The generation of antisera specific for the priming phosphorylation sites on protein kinase Cα (PKCα) has permitted analysis of the dephosphorylation of these sites in relation to the down-regulation of the protein. It was demonstrated that these priming sites are subject to agonist-induced dephosphorylation, consistent with inactivation of the protein. Further, the process is shown to be blocked by a PKC inhibitor, indicating a requirement for PKC catalytic activity. This was corroborated by showing that a constitutively active fragment of PKCα is able to stimulate the dephosphorylation of wild-type PKCα in transfected cells. Consistent with a membrane-traffic event, the process controlled by PKC that leads to dephosphorylation is shown to be temperature-sensitive and to correlate with transient accumulation of PKCα on cytoplasmic vesicular structures. It was established that the dephosphorylation of priming sites in PKCα is not unique and occurs with other conventional PKC isotypes, demonstrating that this is a general desensitization process for this subclass of kinases. The physiological importance of this desensitization is evidenced by the behaviour of PKCβ1 in U937 cells, where dephosphorylation of the activation loop site is shown to be a function of cell density.

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