Phospholipases transfer protein

Additional roles for lipid transfer proteins have been assumed regulation of the acyl-CoA pool within the cytosol and participation in the biosynthesis of llnolenic acid via the cooperative pathway involving the plastids and the endoplasmic reticulum (Dubacq et al., 1984). It has been showed that the chloroplast envelope is a efficient membrane acceptor for phosphatidylcholine molecules transported by lipid transfer proteins ( Miquel et al., 1987). Also, by transporting linoleoy 1-phosphatidylcholine towards chloroplasts by lipid transfer proteins, Ohnishi and Yamada (1982) observed a synthesis of linolenoyl- monogalactosyldiacylglycerol. A phospholipase activity... 

There is a similar situation in all the membranes in contact with the cytosol PC is a major constituent in the outer membrane from plastids and mitochondria and in the peroxisomal membrane (table l). One can question whether this situation has any physiological significance or results from the functionning of the phospholipid transfer proteins that are present in the cytosol. There is almost no redistribution of this PC whithin the chloroplast since phospholipase C treatment destroy almost all the envelope PC without rupture or modification of the outer envelope membrane structure. ... 

Major effector proteins for G-pro-tein-coupled receptors include adenylate cyclase (ATP intracellular messenger cAMP), phospholipase C (phos-phatidylinositol intracellular messengers inositol trisphosphate and di-acylglycerol), as well as ion channel proteins. Numerous cell functions are regulated by cellular cAMP concentration, because cAMP enhances activity of protein kinase A, which catalyzes the transfer of phosphate groups onto functional proteins. Elevation of cAMP levels inter alia leads to relaxation of smooth muscle tonus and enhanced contractility of cardiac muscle, as well as increased glycogenolysis and lipolysis (p. 

Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) pools can be regulated by PI kinases, PtdIns(4,5)P2-phosphatases, phospholipase C (PLC), and lipid transfer and binding proteins. The contribution of each to PtdIns(4,5)P2 pools will depend on the metabolic status of the cell. Enzymes involved in PtdIns(4,5)P2 biosynthesis are shown in Figure 1. Our goal is to convince the reader of the importance of characterizing the metabolic fluxes within the discrete subcellular phospholipid microdomains that make up the lipid signaling pools. 

A GPl-anchor may also allow a protein to be selectively released from the cell surface upon hydrolysis by a GPI-specific phospholipase (e.g., Pl-phospholipase C or GPl-phospholipase D). This has been shown to occur for certain GPI-anchored proteins in mammalian cell culture. One example is GPI-anchored membrane dipeptidase which is released from the adipocyte cell surface by a phospholipase C in response to insulin (S. Movahedi, 2000). Interestingly, other GPI-anchored proteins are not released, indicating a level of regulation in insulin-stimulated hydrolysis of GPI-anchored proteins. GPI-anchored molecules have also been shown to transfer between cells and stably insert in the external leaflet of the acceptor cell s plasma membrane (M.G. Low, 1998). The biological significance of this event is unclear however, the ability of GPI-anchored proteins to transfer between cells has implications for the expression of foreign proteins on the cell surface. 

In the PIP2-Ca signal transduction system, the signal is transferred from the epinephrine receptor to membrane-bound phospholipase C by G proteins. Phospholipase C hydrolyzes PIP2 to form diacylglycerol (DAG) and inositol trisphos-phate (IP3). IP3 stimulates the release of Ca from the endoplasmic reticulum. Ca and DAG activate protein kinase C. The amount of calcium bound to one of the calcium-binding proteins, calmodulin, is also increased. 

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