Phorbol esters cause long term activation of protein kinase C (PKC) and frequently the down-regulation of PKC protein levels in mammalian cells. Mammalian PKC-, -, and - down-regulate in response to phorbol esters when expressed in Schizosaccharomyces pombe. However, PKC- does not down-regulate in S. pombe, in contrast to the behavior of this isotype in mammalian cells. Co-expression of PKC- or - with PKC- in S. pombe renders PKC- susceptible to down-regulation. A protein kinase defective form of PKC- does not down-regulate efficiently in S. pombe but, like PKC-, is susceptible when co-expressed with PKC- or full-length PKC-. Thus, down-regulation is a consequence of the catalytic function of certain PKC isotypes with other isotypes being affected in trans. PKC down-regulation parallels a striking accumulation of vesicles in S. pombe, suggesting a direct relationship between these events.

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