DMA-Tudor interaction modules: a novel approach to Survival Motor Neuron protein (SMN) and Cajal body function

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Survival Motor Neuron protein (SMN) deficiencies cause Spinal Muscular Atrophy (SMA), the most common genetic cause of infant and toddler mortality. Although SMN has been implicated in transcription, RNA processing and translation, the molecular basis of motoneuron loss is still unknown. We recently discovered a novel activity of SMN: biomolecular condensation caused by SMN’s globular tudor domain (SMNTud), which binds ligands modified by dimethylarginine (DMA). Although there are hundreds of DMA-modified proteins in cells, the correspondence between tudor domains and their DMA ligands remains unknown. This understudied post-translational modification is potentially dynamic and has emerging roles in multiple neurological diseases through the altered functions of cellular compartments known as biomolecular condensates (BMCs). Our central hypothesis is SMNTud binding to DMA ligands plays critical roles in cellular organization, which are especially vulnerable in the neuromuscular system. Our findings highlight a critical need to comprehensively determine the DMA ligands of SMNTud and the activities of the interaction modules they form. SMN is diffusely cytoplasmic and present in nuclear BMCs called Cajal bodies, which are essential for embryonic development. Cajal bodies are scaffolded by a known SMNTud ligand and altered in SMA. During stress, SMN forms BMCs in the cytoplasm. Our preliminary results directly implicate DMA binding and biomolecular condensation in SMA, because SMNTud activity was blocked by a single amino acid mutation, E134K, that blocks binding to DMA ligands and causes SMA. Chemical inhibitors of DMA drastically altered the composition and substructure of Cajal bodies, which we determined for the first time with our collaborator and co-I, Dr Joerg Bewersdorf. The identities of the full complement of DMA ligands that bind SMNTud and those that affect Cajal bodies have been unknown until now. Our preliminary data reveal ~70 novel and specific SMNTud ligands, indicating that new insights relevant to SMA are within our grasp. The overall objectives of this new application are to (i) identify the DMA ligands of SMNTud that mediate biomolecular condensation, (ii) understand the dynamicity of asymmetric (aDMA) and symmetric (sDMA) installed by arginine methyltransferases and removed by demethylases with respect to the structure and function of BMCs, and (iii) reveal novel SMN functions by leveraging DMA-SMNTud interaction modules. Our rationale is that objectives concerning DMA-SMNTud interaction modules will be most accessible to rigorous analysis through a combination of biochemistry and state-of-the-art imaging, using mouse and zebrafish cells and tissues, including motoneurons, with which we have expertise. If achieved, our aims will discover novel SMN binding partners and functions of SMNTud in biomolecular condensation. The regulatory potential and dynamicity of the DMA modification, a source of tissue specificity and disease etiology, will be determined. Identification of DMA-SMNTud interaction modules will suggest new therapeutic approaches for SMA.