Cytoplasmic RBM20 granules modulate non-RBM20 target splicing through interactions with other splicing factors.

RNA-binding motif protein 20 (RBM20) is a splicing factor that forms discrete nuclear speckles. Certain pathogenic RBM20 variants disrupt its nuclear localization, leading to cytoplasmic granules formation. The composition of RBM20 nuclear speckles and cytoplasmic granules, how these compartments differ from one another, and how they contribute to splicing regulation remain unclear. Here, we employed in situ proximity labeling proteomics and identified 25 and 12 proteins associated with RBM20 nuclear speckles and cytoplasmic granules, respectively. RBM20 nuclear speckles were enriched in proteins involved in splicing and transcriptional regulation, whereas cytoplasmic granules contained proteins commonly found in other cytoplasmic granule populations. Among these, CELF1 and MBNL2 were detected in both RBM20 nuclear speckles and cytoplasmic granules, as confirmed by co-localization and immunoprecipitation experiments. We further showed that CELF1- and MBNL2-regulated splicing events were disrupted in the hearts of mice carrying pathogenic Rbm20 variants but not in animals with Rbm20 loss-of-function that lack cytoplasmic granules. Moreover, reducing cytoplasmic granule burden through RBM20 knockdown in pathogenic variant knock-in mice showed a trend toward partial restoration of MBNL2-mediated splicing defects. Collectively, these findings define the distinct protein compositions of RBM20 nuclear speckles and cytoplasmic granules and suggest that cytoplasmic RBM20 granules affect the splicing of non-RBM20 target genes.