Regulatory Interplay Between SR Proteins Governs CLK1 Kinase Splice Variants Production

The CLK1 kinase phosphorylates SR proteins to modulate their splicing regulatory activity. Skipping of alternative exon 4 on the CLK1 pre-mRNA produces a CLK1 variant lacking the catalytic site. Here, we aimed to understand how various SR proteins integrate into the regulatory program that controls CLK1 exon 4 splicing. Previously, we observed that the depletion of SRSF10 promoted the inclusion of CLK1 exon 4. Using expression of tagged proteins and CRISPR/Cas9-mediated knockouts in HCT116 cells, we now identify TRA2b, TRA2a, SRSF4, SRSF5, SRSF7, SRSF8 and SRSF9 as activators of exon 4 inclusion. In contrast, SRSF3, SRSF10 and SRSF12 elicit exon 4 skipping. Using CRISPR/dCas13Rx and RNA immunoprecipitation assays, we map an enhancer in exon 4 interacting with TRA2b. Notably, CLK1 kinase inhibitors antagonized the repressor activity of HA-SRSF10, HA-SRSF12 and HA-SRSF3. Our results suggest that CLK1 exon 4 inclusion is determined primarily by a balance between the activities of TRA2 proteins and CLK-phosphorylated SRSF3. CLK-phosphorylated SRSF10 and SRSF12 would interact with TRA2 proteins to prevent their enhancer activity, allowing SRSF3 to enforce exon 4 skipping more efficiently. Our study provides insight into the complex regulatory network controlling the alternative splicing of CLK1, which uses CLK1-mediated phosphorylation of SR proteins to regulate the inclusion of catalytic exon 4 in CLK1 transcripts.