdIns5P Levels to Regulate Tumorigenesis

Although guanine nucleotides are essential for cell growth, how their levels are sensed in mammalian cells is unknown. Sumita et al. show that PIP4K2B, a phosphoinositide kinase, is a molecular sensor that transduces changes in GTP into changes in the levels of the phosphoinositide PtdIns5P to modulate tumour cell growth.

Synergistic modulation of KCNQ1/KCNE1 K(+) channels (IKs) by phosphatidylinositol 4,5-bisphosphate (PIP2) and [ATP]i

Cardiac KCNQ1/KCNE1 channels (IKs) are dependent on the concentration of membrane phosphatidylinositol-4,5-bisphosphate (PIP2) and on cytosolic ATP by two distinct mechanisms. In this study we measured IKs and FRET between PH-PLCδ-based fluorescent PIP2 sensors in a stable KCNQ1/KCNE1 CHO cell line. Effects of activating either a muscarinic M3 receptor or the switchable phosphatase Ci-VSP on IKs were analyzed. Recovery of IKs from inhibition induced by muscarinic stimulation was incomplete despite full PIP2 resynthesis. Recovery of IKs was completely suppressed under ATP-free conditions, but partially restored by the ATP analog AMP-PCP, providing evidence that depletion of intracellular ATP inhibits IKs independent of PIP2-depletion. Simultaneous patch-clamp and FRET measurements in cells co-expressing Ci-VSP and the PIP2-FRET sensor revealed a component of IKs inhibition directly related to dynamic PIP2-depletion. A second component of inhibition was independent of acute changes in PIP2 and could be mimicked by ATP-free pipette solution, suggesting that it results from intracellular ATP-depletion. The reduction of intracellular ATP upon Ci-VSP activation appears to be independent of its activity as a phosphoinositide phosphatase. Our data demonstrate that ATP-depletion slowed IKs activation but had no short-term effect on PIP2 regeneration, suggesting that impaired PIP2-resynthesis cannot account for the rapid IKs inhibition by ATP-depletion. Furthermore, the second component of IKs inhibition by Ci-VSP was reduced by AMP-PCP in the pipette filling solution, indicating that direct binding of ATP to the KCNQ1/KCNE1 complex is required for voltage activation of IKs. We suggest that fluctuations of the cellular metabolic state regulate IKs in parallel with Gq-coupled PLC activation and PIP2-depletion.

PIP4K and the role of nuclear phosphoinositides in tumour suppression

Phosphatidylinositol-5-phosphate (PtdIns5P)-4-kinases (PIP4Ks) are stress-regulated lipid kinases that phosphorylate PtdIns5P to generate PtdIns(4,5)P₂. There are three isoforms of PIP4Ks: PIP4K2A, 2B and 2C, which localise to different subcellular compartments with the PIP4K2B isoform being localised predominantly in the nucleus. Suppression of PIP4K expression selectively prevents tumour cell growth in vitro and prevents tumour development in mice that have lost the tumour suppressor p53. p53 is lost or mutated in over 70% of all human tumours. These studies suggest that inhibition of PIP4K signalling constitutes a novel anti-cancer therapeutic target. In this review we will discuss the role of PIP4K in tumour suppression and speculate on how PIP4K modulates nuclear phosphoinositides (PPIns) and how this might impact on nuclear functions to regulate cell growth. This article is part of a Special Issue entitled Phosphoinositides.

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.

PtdIns5P and Pin1 in oxidative stress signaling

Oxidative signaling is important in cellular health, involved in aging and contributes to the development of several diseases such as cancer, neurodegeneration and diabetes. Correct management of reactive oxygen species (ROS) prevents oxidative stress within cells and is imperative for cellular wellbeing. A key pathway that is regulated by oxidative stress is the activation of proline-directed stress kinases (p38, JNK). Phosphorylation induced by these kinases is often translated into cellular outcome through the recruitment of the prolyl-isomerase Pin1. Pin1 binds to phosphorylated substrates using its WW-domain and can induce conformational changes in the target protein through its prolyl-isomerase activity. We show that exposure of cells to UV irradiation or hydrogen peroxide (H₂O₂), induces the synthesis of the phosphoinositide second messenger PtdIns5P in part by inducing the interaction between phosphatidylinositol-5-phosphate 4-kinase (PIP4K) enzymes that remove PtdIns5P, with Pin1. In response to H₂O₂ exposure, Murine Embryonic Fibroblasts (MEFs) derived from Pin1⁻/⁻ mice showed increased cell viability and an increased abundance of PtdIns5P compared to wild-type MEFs. Decreasing the levels of PtdIns5P in Pin1⁻/⁻ MEFs decreased both their viability in response to H₂O₂ exposure and the expression of genes required for cellular ROS management. The decrease in the expression of these genes manifested itself in the increased accumulation of cellular ROS. These data strongly argue that PtdIns5P acts as a stress-induced second messenger that can calibrate how cells manage ROS.

PIP kinases from the cell membrane to the nucleus

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a membrane bound lipid molecule with capabilities to affect a wide array of signaling pathways to regulate very different cellular processes. PIP(2) is used as a precursor to generate the second messengers PIP(3), DAG and IP(3), indispensable molecules for signaling events generated by membrane receptors. However, PIP(2) can also directly regulate a vast array of proteins and is emerging as a crucial messenger with the potential to distinctly modulate biological processes critical for both normal and pathogenic cell physiology. PIP(2) directly associates with effector proteins via unique phosphoinositide binding domains, altering their localization and/or enzymatic activity. The spatial and temporal generation of PIP(2) synthesized by the phosphatidylinositol phosphate kinases (PIPKs) tightly regulates the activation of receptor signaling pathways, endocytosis and vesicle trafficking, cell polarity, focal adhesion dynamics, actin assembly and 3' mRNA processing. Here we discuss our current understanding of PIPKs in the regulation of cellular processes from the plasma membrane to the nucleus.

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.

Light-induced tyrosine phosphorylation of rod outer segment membrane proteins regulate the translocation, membrane binding and activation of type II α phosphatidylinositol-5-phosphate 4-kinase

Type II phosphatidylinositol 5-phosphate 4-kinase (PIPKIIα) catalyzes the synthesis of phosphatidylinositol-4,5-bisphosphate (PI-4,5-P(2)), an essential lipid second messenger that may be involved in the regulation of phototransduction, neuroprotection, and morphogenesis in the vertebrate retina. Here we report that in rodent and transgenic frogs, the light-mediated activity and membrane binding of PIPKIIα in rod outer segments (ROS) is dependent on tyrosine phosphorylation of ROS proteins. The greater type II α PIP kinase activity in the light-adapted ROS membrane results from light-driven translocation of PIPKIIα from the rod inner segment to ROS, and subsequent binding to the ROS membrane, thus improving access of the kinase to its lipid substrates. These results indicate a novel mechanism of light regulation of the PIPKIIα activity in photoreceptors, and suggest that the greater PIPKIIα activity in light-adapted animals and the resultant accumulation of PI-4,5-P(2) within the ROS membrane may be important for the function of photoreceptor cells.

Nuclear phosphoinositides: a signaling enigma wrapped in a compartmental conundrum

While the presence of phosphoinositides in the nuclei of eukaryotes and the identity of the enzymes responsible for their metabolism have been known for some time, their functions in the nucleus are only now emerging. This is illustrated by the recent identification of effectors for nuclear phosphoinositides. Like the cytosolic phosphoinositide signaling pathway, nuclear phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) is at the center of the pathway and acts both as a messenger and as a precursor for many additional messengers. Here, recent advances in the understanding of nuclear phosphoinositide signaling and its functions are reviewed with an emphasis on PI4,5P(2) and its role in gene expression. The compartmentalization of nuclear phosphoinositide phosphates (PIP(n)) remains a mystery, but emerging evidence suggests that phosphoinositides occupy several functionally distinct compartments.

A novel testis ubiquitin-binding protein gene arose by exon shuffling in hominoids

Most new genes arise by duplication of existing gene structures, after which relaxed selection on the new copy frequently leads to mutational inactivation of the duplicate; only rarely will a new gene with modified function emerge. Here we describe a unique mechanism of gene creation, whereby new combinations of functional domains are assembled at the RNA level from distinct genes, and the resulting chimera is then reverse transcribed and integrated into the genome by the L1 retrotransposon. We characterized a novel gene, which we termed PIP5K1A and PSMD4-like (PIPSL), created by this mechanism from an intergenic transcript between the phosphatidylinositol-4-phosphate 5-kinase (PIP5K1A) and the 26S proteasome subunit (PSMD4) genes in a hominoid ancestor. PIPSL is transcribed specifically in the testis both in humans and chimpanzees, and is post-transcriptionally repressed by independent mechanisms in these primate lineages. The PIPSL gene encodes a chimeric protein combining the lipid kinase domain of PIP5K1A and the ubiquitin-binding motifs of PSMD4. Strong positive selection on PIPSL led to its rapid divergence from the parental genes PIP5K1A and PSMD4, forming a chimeric protein with a distinct cellular localization and minimal lipid kinase activity, but significant affinity for cellular ubiquitinated proteins. PIPSL is a tightly regulated, testis-specific novel ubiquitin-binding protein formed by an unusual exon-shuffling mechanism in hominoid primates and represents a key example of rapid evolution of a testis-specific gene.

PIP5KI gamma is required for cardiovascular and neuronal development

All eukaryotic cells contain the phospholipid phosphatidylinositol 4, 5-bisphosphate (PIP2) that serves multiple roles in signal transduction cascades. Type I phosphatidylinositol-4-phosphate 5-kinase (PIP5KI) catalyzes the synthesis of PIP2 by phosphorylating phosphatidylinositol 4 phosphate. Although the classical isoforms of PIP5KI (designated as alpha, beta, and gamma) all generate the same phospholipid product, they have significantly dissimilar primary structures and expression levels in different tissues, and they appear to localize within different compartments within the cell. Therefore, it appears likely that PIP5KI isoforms have overlapping, but not identical, functions. Here we show that targeted disruption of PIP5KIgamma causes widespread developmental and cellular defects. PIP5KIgamma-null embryos have myocardial developmental defects associated with impaired intracellular junctions that lead to heart failure and extensive prenatal lethality at embryonic day 11.5 of development. Loss of PIP5KIgamma also results in neural tube closure defects that were associated with impaired PIP2 production, adhesion junction formation, and neuronal cell migration. These data, along with those of other PIP5KI isoforms, indicate that individual PIP5KI isoenzymes fulfill specific roles in embryonic development.

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.

Neurotensin receptor binding and neurotensin-induced growth signaling in prostate cancer PC3 cells are sensitive to metabolic stress

Neurotensin (NT) stimulates the proliferation of prostate cancer PC3 cells, which express high levels of its G protein-coupled receptor NTS1. To shed light on mechanisms that might serve to coordinate mitogenic responses to metabolic status, we studied the effects of metabolic inhibitors on NTS1 function. We also related these effects to cellular ATP levels and to the activation of AMP-activated protein kinase (AMPK). Glycolytic and mitochondrial inhibitors, at concentrations that reduced cellular ATP levels, altered NT binding to the cells, inhibited NT-induced inositol phosphate formation, and inhibited NT-induced DNA synthesis. For eight of the nine inhibitors, the potencies to alter NT receptor function correlated to the potencies to decrease cellular ATP levels. In keeping with its known role to oppose metabolic stress, AMPK was activated by the metabolic inhibitors. Accordingly, the AMPK activator AICAR elevated cellular ATP levels and produced effects on NTS1 function that were opposite to those for the metabolic inhibitors. These results indicate that metabolic stress inhibited NTS1 function by a mechanism that involved a fall in cellular ATP levels and that was opposed by activation of AMPK. In a broader context, these findings are compatible with the idea that one means by which cells might coordinate mitogenic signaling to metabolic status could involve changes in growth factor receptor function.

Polyamines, PI(4,5)P2, and actin polymerization

Spermine (SPM) and spermidine (SPD) activate isolated phosphatidylinositol-4-phosphate 5-kinases (PI(4)P5K), enzymes that convert phosphatidylinositol-4-phosphate to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). PI(4,5)P2 formation is known to be involved in cellular actin reorganization and motility, functions that are also influenced by polyamines. It has not been proven that endogenous polyamines can control inositol phospholipid metabolism. We evoked large decreases in SPD and putrescine (PUT) contents in HL60 cells, using the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine (DFMO), which resulted in decreases in PI(4,5)P2 content per cell and inositol phosphate formation to 76.9 +/- 3.5% and 81.5 +/- 4.0% of control, respectively. Accurately reversing DFMO-evoked decreases in SPD content by incubating cells with exogenous SPD for 20 min rescued these decreases. DFMO treatment and SPD rescues also changed the ratio of total cellular PI(4,5)P2 to PIP suggesting involvement of a SPD-sensitive PI(4)P5K. PUT and SPM were not involved in DFMO-evoked changes in cellular PI(4,5)P2 contents. In DFMO-treated HL60 cells, the percent of total actin content that was filamentous was decreased to 59.1 +/- 5.8% of that measured in paired control HL60 cells, a finding that was rescued following reversal of DFMO-evoked decreases in SPD and PI(4,5)P2 contents. In slowly proliferating DMSO-differentiated HL60 cells, inositol phospholipid metabolism was uncoupled from SPD control. We conclude: in rapidly proliferating HL60 cells, but not in slowly proliferating differentiated HL60 cells, there are endogenous SPD-sensitive PI(4,5)P2 pools, probably formed via SPD-sensitive PI(4)P5K, that likely control actin polymerization.

Movin' on up: the role of PtdIns(4,5)P2 in cell migration

Cell migration requires the coordination of many biochemical events, including cell-matrix contact turnover and cytoskeletal restructuring. Recent advances further implicate phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P(2)] in the control of these events. Many proteins that are crucial to the assembly of the migration machinery are regulated by PtdIns(4,5)P(2). Coordinated synthesis of PtdIns(4,5)P(2) at these sites is dependent on the precise targeting of the type I phosphatidylinositol phosphate kinases (PIPKs). Two PIPKI isoforms target to, and generate, PtdIns(4,5)P(2) at membrane ruffles and focal adhesions during cell migration. Here, we discuss our current understanding of PtdIns(4,5)P(2) in the regulation of cell responses to migratory stimuli and how the migrating cell controls PtdIns(4,5)P(2) availability.

Regulation of type II PIP kinase by PKD phosphorylation

The type II PIP kinases phosphorylate the poorly understood inositol lipid PtdIns5P, producing the multi-functional lipid product PtdIns(4,5)P(2). To investigate the regulation of these enzymes by phosphorylation, we partially purified a protein kinase from pig platelets that phosphorylated type IIalpha PIP kinase on an activation loop threonine residue, T376. Pharmacological studies suggested this protein kinase was protein kinase D (PKD), and in vitro experiments confirmed this identification. A phospho-specific antibody was developed and used to demonstrate phosphorylation of T376 in living cells, and its enhancement under conditions in which PKD was activated. Although we were unable to determine the effects of phosphorylation on PIP kinase activity directly, mutation of T376 to aspartate significantly inhibited enzyme activity. We conclude that the type II PIP kinases are physiological targets for PKD phosphorylation, and that this modification is likely to regulate inositol lipid turnover by inhibition of these lipid kinases.

Characterization of the megakaryocyte demarcation membrane system and its role in thrombopoiesis

To produce blood platelets, megakaryocytes elaborate proplatelets, accompanied by expansion of membrane surface area and dramatic cytoskeletal rearrangements. The invaginated demarcation membrane system (DMS), a hallmark of mature cells, has been proposed as the source of proplatelet membranes. By direct visualization of labeled DMS, we demonstrate that this is indeed the case. Late in megakaryocyte ontogeny, the DMS gets loaded with PI-4,5-P(2), a phospholipid that is confined to plasma membranes in other cells. Appearance of PI-4,5-P(2) in the DMS occurs in proximity to PI-5-P-4-kinase alpha (PIP4Kalpha), and short hairpin (sh) RNA-mediated loss of PIP4Kalpha impairs both DMS development and expansion of megakaryocyte size. Thus, PI-4,5-P(2) is a marker and possibly essential component of internal membranes. PI-4,5-P(2) is known to promote actin polymerization by activating Rho-like GTPases and Wiskott-Aldrich syndrome (WASp) family proteins. Indeed, PI-4,5-P(2) in the megakaryocyte DMS associates with filamentous actin. Expression of a dominant-negative N-WASp fragment or pharmacologic inhibition of actin polymerization causes similar arrests in proplatelet formation, acting at a step beyond expansion of the DMS and cell mass. These observations collectively suggest a signaling pathway wherein PI-4,5-P(2) might facilitate DMS development and local assembly of actin fibers in preparation for platelet biogenesis.

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.

Dibasic amino acid residues at the carboxy-terminal end of kinase homology domain participate in the plasma membrane localization and function of phosphatidylinositol 5-kinase gamma

Type I phosphatidylinositol 4-phosphate (PI(4)P) 5-kinases (PIP5Ks) catalyze the synthesis of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), an essential lipid molecule involved in various cellular processes such as regulation of actin cytoskeleton and membrane traffic. The protein localizes to the plasma membrane where its activity has been shown to be regulated by small GTPase ARFs and/or phosphatidic acid. Deletion analysis of amino- or carboxy-terminal sequences of PIP5Kgamma fused with EGFP demonstrated that the presence of central kinase homology domain (KHD), a 380 amino acid-long region highly conserved among PIP5K family, was necessary and sufficient for the plasma membrane localization of PIP5Kgamma. Particularly, the dibasic Arg-Lys sequence located at the carboxy-terminal end of KHD was shown to be crucial for the plasma membrane targeting of PIP5Kgamma, since the deletion or charge-reversal mutation of this dibasic sequence resulted in the mislocalization of the protein to the cytoplasm. Mislocalized mutants also failed to complement the temperature-sensitive growth of Saccharomyces cerevisiae mss4-1 mutant defective in PIP5K function. The presence of dibasic residues at the C-terminal end of KHD was conserved among mammalian as well as invertebrate PIP5K family members, but not in the type II PIPKs that are not targeted to the plasma membrane, suggesting that the conserved dibasic motif provides a mechanism essential for the proper localization and cellular function of PIP5Ks.

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.

Reassessment of the effects of cycloheximide on mossy fiber sprouting and epileptogenesis in the pilocarpine model of temporal lobe epilepsy

A feature of animal models of temporal lobe epilepsy and the human disorder is hippocampal sclerosis and Timm stain in the inner molecular layer (IML) of the dentate gyrus, which represents synaptic reorganization and may be important in epileptogenesis. We reassessed the hypothesis that pre-treatment with cycloheximide (CHX) prevents Timm staining in the IML following pilocarpine (PILO)-induced status epilepticus (a multifocal model of temporal lobe epilepsy), but allows epileptogenesis (i.e., chronic spontaneous seizures) after a latent period. Hippocampal slices from PILO-treated rats without Timm stain in the IML after CHX treatment were hypothesized to lack the electrophysiological abnormalities suggestive of recurrent excitation. The primary experimental groups were as follows: 1) CHX (1 mg/kg) 30-45 min prior to administration of PILO (320 mg/kg ip, 2) only PILO, and 3) only saline (0.5 ml, IP). The CHX pre-treatment significantly decreased the number of rats that responded to PILO with status epilepticus compared to rats that received only PILO. Pre-treatment with CHX did not significantly alter the spontaneous motor seizure rate post-treatment compared to treatment with PILO alone in those animals from each group that developed status epilepticus during PILO treatment. Timm stain in the IML was not significantly different between the PILO- and PILO+CHX-treated rats. Using quantitative methods, CHX did not prevent hilar, CA1, or CA3 neuronal loss compared to the PILO-treated rats. Extracellular responses to hilar stimulation in 30 microM bicuculline and 6 mM [K(+)](o) demonstrated all-or-none bursting in both the CHX+PILO- and PILO-treated rats but not in control rats. Whole cell recordings from granule cells, using glutamate flash photolysis to activate other granule cells, showed that both the CHX+PILO- and PILO-treated rats had excitatory synaptic interactions in the granule cell layer, which were not found after saline treatment. Some rats responded to PILO (with or without CHX pre-treatment) with only one or a few seizures at treatment, and some of these animals (n = 4) demonstrated spontaneous motor seizures within 2 mo after treatment. Timm staining and neuron loss in this group were not clearly different from saline-treated rats. These results suggest that in the PILO model, pre-treatment with CHX does not affect mossy fiber sprouting in the IML of epileptic rats and does not prevent the formation of recurrent excitatory circuits. However, the develoment of spontaneous motor seizures, in a small number of rats, could occur without detectable hippocampal neuron loss or mossy fiber sprouting, as assessed by the Timm stain method.

Spermine increases phosphatidylinositol 4,5-bisphosphate content in permeabilized and nonpermeabilized HL60 cells

The polyamine spermine (N,N'bis[3-aminopropyl]-1,4-butanediamine) activates phosphatidylinositol-4-phosphate 5-kinase (PtdIns(4)P5K) and phosphatidylinositol 4-kinase (PtdIns4K) in vitro. Spermine concentration increases that occur in proliferating cells were approximated in streptolysin O-permeabilized HL60 cells. When phospholipase C was activated by GTPgammaS in the presence of PITPalpha, 0.1-1.2 mM spermine evoked increases in PtdIns(4,5)P(2) contents in a dose-dependent manner to 110-170% of control and concomitantly decreased inositol phosphate formation by 10-50%. Spermine-induced increases in PtdIns(4,5)P(2) content in permeabilized cells also occurred during GTPgammaS stimulation in the absence of PITPalpha, were augmented in the presence of PITPalpha, occurred in unstimulated cells and were additive to PtdIns(4,5)P(2) formation evoked by ARF1, another activator of phosphoinositide kinases. Slowly developing spermine-evoked increases in PtdIns(4,5)P(2) contents occurred in nonpermeabilized cells that were abolished in the presence of a spermine transport inhibitor. Data are consistent with spermine at physiological concentrations evoking a PITPalpha-dependent shift in formation of PtdIns(4,5)P(2) from compartments that contained an active phospholipase C to compartments that were separated from an active PLC and from PtdIns(4,5)P(2) formed by ARF1.

Autophosphorylation of type I phosphatidylinositol phosphate kinase regulates its lipid kinase activity

Phosphatidylinositol phosphate kinases (PIPKs) have important roles in the production of various phosphoinositides. For type I PIP5Ks (PIP5KI), a broad substrate specificity is known. They phosphorylate phosphatidylinositol 4-phosphate most effectively but also phosphorylate phosphatidylinositol (PI), phosphatidylinositol 3-phosphate, and phosphatidylinositol (3,4)-bisphosphate (PI(3, 4)P(2)), resulting in the production of phosphatidylinositol (4, 5)-bisphosphate (PI(4,5)P(2)), phosphatidylinositol 3-phosphate, phosphatidylinositol (3,4)-bisphosphate (PI(3,4)P(2)), phosphatidylinositol (3,5)-bisphosphate (PI(3,5)P(2)), and phosphatidylinositol (3,4,5)-trisphosphate. We show here that PIP5KIs have also protein kinase activities. When each isozyme of PIP5KI (PIP5KIalpha, -beta, and -gamma) was subjected to in vitro kinase assay, autophosphorylation occurred. The lipid kinase-negative mutant of PIP5KIalpha (K138A) lost the protein kinase activity, suggesting the same catalytic mechanism for the lipid and the protein kinase activities. PIP5KIbeta expressed in Escherichia coli also retains this protein kinase activity, thus confirming that no co-immunoprecipitated protein kinase is involved. In addition, the autophosphorylation of PIP5KI is markedly enhanced by the addition of PI. No other phosphoinositides such as phosphatidylinositol phosphate, phosphatidylinositol bisphosphate, or phosphatidylinositol trisphosphate have such an effect. We also found that the PI-dependent autophosphorylation strongly suppresses the lipid kinase activity of PIP5KI. The lipid kinase activity of PIP5KI was decreased to one-tenth upon PI-dependent autophosphorylation. All these results indicate that the lipid kinase activity of PIP5KI that acts predominantly for PI(4,5)P(2) synthesis is regulated by PI-dependent autophosphorylation in vivo.

Modulation of nucleotide sensitivity of ATP-sensitive potassium channels by phosphatidylinositol-4-phosphate 5-kinase

ATP-sensitive potassium channels (K(ATP) channels) regulate cell excitability in response to metabolic changes. K(ATP) channels are formed as a complex of a sulfonylurea receptor (SURx), a member of the ATP-binding cassette protein family, and an inward rectifier K(+) channel subunit (Kir6.x). Membrane phospholipids, in particular phosphatidylinositol (PI) 4,5-bisphosphate (PIP(2)), activate K(ATP) channels and antagonize ATP inhibition of K(ATP) channels when applied to inside-out membrane patches. To examine the physiological relevance of this regulatory mechanism, we manipulated membrane PIP(2) levels by expressing either the wild-type or an inactive form of PI-4-phosphate 5-kinase (PIP5K) in COSm6 cells and examined the ATP sensitivity of coexpressed K(ATP) channels. Channels from cells expressing the wild-type PIP5K have a 6-fold lower ATP sensitivity (K(1/2), the half maximal inhibitory concentration, approximately 60 microM) than the sensitivities from control cells (K(1/2) approximately 10 microM). An inactive form of the PIP5K had little effect on the K(1/2) of wild-type channels but increased the ATP-sensitivity of a mutant K(ATP) channel that has an intrinsically lower ATP sensitivity (from K(1/2) approximately 450 microM to K(1/2) approximately 100 microM), suggesting a decrease in membrane PIP(2) levels as a consequence of a dominant-negative effect of the inactive PIP5K. These results show that PIP5K activity, which regulates PIP(2) and PI-3,4,5-P(3) levels, is a significant determinant of the physiological nucleotide sensitivity of K(ATP) channels.

Coupled inositide phosphorylation and phospholipase D activation initiates clathrin-coat assembly on lysosomes

Adaptors appear to control clathrin-coat assembly by determining the site of lattice polymerization but the nucleating events that target soluble adaptors to an appropriate membrane are poorly understood. Using an in vitro model system that allows AP-2-containing clathrin coats to assemble on lysosomes, we show that adaptor recruitment and coat initiation requires phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis. PtdIns(4,5)P2 is generated on lysosomes by the sequential action of a lysosome-associated type II phosphatidylinositol 4-kinase and a soluble type I phosphatidylinositol 4-phosphate 5-kinase. Phosphatidic acid, which potently stimulates type I phosphatidylinositol 4-phosphate 5-kinase activity, is generated on the bilayer by a phospholipase D1-like enzyme located on the lysosomal surface. Quenching phosphatidic acid function with primary alcohols prevents the synthesis of PtdIns(4, 5)P2 and blocks coat assembly. Generating phosphatidic acid directly on lysosomes with exogenous bacterial phospholipase D in the absence of ATP still drives adaptor recruitment and limited coat assembly, indicating that PtdIns(4,5)P2 functions, at least in part, to activate the PtdIns(4,5)P2-dependent phospholipase D1. These results provide the first direct evidence for the involvement of anionic phospholipids in clathrin-coat assembly on membranes and define the enzymes responsible for the production of these important lipid mediators.

Regulation of phosphatidylinositol 4-phosphate 5-kinase from Schizosaccharomyces pombe by casein kinase I

Phosphatidylinositol ()P 5-kinase (PtdIns(4)P 5-kinase) catalyzes the last step in the synthesis of phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2). PtdIns(4,5)P2 is a precursor of diacylglycerol and inositol 1,4,5-trisphosphate and is also involved in regulation of actin cytoskeleton remodeling and membrane traffic. To satisfy such varied demands in several aspects of cell physiology, synthesis of PtdIns(4,5)P2 must be stringently regulated. In this paper we describe extraction, purification, and characterization of PtdIns(4)P 5-kinase from the plasma membranes of Schizosaccharomyces pombe. We also provide evidence that PtdIns(4)P 5-kinase is phosphorylated and inactivated by Cki1, the S. pombe homolog of casein kinase I. Phosphorylation by Cki1 in vitro decreases the activity of PtdIns(4)P 5-kinase. In addition, and most importantly, overexpression of Cki1 in S. pombe results in a reduced synthesis of PtdIns(4,5)P2 and in a lower activity of PtdIns(4)P 5-kinase associated with the plasma membrane. These results suggest that PtdIns(4)P 5-kinase is a target of Cki1 in S. pombe and that Cki1 is involved in regulation of PtdIns(4, 5)P2 synthesis by phosphorylating and inactivating PtdIns(4)P 5-kinase.

PIPkins1, their substrates and their products: new functions for old enzymes

The phosphatidylinositolphosphate kinases (PIPkins) are a unique family of enzymes that catalyse the production of phosphorylated inositol lipids. Recent advances have revealed that, due to their ability to utilise a number of different lipid substrates (at least in vitro), this family is potentially able to generate several distinct, physiologically important inositol lipids. Despite their importance, however, our understanding of the regulation of the PIPkins and of their physiological role in cellular signalling and regulation is still poor. Here we describe in turn the diverse physiological functions of the known substrates and major products of the PIPkins. We then examine what is known about the members of the PIPkin family themselves, and their characteristics and regulation.

Phosphoinositide signaling pathways in nuclei are associated with nuclear speckles containing pre-mRNA processing factors

Phosphoinositide signal transduction pathways in nuclei use enzymes that are indistinguishable from their cytosolic analogues. We demonstrate that distinct phosphatidylinositol phosphate kinases (PIPKs), the type I and type II isoforms, are concentrated in nuclei of mammalian cells. The cytosolic and nuclear PIPKs display comparable activities toward the substrates phosphatidylinositol 4-phosphate and phosphatidylinositol 3-phosphate. Indirect immunofluorescence revealed that these kinases were associated with distinct subnuclear domains, identified as "nuclear speckles," which also contained pre-mRNA processing factors. A pool of nuclear phosphatidylinositol bisphosphate (PIP2), the product of these kinases, was also detected at these same sites by monoclonal antibody staining. The localization of PIPKs and PIP2 to speckles is dynamic in that both PIPKs and PIP2 reorganize along with other speckle components upon inhibition of mRNA transcription. Because PIPKs have roles in the production of most phosphatidylinositol second messengers, these findings demonstrate that phosphatidylinositol signaling pathways are localized at nuclear speckles. Surprisingly, the PIPKs and PIP2 are not associated with invaginations of the nuclear envelope or any nuclear membrane structure. The putative absence of membranes at these sites suggests novel mechanisms for the generation of phosphoinositides within these structures.

Phosphoinositide kinases

Phosphatidylinositol, a component of eukaryotic cell membranes, is unique among phospholipids in that its head group can be phosphorylated at multiple free hydroxyls. Several phosphorylated derivatives of phosphatidylinositol, collectively termed phosphoinositides, have been identified in eukaryotic cells from yeast to mammals. Phosphoinositides are involved in the regulation of diverse cellular processes, including proliferation, survival, cytoskeletal organization, vesicle trafficking, glucose transport, and platelet function. The enzymes that phosphorylate phosphatidylinositol and its derivatives are termed phosphoinositide kinases. Recent advances have challenged previous hypotheses about the substrate selectivity of different phosphoinositide kinase families. Here we re-examine the pathways of phosphoinositide synthesis and the enzymes involved.

Structure of type IIbeta phosphatidylinositol phosphate kinase: a protein kinase fold flattened for interfacial phosphorylation

Phosphoinositide kinases play central roles in signal transduction by phosphorylating the inositol ring at specific positions. The structure of one such enzyme, type IIbeta phosphatidylinositol phosphate kinase, reveals a protein kinase ATP-binding core and demonstrates that all phosphoinositide kinases belong to one superfamily. The enzyme is a disc-shaped homodimer with a 33 x 48 A basic flat face that suggests an electrostatic mechanism for plasma membrane targeting. Conserved basic residues form a putative phosphatidylinositol phosphate specificity site. The substrate-binding site is open on one side, consistent with dual specificity for phosphatidylinositol 3- and 5-phosphates. A modeled complex with membrane-bound substrate and ATP shows how a phosphoinositide kinase can phosphorylate its substrate in situ at the membrane interface.

A gene encoding phosphatidylinositol-4-phosphate 5-kinase is induced by water stress and abscisic acid in Arabidopsis thaliana

Phosphatidylinositol-4-phosphate 5-kinase (PIP5K) phosphorylates phosphatidylinositol-4-phosphate to produce phosphatidylinositol-4,5-bisphosphate as a precursor of two second messengers, inositol-1,4,5-triphosphate and diacylglycerol, and as a regulator of many cellular proteins involved in signal transduction and cytoskeletal organization. Despite PIP5K playing such an essential role in a number of physiological processes, much still remains to be made clear about its association with plants. Searching the Arabidopsis expression sequence tag database against already known yeast and mammalian PIP5K cDNAs, we identified two clones which partly encode the same Arabidopsis PIP5K and isolated a corresponding full-length cDNA encoding a protein that we designated AtPIP5K1. Recombinant AtPIP5K1 expressed in Escherichia coli possessed a PIP5K activity in vitro. Due to some structural and biochemical differences, AtPIP5K1 was not categorized as either a type I or type II PIP5K. The expression of the AtPIP5K1 mRNA was induced rapidly by treating Arabidopsis plants with drought, salt and abscisic acid, which suggests that AtPIP5K11 is involved in water-stress signal transduction. These data give evidence for a close link between phosphoinositide signaling cascades and water-stress responses in plants.