|Ph.D Student||Savulescu Anca|
|Subject||Differential Nuclear Targeting or Import of Proteasome|
|Department||Department of Biology||Supervisors||Professor Amnon Harel|
|Professor Michael Glickman|
|Full Thesis text|
The 26S proteasome is a conserved 2.5 MDa protein degradation machine that localizes to different cellular compartments, including the nucleus. Little is known about the specific targeting mechanisms of proteasomes in eukaryotic cells. We used a cell-free nuclear reconstitution system to test for nuclear targeting and import of distinct proteasome species. Three types of stable, proteolytically active proteasomes particles were purified from Xenopus egg cytosol. Two of these, the 26S holoenzyme and the 20S core particle, are targeted to the nuclear periphery, but are unable to reach the nucleoplasm. This targeting depends on the presence of mature nuclear pore complexes (NPCs) in the nuclear envelope. A third, novel form designated here as 20S, is actively imported through NPCs, indicating that an intact, active proteasome particle can be imported into the nucleus. The 20S proteasome particle resembles recently described structural intermediates from other systems. Nuclear import of this particle requires functional NPCs, but is not directly regulated by the Ran GTPase cycle. The presence of a stable 20S particle in unfertilized eggs may provide a means for the quick mobilization of existing proteasome particles into newly formed nuclear compartments during early development.