Blm10 facilitates nuclear import of proteasome core particles

Abstract

Short-lived proteins are degraded by proteasome complexes, which contain a proteolytic core particle (CP) but differ in the number of regulatory particles (RPs) and activators. A recently described member of conserved proteasome activators is Blm10. Blm10 contains 32 HEATlike modules and is structurally related to the nuclear import receptor importin/karyopherin b. In proliferating yeast, RP-CP assemblies are primarily nuclear and promote cell division. During quiescence, RP-CP assemblies dissociate and CP and RP are sequestered into motile cytosolic proteasome storage granuli (PSG). Here, we show that CP sequestration into PSG depends on Blm10, whereas RP sequestration into PSG is independent of Blm10. PSG rapidly clear upon the resumption of cell proliferation and proteasomes are relocated into the nucleus. Thereby, Blm10 facilitates nuclear import of CP. Blm10-bound CP serves as an import receptor-cargo complex, as Blm10 mediates the interaction with FG-rich nucleoporins and is dissociated from the CP by Ran-GTP. Thus, Blm10 represents the first CP-dedicated nuclear import receptor in yeast.

Intracellular Dynamics of the Ubiquitin-Proteasome-System

“…Later investigations reported a shift towards cytoplasmic proteasomes dependent on the type of the cell line and the density of the cells ( Wojcik & DeMartino, 2003). Proteasome localization also varies with the growth phase in yeast ( Laporte et al , 2008; Weberruss et al , 2013). In growing yeast at logarithmic phase (OD~1), proteasomes are primarily nuclear.…”

“…However, this nuclear uptake mechanism cannot explain the predominant nuclear localization of proteasomes in yeast cells which divide without mitotic breakdown of the NE. Proteasomes are the second most abundant protein complexes in eukaryotic cells and require continuous synthesis within the cytoplasm and nuclear import during cell division ( Weberruss et al , 2013). The most common route for protein complexes to cross the NE in an organism with closed mitosis is through nuclear pore complexes (NPC).…”

“…In spite of the differences between chemically-induced cell cycle arrest and quiescence, inhibited proteasomes are sequestered into juxta nuclear quality control compartments (JUNQs), situated at the cytoplasmic side of the NE and behaving similar to PSGs ( Kaganovich et al , 2008; Weberruss et al , 2013). When the cell cycle-arrested mutants were allowed to resume growth at permissive temperatures or upon withdrawal of proteasome inhibition, JUNQs were seen to dissolve like the PSG.…”

“…Their presence protects cells against proteo- and genotoxic stress and confers cell fitness during aging. Post-translation modifications such as N-acetylation also play a role in PSG organization, but their targets are unknown ( Saunier et al , 2013; van Deventer et al , 2015; Weberruss et al , 2013). …”

“…Conflicting reports exist about the stability of RP-CP assemblies in lysates of quiescent cells ( Bajorek et al , 2003; Hanna et al , 2012; Weberruss et al , 2013). The finding that RP-CP assemblies are less stable coincides with the decline in ATP during quiescence as well as the reduced proclivity of the proteasome to degrade poly-ubiquitinated substrate.…”