Phospho-inositol-5 kinase

In the kiss-and-run mode exocytosis and endocytosis are directly coupled to each other, while in the case of classical complete vesicle fusion, exocytosis and slow clathrin-mediated endocytosis are timely and spatially separated. However, it appears that also in the latter case exocytosis and endocytosis occur coordinated, as both are stimulated by an increase of the cytoplasmic calcium concentration. It has been shown that after calcium entry the enzyme phospho-inositol-5 kinase Iy, which is enriched in the synapse, catalyzes the synthesis of phosphatidylinos-itol (4,5)-bisphosphate and that this mechanism is important for synaptic vesicle trafficking (Di Paolo et al. 2004). As many proteins involved in clathrin-mediated endocytosis are recruited to the plasma membrane by binding to phosphatidylinosi-tol (4,5)-bisphosphate (e.g., amphiphysin, dynamin, epsin, AP-180, and AP-2) it is attractive to speculate that elevated levels of calcium mediate the recruitment of en-docytic proteins to the plasma membrane by this mechanism. The increased level of phosphatidylinositol (4,5)-bisphosphate could be in part degraded by synaptojanin that thereby initiates the disassembly of the clathrin coat. Hence, calcium-induced transient increases in the level of phosphatidylinositol (4,5)-bisphosphate appear to play a central role for coupling exocytosis to clathrin-mediated endocytosis. In addition, it has been demonstrated that calcium also leads to the dephosphorylation of endocytic proteins as amphiphysin, dynamin, and synaptojanin, which in vitro is important for efficient coat assembly (Cousin and Robinson 2001). 

One of the most studied examples of signaling with membrane lipids is provided by the phospho-inositide cascade, which is pictured in Fig. 11-9. Six or more phosphate esters of phosphatidylinositol (PI) are generated by the action of kinases.219 220 More than 100 extracellular signaling molecules activate specific isozyme forms of phospholipase C,221-224 releasing 20 or more different inositol phosphates from these... 

Ptdlns 4,5-P2 is formed by a two-stage phosphorylation of Ptdlns, which is first phosphorylated at the 4-position of its inositol headgroup by a specific kinase to form Ptdlns 4-P this is in turn phosphorylated at the 5-position to give Ptdlns 4,5-P2, the substrate for receptor-mediated lipid hydrolysis. As well as the two kinases responsible for the stepwise phosphorylation of Ptdlns there are corresponding phospho-... 

Before we leave our brief survey of intracellular Ca -binding proteins, we must write a few lines about an important Ca " -regulated kinase (a phospho-rylating enzyme), i.e., protein kinase C (PKC). The activity of this enzyme, or rather family of enzymes,appears to be regulated by three factors phospholipids, in particular phosphatidylserine diacyl-glycerols, one of the products of inositol lipid breakdown and Ca " ions. The high-activity form of PKC, which appears responsible for much of the phosphorylation activity of many cells, is presumably membrane-bound, whereas the low-activity form may be partly cytosolic (Figure 3.27). The schematic structure of rabbit PKC (Mr 11 kDa)... 

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