Proteasome response to folding stress in fission yeast

Abstract

The proteasome is an essential multiprotein complex responsible for degrading a wide range of proteins, including mitotic substrates, whose degradation enables cells to progress through mitosis, and misfolded proteins, which can become toxic at high concentrations. During protein folding stress, cells face elevated levels of misfolded proteins, yet the mechanisms by which they adapt and enhance protein degradation under such conditions are not fully understood. In this study, we analyze the proteasome response to protein folding stress using the fission yeast Schizosaccharomyces pombe as model organism. We demonstrate that folding stress, caused by heat shock or the amino acid analogue canavanine, promotes the assembly of 26S/30S proteasomes from preexisting 20S and 19S complexes. This proteasome assembly response, which reduces the pool of free 20S particles, serves as a first line of defense when misfolded proteins accumulate due to stress. Interestingly, folding stress also triggers the upregulation of the 20S assembly chaperone Ump1, which drives the assembly of new 20S proteasomes, thereby maintaining their steady-state levels. We show that limiting Ump1 function, either by deletion of the ump1 gene or through a conditional ump1 mutant (ump1-ts), leads to a significant reduction in both 20S and 26S/30S particles. Under these conditions, the proteasome assembly response is insufficient to generate enough 26S/30S proteasomes to counteract folding stress, ultimately compromising cell survival at high temperatures. Remarkably, in ump1-deficient cells, the reduced pool of 26S/30S proteasomes predominantly accumulates in the nucleus, allowing successful progression through mitosis. However, the concomitant reduction in cytoplasmic proteasomes in these cells results in chronic cytoplasmic proteotoxicity, leading to the activation of the heat shock response (HSR). This toxicity can be partially alleviated by impairing proteasome import into the nucleus. Our findings highlight the importance of proteasome assembly in response to folding stress and the functional relevance of maintaining a proper balance of nuclear and cytoplasmic proteasomes for both cell-cycle progression and cytoplasmic proteostasis. Our data also reveal a role of the HSR in compensating for deficient cytoplasmic proteasome activity, thus highlighting the robustness of the proteostasis network. This research may provide valuable insights into the role of proteostasis networks in aging, and it may aid in developing new pharmacological strategies for manipulating proteasome function or subcellular localization in the treatment of neurodegenerative diseases or cancer.