Uncovering a Mitochondrial Unfolded Protein Response

Science's STKE  16 Oct 2007:
Vol. 2007, Issue 408, pp. tw374
DOI: 10.1126/stke.4082007tw374

Proteins are synthesized and folded and assembled in the cytosol, endoplasmic reticulum (ER), and mitochondria. For the cytosol and ER, pathways associated with excess unfolded or misfolded proteins have been characterized and are called the heat shock response and the ER unfolded protein response (UPRer). Haynes et al. applied a genome-wide RNAi screen to identify four genes involved in the mitochondrial UPR (UPRmt) in Caenorhabditis elegans. Three of the genes were characterized in detail: dve-1, which encodes a homolog of the Drosophila homeobox transcription factor defective proventriculus and mammalian SatB2; ubl-5, which encodes a ubiquitin-like protein; and clpp-1, which encodes a protein homologous to the bacterial protease ClpP. (The fourth encoded guanosine triphosphatase homologous to mammalian Rheb.) Reduction in the expression of any of these genes compromised the expression of the chaperone Hsp60 in response to mitochondrial unfolded protein stress without interfering with UPRer and caused perturbed mitochondrial morphology, consistent with persistent UPRmt. DVE-1 showed a nuclear localization and redistributed to nuclear puncta in response to conditions of mitochondrial unfolded protein stress. In worms deficient in, but not completely lacking, DVE-1 (the latter was lethal), mitochondrial mass was decreased (as detected by MitoTracker staining and ligand blot assay). In an affinity purification assay, a tagged form of DVE-1 and a tagged form of UBL-5 were found to form a complex when the worms were subjected to conditions causing mitochondrial unfolded protein stress. A similar interaction was observed for the mammalian homologs. RNAi experiments suggested that CLPP-1 acted upstream of UBL-5 (the expression of which is stimulated in response to conditions of mitochondrial unfolded protein stress) and of DVE-1 in the UPRmt. CLPP-1 was localized to the matrix of the mitochondria, where it appeared to be part of a large multisubunit complex. When CLPP-1 activity was decreased by RNAi, increased expression of the ubl-5 and hsp60 genes in response to conditions that cause mitochondrial unfolded protein stress was compromised, and the redistribution of DVE-1 to nuclear puncta was also blocked. Protease inhibitors also prevented the stimulation of ubl-5 expression in response to mitochondrial unfolded protein stress, suggesting that the protease activity of CLPP-1 is important for its function in the UPRmt.

C. M. Haynes, K. Petrova, C. Benedetti, Y. Yang, D. Ron, ClpP mediates activation of a mitochondrial unfolded protein response in C. elegans. Dev. Cell 13, 467-480 (2007). [PubMed]