RBM20 phosphorylation on serine/arginine domain is crucial to regulate pre-mRNA splicing and protein shuttling in the heart

Molecular and cellular mechanisms of mutations of splicing factors in heart function are not well understood. The splicing of precursor mRNA is dependent on an essential group of splicing factors containing serine-arginine (SR) domain(s) that are critical for protein-RNA and protein-protein interaction in the spliceosome assembly. Phosphorylation of SR domains plays a key role in splicing control and the distribution of splicing factors in the cell. RNA binding motif 20 (RBM20) is a splicing factor predominantly expressed in muscle tissues with the highest expression level in the heart. However, its phosphorylation status is completely unknown up-to-date. In this study, we identified sixteen amino acid residues that are phosphorylated by middle-down mass spectrometry. Four of them are located in the SR domain, and two out of the four residues, S638 and S640, play an essential role in splicing control and facilitate RBM20 shuttling from the nucleus to the cytoplasm. Re-localization of RBM20 promotes protein aggregation in the cytoplasm. We have also verified that SR-protein kinases (SRPKs), cdc2-like kinases (CLKs) and protein kinase B (PKB or AKT) phosphorylate S638 and S640. Mutations of S638A and S640G reduce RBM20 phosphorylation and disrupt the splicing. Taken together, we determine the phosphorylation status of RBM20 and provide the first evidence that phosphorylation on SR domain is crucial for pre-mRNA splicing and protein trafficking. Our findings reveal a new role of RBM20 via protein shuttling in cardiac function.