Nuclear transport of yeast proteasomes

Abstract

Proteasomes are conserved protease complexes enriched in the nuclei of dividing yeast cells, a major site for protein degradation. If yeast cells do not proliferate and transit to quiescence, metabolic changes result in the dissociation of proteasomes into proteolytic core and regulatory complexes and their sequestration into motile cytosolic proteasome storage granuli. These granuli rapidly clear with the resumption of growth, releasing the stored proteasomes, which relocalize back to the nucleus to promote cell cycle progression. Here, I report on three models of how proteasomes are transported from the cytoplasm into the nucleus of yeast cells. The first model applies for dividing yeast and is based on the canonical pathway using classical nuclear localization sequences of proteasomal subcomplexes and the classical import receptor importin/karyopherin αβ. The second model applies for quiescent yeast cells, which resume growth and use Blm10, a HEAT-like repeat protein structurally related to karyopherin β, for nuclear import of proteasome core particles. In the third model, the fully-assembled proteasome is imported into the nucleus. Our still marginal knowledge about proteasome dynamics will inspire the discussion on how protein degradation by proteasomes may be regulated in different cellular compartments of dividing and quiescent eukaryotic cells.

Figure 1

Current models of the nuclear import of yeast proteasomes (A) In highly proliferating cells, nuclear proteasomes are assembled from proteasomal modules that are recognized by the canonical import receptor, importin/karyopherin αβ. Classical nuclear localization sequences (NLS) are accessible in α subunits of CP precursor complexes with disordered α rings [39,40,41]. The dimerization of two half-CP yields the pre-holo-CP, a labile CP precursor complex. In the pre-holo-CP, the active sites are freed by β-propeptide processing, which is guided by the CP-dedicated maturation factor Ump1. Ump1 is degraded upon CP maturation. The α rings are closed, and the NLS is no longer accessible [17,35]. (B) The RP is imported into the nucleus by the canonical NLS receptor pathway. Rpt2 and Rpn2 confer NLS to the RP base and Sts1 to the RP lid. Sts1 is a short-lived protein and degraded by RP-CP assemblies [34,36,38]. (C) Proteasomes are imported into the nucleus as holo-enzymes independent of the canonical importin/karyopherin αβ pathway. The regulation of this recently discovered import pathway is not yet understood [47]. (D) When quiescent cells resume growth, nuclear import of mature CP is facilitated by Blm10, a conserved 240-kDa HEAT-like repeat protein with structural similarity to karyopherin β [48]. Blm10 preferentially binds to CP with disordered α rings, which is comparable with α ring conformations in the pre-holo-CP and half-CP. Open or disordered α rings represent an import-competent conformation [49]. CP precursor complexes are not available during quiescence, due to stalled protein synthesis. Proteasome structures were drawn according to cryo-electron microscopy studies with a license from Cell Press Elsevier [50].