Phosphorylated phosphoinositides

Akt is a downstream target of PI-3K activation (43-45). PI-3K phosphorylates phosphoinositides (Ptdlns) at the 3-position of the inositol ring, generating PtdlnsJP. PtdIns(3,4)P2, and PtdIns(3,4,5)P3. The studies that elucidated these mechanisms were primarily based on the observation that growth-factor-induced Akt activation could be... 

KUCERA GE RIITENHXISE SE. Human platelets form 3-phosphorylated phosphoinositides in response to a-thrombin, U 46619 or GTPyS. JBiol Chem 265 5345-5348,1990. 

In the synthesis of highly phosphorylated phosphoinositide derivatives regioselective hydrolysis of one out of three butyrates by a lipase in pH 7.8 buffer containing... 

Addic Phospholipids Phosphatidic Acid, Cardiolipin, Phosphatidylglycerol, Phosphatidylserine, Phosphatidylinositol, and Phosphorylated Phosphoinositides... 

PATHWAY OF SYNTHESIS OF 3- AND 4-PHOSPHORYLATED PHOSPHOINOSITIDES IN STOMATAL GUARD CELLS OF COMMELINA COMMUNIS L. 

In the plant kingdom, the pathway of synthesis of 3- and 4-phosphorylated phosphoinositides has been described only in Spirodela polyrhiza L. [1]. If animal models of phosphoinositide metabolism are to be applied to plants, it is imperative to determine the route of synthesis of phosphoinositides in other members of the plant kingdom in which signaling functions for phosphoinositides and inositol phosphates have been implicated. Recently, we have rigorously identified 3- and 4-phosphorylated phosphoinositides in stomatal guard cells of Commelina communis L. [2]. In the present work we have set out to determine the metabolic route by which PtdIns(3,4)P and PtdIns(4,5)P synthesis occurs and to examine the metabolic relationship between 3- and 4-phosphoryrated lipids in stomatal guard cells. 

Parmar PN, Brearley CA. Identification of 3- and 4-phosphorylated phosphoinositides and inositol phosphates in stomatal guard cells of Commelina communis L. Plant J 1993 4 255-263. 

Cellular phosphoinositide concentrations are under tight control by phospholipid kinases and phosphatases. Phospholipid kinases preferentially phosphorylate distinct positions of the inositol ring and hence are subdivided into phosphoinositide 3-kinases (PI3Ks), phosphoinositide 4-kinases (Pl4Ks), and phosphoinositide 5-kinases (PI5Ks) that phosphorylate Pis on position 3, 4 and 5, respectively. In a canonical pathway, Ptdlns... 

Pleckstrin homology domain (PH-domain) was first identified at the amino and carboxyl termini of a haematopoietic protein called pleckstrin. PH-domain, a protein region of approximately 120 amino acids, by binding to phosphatidylinositol lipids of the biological membranes induces the translocation of the PH-domain containing protein to membrane compartment. Various PH-domains possess specificities for phosphoinositides phosphorylated at different sites within the inositol ring. 

The current understanding on activation of Tec kinases fits into a two-step model. In the first step an intramolecular interaction between the SH3 domain and aproline-rich region in the TH domain is disrupted by binding ofthe PH domain to phosphoinositides, G protein subunits, or the FERM domain of Fak. These interactions lead to conformational changes of Tec and translocation to the cytoplasmic membrane where, in a second step, Src kinases phosphorylate a conserved tyrosine residue in the catalytic domain thereby increasing Tec kinase activity. Autophosphorylation of a tyrosine residue in the SH3 domain further prevents the inhibitory intramolecular interaction resulting in a robust Tec kinase activation. 

The major 3 -phosphoinositide products of class I PI3Ks are phosphati-dylinositol 3,4,5-trisphosphate [PI(3,4,5)P3, which is formed primarily from phosphorylation of PI(4,5)P2) and its metabolite phosphatidylinositol 3,4-bisphosphate, PI(3,4)P2]. The basal levels of PI(3,4)P2 and PI(3,4,5)P3 in cells are usually in low abundance but can rise sharply after cell stimulation to interact with an array of protein effectors via pleckstrin homology (PH) domains, modular segments of about 100 amino acids found in many signaling proteins. It is these PH-domain-containing proteins that are able to propagate and drive downstream signaling events. 

Phorbol esters are promoters that interact with cellular receptors and activate protein kinase C. Usually protein kinase C is activated by Ca++ and diacylglycerol, both of which result from the hydrolysis of phosphoinositides catalyzed by phospholipase C. Phospholipase C is normally activated by several different growth factors. Thus phorbol esters bypass a tightly regulated step in the control of cell growth. Since protein kinase C phosphorylates various proteins, it is not known how this activity participates in establishing a cancerous line of cells. 

Lali, F. V., Hunt, A. E., Turner, S. J., and Foxwell, B. M. (2000). The pyridinyl imidazole inhibitor SB203580 blocks phosphoinositide-dependent protein kinase activity, protein kinase B phosphorylation, and retinoblastoma hyperphosphorylation in interleukin-2-stimulated T cells independendy ofp38 mitogen-activated protein kinase. J. Biol. Chem. 275, 7395-7402. 

Figure 11.2 Structure of the insulin receptor (a). Binding of insulin promotes autophosphorylation of the (3-subunits, where each (3-subunit phosphorylates the other (3-subunit. Phosphate groups are attached to three specific tyrosine residues (tyrosines 1158, 1162 and 1163), as indicated in (b). Activation of the (3-subunit s tyrosine kinase activity in turn results in the phosphorylation of various intracellular (protein) substrates which trigger the mitogen-activated protein kinase and/or the phosphoinositide (PI-3) kinase pathway responsible for inducing insulin s mitogenic and metabolic effects. The underlying molecular events occurring in these pathways are complex (e.g. refer to Combettes-Souverain, M. and Issad, T. 1998. Molecular basis of insulin action. Diabetes and Metabolism, 24, 477-489)...

All isoforms of PKC are predominantly localized to the cytosol and, upon activation, undergo translocation to either plasma or nuclear membranes. However, newly synthesized PKCs are localized to the plasmalemma and are in an open conformation in which the auto inhibitory pseudosubstrate sequence is removed from the substrate binding domain. The maturation of PKC isoforms is effected by phosphoinositide-dependentkinase-I (PDK-I), which phosphorylates a conserved threonine residue in the activation loop of the catalytic (C4) domain [24]. This in turn permits the autophosphorylation of C-terminus threonine and serine residues in PKC, a step which is a prerequisite for catalytic activity (see also Chs 22 and 23). The phosphorylated enzyme is then released into the cytosol, where it is maintained in an inactive conformation by the bound pseudosubstrate. It was originally thought that 3-phosphoinositides such as PI(3,4)P2 and PI(3,4,5)P3 could directly activate PKCs. However, it now seems more likely that these lipids serve to activate PDK-1 (a frequent contaminant of PKC preparations). 

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

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