The proteins SFPQ (splicing Factor Proline/Glutamine rich), NONO (Non-POU domain-containing octamer-binding protein) and PSPC1 (paraspeckle protein component 1), are mammalian members of the Drosophila Behaviour/Human Splicing (DBHS) family. SFPQ, NONO and PSPC1 are involved in all steps of post-transcriptional gene regulation, as well as DNA repair and are primarily located in paraspeckles in mammals: liquid phase-separated, ribonucleoprotein sub-nuclear bodies scaffolded by NEAT1 long non-coding RNA. Structured and low-complexity regions provide polyvalent interaction interfaces which facilitate homo- and heterodimerisation, polymerisation, interaction with RNA, and liquid phase-separation. The strength and competition of these interaction modes defines their ability to dissociate from paraspeckles to fulfil functional roles throughout the nucleus or the cytoplasm.
Despite the importance of the RNA-related functions of DBHS proteins, the potential for therapeutic intervention in RNA-binding is underexplored. While the location of RNA-binding in DBHS proteins has been identified in previous studies, and RNA-binding is critical to the cancer pathology of NONO, the binding site has long been considered “undruggable”: a relatively open, flat, hydrophobic landing pad. However, the recent discovery of a family of thiol-reactive small molecules which stereospecifically target NONO has opened a new door to modulating NONO’s gene regulatory activity. These compounds appear to trap unspliced oncogene transcripts in complex with NONO. We have determined the crystal structure of NONO bound covalently to one of these thiol-reactive compounds compound (R)-SKBG-1 and demonstrated using microscale thermophoresis that SKBG-1 increases the affinity of NONO for RNA. The structure offers an explanation of how the compound may increase affinity for RNA by acting as a “super-sidechain”, and suggests a path to selective retention of unspliced mRNA. [1]