Proteasome dysfunction triggers activation of SKN-1A/Nrf1 by the aspartic protease DDI-1

• Published in: eLife Vol. 5
• Main Authors: Lehrbach, Nicolas J, Ruvkun, Gary
• Format: Journal Article
• Language: English
• Published: England eLife Sciences Publications Ltd 16.08.2016; eLife Sciences Publications, Ltd
• Subjects: Animals; Aspartic Acid Proteases - metabolism; Aspartic endopeptidase; aspartic protease; Bacteria; Caenorhabditis elegans - enzymology; Caenorhabditis elegans - metabolism; Caenorhabditis elegans Proteins - metabolism; Cancer; Cell Biology; Developmental Biology and Stem Cells; DNA-Binding Proteins - metabolism; E coli; ER traffic; Gene Expression Regulation; Genes; Homeostasis; Mutation; N-glycanase; Nematodes; peptide N-glycanase; proteasome; Proteasome Endopeptidase Complex - metabolism; Proteasomes; Protein interaction; Proteins; Proteolysis; SKN-1A/Nrf1; Software; Transcription activation; Transcription factors; Transcription Factors - metabolism; proteasome; stem cells; C. elegans; cell biology; ER traffic; developmental biology; SKN-1A/Nrf1; peptide N-glycanase; aspartic protease
• ISSN: 2050-084X; 2050-084X
• DOI: 10.7554/eLife.17721

Proteasomes are essential for protein homeostasis in eukaryotes. To preserve cellular function, transcription of proteasome subunit genes is induced in response to proteasome dysfunction caused by pathogen attacks or proteasome inhibitor drugs. In Caenorhabditis elegans, this response requires SKN-1, a transcription factor related to mammalian Nrf1/2. Here, we use comprehensive genetic analyses to identify the pathway required for C. elegans to detect proteasome dysfunction and activate SKN-1. Genes required for SKN-1 activation encode regulators of ER traffic, a peptide N-glycanase, and DDI-1, a conserved aspartic protease. DDI-1 expression is induced by proteasome dysfunction, and we show that DDI-1 is required to cleave and activate an ER-associated isoform of SKN-1. Mammalian Nrf1 is also ER-associated and subject to proteolytic cleavage, suggesting a conserved mechanism of proteasome surveillance. Targeting mammalian DDI1 protease could mitigate effects of proteasome dysfunction in aging and protein aggregation disorders, or increase effectiveness of proteasome inhibitor cancer chemotherapies. Proteins perform many important roles in cells, but these molecules can become toxic if they are damaged or are no longer needed. A molecular machine called the proteasome destroys ‘unwanted’ proteins in animal and other eukaryotic cells. If the proteasome stops working properly, unwanted proteins start to accumulate and cells respond by increasing the activity of genes that make proteasomes. A protein called SKN-1 is involved in this response and activates the genes that encode proteasome proteins, but it is not understood how SKN-1 “senses” that proteasomes are not working properly. Here, Lehrbach and Ruvkun used a roundworm called Caenorhabditis elegans to search for new genes that activate SKN-1 when the proteasome’s activity is impaired. The roundworms were genetically engineered to produce a fluorescent protein that indicates when a particular gene needed to make proteasomes is active. Lehrbach and Ruvkun identified some roundworms with mutations that cause the levels of fluorescence to be lower, indicating that SKN-1 was less active in these animals. Further experiments showed that some of these mutations are in genes that encode enzymes called DDI-1 and PNG-1. DDI-1 is able to cut certain proteins, while PNG-1 can remove sugars that are attached to proteins. Therefore, it is likely that these enzymes directly interact with SKN-1 and alter it to activate the genes that produce the proteasome. More work is now needed to understand the details of how modifying SKN-1 changes its activity in cells. In the future, drugs that target DDI-1 or PNG-1 might be used to treat diseases in which proteasome activity is too high or low, including certain cancers and neurodegenerative diseases.