Protein phosphatase 2A suppresses MAP kinase signalling and ectopic protein expression.

Signalling by MAP kinase was examined in COS-7 cells by transiently expressing a transcription reporter system plus epitope-tagged protein phosphatase 2A catalytic subunit [(HA)3-PP2Ac]. Transactivation of a luciferase gene by GAL4-Elk-1 in serum-stimulated cells was reduced 20-fold by co-expression of wild type (HA)3-PP2Ac. This reduction of MAP kinase signalling required specific type-2A phosphatase activity, because the effects were not mimicked by co-expression of either a mutated, inactive (HA)3-PP2Ac or wild-type PP1Cdelta. Expression of (HA)3-PP2Ac was severely restricted by its own activity because 3-fold more inactive (HA)3-PP2Ac was produced. In a different assay the kinase activity of FLAG-ERK2 was 4-fold lower when co-transfected with (HA)3-PP2Ac, compared to controls. Unexpectedly, mRNA of the reporter constructs were nearly eliminated by even low level expression of (HA)3-PP2Ac in either COS7 or HEK293 cells. The results show that PP2A activity is strictly regulated and can be a limiting factor in ectopic expression of various proteins.