Proteasomes are proteolytic complexes involved in non-lysosomal degradation 
which are localized in both the cytoplasm and the nucleus. The dynamics of 
proteasomes in living cells is unclear, as is their targeting to proteins 
destined for degradation. To investigate the intracellular distribution and 
mobility of proteasomes in vivo, we generated a fusion protein of the proteasome 
subunit LMP2 and the green fluorescent protein (GFP). The LMP2-GFP chimera was 
quantitatively incorporated into catalytically active proteasomes. The 
GFP-tagged proteasomes were located within both the cytoplasm and the nucleus. 
Within these two compartments, proteasomes diffused rapidly, and bleaching 
experiments demonstrated that proteasomes were transported slowly and 
unidirectionally from the cytoplasm into the nucleus. During mitosis, when the 
nuclear envelope has disintegrated, proteasomes diffused rapidly throughout the 
dividing cell without encountering a selective barrier. Immediately after cell 
division, the restored nuclear envelope formed a new barrier for the diffusing 
proteasomes. Thus, proteasomes can be transported unidirectionally over the 
nuclear membrane, but can also enter the nucleus upon reassembly during cell 
division. Since proteasomes diffuse rapidly in the cytoplasm and nucleus, they 
may perform quality control by continuous collision with intracellular proteins, 
and degrading those proteins that are properly tagged or misfolded.