DMA-tudor interaction modules control the specificity of in vivo condensates

Biomolecular condensation is a widespread mechanism of cellular compartmentalization. Because the survival of motor neuron protein (SMN) is required for the formation of three different membraneless organelles (MLOs), we hypothesized that at least one region of SMN employs a unifying mechanism of condensation. Unexpectedly, we show here that SMNs globular tudor domain was sufficient for dimerization-induced condensation in vivo, while its two intrinsically disordered regions (IDRs) were not. The condensate-forming property of the SMN tudor domain required binding to its ligand, dimethylarginine (DMA), and was shared by at least seven additional tudor domains in six different proteins. Remarkably, asymmetric versus symmetric DMA determined whether two distinct nuclear MLOs - gems and Cajal bodies - were separate or overlapping. These findings show that the combination of a tudor domain bound to its DMA ligand - DMA-tudor - represents a versatile yet specific interaction module that regulates MLO assembly and defines their composition.