A nonproteolytic proteasome activity controls organelle fission in yeast.

To understand the processes underlying organelle function, dynamics and inheritance, it is necessary to identify and characterize the regulatory components involved. Recently in yeast and mammals, proteins of the membrane fission machinery (Dnm1-Mdv1-Caf4-Fis1 in yeast and DLP1-FIS1 in human) have been shown to have a dual localization on mitochondria and peroxisomes, where they control mitochondrial fission and peroxisome division. Here, we show that whereas vacuole fusion is regulated by the proteasome degradation function, mitochondrial fission and peroxisomal division are not controlled by the proteasome activity but rather depend on a new function of the proteasomal lid subunit Rpn11. Rpn11 was found to regulate the Fis1-dependent fission machinery of both organelles. These findings indicate a unique role of the Rpn11 protein in mitochondrial fission and peroxisomal proliferation that is independent of its role in proteasome-associated deubiquitylation.

Integrity of the Saccharomyces cerevisiae Rpn11 protein is critical for formation of proteasome storage granules (PSG) and survival in stationary phase.

Decline of proteasome activity has been reported in mammals, flies and yeasts during aging. In the yeast Saccharomyces cerevisiae, the reduction of proteolysis in stationary phase is correlated with disassembly of the 26S proteasomes into their 20S and 19S subcomplexes. However a recent report showed that upon entry into the stationary phase, proteasome subunits massively re-localize from the nucleus into mobile cytoplasmic structures called proteasome storage granules (PSGs). Whether proteasome subunits in PSG are assembled into active complexes remains an open question that we addressed in the present study. We showed that a particular mutant of the RPN11 gene (rpn11-m1), encoding a proteasome lid subunit already known to exhibit proteasome assembly/stability defect in vitro, is unable to form PSGs and displays a reduced viability in stationary phase. Full restoration of long-term survival and PSG formation in rpn11-m1 cells can be achieved by the expression in trans of the last 45 amino acids of the C-terminal domain of Rpn11, which was moreover found to co-localize with PSGs. In addition, another rpn11 mutant leading to seven amino acids change in the Rpn11 C-terminal domain, which exhibits assembled-26S proteasomes, is able to form PSGs but with a delay compared to the wild type situation. Altogether, our findings indicate that PSGs are formed of fully assembled 26S proteasomes and suggest a critical role for the Rpn11 protein in this process.