4EBP1 phosphorylation is included as a control for starvation (representative from 2 independent experiments). Cells were incubated in HBSS solution and harvested at different time points for immunoblotting. b Protein levels of proteasome components and other factors following nutrient deprivation in IMR90 cells. 1a) 9.Ī Schematic representation of the mammalian proteasome composed by the 20S catalytic particle (CP) and 19S or 11S regulatory particles (RP). The CP can also associate with other regulatory complexes including the homoheptameric ring-shaped 11S complex composed of PSME3 (PA28γ or REGγ), which targets proteins for ubiquitin-independent degradation (Fig. The RP is responsible for the recognition and unfolding of polyubiquitinated proteins, as well as their translocation inside the CP.
The RP is also a large multi-protein complex that binds the CP to assemble a competent proteasome. This particle is the target of the widely used proteasome inhibitors (e.g., MG132 and Bortezomib) 7, 8. The CP contains the proteases with CASPASE-like, trypsin-like and chymotrypsin-like activities that are responsible for substrate degradation into small peptides 7, 8. The 26S proteasome is composed of two sub-complexes, the 20S cylinder-like catalytic particle (CP) and the 19S regulatory particle (RP) (Fig. The proteasome is an evolutionarily conserved protein degradation machinery that generally recognizes substrates that are polyubiquitinated through the concerted action of E2 ubiquitin-conjugating enzymes and E3 ubiquitin ligases. On the other hand, the proteasome catalyzes the degradation of proteins that are in surplus, improperly folded or unwanted at a given time or in a specific subcellular location 4, 5, 6. The autophagy system targets large macromolecule complexes, protein aggregates and organelles, all of which are first engulfed within double membrane-delimited structures and subsequently delivered to the lysosome for degradation 1, 2, 3. Protein degradation and subsequent recycling of amino acids is fundamental for normal cell physiology. Finally, SIPAN formation is associated with decreased cell survival and p53-mediated apoptosis, which might contribute to tissue fitness in diverse pathophysiological conditions. We further show that: (i) SIPAN contain K48-conjugated ubiquitin, (ii) proteasome inhibition accelerates SIPAN formation, (iii) deubiquitinase inhibition prevents SIPAN resolution and (iv) RAD23B proteasome shuttling factor is required for SIPAN formation. SIPAN undergo fusion events, rapidly exchange proteasome particles with the surrounding milieu and quickly dissolve following amino acid replenishment. We termed these proteasome condensates SIPAN (Starvation-Induced Proteasome Assemblies in the Nucleus) and show that these are a common response of mammalian cells to amino acid deprivation. Here we show that the mammalian proteasome undergoes liquid-liquid phase separation in the nucleus upon amino acid deprivation. However, how protein degradation is coordinated with amino acid supply and protein synthesis has remained largely elusive. Nature Communications volume 12, Article number: 6984 ( 2021)Įukaryotic cells have evolved highly orchestrated protein catabolic machineries responsible for the timely and selective disposal of proteins and organelles, thereby ensuring amino acid recycling. Starvation-induced proteasome assemblies in the nucleus link amino acid supply to apoptosis