Phosphatidylinositol transport protein

Figure 2. The so-called canonical phosphoinositide pathway . The continuous phosphorylation/dephosphorylation reactions allow a steady-state level of Ptdins, PtdIns(4)P and PtdIns(4,5)P2 in the plasma membrane (PM). Cleavage of PtdIns(4,5)P2 by phospholipase C (PLC) generates the two well-known second messengers, inositol 1,4,5-trisphosphate (Ins(l,4,5)P3) and diacylglycerol (DAG). Besides its role as a protein kinase C (PKC) activator, DAG can be phosphorylated to phosphatidic acid (PA). The resynthesis of Ptdins from inositol and PA occurs mainly in the endoplasmic reticulum (ER). PPi, inorganic phosphate. PA-Pase, phosphatidic acid phosphatase. PA-TP, phosphatidic acid transport protein. PtdIns-TP, phosphatidylinositol transport protein. CDP-DAG, cytidine diphosphate-diacylglycerol. CMP, CDP and CTP, cytidine mono-, di- and triphosphate, respectively.

One of the most important effects of insulin is to increase glucose uptake by cells 373/389 The mechanism is thought to depend upon the transporter protein GLUT4, which is stored within the membranes of small cytoplasmic vesicles. Binding of insulin to its receptors induces movement of these vesicles to the plasma membrane where fusion with the plasma membrane makes the GLUT4 molecules available for glucose transport.390 Phosphatidylinositol 3-kinase also plays an important role. The PtdIns(3,4)P2 and PtdIns(3,4,5)P3 generated by this enzyme (Fig. 11-9)... 

Figure 4 Schematic representation of the Ca2+-transporting systems affecting cellular calcium homeostasis during hormonal stimulation, oq = oq-adrenergic receptor VP = vasopressin receptor PLC = phospholipase C PI = phosphatidylinositol PIP = phospha-tidylinositol-4-phosphate PIP2 = phosphatidylinositol-4,5-biphosphate IP3 = inositol-1,4,5-triphosphate DG = diacylglycerol PKC = protein kinase C. (Modified from Refs. 125 and 285.)...

IP3 = 1,4,5-inositol triphosphate PI = phosphatidylinositol PIP = phosphatidylinositol phosphate PIP2 = phosphatidylinositol 4,5-biphosphate PKC = protein kinase C PLC = phosphohpase C R = receptor T = transporter. 

Phospholipids have been shown to exchange between different membranes, e.g., of mitochondria and the ER. Exchanges of phosphatidylcholine, phosphatidylinositol, and sphingomyelin are catalyzed by specific exchange proteins (Box 21-A).238 2383 These proteins may also participate in signaling, but their major function may be to transport the phospholipids from their sites of synthesis to the various membranes of the cell. 

---