Activation of protein kinase C by the 14‐3‐3 proteins homologous with Exol protein that stimulates calcium‐dependent exocytosis

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Abstract
The 14‐3‐3 proteins are a family of acidic proteins found mainly in the brain and are suggested to have a role in monoamine synthesis based on their ability to activate tyrosine and tryptophan hydroxylases in the presence of type II Ca2+/calmodulin‐dependent protein kinase. Recently, however, it has been demonstrated that a member of the 14‐3‐3 family, termed Exol, stimulates Ca2+‐dependent exocytosis in permeabilized adrenal chromaffin cells, suggesting that this protein family may influence the protein kinase C‐mediated control of Ca2+‐dependent exocytosis. Here we show that the 14‐3‐3 proteins activate protein kinase C at about 2‐fold more than the known level of the activated protein kinase, i.e. the activity of protein kinase C in the presence of Ca2+ and phospholipids. This raises the possibility that the cellular activity of protein kinase C is regulated by diverse members of the 14‐3‐3 family and that the reported ability of Exol to reactivate Ca2+‐dependent exocytosis is based on its stimulatory effect on protein kinase C activity. The 14‐3‐3 family, therefore, appears to be a multifunctional regulator of cell signalling processes mediated by two types of Ca2+‐dependent protein kinase, protein kinase C and type II calmodulin‐dependent protein kinase.