Phosphatidylinositol lipid-dependent protein

Welters, P., Takegawa, K., Emr, S.D., and Chrispeels, M.J., 1994, ATVPS34, a phosphatidylinositol 3-kinase of Arabidopsis thaliana, is an essential protein with homology to a calcium-dependent lipid binding protein. Proc. Natl. Acad. Sci. U.S.A. 91 11398-11402. 

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. 

Extraction parameters such as solvent type, mixture ratios, metal ion concentration, pH of the aqueous phase, extraction time, and temperature influence the recovery of extracted lipids and must be validated to ensure reliable results. For example, the recovery of the acidic lipids PA and phosphatidylglycerol (PG) can be less than 30% in classic Folch and Bligh Dyer extraction, where these lipids can become bound to proteins tightly (17). Lipids bound to proteins covalently are only released under appropriate conditions, which depend on the type of lipid-protein linkage. For example, ceramides bound to protein of the comi-fled envelop in the human skin (18) can be extracted after mild alkaline hydrolysis of the ester linkage between hpid and protein. Special conditions are required for extraction of more polar lipids such as gangliosides, lysophospholipids and lysosphin-golipids, or phosphatidylinositol-phosphates. 

The C-terminal domain is a Ca -dependent C-type lechn (Chapter 4), while the N-terminal domain is involved in oligomer formation through disulfide bridges. The overall structure is similar to that of the complement protein Clq (Chapter 31). Protein D is also collagen-like but evidently plays a very different funchonal role than SP-A. The latter associates with the major surfactant lipids but SP-D does not. It does bind phosphatidylinositol and gluco-sylceramide, lipids that are present in small amounts. Perhaps SP-D helps to remove these polar lipids which might interfere wifh surfactant action. Both proteins A and D may also have functions in the immune system. ... 

Many hormones, such as the hormonal amines and all pep-tidic hormones, are unable to penetrate the lipid matrix of the cell membrane, and thus depend on the presence of receptor sites at the surface of target cells. As listed in Table 30-4, there are several types of cell membrane receptors for these hormones, each of which is coupled to a distinct set of intracellular postreceptor pathways. The surface receptors all initiate postreceptor events that involve the phosphorylation of one or more intracellular proteins, some of which are enzymes whose activities depend on the state of phosphorylation. In two of these cases, an intracellular second messenger is utilized to implement the hormonal action and involves G-protein-coupled receptors. One is coupled to the adenylate cyclase-cAMP system and the other is associated with the phosphatidylinositol-Ca + pathway (IP3 pathway). 

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