RBM14 interacts with STIL and prevents a complex formation of STIL and CPAP.Figure 3A HeLa cells immunoprecipitated with control IgG or STIL antibodies. Soluble cytosolic fractions (input) and immunoprecipitates (IPs) were analyzed by Western blotting using RBM14, STIL or CPAP antibodies. B GST pull-down assay testing interactions between purified STIL[N] (˜5 μg, aa1-1018) and GST-RBM14 [N] or [C]. The asterisks indicate non-specific bands. C Schematic of our analyses of Y2H, GST pull-down and co-immunoprecipitation of the interaction between RBM14 and STIL (see also Supplementary Fig S3). Brackets indicate the fragments tested in this study, and the interaction detected is shown with arrows. A previous study reported that the C-terminus of CPAP interacts with the fragment of STIL aa231-781, as indicated (Tang et al, 2011). D Mitotic U2OS cells treated with control or RBM14 siRNA were stained with antibodies against centrin-2 (green) and STIL (magenta). Insets show approximately twofold magnified views of fluorescent foci around the centrosome. Scale bar, 5 μm. Histograms represent frequency of mitotic cells with excess STIL foci co-localized with centrin foci. Values are mean percentages ± SEM from three independent samples (n = 30 for each condition). **P < 0.01 (one-tailed t-test). E In vitro competitive binding assay. GST pull-down experiment was performed as in (B), with purified STIL[N] and GST-RBM14[C] in the presence of the indicated amount of purified His-CPAP[SBD], His-tagged STIL-Binding Domain of CPAP. The fraction of STIL[N] bound to GST-RBM14[C] in such conditions was monitored by Western blotting using STIL antibodies which recognize the N-terminal region of STIL. Input materials were analyzed by Western blotting using the STIL or CPAP antibodies. The precipitated GST-RBM14[C] was analyzed by SDS-PAGE, stained with SimplyBlue™ Safe (Invitrogen). F Interaction between STIL and GFP-CPAP in the presence of FLAG-RBM14 FL or fragments. U2OS cells expressing GFP-CPAP were transfected with empty vector as a control, FLAG-RBM14[N], FLAG-RBM14[C] or FLAG-RBM14 FL constructs, immunoprecipitated with STIL antibodies, and the resulting IPs were analyzed by Western blotting using STIL, CPAP or FLAG antibodies. Quantification of relative protein amounts of co-immunoprecipitated GFP-CPAP in each STIL-IP fraction, normalized with the amount of STIL precipitated, is shown below the panels. Means ± SEM were calculated from three independent experiments. **P < 0.01 (one-tailed t-test). Note that we did not find any significant effect of exogenous expression of RBM14 full-length protein or deletion mutants on the expression levels of GFP-CPAP or endogenous STIL proteins in this experiment. G U2OS cells treated with the indicated siRNAs were stained with antibodies against centrin-2 (green) and C-Nap1 (magenta). Histograms represent frequency of mitotic cells with excess centrin foci at spindle poles in each condition. Values are mean percentages ± SEM from three independent experiments (n = 30 for each condition). **P < 0.01 (one-tailed t-test). Representative mitotic cells treated with siRNAs against RBM14 alone, RBM14 + STIL or RBM14 + CPAP are shown below. Insets show approximately twofold magnified views of fluorescent foci around the centrosome. Scale bar, 5 μm.