Association of protein phosphatase 2A with its substrate vimentin intermediate filaments in 9L rat brain tumor cells

The importance of protein phosphatases in maintaining the integrity of intermediate filaments is supported by the fact that intermediate filaments would undergo a massive reorganization in cells treated with inhibitors of protein phosphatases 1 and 2A. Herein we used okadaic acid to investigate the differential roles of protein phosphatases 1 and 2A in the maintenance of intermediate filament integrity in 9L rat brain tumor cells. Protein phosphatase 2A activity was substantially inhibited after treatment with 400 nM okadaic acid for 2 h, whereas the activity of protein phosphatase 1 was only slightly affected. Furthermore, protein phosphatase 2A shows selective specificity toward phosphovimentin, which was immunologically precipitated from isotopically labeled and okadaic acid-treated cells. Further biochemical fractionation and microscopic studies revealed that vimentin intermediate filaments were colocalized with protein phosphatase 2A, but not protein phosphatase 1, in control cells. On okadaic acid treatment, vimentin filament disassembled and protein phosphatase 2A redistributed throughout the cytoplasm, suggesting that these two proteins separate from each other, whereas protein phosphatase 2A was inhibited. This working hypothesis was further supported by treatment with a low concentration (40 nM) of okadaic acid, which causes the same phenomenon. Taken together, our results showed that protein phosphatase 2A could be assigned to the intermediate filaments to serve the physiological role in maintaining the proper phosphorylation level of intermediate filaments in normal cells. This finding should pave the way for the elucidation of the regulatory mechanism of intermediate filament organization governed by protein phosphorylation.