Muscleblind-like proteins dimerize by forming disulfide bonds to influence regulation of alternative splicing

Muscleblind-like (MBNL) RNA-binding proteins (RBPs) possess modular domains that contribute to their regulation of alternative splicing and RNA localization. MBNL dimerization may play a role in these functions, but the mechanism is not well understood. Here we identify a cysteine residue in the unstructured carboxy-terminal domain of MBNL1 that enables dimerization by forming intermolecular disulfide bonds. We also demonstrate that MBNL2 likewise dimerizes by forming disulfide bonds between multiple cysteines in its carboxy-terminal tail. Nucleocytoplasmic fractionation revealed a greater proportion of MBNL1 dimer in the nucleus, suggesting a nuclear function for the MBNL1 dimer. We investigated a connection between MBNL1 dimerization and MBNL1-mediated regulation of alternative splicing. To accomplish this, we mutated the MBNL1 cysteine in question to alanine (C325A) and performed RNAseq. We uncovered numerous novel splicing events sensitive to MBNL1 dimerization.