An Inducible Chaperone Adapts Proteasome Assembly to Stress

The proteasome is essential for the selective degradation of most cellular proteins. To survive overwhelming demands on the proteasome arising during environmental stresses, cells increase proteasome abundance. Proteasome assembly is known to be complex. How stressed cells overcome this vital challenge is unknown. In an unbiased suppressor screen aimed at rescuing the defects of a yeast Rpt6 thermosensitive proteasome mutant, we identified a protein, hereafter named Adc17, as it functions as an ATPase dedicated chaperone. Adc17 interacts with the amino terminus of Rpt6 to assist formation of the Rpt6-Rpt3 ATPase pair, an early step in proteasome assembly. Adc17 is important for cell fitness, and its absence aggravates proteasome defects. The abundance of Adc17 increases upon proteasome stresses, and its function is crucial to maintain homeostatic proteasome levels. Thus, cells have mechanisms to adjust proteasome assembly when demands increase, and Adc17 is a critical effector of this process.