MISTERMINATE Mechanistically Links Mitochondrial Dysfunction with Proteostasis Failure

Abstract

Mitochondrial dysfunction and proteostasis failure frequently coexist as hallmarks of neurodegenerative disease. How these pathologies are related is not well understood. Here we describe a phenomenon termed MISTERMINATE (mitochondrial stress-induced translational termination impairment and protein carboxyl terminal extension), which mechanistically links mitochondrial dysfunction with proteostasis failure. We show that mitochondrial dysfunction impairs translational termination of nuclear-encoded mitochondrial mRNAs including complex-I 30kD subunit (C-I30) mRNA, occurring on mitochondrial surface in Drosophila and mammalian cells. Ribosomes stalled at the normal stop codon continue to add to the C-terminus of C-I30 certain amino acids non-coded by mRNA template. C-terminally-extended C-I30 is toxic when assembled into C-I and forms aggregates in the cytosol. Enhancing co-translational quality control prevents C-I30 C-terminal extension and rescues mitochondrial and neuromuscular degeneration in a Parkinson's disease model. These findings emphasize the importance of efficient translation termination and reveal unexpected link between mitochondrial health and proteome homeostasis mediated by MISTERMINATE. translation, attenuated disease progression (Johnson et al., 2013). These studies strongly support an intimate connection between mitochondria and cytosolic translation, although the underlying mechanism is poorly defined.Here we provide a molecular link between mitochondrial function and cytosolic translation/proteostatsis. Mitochondrial damage induces translational stalling of mitochondrial outer membrane (MOM)-associated C-I30 mRNA by impairing the termination and ribosomerecycling factors eRF1 and ABCE1. Stalled ribosomes continue to add certain AAs to the Cterminus of C-I30, in a process analogous to CAT-tailing but with distinct features. C-I30 with C-terminal extension (CTE) inhibits oxidative phosphorylation (OxPhos) when assembled into C-I, and disrupts proteostasis when aggregating in the cytosol. Genetic manipulations of eRF1, ABCE1, or conserved CAT-tailing machinery prevent C-I30 CTE formation and rescue neuromuscular degeneration in PINK1 flies. PINK1/Parkin mechanistically regulates the RQC and CTE processes. These results identify C-I30 and possibly ATP5a as the first endogenous metazoan substrates of the CTE process, revealing intimate connections between mitochondrial health, co-translational QC, and proteostasis in neuromuscular tissues. RESULTS A Novel Form of C-I30 in PINK1 Neuromuscular TissuesHuman C-I is the largest enzyme of the respiratory chain, comprising 45 nuclear-or mitochondrial-encoded subunits, with C-I30 being part of the core assembly (Formosa et al., 2018). Mutations in C-I30 cause OxPhos and neuromuscular defects in Leigh syndrome. C-I30mRNA is translationally repressed in the cytosol before being recruited to MOM and reactivated by PINK1 and Parkin (Gehrke et al., 2015), two factors linked to familial PD.In muscle and brain tissues of PINK1 flies, we detected canonical C-I30...

Aggregation of CAT tails blocks their degradation and causes proteotoxicity in S. cerevisiae

“…In mice, a hypomorphic LTN1 allele induces a progressive neurodegeneration that shares phenotypic similarities with amyotrophic lateral sclerosis (ALS) (Chu et al, 2009). It is possible that degenerating neurons in these LTN1 -hypomorphic mice harbor toxic CAT tail aggregates, as has been observed in a Drosophila model of Parkinson Disease (Wu Z, Tantray I, Lim J, Chen S, Li Y, Davis Z, Sitron C, Dong J, Gispert S, Auburger G, Brandman O, Bi X, Snyder M, Lu B., 2019). As the connection between the phenotypes in these disease models and human disease becomes more clear, future studies can focus on strategies to mitigate toxicity associated with compromised Ltn1.…”

“…To preserve threonine content, the fitness benefit from this threonine-mediated CAT tail function would need to outweigh any fitness deficits arising from CAT tail aggregation. Given the recent discovery of Rqc2 homologs that incorporate different amino acids into CAT tails (Lytvynenko et al, 2019; Wu Z, Tantray I, Lim J, Chen S, Li Y, Davis Z, Sitron C, Dong J, Gispert S, Auburger G, Brandman O, Bi X, Snyder M, Lu B., 2019), domain-swapping between these homologs to bias CAT tail amino acid content may assist in determining the function of threonine in CAT tails in future studies. Broadly, understanding how the variable amino acid composition of CAT tails has evolved to meet physiological demands in different organisms may prove a fruitful area of research.…”

“…Loss of Ltn1 function results in a buildup of CAT-tailed (“CATylated”) proteins (Shen et al, 2015). The CAT tails on these proteins can homo-typically associate and form aggregates of CATylated proteins, which have been observed in yeast and metazoans (Choe et al, 2016; Defenouillère et al, 2016; Wu Z, Tantray I, Lim J, Chen S, Li Y, Davis Z, Sitron C, Dong J, Gispert S, Auburger G, Brandman O, Bi X, Snyder M, Lu B., 2019; Yonashiro et al, 2016). The presence of threonine in yeast CAT tails increases their aggregation propensity (Yonashiro et al, 2016).…”

“…Salmonella enterica and Streptococcus pneumoniae ) (Chaudhuri et al, 2009; Glass et al, 2006; Julio et al, 2000; Mann et al, 2012). In metazoan eukaryotes, mutations to LTN1 or perturbations that introduce large influxes of RQC substrates lead to neurodegeneration (Chu et al, 2009; Ishimura et al, 2014; Wu Z, Tantray I, Lim J, Chen S, Li Y, Davis Z, Sitron C, Dong J, Gispert S, Auburger G, Brandman O, Bi X, Snyder M, Lu B., 2019). Each of these phenotypes highlights the central role that tmRNA and Ltn1 play in maintaining protein homeostasis and avoiding the toxicity associated with compromised co-translational quality control.…”

“…Rqc2 homologs bind the large ribosomal subunit and direct it to elongate the incomplete polypeptide’s C-terminus with either alanine (“Ala tails” in prokaryotes) or both alanine and threonine residues (“CAT tails” in yeast) (Lytvynenko et al, 2019; Osuna et al, 2017; Shen et al, 2015). Metazoan CAT tails may include a more diverse repertoire of amino acids (Wu Z, Tantray I, Lim J, Chen S, Li Y, Davis Z, Sitron C, Dong J, Gispert S, Auburger G, Brandman O, Bi X, Snyder M, Lu B., 2019). These extensions, made without a small subunit or mRNA, act as “degrons” to mark incomplete polypeptides for degradation by the bacterial protease ClpXP or the eukaryotic proteasome (Lytvynenko et al, 2019; Sitron and Brandman, 2019).…”