The spliceosome is a sophisticated molecular machine that guarantees the accurate assembly of genetic information from the genome into mature mRNA after the transcription of mRNA precursors. This splicing process is essential for producing proteins necessary for an organism’s crucial functions. When the spliceosome malfunctions, it can result in serious illnesses. For the first time, researchers at the Heidelberg University Biochemistry Center (BZH) have successfully visualized a dysfunctional ‘blocked’ spliceosome in high resolution and reconstructed its recognition and removal processes within the cell. This research was a collaboration with scientists from the Australian National University.
All living organisms have their genetic information stored in DNA, with a majority of genes in complex organisms arranged in a mosaic pattern. To access the instructions for protein synthesis embedded in these genetic fragments, cells first create precursors of mRNA, or messenger RNA. The spliceosome’s role is to transform these precursors into mature, functioning mRNA by removing the non-coding sections (introns) and connecting the coding sections (exons) to create a continuous information strand. Mistakes during splicing are a leading cause of hereditary genetic disorders and are linked to neurodevelopmental conditions and cancers. While it was known that spliceosomes have quality control mechanisms, the specific mechanisms were not previously understood.
In their experiments, BZH researchers, led by director Prof. Dr. Irmgard Sinning, utilized the fission yeast Schizosaccharomyces pombe, a common model organism in cell biology. By applying molecular markers, they identified and purified defective spliceosomes for structural analysis using cryo-electron microscopy. “The mostly stable structure at the core of the spliceosome allowed us to obtain high-resolution data, enabling us to visualize a spliceosome discarded during cellular quality control at the atomic level for the first time,” explains the structural biologist. Nevertheless, analyzing the components that are flexibly attached to the outer edge of the spliceosome posed a significant challenge, as noted by Dr. Komal Soni from the BZH.
With this structural information, the researchers gained insights into the errors that arise during splicing, how the spliceosome identifies faulty processes, and how it subsequently aborts splicing, effectively discarding the faulty complex. Leveraging these detailed structures, the team was able to model the underlying molecular mechanisms. The proteins responsible for this cellular quality control are conserved across eukaryotic organisms, from fission yeast to humans. Thus, the researchers believe that the mechanisms for identifying and removing defective spliceosomes have remained largely unchanged throughout evolution.
This research was part of a long-term collaboration between the teams of Prof. Sinning and Prof. Dr. Tamas Fischer, an expert in RNA surveillance at the Australian National University in Canberra. The study also involved Prof. Dr. Henning Urlaub’s research group at the Max Planck Institute for Multidisciplinary Sciences in Göttingen. The project was supported by the German Research Foundation and the Australian Research Council. The findings were published in “Nature Structural & Molecular Biology.”
