On April 18, 2017, Prof. Kiyoshi Nagai visited Institute of Biophysics and delivered a report entitled "CryoEM snapshots of the spliceosome at work: Towards Understanding its Catalytic Mechanism". The lecture was hosted by Prof. Zihe Rao and was part of the special lecture series named “Shizhang Bei Lecture” of Institute of Biophysics.  

Professor Nagai worked on the structure biology of RNA splicing molecular mechanisms for a lone time. In the past 20 years, only some subunits or sub-complex of the spliceosome structure has been dissected, while lacking high-resolution overall structure, as the spliceosome is a huge and complicated dynamic molecular machine. With the rapid development of cryo-EM technology in recent years, it is possible to solve high-resolution cryo-EM structure of spliceosome. Since 2015, research groups such as Nagai, Luhrmann and Yigong Shi have published a number of papers in Nature and Science to report the structure of the spliceosome in different states.  

In the presentation, Professor Nagai first introduced the chemical mechanism of spliceosome, indicating the splicing process was mainly a two-step transesterification reaction, then he introduced the cryo-EM structure of U4/U6.U5 tri-snRNP in Saccharomyces cerevisiae at 5.9 ？ resolution, which contains over 30 proteins and 3 snRNAs. Prp8 located in the center of the tri-snRNP region and assembled the complex by protein-protein interactions and protein-RNA interactions. Brr2 helicase activated the spliceosome by opening U4/U6 snRNA duplex. Professor Nagai's group improved the U4/U6.U5 tri-snRNP resolution to 3.7 ？ within one year, making the structure to be determined at near-atomic resolution. Professor Nagai also talked about the solving process of the spliceosome C complex in different states. They assembled and purified C complex of yeast spliceosome from 120 liter fermentor by the MS2-MBP tag pre-bound to pre-mRNA. The purified complex was then analyzed by biochemical method. They finally obtained a core complex C structure at resolution of 3.8 ？ by sorting, single particles picking and 3D reconstructions. In the structure, two metal ions were found in the RNA catalytic core. Professor Nagai also described their work that was recently published on the C* complex remodelling. This work reveals the transition of spliceosome from branching to exon ligation. Prp16, a DEAH-box helicase, plays a key role in this process.  

After the lecture, Prof. Nagai and the audience shared their views on the issues of common interest.