Mechanism of the proteasome nuclear localization

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The 26S proteasome is a highly organized protease complex that is responsible for targeted protein degradation in both the cytosolic and nuclear compartment. In budding yeast and several cancer cells, it is known that the 26S proteasome is mainly localized in the nucleus. Although several studies have described the mechanism of the nuclear translocation, it is still unclear when the proteasomes enter the nucleus upon their assembly process. Using dual color fluorescence cross correlation spectrometry(FCCS), we determined local concentration and dynamics of the 26S proteasome in living yeast cells. Surprisingly, almost all the proteasome subunits were incorporated into the 26S proteasome in both the cytosol and nucleus. Importantly, the proteasome dynamics was not changed in the importin.(srp1-49) mutant cells in which the 26S proteasome retains the cytosol. These results suggest that the 26S proteasome is fully assembled in the cytoplasm prior to the nuclear translocation.