Protein kinase C (PKC) is a key enzyme in the intracellular signaling network. Upon activation by 12-O-tetradecanoylphorbol 13-acetate, the alpha-isoform of PKC translocates to the detergent-soluble and the detergent-insoluble fractions. Besides cofactors, the activity and stability of this protein is critically regulated by multisite phosphorylations. At least three distinct sites, Thr497, Thr638 and Ser657, are involved. We have previously shown that the replacement of Ser657 by alanine leads to a premature down-regulation in the detergent soluble compartment of LLC-PK1 cells [Gysin, S. & Imber, R. (1996) Eur. J. Biochem. 240, 747-750]. More detailed analysis revealed that, in contrast to the wild-type molecule, the down-regulation of the mutant protein is in vivo preceded by a rapid dephosphorylation after phorbol-ester-induced translocation to both the detergent-soluble and insoluble compartments. The [Ala657]PKC-alpha mutant protein molecule showed in vitro a strongly increased sensitivity towards protein phosphatase 2A whereas its overall proteolytic sensitivity remained unchanged when compared to wild type. The in vitro studies led to the suggestion that further dephosphorylation of the mutant protein is a prerequisite in order to become proteolytically down-regulated. Therefore phosphorylation of Ser657 controls the duration of activation of this PKC isozyme upon agonist-induced translocation by preventing premature proteolytic down-regulation via protecting the protein from dephosphorylation.