The evidence that nuclear proteins can be degraded by cytosolic proteasomes has received considerable experimental support. However, the presence of proteasome subunits in the nucleus also suggests that protein degradation could occur within this organelle. We determined that Sts1 can target proteasomes to the nucleus and facilitate the degradation of a nuclear protein. Specific sts1 mutants showed reduced nuclear proteasomes at the nonpermissive temperature. In contrast, high expression of Sts1 increased the levels of nuclear proteasomes. Sts1 targets proteasomes to the nucleus by interacting with Srp1, a nuclear import factor that binds nuclear localization signals. Deletion of the NLS in Sts1 prevented its interaction with Srp1 and caused proteasome mislocalization. In agreement with this observation, a mutation in Srp1 that weakened its interaction with Sts1 also reduced nuclear targeting of proteasomes. We reported that Sts1 could suppress growth and proteolytic defects of rad23⌬ rpn10⌬. We show here that Sts1 suppresses a previously undetected proteasome localization defect in this mutant. Taken together, these findings explain the suppression of rad23⌬ rpn10⌬ by Sts1 and suggest that the degradation of nuclear substrates requires efficient proteasome localization.Many factors that regulate the cell cycle, DNA repair, transcription, and tumor suppression are nuclear proteins that are degraded by the ubiquitin/proteasome system (1). However, the mechanism that mediates their turnover and the subcellular location of degradation are often not known. Nuclear proteasomes (2, 3) may perform both proteolytic and nonproteolytic functions (4, 5). The evidence that the hydrolytic activities of proteasomes are present in the nucleus is limited. Although some proteins are degraded within the nucleus (4), others are exported from the nucleus and degraded by cytosolic proteasomes (6 -8). It is unknown if nuclear degradation is restricted to specific proteins, while others are exported and degraded by cytoplasmic proteasomes.Rad23 is a substrate shuttle factor that can transfer ubiquitinated proteins to the proteasome (9, 10), whereas Rpn10 is a major proteasome receptor for multiubiquitinated proteins (11-15). The loss of both proteins in rad23⌬ rpn10⌬ caused severe growth and proteolytic defects, including sensitivity to drugs, stabilization of substrates, and accumulation of ubiquitinated proteins (16). Sts1 is a dosage suppressor of these pleiotropic defects of rad23⌬ rpn10⌬ (17), indicating that it plays a role in the ubiquitin/proteasome system. In agreement, we found that Sts1 protein can bind the proteasome, and an sts1-2 mutant (C194Y) was defective in protein degradation and accumulated high levels of ubiquitinated proteins. Significantly, the interaction between multiubiquitinated proteins and proteasomes was reduced in sts1-2.We report here that Sts1 is required for efficient translocation of proteasomes to the nucleus. We propose that the failure of proteasomes to bind multiubiquitinated substrates in sts1-2...

Multicellular organisms encounter environmental conditions that adversely affect protein homeostasis (proteostasis), including extreme temperatures, toxins, and pathogens. It is unclear how they use sensory signaling to detect adverse conditions and then activate stress response pathways so as to offset potential damage. Here, we show that dopaminergic mechanosensory neurons in C. elegans release the neurohormone dopamine to promote proteostasis in epithelia. Signaling through the DA receptor DOP-1 activates the expression of xenobiotic stress response genes involved in pathogenic resistance and toxin removal, and these genes are required for the removal of unstable proteins in epithelia. Exposure to a bacterial pathogen (Pseudomonas aeruginosa) results in elevated removal of unstable proteins in epithelia, and this enhancement requires DA signaling. In the absence of DA signaling, nematodes show increased sensitivity to pathogenic bacteria and heat-shock stress. Our results suggest that dopaminergic sensory neurons, in addition to slowing down locomotion upon sensing a potential bacterial feeding source, also signal to frontline epithelia to activate the xenobiotic stress response so as to maintain proteostasis and prepare for possible infection.

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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.

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