Fig. 1.

USP15 regulates the cellular level of TOP2A. a TOP2A was identified from a data set of proteins that differed in abundance on USP15 depletion. SILAC-labelled A549 cells were transfected with pooled siRNA targeting USP15 or a non-targeting control (siCON1) for mass spectrometry analysis. Multiple TOP2A peptides were downregulated in USP15 depleted cells across three independent experiments (left); spectra for an example peptide shown (right). b, c TOP2A protein expression correlates with total USP15 levels. A549 cells were transfected with siRNAs as indicated and whole-cell lysates analysed 72 h later. b Representative immunoblot, with quantification of TOP2A protein expression relative to actin below (mean of three independent experiments, error bars SD, one-way ANOVA with Tukey’s multiple comparison test **P ≤ 0.01, ***P ≤ 0.005). c Correlation of TOP2A protein and total USP15 protein in these experiments. d TOP2A mRNA level was not affected by USP15 depletion. qRT-PCR analysis of TOP2A relative to actin (mean of four independent experiments, error bars SD, ns = not significant by one-way ANOVA with Tukey’s multiple comparison test)