Separation and identification of two phosphatidylinositol 4-kinase activities in bovine uterus

A homogeneous and nonisotopic assay for phosphatidylinositol 4-kinases

Phosphatidylinositol 4-kinases (PI 4-kinases) catalyze the conversion of phosphatidylinositol to phosphatidylinositol 4-phosphate (PtdIns4P). The four known mammalian PI 4-kinases, PI4KA, PI4KB, PI4K2A, and PI4K2B have roles in intracellular lipid and protein trafficking. PI4KA and PI4KB also assist in the replication of several positive-sense RNA viruses. The identification of selective inhibitors of these kinases would be facilitated by assays suitable for high-throughput screening. We describe a homogeneous and nonisotopic assay for PI 4-kinase activity based on the bioluminescent detection of the ADP produced by kinase reactions. We have evaluated this assay with known nonselective inhibitors of PI 4-kinases and show that it performs similar to radiometric assay formats previously described in the literature. In addition, this assay generates Z-factor values of >0.7 for PI4KA in a 384-well format, demonstrating its suitability for high-throughput screening applications.

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.

Phosphatidylinositol 4-kinase of Torpedo californica electrocytes: physico-chemical characterization and regulation by calcium and vicinal molecules of phosphatidylinositol

A phosphatidylinositol 4-kinase (Ptdlns 4-kinase, M(r) approximately 95,000) from the membranes of the electric organ of Torpedo californica was purified to apparent homogeneity. The Michaelis constant for ATP (KM = 280 +/- 60 microM at 20 degrees C) and the inhibition constant for adenosine (Ki = 0.4 mM at 20 degrees C) qualify the electrocyte Ptdlns 4-kinase as a type III kinase. The Ptdlns 4-kinase phosphorylates preferentially exogenous Ptdlns, added in the form of mixed Ptdlns/Triton X-100 micelles, whereas endogenously bound Ptdlns in the membrane fragments of electrocytes is a very poor substrate. It is important that the enzyme and the substrate Ptdlns are situated in different lipid bilayers. The catalytic turnover constant for exogenous Ptdlns is k = 55.3 +/- 6 min-1 at 20 degrees C and the molar Triton X-100/Ptdlns ratio of 16:1. For the substrate Ptdlns in the 'micellar solvent' Triton X-100, steady state kinetics were analysed in terms of the mole fraction X = n(Ptdlns)/[n(Ptdlns) + n(Triton X)] yielding the characteristic Michaelis mole fraction XM = 0.019 +/- 0.005 at 20 degrees C. The activity of the enzyme was enhanced about 5-fold in the presence of Triton X-114, yielding k = 277 +/- 30 min-1 at 20 degrees C. Triton X-114 has a shorter head-group, indicating that the vicinity of the Ptdlns head group in the mixed micelles should not be screened by bulky neighbours. The inhibition of the enzyme activity by Ca2+ is highly cooperative yielding the Hill inhibition constant Ki = 0.47 +/- 0.1 mM and the Hill coefficient h = 3.6 +/- 0.5. The enthalpy of activation is 100 +/- 10 kJ/mol between 0 degree C and 20 degrees C. Although the Ptdlns 4-kinase can be affinity-chromatographically copurified with the nicotinic acetylcholine (AcCho) receptor, suggesting tight association between the two proteins. AcCho does not affect the activity of the Ptdlns 4-kinase in the presence of the AcCho receptor.

Cloning, expression, and localization of 230-kDa phosphatidylinositol 4-kinase

A phosphatidylinositol (PI) 4-kinase cDNA was cloned from a rat brain cDNA library. This cDNA encoded a protein of 2041 amino acids with a calculated molecular weight of 231,317. The deduced amino acid sequence shared the identity of 52.3 and 34.4% in the presumed catalytic domain with two yeast PI 4-kinases, STT4 and PIK1, respectively, and showed 31.7% identity to p110alpha subunit of rat PI 3-kinase in the same domain. In addition, a 3' half coding region of the present cDNA was 89.6% identical to and its deduced amino acid sequence was 98.2% identical to the sequence for P14Kalpha, a recently reported human PI 4-kinase of type II, suggesting that P14Kalpha is an alternative form of the present PI 4-kinase molecule. The present cDNA contained sequences encoding the ankyrin repeat domain, lipid kinase unique domain, pleckstrin homology domain, presumed lipid kinase/protein kinase homology domain, proline-rich region, and SH3 domain. By examining PI kinase activity in transfected COS-7 cells using the epitope tag immunoprecipitation as well as the conventional way, the product phosphatidylinositol phosphate was identified as phosphatidylinositol 4-phosphate but not phosphatidylinositol 3-phosphate. This PI 4-kinase activity was markedly enhanced in the presence of Triton X-100 but relatively insensitive to inhibition by adenosine. By epitope tag immunohistochemistry, the immunoreactivity for this PI 4-kinase molecule was largely localized in close association with the membranes of the Golgi vesicles and vacuoles. By in situ hybridization analysis, the expression of mRNA for this PI 4-kinase was evident throughout the gray matter of entire brain with higher expression intensity in fetal brain. These data imply that this novel PI 4-kinase is involved in some processes essential to neuronal differentiation and maturation including the synaptogenesis and synaptic plasticity.

Phosphatidylinositol 4-kinase: gene structure and requirement for yeast cell viability

Phosphatidylinositol (PtdIns) 4-kinase catalyzes the first step in the biosynthesis of PtdIns-4,5-bisphosphate (PtdIns[4,5]P2). Hydrolysis of PtdIns[4,5]P2 in response to extracellular stimuli is thought to initiate intracellular signaling cascades that modulate cell proliferation and differentiation. The PIK1 gene encoding a PtdIns 4-kinase from the yeast Saccharomyces cerevisiae was isolated by polymerase chain reaction (PCR) with oligonucleotides based on the sequence of peptides derived from the purified enzyme. The sequence of the PIK1 gene product bears similarities to that of PtdIns 3-kinases from mammals (p110) and yeast (Vps34p). Expression of PIK1 from a multicopy plasmid elevated PtdIns 4-kinase activity and enhanced the response to mating pheromone. A pik1 null mutant was inviable, indicating that PtdIns4P and presumably PtdIns[4,5]P2 are indispensable phospholipids.

The plant phosphoinositide system

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Purification and reconstitution of phosphatidylinositol 4-kinase from human erythrocytes

A membrane-bound phosphatidylinositol 4-kinase (PtdIns kinase) has been purified to apparent homogeneity from human erythrocytes. Enzyme activity was solubilized from urea-KCl-stripped, inside-out membrane vesicles by 3% Triton X-100. Purification to apparent homogeneity was accomplished by cation-exchange chromatography on phosphocellulose, followed by heparin-acrylamide chromatography. This resulted in a nearly 3900-fold purification of PtdIns kinase activity to a specific activity of 44 nmol min-1 mg-1. The purified enzyme has an Mr of 59,000 on silver-stained SDS-PAGE; however, many preparations also contain 54 kDa and 50 kDa proteins which are related to the 59 kDa protein and have PtdIns kinase activity. Kinetic analysis of the PtdIns kinase indicate apparent Km values of 40 and 35 microM for phosphatidylinositol and ATP, respectively. The purified enzyme has been reconstituted into phospholipid liposomes and shown to phosphorylate phosphatidylinositol.

Purification and chemical modification of a phosphatidylinositol kinase from sheep brain

A membrane-bound phosphatidylinositol (PtdIns) kinase has been purified approximately 9500-fold to apparent homogeneity from sheep brains. The purification procedure involves: solubilisation of the membrane fraction with Triton X-100, ammonium sulphate fractionation and a number of ion-exchange and gel-filtration chromatography steps. The purified enzyme exhibited a final specific activity of 1149 nmol.min-1.mg-1. The molecular mass of the enzyme was estimated to be 55 kDa by SDS/PAGE and 150 +/- 10 kDa by HPLC gel filtration in the presence of Triton X-100. Kinetic measurements have shown that the apparent Km value of PtdIns kinase for the utilisation of PtdIns is 22 microM and for ATP 67 microM. Mg2+ was the most effective divalent cation activator of PtdIns kinase, with maximal enzymatic activity reached at a concentration of 10 mM Mg2+. In addition to adenosine and ADP, the 2'(3')-O-(2,4,6-trinitrophenyl) derivative of ATP was found to be a strong competitive inhibitor of the enzyme, with a Ki of 32 microM. Enzymatic activity was found to be stimulated by Triton X-100 but inhibited by deoxycholate.

Purification and characterization of phosphatidylinositol 4-kinase from human erythrocyte membranes

Two species of PtdIns 4-kinase with molecular masses of 50 kDa and 45 kDa were detected in human erythrocyte membranes using SDS/PAGE. These enzymes were purified to near homogeneity and found to display very similar enzymatic characteristics. The purification scheme consisted of solubilization from erythrocyte membranes in the presence of Triton X-100, followed by Cibacron-blue-Sephadex, phosphocellulose and Mono Q anion-exchange chromatography. The final step in the purification protocol was preparative SDS/PAGE, followed by electroelution and renaturation of the enzyme. This procedure afforded an about 4000-fold purification of the enzyme from erythrocyte membranes. Characterization of the [32P]PtdInsP products formed by the purified PtdIns kinases indicated that these enzymes specifically phosphorylated the D-4 position of the inositol ring. The Km values of both PtdIns 4-kinase species for PtdIns and ATP were found to be 0.2 mM and 0.1 mM, respectively. The enzymes are both activated by Mg2+, and inhibited by Ca2+ and by adenosine. The potential importance of these effectors for the regulation of PtdIns phosphorylation in cells is discussed.

A monoclonal antibody distinguishes two types of phosphatidylinositol 4-kinase

A monoclonal antibody has been developed against the type II PtdIns 4-kinase from bovine brain. This antibody, 4C5G, causes greater than 90% inhibition of the type II PtdIns 4-kinase from bovine brain, rat brain and human erythrocytes. However, it fails to inhibit type III PtdIns 4-kinase from bovine brain or PtdIns 3-kinase from rat liver. These results suggest that type II and type III PtdIns 4-kinases are distinct gene products, and that 4C5G will be useful in studying the function of the type II PtdIns 4-kinase.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.