Sts1 Plays a Key Role in Targeting Proteasomes to the Nucleus

Abstract

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...

Nuclear Import Factor Srp1 and Its Associated Protein Sts1 Couple Ribosome-bound Nascent Polypeptides to Proteasomes for Cotranslational Degradation

“…Thus, Srp1 is involved in the coupling of proteasomes to ribosomebound nascent chains, and this activity is compromised in Srp1 E145K . It has been suggested that the interaction between Srp1 and the proteasome may be mediated or facilitated by Sts1, a multicopy suppressor of srp1-49 (45,46). We found that the cosedimentation of proteasomes with polysomes significantly decreased in the sts1-2 mutant (Fig.…”

Quantitative live-cell imaging reveals spatio-temporal dynamics and cytoplasmic assembly of the 26S proteasome

“…These observations imply that the yeast 26S proteasome is assembled inside the nucleus from independently imported modules. By contrast, other studies have suggested that the holoenzyme or novel form of the preassembled proteasome undergoes nuclear translocation [22][23][24] . Because the channel of the nuclear pore complex (NPC) can expand to accommodate cargoes with a diameter of up to 39 nm, as revealed by studies using gold particles 25 , it is theoretically possible for intact CP or even the entire 26S proteasome to pass through the NPC.…”

“…The direction and orientation of translocation might be governed by Sts1, an NLS-containing protein that binds the lid subunit Rpn11 (ref. 22). …”

Intracellular Dynamics of the Ubiquitin-Proteasome-System

“…Isono et al (2007) later confirmed that Rpn2 provides a crucial NLS to aid nuclear import of the RP base and that the lid is separately imported. The nuclear import of the RP lid also requires importin α, though no classical NLS has been identified within RP lid subunits; rather Sts1, a short-lived protein that itself contains a classical NLS, associates with Rpn11 to facilitate nuclear import of the RP lid by importin αβ ( Chen et al , 2011). In accordance, deletion of the Sts1 NLS has downstream effects on the nuclear localization of RP lid in addition to RP base and CP, which suggests that proteasomes could also be transported as holo-enzymes ( Chen & Madura, 2014b).…”