Figure 7A, B Validation of the effect of SRSF9 on retention/splicing of HSATIII target introns by qRT-PCR. The relative expression levels of intron-retaining (IR) forms (A) and spliced forms (B) of newly transcribed RNAs during the recovery period are shown. Expression levels were calculated as the ratio of each RNA to GAPDH mRNA and were normalized to the control. C, D Time course analysis of the splicing patterns of CLK1 and TAF1D intron-retaining RNAs in control and HSATIII knockdown cells by semi-quantitative RT-PCR. Arrows indicate the positions of primers for RT-PCR. GAPDH mRNA was used as an internal control. The experiment was performed in triplicate. Quantification of the data shown in (C), E Scheme for analyzing the effect of CLK1 inhibition on intron retention by newly transcribed RNAs after stress removal. CHX, Cycloheximide; EU, 5-Ethynyl uridine. F Validation of the requirement of CLK1 catalytic activity for promotion of intron retention by qRT-PCR. Data are shown as the mean±SD (n=3). G The diagrams show the domain structures of wild-type (WT) and mutant (mut) SRSF9, in which all SR/SP dipeptides were replaced by AR/AP. FLAG-SRSF9-WT and FLAG-SRSF9-mut were extracted from transfected HeLa cells under normal (37°C) and thermal stress (42°C for 2 h) conditions, separated in a Phos-tag gel, and detected by western blot using an anti-FLAG antibody. The arrowhead indicates the thermal stress-induced de-phosphorylated form of FLAG-SRSF9-WT. The role of phosphorylation of SRSF9 on splicing of the TAF1D (H) splicing reporter plasmid was transfected into HeLa cells along with a siRNA (control or SRSF9-specific) and a siRNA-resistant SRSF9-WT or SRSF9-mut expression plasmid (or pEGFP-C1 vector as the empty vehicle (-)) 48 h before the assay. Semi-quantitative RT-PCR analyses revealed the splicing patterns of the reporter RNAs purified from the thermal stress-exposed cells (42°C for 2 h and 37°C for 1 h). The positions of the PCR primers are indicated by arrows. Graphs represent the relative changes in the spliced/unspliced ratios, normalized to the control sample with control siRNA and empty plasmid (-). I The role of phosphorylation of SRSF9 on splicing of the DNAJB9 (I) reporters. Each splicing reporter plasmid was transfected into HeLa cells along with a siRNA (control or SRSF9-specific) and a siRNA-resistant SRSF9-WT or SRSF9-mut expression plasmid (or pEGFP-C1 vector as the empty vehicle (-)) 48 h before the assay. Semi-quantitative RT-PCR analyses revealed the splicing patterns of the reporter RNAs purified from the thermal stress-exposed cells (42°C for 2 h and 37°C for 1 h). The positions of the PCR primers are indicated by arrows. Graphs represent the relative changes in the spliced/unspliced ratios, normalized to the control sample with control siRNA and empty plasmid (-). J A model for the function of nSBs in splicing control. Thermal stress-induced HSATIII integrates subsets of de-phosphorylated SRSFs and SR-related proteins. After stress removal, CLK1 relocates to nSBs to promote rapid re-phosphorylation of the sequestrated SRSFs, especially SRSF9. Re-phosphorylated SRSF9 suppresses splicing of target introns to accumulate intron-retaining RNAs in the nucleus.