Figure 2

Hypotheses of PP2A activity. (A) The IKK phosphorylation curve consists of two phases. Phase I is defined by an increase of phosphorylated IKK, indicating a predominant kinase activity. Phosphorylated IKK decreases in phase II, indicating a predominant phosphatase activity. Both activation of PP2A by IL-1 and constitutive PP2A activity could explain this behaviour. Scaling is different for PP2A and the IKK kinase. (B) Cells were left untreated or were stimulated with IL-1 for 15 min and 2 h respectively. Subsequently, PP2Ac was immunoprecipitated and subjected to an in vitro phosphatase assay using a threonine-phosphopeptide as a substrate. Pooled data of three independently performed experiments are depicted. (C) Cells were pre-stimulated or not with 20 μM MG132 for 1 h. Subsequently cells were left untreated or stimulated with IL-1 for 15 min or 2 h. Phosphorylation status of IKKβ and protein status of IκBα were determined by Western-blot analysis using α-tubulin as loading control.