RNA-binding protein that acts as a regulator of mRNA splicing of a subset of genes encoding key structural proteins involved in cardiac development, such as TTN (Titin), CACNA1C, CAMK2D or PDLIM5/ENH (PubMed:33188278). Acts as a repressor of mRNA splicing: specifically binds the 5'UCUU-3' motif that is predominantly found within intronic sequences of pre-mRNAs, leading to the exclusion of specific exons in target transcripts (By similarity). RBM20-mediated exon skipping is hormone-dependent and is essential for TTN isoform transition in both cardiac and skeletal muscles (By similarity). RBM20-mediated exon skipping of TTN provides substrates for the formation of circular RNA (circRNAs) from the TTN transcripts (By similarity). Together with RBM24, promotes the expression of short isoforms of PDLIM5/ENH in cardiomyocytes (By similarity). Associates with components of the U1 and U2 U1 small nuclear ribonucleoprotein complexes. The active form that regulates alternative splicing localizes to the nucleus (PubMed:33188278). Also localizes to cytoplasmic ribonucleoprotein granules; localization to cytoplasmic ribonucleoprotein granules plays an important regulatory role (PubMed:33188278). Subcellular localization is regulated by phosphorylation of different parts of the protein: while phosphorylation of the RS (arginine/serine-rich) region promotes nuclear localization, phosphorylation of the C-terminal disordered region promotes localization to cytoplasmic ribonucleoprotein granules (By similarity). Phosphorylation regulates the subcellular localization (By similarity). Phosphorylation of Ser-637 and Ser-639 in the RS (arginine/serine-rich) region promotes nuclear localization of the protein (By similarity). In contrast, phosphorylation of the C-terminal disordered region promotes localization to cytoplasmic ribonucleoprotein granules (By similarity).