Figure 6.

S62- or S104-phosphorylated GST-ARPP19 proteins are substrates of platelet PP2A, but only S62-phosphorylated GST-ARPP19 is a potent PP2A inhibitor. (a) GST-ARPP19 phosphorylated at S104 (up) or S62 (down) were incubated with human platelet lysates (obtained from 1.5 × 10⁹ platelets/mL) alone or in the presence of thio-pS62 GST-ARPP19, 2 nM OA, or non-phosphorylated GST-ARPP19 at 37 °C. Samples were taken in Laemmli buffer before (0 min) and after 5, 15 and 30 min of ARPP19 addition to lysate and analysed by immunoblotting using phosphospecific antibodies. Representative western blots are shown. (b) Dephosphorylation of PKA-phosphorylated (S104) GST-ARPP19 in human platelet lysates. The level of phosphorylation was densitometrically quantified and normalized to the 0 min control sample. Data are presented as means +/− SD of three independent experiments. (c) Dephosphorylation of MASTL-phosphorylated (S62) GST-ARPP19 in human platelet lysate. The level of phosphorylation was densitometrically quantified and normalized to the 0 min control sample. Data are presented as means +/− SD of three independent experiments.