A study of the proteasome — a protein-degradation complex — reveals an evolutionarily conserved pathway that acts through the protein kinase TORC1 to adjust proteasome levels in response to cellular needs. See Letter p.184
To maintain amino-acid and protein levels, cells must couple nutrient availability to protein synthesis and turnover. Central to this process is the enzyme called target of rapamycin complex 1 (TORC1) kinase, a master growth controller that integrates diverse environmental inputs to coordinate many metabolic processes1. Rousseau and Bertolotti2 reveal on page 184 that inhibition of TORC1 increases levels of the proteasome — a large protein complex involved in cellular protein degradation — to promote cell survival under stressful conditions. Consistent with previous reports3,4,5, the new work identifies TORC1 as a central regulator of proteasome homeostasis. However, the relationship between TORC1 and the control of proteasome function seems to be complex, because TORC1 can regulate the proteasome through multiple mechanisms that depend on the particular cellular context3,4,5.
The proteasome functions in one of the main protein-degradation pathways in cells, the ubiquitin–proteasome system6. In this pathway, a multi-enzymatic cascade covalently links the small polypeptide ubiquitin to proteins. This modification is recognized by the proteasome, which degrades ubiquitinated proteins to produce peptide mixtures that can then replenish the intracellular pool of amino acids6.
The proteasome comprises a multisubunit core particle, which carries out protein degradation, and up to two additional regulatory particle components that facilitate substrate recognition, removal of ubiquitin, and protein unfolding and translocation into the proteasome6. Inhibition of the proteasome results in a lethal shortage of amino acids7; therefore cells must maintain adequate proteasome levels to survive. However, the mechanisms that govern the assembly and regulation of this complex molecular machine, particularly under stressful conditions, are not fully understood.
The discovery8 in yeast of Adc17, a stress-induced regulatory particle assembly chaperone protein (RAC), offers an insight into the mechanism of proteasome regulation. Rousseau and Bertolotti used Adc17 as a starting point to investigate the proteasome. They treated yeast with the antibiotic tunicamycin to induce the unfolded-protein response, a cellular stress response to the presence of misfolded or unfolded proteins. They found that yeast upregulates Adc17 levels in the presence of tunicamycin, and that loss of the protein Sfp1, a negative regulator of TORC1, abrogates this increase of Adc17. The authors established that the increase in Adc17 requires inhibition of TORC1. Pharmacological suppression of TORC1 by the compound rapamycin or genetic inhibition of TORC1 by inactivation of KOG1, which encodes an essential TORC1 subunit, are sufficient to increase the expression not only of Adc17, but also of all other known RACs and of proteasome subunits.
To understand how TORC1 might mediate an increase in proteasome abundance, the authors focused on Mpk1, a yeast enzyme known as a mitogen-activated protein kinase (MAPK) that functions downstream of TORC1, and which is essential for the survival of cells in which tunicamycin has induced a stressful increase of unfolded proteins. Rousseau and Bertolotti found that Mpk1 is required for the TORC1-mediated increase in RACs and proteasome subunits (Fig. 1a). Neither the abundance of their messenger RNAs nor the protein stability of these RACs and proteasome subunits was altered in response to rapamycin-mediated inhibition of TORC1, which indicates that the increased levels of these proteins probably occur though regulation of mRNA translation.
An enhanced proteasomal capacity enables cells to adapt to the rising demand for protein degradation that accompanies stress. The absence of proteasome induction, as tested by Rousseau and Bertolotti using cells in which the gene for Mpk1 had been deleted, severely impairs the clearance of ubiquitinated proteins and of well-characterized reporter substrates used to monitor proteasomal activity.
The authors found that in mammalian cells, ERK5, the mammalian equivalent of Mpk1, also facilitates a rapid rise in RAC and proteasome levels when mTORC1 (the mammalian equivalent of TORC1) is inhibited (Fig. 1b). Thus, the TORC1 and Mpk1 pathway is an evolutionarily conserved regulator of proteasomal homeostasis.
Rousseau and Bertolotti's work contributes an additional perspective to the current debate about the exact relationship between TORC1/mTORC1 and the regulation of proteasome function. Consistent with the model proposed by Rousseau and Bertolotti, a study by Zhao et al.4 found that acute pharmacological inhibition of mTORC1 in the HEK293 mammalian cell line upregulates protein degradation by the proteasome. However, a report by Zhang et al.3 reveals nuances in the regulation of the proteasome by mTORC1, and finds that in the absence of the protein TSC2, a major inhibitor of the mTORC1 pathway, the transcription factor NRF1 mediates an increase in levels of the proteasome and of intracellular amino acids.
The differences between these three studies2,3,4 probably arise from variations in the extent to which mTORC1 is perturbed. Under acute mTORC1 inhibition, as studied by Rousseau and Bertolotti2 and Zhao et al.4, upregulation of the proteasome would increase amino-acid pools and permit the translation of proteins necessary for survival. mTORC1 inhibition induces autophagy, another major intracellular protein-degradation pathway that removes proteins in bulk from the cytoplasm and delivers them to an organelle called the lysosome for breakdown1. In combination, the rapid and coordinated activation of both the autophagic and proteasomal arms of protein degradation would be beneficial to cells as a mechanism to increase amino-acid levels under stress or nutrient deprivation.
However, under states of prolonged mTORC1 hyperactivation — for example, when TSC2 is lost, as investigated by Zhang et al.3 — cells may also need to increase proteasomal capacity to counteract unrestrained consumption of resources driven by sustained mTORC1 activity. It will be informative to compare the regulation of the proteasome in genetic models in which mTORC1 is constitutively active but not hyperactivated — for example, in mice that have a constitutively active Rag GTPase enzyme9.
How TORC1 inhibition increases proteasome-dependent degradation is another question requiring further investigation. Rousseau and Bertolotti found that this upregulated proteolysis depends on elevated proteasome levels, whereas the study by Zhao et al.4 found that enhanced ubiquitination drives protein breakdown without a change in proteasome content or activity. It will also be of interest to determine whether specific proteins are preferentially targeted for proteasomal degradation when TORC1 is inhibited. Consistent with this possibility, Zhao et al.4 found evidence for the selective proteasomal breakdown of growth-related proteins. Finally, given the integral link between ubiquitination and the proteasome, it is probable that both systems are concomitantly regulated under stress. The identification of enzymes called ubiquitin ligases and deubiquitinases, which are necessary to target substrates specifically to the proteasome, may provide a way to address this question.
From all these studies2,3,4, it is clear that the TORC1/mTORC1 pathway is a central regulator of proteasome homeostasis. It will be necessary to resolve the differences in current models of how this pathway affects the proteasome, especially given that modulation of the proteasome might be a therapeutic approach for diseases such as cancer and neurodegeneration.
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Frontiers in Molecular Neuroscience (2017)