Vms1 Relieves a Mitochondrial Import Chokehold

Ribosome stalling results in the production of truncated proteins that can cause proteotoxic stress if not efficiently degraded. A recent paper by Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar identifies Vms1 as a key player in the regulation of ribosome quality control specifically on mitochondria-localized ribosomes, ultimately preventing protein aggregate accumulation within mitochondria. Ribosome stalling results in the production of truncated proteins that can cause proteotoxic stress if not efficiently degraded. A recent paper by Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar identifies Vms1 as a key player in the regulation of ribosome quality control specifically on mitochondria-localized ribosomes, ultimately preventing protein aggregate accumulation within mitochondria. The accumulation of flawed or truncated proteins can perturb diverse aspects of the proteostasis environment by aggregating and occupying molecular chaperones and proteases. To ensure efficient protein synthesis and processing, cells have evolved numerous quality control checkpoints that evaluate mRNA, translation, and protein folding, which, combined, maintain proteostasis. One such pathway is the ribosome quality control (RQC) pathway that clears incomplete or erroneous proteins produced by stalled ribosomes. Stalling may be caused by truncated mRNAs, mRNAs lacking stop codons, stable mRNA secondary structures, or tRNA deficiencies. Upon stalling, the ribosomes are split and the ubiquitin ligase, Ltn1 (Listerin in mammals), is recruited to the 60S subunit-nascent protein chain complex by Rqc2. Rqc2 also modifies the nascent chain by adding C-terminal alanine and threonine residues (CAT tails) via non-canonical translation (Shen et al., 2015Shen P.S. Park J. Qin Y. Li X. Parsawar K. Larson M.H. Cox J. Cheng Y. Lambowitz A.M. Weissman J.S. et al.Science. 2015; 347: 75-78Crossref PubMed Scopus (180) Google Scholar). CAT tail elongation pushes nascent chains out of the ribosome tunnel, promoting lysine exposure and polyubiquitination by Ltn1 (Kostova et al., 2017Kostova K.K. Hickey K.L. Osuna B.A. Hussmann J.A. Frost A. Weinberg D.E. Weissman J.S. Science. 2017; 357: 414-417Crossref PubMed Scopus (71) Google Scholar; reviewed in Joazeiro, 2017Joazeiro C.A.P. Annu. Rev. Cell Dev. Biol. 2017; 33: 343-368Crossref PubMed Scopus (109) Google Scholar). The ATPase Cdc48 recognizes the polyubiquitinated nascent chain and extracts it from the 60S ribosome for proteasomal degradation. Importantly, CAT-tailed proteins are aggregation prone if not degraded in a timely manner. Interestingly, evidence suggests that the RQC machinery evolved prior to the endosymbiotic event that led to mitochondria (Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar). Thus, the impact of RQC on ribosomes and nascent chains that are targeted to mitochondria or on those actively engaged in co-translational mitochondrial protein import (Williams et al., 2014Williams C.C. Jan C.H. Weissman J.S. Science. 2014; 346: 748-751Crossref PubMed Scopus (223) Google Scholar) is unclear. Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar set out to understand how ribosome quality is monitored if the nascent chain is inaccessible to the cytosol due to tight association between the ribosome and the mitochondrial import machinery during co-translational import. The authors focused on Vms1, a protein previously shown to accumulate on damaged mitochondria during oxidative stress and to recruit Cdc48 (Heo et al., 2013Heo J.-M. Nielson J.R. Dephoure N. Gygi S.P. Rutter J. Mol. Biol. Cell. 2013; 24: 1263-1273Crossref PubMed Scopus (28) Google Scholar). Curiously, Vms1 was found to associate with multiple components of the 60S ribosome and all of the components of the RQC complex, implicating it in some aspect of ribosome quality control, potentially at mitochondria (Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar). To determine Vms1's role in RQC, Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar generated double-mutant strains combining VMS1 and RQC gene deletions. Whereas single-mutant ltn1Δ or vms1Δ cells were relatively healthy, cells lacking both VMS1 and LTN1 grew poorly when forced to rely on oxidative phosphorylation for ATP production, suggestive of mitochondrial dysfunction. Importantly, the growth defect and mitochondrial dysfunction was completely rescued when RQC2 was deleted, suggesting that CAT-tail extension can perturb mitochondrial function. Because CAT tails can drive aggregation, the authors took a closer look at aggregate formation in their RQC mutants. Strikingly, cells lacking both VMS1 and LTN1 accumulated detergent-insoluble aggregates that co-localized with mitochondria, which were also significantly reduced when RQC2 was deleted. The authors then asked what proteins were found in these mitochondrial-localized aggregates. Consistent with a role for Vms1 in RQC of proteins destined for mitochondria, the aggregates were comprised almost entirely of mitochondrial proteins and localized within the mitochondrial matrix. Furthermore, Izawa et al., 2017Izawa T. Park S.-H. Zhao L. Hartl U.F. Neupert W. Cell. 2017; 171: 890-903Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar found that mitochondrial protein aggregates sequester mitochondrial chaperones, proteases, and tRNA ligases, providing a satisfying explanation for the synthetic toxicity of VMS1 and LTN1 double deletion. The accumulation of CAT-tailed proteins within mitochondria, an environment that evolved independently of RQC and apparently lacks the activity necessary to efficiently clear them, results in aggregation that sequesters essential mitochondrial proteostasis factors. Because CAT-tail-dependent aggregation occurs only after import into mitochondria, the authors hypothesized that Vms1 antagonizes Rqc2 on stalled mitochondrial-localized ribosomes to prevent CAT-tail extension of proteins destined for the mitochondrial matrix that have managed to escape Ltn1 ubiquitination. Consistent with this idea, Vms1 overexpression counteracted mitochondrial-localized aggregation caused by Rqc2 and reduced the amount of Rqc2 that bound 60S ribosomes. This reduction of Rqc2 binding suggests that Vms1 physically competes with Rqc2 for binding to stalled 60S ribosomes. However, the precise mechanism for how Vms1 affects Rqc2 CAT-tailing activity requires further study. Combined, these data suggest a dynamic model for RQC of nascent peptides headed to mitochondria and nascent peptides undergoing co-translational import. Because VMS1 deletion alone is non-toxic, it is likely that Ltn1 can access a significant fraction of mitochondrial-targeted proteins on stalled ribosomes, and it likely does so by acting prior to their engagement with the mitochondrial import machinery. These proteins are then degraded by cytosolic proteasomes, preventing the mitochondrial import of CAT-tailed nascent chains (Figure 1). However, if mitochondrial import of the nascent peptide has been initiated when ribosome stalling occurs—as would occur during co-translational import—Ltn1 may not be able to access the nascent chain. The reason may be the ratcheting of the nascent peptide by matrix-localized chaperones, pulling the 60S ribosome next to the mitochondrial TOM channel. In such cases, Vms1 is present to counteract Rqc2 activity in order to reduce CAT-tailing of nascent chains destined for the mitochondrial matrix. An intriguing idea brought up by the data is that ribosome stalling happens frequently on mitochondrial-targeted proteins, even in otherwise healthy cells, and that there is likely more co-translational mitochondrial import than previously thought. Going forward, it will be interesting to examine the relationship between mitoRQC and mitochondrial stress. For example, damaged mitochondria may also cause increased ribosome stalling due to reduced protein import efficiency (Nargund et al., 2012Nargund A.M. Pellegrino M.W. Fiorese C.J. Baker B.M. Haynes C.M. Science. 2012; 337: 587-590Crossref PubMed Scopus (624) Google Scholar). It also remains to be determined how Vms1 accumulates specifically on mitochondria with stalled ribosomes. Intriguingly, previous studies demonstrated that Vms1 localizes to mitochondria upon treatment with rapamycin, which potentially increases ribosome stalling via mTOR inhibition (Heo et al., 2010Heo J.-M. Livnat-Levanon N. Taylor E.B. Jones K.T. Dephoure N. Ring J. Xie J. Brodsky J.L. Madeo F. Gygi S.P. et al.Mol. Cell. 2010; 40: 465-480Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar). These studies demonstrate an essential role for Vms1 and RQC in maintaining mitochondrial function. Interestingly, loss-of-function mutations in the gene encoding Listerin (the mammalian Ltn1 homolog) result in severe neurodegeneration (Bengtson and Joazeiro, 2010Bengtson M.H. Joazeiro C.A.P. Nature. 2010; 467: 470-473Crossref PubMed Scopus (313) Google Scholar). However, the role of mitochondrial dysfunction remains to be evaluated. Because nearly all neurodegenerative diseases, including Parkinson's, Alzheimer's, and amyotrophic lateral sclerosis (ALS), are associated with mitochondrial dysfunction, it will be interesting to determine the role of RQC and Vms1 in these scenarios. Cytosolic Protein Vms1 Links Ribosome Quality Control to Mitochondrial and Cellular HomeostasisIzawa et al.CellOctober 26, 2017In BriefA quality-control pathway comprising the cytosolic protein Vms1 protects mitochondria from the toxic effects of ribosome-stalled polypeptides. Full-Text PDF Open Archive