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. Author manuscript; available in PMC: 2020 Sep 19.
Published in final edited form as: Cell Chem Biol. 2019 Jul 25;26(9):1295–1305.e6. doi: 10.1016/j.chembiol.2019.07.001

Figure 4: PG3 compounds inhibit intracellular C6 activity, PG3d is an irreversible, non-competitive C6 inhibitor.

Figure 4:

(A) COS-7 cells were transfected with C6 and treated with different amounts of PG3 compounds in the media. The amount of intracellularly cleaved lamin A was analysed by ELISA in cell lysates (Ehrnhoefer, et al., 2011). (B) C6 was co-transfected with mHTT1–1212 into COS-7 cells, and cells were treated with PG3d or the cell-permeable pan-caspase inhibitor Q-VD-OPh in the media. Lysates were analysed by Western blotting with HTT antibody MAB2166, and the amount of cleaved HTT was quantified. (C) Recombinant C6 enzyme was incubated for 1h with 10 μM inhibitor, followed by two wash steps with ultrafiltration spin columns and a TR-FRET-based assay for HTT cleavage. While the reversible inhibitor VDVAD-CHO loses inhibition after washout, PG3d maintains its activity similar to the irreversible inhibitor zVAD-fmk. (D) 60 min pre-incubation of C6 with inhibitor leads to a shift in dose-response curves for PG3d, but not for the reversible inhibitor VDVAD-CHO in a TR-FRET based assay for HTT cleavage. (E) Recombinant C6 enzyme was incubated with 10 μM inhibitor or 5% SDS, followed by LE22 labeling and Western blotting. The irreversible active-site inhibitor zVAD-fmk and the denaturing SDS treatment prevent LE22 labeling, while the reversible inhibitor VDVAD-CHO is replaced by the irreversible binding of the LE22 probe. This leads to a signal in the LE22 channel as well as a band shift detected with the C6 antibody HD91. Although PG3d demonstrated irreversibility in (C), it does not prevent labelling of the active site with LE22, suggesting it is a non-competitive inhibitor.

IC50 values for (A) are listed in Suppl. Table S1. Graphed data are pooled results of three independent experiments graphed with S.E.M., Western blots are representative images. Statistical significance was determined by Student’s t-test in (B), and 2-way ANOVA (inhibitor p <0.0001, washout p <0.0001) with Tukey’s post-hoc correction in (C). **: p <0.01, ***: p <0.001.