Speaker : Chan-Gi Pack, Ph.D. (Research Scientist, Cellular Informatics Laboratory, ASI, RIKEN, 2-1 Hirosawa, Japan)
Date : 2013-02-25
Location: Room 106， Pharmacy Hall, Dankook University
Abstract : The 26S proteasome is a 2.5-MDa multi-subunit protease complex that degrades ubiquitinated proteins in eukaryotic cells. The proteasome is composed of 66 subunits, assisted by multiple dedicated chaperones. In rapidly proliferating yeast and some cancer cells, the 26S proteasome can be localized in the nucleus, in order to ensure the degradation of nuclear proteins. However, it is unclear how and when during the assembly process the proteasome enters the nucleus. Here, we for the first time measured the absolute concentration, dynamics, and complex formation of the 26S proteasome in living yeast cells by fluorescence correlation methods (FCM), a highly sensitive and quantitative method. Interestingly, we found that the 26S proteasome is a highly mobile complex, and that almost all proteasome subunits were stably incorporated into the mega-complex of 26S proteasome in both the cytosol and nucleus. Complex formation was not significantly changed even in an importin-α mutant, suggesting that the proteasome is assembled in the cytosol. Consistent with this, a genetically stabilized 26S proteasome did not exhibit any obvious defects and was distributed normally in the nucleus. These results suggest that the 26S proteasome completes its assembly process in the cytoplasm and translocates into the nucleus through the channel of nuclear pore as 26S large complex. Considering that the 26S proteasome is a fascinating target for pharmaceutical development, the proteasome dynamics revealed by FCM provides a new perspective for developing next-generation proteasome inhibitors.