Academician Dr Tao-shih Hsieh (deceased), Distinguished Research Fellow at the Institute of Cellular and Organismic Biology (ICOB), have discovered the biochemical mechanism on how Tudor domain-containing protein 3 (TDRD3) plays a regulatory role for the enzyme Topoisomerase 3β (Top3β). This finding also uncovered a novel RNA topoisomerase activity of Top3β, the generation of double-stranded RNA circles, therefore providing new insights to the role of circular RNAs in neurodevelopment and diseases in the future. The research was published on September 20th, 2016 in Proceedings of the National Academy of Sciences of the United States of America (PNAS).
　　Top3β is a type 1A topoisomerase that works on both DNA and RNA molecules. This enzyme can associate with a multi-domain scaffolding protein, TDRD3, to participate in transcription and translation. TDRD3 has been shown to function as an epigenetic reader on nuclear chromatin and binds with Fragile X Mental Retardation Protein (FMRP) on mRNA. The mechanism of how TDRD3 acts as Top3β’s partner in regulating these cellular processes is unknown. The research team demonstrated that TDRD3 is able to stimulate Top3β’s DNA and RNA topoisomerases catalytic activities, through binding and stabilizing single-stranded regions in DNA and RNA substrates.
　　Since these regions are the preferred site for Top3β, TDRD3 therefore acts as a regulator to provide access of the enzyme to these nucleic acid substrates and to act upon them. A novel RNA topoisomerase activity of Top3β was also discovered, which is the strand annealing activity that results in the generation of double-stranded circular RNA from two single-stranded circular RNA. The formation of these Top3β’s reaction products, and also of the more complex multimeric RNA circles, was enhanced by TDRD3. The RNA topoisomerase assay developed here provides a facile biochemical method to produce double-stranded circular RNA to study its role in gene regulation. The work thus reveals the mechanistic insights on how TDRD3 acts as a mediator for Top3β in executing its topoisomerase activities.
　　This research was carried out in ICOB, under the direction of Dr Tao-shih Hsieh, with collaboration with Dr Michael D. Been from Department of Biochemistry, Duke University. The co-first authors are Grace Ee-Lu Siaw, a PhD candidate of Taiwan International Graduate Program-Molecular Cell Biology and National Defense Medical Center, and Dr I-Fen Liu, a postdoctoral fellow in Dr Hsieh’s laboratory. The research project was funded by grants from the Ministry of Science and Technology of Taiwan and Academia Sinica.
The full article entitled “DNA and RNA topoisomerase activities of Top3β are promoted by mediator protein Tudor domain-containing protein 3”is available at the PNAS website at:
http://www.pnas.org/content/113/38/E5544.long