Phosphatidylinositols Ptdlns

Family of enzymes phosphorylating phosphatidylinositol (Ptdlns), PtdIns(4)phosphate, and PtdIns(4,5)phosphate in the 3-position. The Ptdlns(3 phospholipids are second messengers in processes like cell growth, cytoskeletal rearrangement, and vesicular transport. PI 3-kinases are heterodimers composed of a catalytic and a regulatory subunit. The enzymes are activated by insulin, many growth factors, and by a variety of cytokines. Their activity can be inhibited by wortmannin and LY294002. 

The relatively small pool of the precursor for Ins(l,4,5)P3, the phospholipid PtdIns(4,5)P2, is synthesized from the larger reservoir of phosphatidylinositol (Ptdlns) and, therefore, the supply of PtdIns(4,5)P2... 

Figure 6.6. Structure and formation of phosphatidylinositol (Ptdlns), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2). See text for details.

Substitution of the CMP residue by inositol then provides phosphatidylinositol (Ptdlns enzyme CDPdiacylglycerolinositol-3-phosphatidyl transferase 2.7.8.11). 

Phosphatidylinositol (Ptdlns) 563, 565, 566s in signalling 563 - 566 Phosphatidylinositol 3-kinase 565 Phosphatidylinositol 4,5-bisphosphate 563 Phosphatidylserine 383s, 564 decarboxylation of 753 Phosphatidylserine decarboxylase 409, 755 3 -Phosphoadenosine 5 -phosphosulfate (PAPS) 659 Phosphoadenylation... 

Phosphatidylinositol (Ptdlns) and phosphatidylserine (PtdSer) in solid form phosphatidylinositol (4)-monophosphate and phosphatidylinostiol (4,5)bisphos-phate (1 mg/mL in chloroform stored for up to 6 mo at -20°C). 

Fig. 1. The major inositol lipids. Phosphatidylinositol (Ptdlns), the major membrane inositol phospholipid, is phosphorylated to phosphatidylinositol 4-phosphate (Ptdlns 4-P) by a phosphatidylinositol kinase (a). Ptdlns 4-P is further phosphorylated to phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5-P2) by a phosphatidylinositol 4-phosphate kinase (b). Ptdlns 4,5-P, is converted back to Ptdlns 4-P by phosphatidylinositol 4,5-bisphosphate phosphomonoesterase (c) and then to Ptdlns by phosphatidylinositol 4-phosphate monoesterase (d). The pathway of phosphorylation and dephosphorylation constitutes a futile cycle and is only interrupted by agonist-induced hydrolysis of Ptdlns 4,5-P,.

Figure 2. Biosynthetic pathways from glucose 6-P to myo-inositol tetra/r/.vphosphates. The six carbons of the Ins ring are numbered according to the D-numbering convention. The pathways to the center-left proceed entirely via soluble myo-inositol (Ins) phosphates. Pathways to the center-right proceed via phosphatidylinositol (Ptdlns) phosphates. Questionmarks indicate synthetic steps that have not been confirmed in chemical, molecular or genetic analyzes. In addition, the hydrolysis of PtdIns(3,4,5)P3 to yield Ins(l,3,4,5)P4 has not been observed in any experiments, and is entirely speculative. P = PH204.

The ability to sense and respond to the environment is essential for the survival of all living organisms. The PI pathway is involved in sensing environmental stimuli and is one of the most conserved signaling pathways. Components of the PI pathway are present in both prokaryotes and eukaryotes. For example, membranes from the hyperthermophilic archaeon Pyrococcus furiosus will phosphorylate phosphatidylinositol (Ptdlns) to form PtdIns3P (Figure 2). 

Phosphatidylinositol (abbreviated Ptdlns, or PI) is a minor class of phospholipids composed of glycerol, fatty acids and inositol. Pis are found in the cytosolic side of eukaryotic cell membranes. They are substrates fora large number of enzymes which are involved in cell signalling. 

Adapted from [3]. Ptdlns, phosphatidylinositol PtdIns(4,5)P2, phosphatidylinositol-4,5-insphosphate PtdCho, phosphatidylcholine PtdEtn, phosphatidylethanolamine PtdSer, phosphatidylserine. 

The Chilton Conference nomenclature for inositol lipids is used throughout this chapter, e.g. PI, PI4P, PI(4,5)P2 for phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate respectively. Note that the IUB-recommended nomenclature for these lipids is Ptdlns, PtdIns4P and PtdIns(4,5)P2 (see Ch. 3). 

PAL peptidyl-a-hydroxyglycine a-amidating lyase Ptdlns 4P phosphatidylinositol 4-phosphate... 

PAM peptidylglycine a-amidating monooxygenase Ptdlns 4,5P2 phosphatidylinositol 4,5-bisphosphate... 

More recently, the importance of a group of highly polar inositol lipids, present in neutrophils and many other cell types, has been recognised. Activation of neutrophils by fMet-Leu-Phe results in the transient accumulation of phosphatidylinositol 3-phosphate (Ptdlns 3-P), phosphatidylinositol 3,4-bisphosphate (Ptdlns 3,4-P2) and phosphatidylinositol 3,4,5-trisphosphate (Ptdlns 3,4,5-P3). Apparently, the enzyme phosphatidylinositol 3-hydroxy (3-OH) kinase plays a key role in the formation of these novel lipids. This enzyme can catalyse the formation of these lipids from phosphatidylinositol, phosphatidylinositol 4-phosphate (Ptdlns 4-P) and phosphatidylinositol 4,5 bisphosphate (Ptdlns 4,5-P2) in vitro (Fig. 6.10). Alternatively, it is possible that Ptdlns 3,4-P2 and Ptdlns 3-P are derived from the sequential dephosphorylation of Ptdlns 3,4,5-P3. 

Figure 6.10. Phosphatidylinositol 3-hydroxy kinase activity. The enzyme phosphatidylinositol 3-hydroxy (Ptdlns 3-OH) kinase may phosphorylate inositol lipids as shown, to generate a series of novel polar lipids that may function in cell activation. See text for details.

An additional phosphorylation (enzyme phosphatidylinositol-4-phosphate kinase 2.7.1.68) finally provides phosphaditylino-sitol-4,5-bisphosphate (PIP2, Ptdlns(4,5)P2). PIP2 is the precursor for the second messengers 2,3-diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (InsPa, IP3 see p. 367). 

Activation of CC- responses. Stimulation of 0 receptors by catecholamines leads to the activation of a Gq-coupling protein. The activated a subunit (Kq ) of this G protein activates the effector, phospholipase C, which leads to the release of IP3 (inositol 1,4,5-trisphosphate) and DAG (diacylglycerol) from phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5P2). IP3 stimulates the release of sequestered stores of calcium, leading to... 

Phosphorylated IRS-1 activates a second signaling pathway by interacting with an 85-kDa SH2-containing protein that is a subunit of phophatidylinositol 3-kinase.384 386 This activates the 110-kDa catalytic subunit of the 3-kinase, which catalyzes formation of phosphatidylinositol 3-phosphate as well as Ptdlns (3,4)P2 and Ptdlns (3,4,5)P3.387/387a These compounds, which remain within membranes, activate other branches of the signaling cascade, some of which may converge with those of the MAP kinase cascade. However, there appears to be specific activation of a ribosomal Ser/Thr kinase that, among other activities, phosphorylates ribosomal protein S6, a component of the small ribosomal subunit.388 It also phosphorylates some isoforms of protein kinase C and other enzymes. Ptdlns 3-kinase may also activate 6-phosphofructo-2-kinase (Fig. 11-2, step ti).384/388... 

Ptdlns (or PI) Phosphatidylinositol 3-kinase V2R Type 2 vasopressin receptor... 

Glutamate-mediated Ca2+ entry through NMDA at the plasma membrane level and mobilization of Ca2+from intracellular stores through PLC-mediated generation of PtdIns-3/J is indispensable for the basal NF-kB activity. Three cytosolic Ca2+ sensors, calmodulin, protein kinases C (PKC), and the p2 l(ras)/phosphatidylinositol 3-kinase (Ptdlns-3K)/Akt pathways, are simultaneously involved in the steps linking the Ca2+ to NF-kB activity (Lilienbaum and Israel, 2003 Marchetti et al., 2004 Lubin et al., 2005). Calmodulin modulates calcineurin, a Ca2+-dependent protein phosphatase, which plays a role in the basal NF-kB activity, whilst stimulation of both the calmodulin kinase II and Akt kinase pathways results in the up-regulation of the transcriptional potential of the p65 subunit of NF-/cB (Lilienbaum and Israel,... 

In primary cultures of neonatal cerebellar granule neurons, all Ca2+ sensors, calmodulin, protein kinases C (PKC), and the p21(ras)/phosphatidylinositol 3 -kinase (Ptdlns-3K)/Akt pathway, converge towards NF-kB at the levels of nuclear translocation as well as transcription. The duration of NF-kB activation is a critical determinant for sensitivity toward excitotoxic stress and is dependent on the different upstream and downstream signaling associated with various kinases. This is in contrast to studies in non-neuronal cells, which either do not respond to Ca2+ or do not simultaneously activate all three cascades (Lilienbaum and Israel, 2003). Collective evidence suggests that brain inflammatory processes differ from systemic inflammation not only in the involvement of various types of neural cells but also in differences in response to second messengers. 

In animal cells there are three major myo-inositol-containing phospholipids (Fig. 1) phosphatidylinositol [l-(3-sn-phosphatidyl)-D-myo-inositol) (Ptdlns)], which usually accounts for over 90% of the total inositol lipid, phosphatidylinositol-4-phosphate (Ptdlns 4-P) and phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5-P2). The polyphosphorylated inositides generally constitute 1-10% of total inositol lipids, which are themselves 6-10% of total phospholipids. The polyphosphoinositides are considered to be located on the cytoplasmic side of the plasma membrane, which places them in an ideal position to play a role in signal transduction. Ptdlns, on the other hand, is found in all cellular membranes. In some cells there is evidence for separate hormone sensitive and hormone insensitive pools of the inositol lipids [3],... 

Figure 5. Components and processes of a membrane-bound organelle, representing either a Protein Storage Vacuole (PSV) or a membrane-bound globoid found within a compound PSV V-PPase = vacuolar inorganic pyrophosphatase. V-ATPase = vacuolar ATPase. TIP = Tonoplast Intrinsic Protein. Ptdlns = phosphatidylinositol. DAG = diacylglycerol. Questionmarks indicate purely speculative aspects of the diagram.

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