Regulation of inositol phospholipid-pelated 2nd messengers

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Phosphoinositides (PI) turnover produces second messengers such as diacylglycerol (DG) and inositol triphosphate in response to external stimuli. DG acts as an activator of several forms of protein kinase C and is converted to phosphatidic acid (PA) by DG kinase. On the other hand, PA is cleaved from phosphatidylcholine and converted to DG by PA phosphatase. PA and its metabolic derivative, lysophosphatidic acid, are potent mitogens and activators. Therefore, DG kinase and PA phosphatase play crucial roles in regulation of the relative concentration of DG and PA,both of which serve as signal mediators as well as intermediates of glycerolipid synthesis.The present study disclosed for the first time the heterogeneity of DG kinase in terms of molecular structure, biochemical characteristics and gene expression localization in the brain and clarified the molecular identity of PA phosphatase : Four subtypes of DG kinase (termed type I-IV) were identified by H.Kondo (Head investigator) and h … More is group, its fifth subtype (type delta) was identified by H.Kanoh (co-investigator), its sixth subtype (type thita) by W.J.van Blitterswijk (co-investigator), its seventh by J.A.Glomset (co-investigator), while PA phosphatase (type II) was identified by H.Kanoh. The DG kinase type I is of soluble form and localized in the oligodendrocyte, but not neurons, and the absence of its expression is seen in the brain of myelin-deficient (shiverer) mice, suggesting the role in formation and maintenance of the myelin. The type II DG kinase is of membrane-associated from and localized in the medium-spiny neurons, neurons in the nucleus accumbens and olfactory tubercle, and in the endocrine cells of the pituitary inermediate lobe, suggesting the involvement in the dopaminergic transmission. The type III is localized dominantly in the Purkinje neurons and substantially in the granule cells of the cerebellum, suggesting its role in the cerebellar motor-control. The type IV is localized in the cerebral and cebellar cortical neurons, but peculiar in the molecular structure : while the former three contain two EF-hand and zinc-finger motifs in addition to the ATP-binding domain, the type IV contains no EF-hand motifs, but possesses ankyrin-like repeats. This structure has a high homology to rdgA (a gene of Drosophila inducing the retinal degeneration). A nuclear targeting motif is also contained and the transfection of this cDNA into the fibroblast results in localization of its protein in the nucleus, suggesting its role in the regulation of transcription. The delta type contains a pleckstrin homology domain and a DPH C-terminal tail homology domain, while the thita type contains three cysteine-rich domains, a proline-rich region and a pleckstrin homology domain with overlapping ras-associating domain. The DG kinase subtype by J.A.Glomset is a membrane-bound and selectively phosphrylates arachidonoyl-DG,indicating that this is specific in the PI turnover. The type II PA phosphatase is membrane-bound and its N-terminal sequence is conserved in the partial cDNA of hic53, a H O -inducible gene. Less