Structure and properties of a ubiquitously expressed protein kinase C, nPKCδ

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Abstract
CDNA clones coding for novel protein kinase C δ (nPKCδ) were isolated from a mouse brain cDNA library. Mouse nPKCδ consists of 674 amino acid residues and has sequence identity of 95% with rat nPKCδ. Antiserum raised against a C‐terminal peptide of rat nPKCδ identified a 79‐kDa protein in COS cells transfected with a mouse nPKCδ cDNA expression plasmid. nPKCδ expressed in COS1 cells had phorbol‐ester‐binding activity and protein kinase activity in a phorbol‐ester‐or diacylglycerol‐dependent manner, like conventional protein kinase C (cPKC) isozymes and nPKCδ. However, nPKCδ, like nPKCδ, is not activated by Ca2+, a known activator of cPKCs, and requires lower concentrations of Mg2+ for full activation than cPKCs. Moreover, apparent kinetic constants for synthetic oligopeptides (MBP4–14, EGFR peptide and ɛ‐peptide) were quite different between nPKCδ and cPKC in two different conditions. Among various phospholipids tested, phosphatidylinositol is the most potent activator of nPKCδ, in clear contrast to cPKCs and nPKCδ. Limited proteolysis of nPKCδ generated a C‐terminal active fragment with a cofactor‐independent kinase activity. Northern blot analysis indicated that nPKCδ, like cPKCα, is widely distributed in almost all the tissues and cells examined and, in some cases such as fibroblast cells, exists as a major PKC type. These results suggest that nPKCδ is involved in fundamental cellular functions regulated by diacylglycerols and mimicked by phorobol esters.

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