A proteasome assembly defect in rpn3 mutants is associated with Rpn11 instability and increased sensitivity to stress

Rpn11 is a proteasome-associated deubiquitinating enzyme that is essential for viability. Recent genetic studies showed that Rpn11 is functionally linked to Rpn10, a major multiubiquitin chain binding receptor in the proteasome. Mutations in Rpn11 and Rpn10 can reduce the level and/or stability of proteasomes, indicating that both proteins influence its structural integrity. To characterize the properties of Rpn11, we examined its interactions with other subunits in the 19S regulatory particle and detected strong binding to Rpn3. Two previously described rpn3 mutants are sensitive to protein translation inhibitors and an amino acid analog. These mutants also display a mitochondrial defect. The abundance of intact proteasomes was significantly reduced in rpn3 mutants, as revealed by strongly reduced binding between 20S catalytic with 19S regulatory particles. Proteasome interaction with the shuttle factor Rad23 was similarly reduced. Consequently, higher levels of multiUb proteins were associated with Rad23, and proteolytic substrates were stabilized. The availability of Rpn11 is important for maintaining adequate levels of intact proteasomes, as its depletion caused growth and proteolytic defects in rpn3. These studies suggest that Rpn11 is stabilized following its incorporation into proteasomes. The instability of Rpn11 and the defects of rpn3 mutants are apparently caused by a failure to recruit Rpn11 into mature proteasomes.