Molecular Switches In Proteasome Assembly : Tales Of The Tails And Chaperones

Abstract

In the proteasome, the proteolytic 20S core particle (CP) associates with the 19S regulatory particle (RP) to degrade polyubiquitinated proteins. Six ATPases (Rpt1‐Rpt6) of the RP form a hexameric Rpt ring and interact with the heptameric α ring (α1–α7) of the CP via the Rpt C‐terminal tails individually binding to the α subunits. Importantly, the Rpt6 tail has been suggested to be crucial for RP assembly. Here, we show that the interaction of the CP and Rpt6 tail promotes a CP‐Rpt3 tail interaction, and that they jointly mediate proteasome activation via opening the CP gate for substrate entry. The Rpt6 tail forms a novel relationship with the Nas6 chaperone, which binds to Rpt3 and regulates the CP‐Rpt3 tail interaction, critically influencing cell growth and turnover of polyubiquitinated proteins. CP‐Rpt6 tail binding promotes the release of Nas6 from the proteasome. Based on disulfide crosslinking that detects cognate α3‐Rpt6 tail and α2‐Rpt3 tail interactions in the proteasome, decreased α3‐Rpt6 tail interaction facilitates robust α2‐Rpt3 tail interaction that is also strongly ATP‐dependent. Together, our data support the reported role of Rpt6 during proteasome assembly, and suggest that its function switches from anchoring for RP assembly into promoting Rpt3‐dependent activation of the mature proteasome.Support or Funding InformationThis study was supported by the University of Colorado start‐up fund and Boettcher Web‐Warring Biomedical Research Award to S.P.