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.