Protein kinase C activation by acidic proteins including 14-3-3

14-3-3 proteins may function as adapter or scaffold proteins in signal transduction pathways. We reported previously that several 14-3-3 isotypes bind to protein kinase C (PKC)-zeta and facilitate coupling of PKC-zeta to Raf-1 [van der Hoeven, van der Wal, Ruurs, van Dijk and van Blitterswijk (2000) Biochem. J. 345, 297-306], an event that boosts the mitogen-activated protein kinase (ERK) pathway in Rat-1 fibroblasts. The present work investigated whether bound 14-3-3 would affect PKC-zeta activity. Using recombinant 14-3-3 proteins and purified PKC-zeta in a convenient, newly developed in vitro kinase assay, we found that 14-3-3 proteins stimulated PKC-zeta activity in a dose-dependent fashion up to approx. 2.5-fold. Activation of PKC-zeta by 14-3-3 isotypes was unrelated to their mutual affinity, estimated by co-immunoprecipitation from COS cell lysates. Accordingly, PKC-zeta with a defective (point-mutated) 14-3-3-binding site, showed the same 14-3-3-stimulated activity as wild-type PKC-zeta. As 14-13-3 proteins are acidic, we tested several other acidic proteins, which turned out to stimulate PKC-zeta activity in a similar fashion, whereas neutral or basic proteins did not. These effects were not restricted to the atypical PKC-zeta, but were also found for classical PKC. Together, the results suggest that the stimulation of PKC activity by 14-3-3 proteins is non-specific and solely due to the acidic nature of these proteins, quite similar to that known for acidic lipids.