Figure 1A 293FT cells were transfected with 3xFlag-Usp27x (pFCMV7.1 vector backbone) together with a construct driving expression of untagged BimEL or 3xHA-BimEL(both pMIG vector backbone). Cells were lysed, and 3xHA-BimEL was immunoprecipitated with anti-HA-antibodies. IP products were tested by Western blotting for the presence of Bim and FLAG-Usp27x probing with antibodies against Bim or against the FLAG-peptide. See also Appendix Fig. S1B. Western blots show representative of n≥3 independent experiments.B Usp27x binds a mutant of Bim incapable of binding to anti-apoptotic Bcl-2-proteins. 293FT cells transfected with constructs encoding 3xFLAG-Usp27x and 3xHA-tagged BimEL (see A, 2µg each) or 3xHA-tagged BimEL∆∆(a mutant with two mutations in the BH3-domain, incapable of binding antiapoptotic Bcl-2proteins [50]) were immunoprecipitated from whole cell extracts using anti-HA-resin. Bim and Usp27x were detected with anti-HA- and anti-FLAG-antibodies as indicated; antiapoptotic proteins: Mcl-1, Bcl-XL, Bcl-2. The caspase-inhibitor Q-VD-OPh (QVD) was added to the cultures described in A and B to inhibit Bim-induced apoptosis. Western blots are representative of n=3 independent experiments.C Usp27x interacts with endogenous BimEL independently of its catalytic activity293FT cells either carrying 3xFlag-Usp27x (293FT-TetR-3xFlag-usp27x) or the catalytic inactive mutant 3xFlag-Usp27xC87A under the control of the Tet-repressor were treated for 24h with dox to induce expression of Usp27x or Usp27xC87A. In all conditions PMA (to induce Bim-ubiquitination, 16.2 nM), and Q-VD-OPh (to inhibit apoptosis, 10 µM, see Fig. 3) were added at the time of Usp27x-induction. MG132 (to prevent Bim-degradation, 40 µM) was added in all conditions 4h prior to cell lysis. 3xFlag tagged Usp27x or Usp27xC87A was immunoprecipitated from whole cell lysates using anti-Flag resin. Interaction with BimEL or β-TrCP was detected by Western blotting using anti-Bim or anti-β-TrCP antibodies. Western blots are representative of n≥3 independent experiments.D Usp27x expression does not inhibit interaction of BimEL to β-TrCP293FT-TetR-3xFlag-Usp27x cells were transfected with pMIG-3xHA-BimEL in the presence of both PMA and QVD. At the same time dox (to induce 3xFlag-Usp27x) was added as indicated and 20h later cells were treated with MG132 [40µM] for additional 4h.Cells were lysed and 3xHA-BimEL was immunoprecipitated as described above. As a control HA-matrix was used without lysate to rule out any unspecific signal that might come from the immobilized anti HA-antibody (beads). Western blots show representative of n=2 independent experiments.E Binding of Usp27x to BimEL can be blocked by the MEK-inhibitor UO126293FT-TetR-3xFlag-Usp27x cells (see C) were treated for 24h with dox to induce expression of Usp27x. In all conditions PMA [16.2 nM], and Q-VD-OPh [10 µM] were added at the time of Usp27x-induction. As a control, cells were pretreated with UO126 [10µM] for 30 minutes to block the PMA stimulated ERK pathway before the addition of dox plus PMA plus QVD. 3xFlag tagged Usp27x was immuno-precipitated from whole cell lysates using anti-Flag resin. Interaction with BimEL was detected by Western blotting using anti-Bim antibodies. As a control Flag-matrix (beads) was used alongside the IP under same conditions but without addition of protein lysates. Western blots are representative of n=4 independent experiments (see also Appendix Fig. S2B).F Binding of Usp27xto BimEL does not require β-TrCP293FT-TetR-3xFlag-Usp27x cells were transfected with control siRNA (siCo3) or siRNA specific for β-TrCP. 48h later cells were stimulated with PMA plus dox plus QVD for additional 24h. After cell lysis, Flag-Usp27x was immuno-precipitated using anti-Flag-matrix, and BimEL and β-TrCP bound to Usp27x were identified by Western blotting. Blots are representative of n=2 independent experiments.G Usp27x binds specifically to BimEL293FT-TetR-3xFlag-Usp27x cells or 293FT-3xFlag-Usp27x cells with a specific deletion of the BimELprotein were treated with dox, PMA and QVD as in C. Cells were lysed, and lysates were subjected to anti-Flagimmunoprecipitation. 3xFlag-Usp27x was detected using anti-FLAG antibodies. Blots are representative of n=3 independent experiments.