Ca, cAMP & Lipid Signaling

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Activation of protein kinase C (PKC) is one of the earliest events in a cascade that controls a variety of cellular responses, including secretion, gene expression, proliferation, and muscle contraction (1,2). PKC isoforms belong to three groups based on calcium dependency and activators. Classical PKCs are calcium-dependent via their C2 domains and are activated by phosphatidylserine (PS), diacylglycerol (DAG), and phorbol esters (TPA, PMA) through their cysteine-rich C1 domains. Both novel and atypical PKCs are calcium-independent, but only novel PKCs are activated by PS, DAG, and phorbol esters (3-5). Members of these three PKC groups contain a pseudo-substrate or autoinhibitory domain that binds to substrate-binding sites in the catalytic domain to prevent activation in the absence of cofactors or activators. Control of PKC activity is regulated through three distinct phosphorylation events. Phosphorylation at Thr500 in the activation loop, the autophosphorylation site at Thr641, and at carboxy-terminal hydrophobic site Ser660 occurs in vivo (2). Atypical PKC isoforms lack hydrophobic region phosphorylation, which correlates with the presence of glutamic acid rather than the serine or threonine residues found in more typical PKC isoforms. Either the enzyme PDK1 or a close relative is responsible for PKC activation. A recent addition to the PKC superfamily is PKCμ (PKD), which is regulated by DAG and TPA through its C1 domain. PKD is distinguished by the presence of a PH domain and by its unique substrate recognition and Golgi localization (6). PKC-related kinases (PRK) lack the C1 domain and do not respond to DAG or phorbol esters. Phosphatidylinositol lipids activate PRKs and small Rho-family GTPases bind to the homology region 1 (HR1) to regulate PRK kinase activity (7).

Phosphorylatioin of tyrosine residues in PKCdelta are suggested to play a role in determining its functional properties. Phosphorylated tyrosine residues have been identified in the catalytic domain, regulatory domain, and the hinge of PKCdelta (8). While no clear designation of regulatory specificity has been deciphered based on phosphorylated tyrosine patterns, these various phosphorylations have been shown to decrease PKCdelta protein level, increase kinase activity or increase selectivity of substrate specificity (8-10).

1. Nishizuka, Y. (1984) Nature 308, 693-698.
2. Keranen, L.M. et al. (1995) Curr. Biol. 5, 1394-1403.
3. Mellor, H. and Parker, P.J. (1998) Biochem J. 332 (Pt 2), 281-292.
4. Ron, D. and Kazanietz, M.G. (1999) FASEB J. 13, 1658-1676.
5. Moscat, J. and Diaz-Meco, M.T. (2000) EMBO Rep. 1, 399-403.
6. Baron, C.L. and Malhotra, V. (2002) Science 295, 325-328.
7. Flynn, P. et al. (2000) J. Biol. Chem. 275, 11064-11070.
8. Steinberg, S.F. (2004) Biochem J 384, 449-59.
9. Blake, R.A. et al. (1999) Cell Growth Differ 10, 231-41.
10. Konishi, H. et al. (2001) Proc Natl Acad Sci U S A 98, 6587-92.

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PKCδ Antibody

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