scholarly journals The nucleus is a quality control center for non-imported mitochondrial proteins

2020 ◽  
Author(s):  
Viplendra P.S. Shakya ◽  
William A. Barbeau ◽  
Tianyao Xiao ◽  
Christina S. Knutson ◽  
Adam L. Hughes
Keyword(s):  
Quality Control ◽  
Steady State ◽  
Nuclear Protein ◽  
Protein Quality ◽  
Mitochondrial Proteins ◽  
Mitochondrial Targeting ◽  
Mitochondrial Import ◽  
Control Center ◽  
Mitochondrial Impairment ◽  
Precursor Proteins

AbstractMitochondrial import deficiency causes cellular stress due to the accumulation of non-imported mitochondrial precursor proteins. Despite the burden mis-localized mitochondrial precursors place on cells, our understanding of the systems that dispose of these proteins is incomplete. Here, we catalog the location and steady-state abundance of mitochondrial precursor proteins during mitochondrial impairment in S. cerevisiae. We find that a number of non-imported mitochondrial proteins localize to the nucleus, where they are eliminated by proteasome-based nuclear protein quality control. Recognition of mitochondrial precursors by the nuclear quality control machinery requires the presence of an N-terminal mitochondrial targeting sequence (MTS), and impaired breakdown of precursors leads to their buildup in nuclear-associated foci. These results identify the nucleus as a key destination for the disposal of non-imported mitochondrial precursors.

scholarly journals A nuclear-based quality control pathway for non-imported mitochondrial proteins

eLife ◽  
2021 ◽  
Vol 10 ◽  
Author(s):  
Viplendra PS Shakya ◽  
William A Barbeau ◽  
Tianyao Xiao ◽  
Christina S Knutson ◽  
Max H Schuler ◽  
...  
Keyword(s):  
Quality Control ◽  
Steady State ◽  
Mitochondrial Proteins ◽  
Mitochondrial Targeting ◽  
Mitochondrial Import ◽  
Cellular Toxicity ◽  
Mitochondrial Impairment ◽  
Induced Stress ◽  
Precursor Proteins ◽  
Combined Action

Mitochondrial import deficiency causes cellular toxicity due to the accumulation of non-imported mitochondrial precursor proteins, termed mitoprotein-induced stress. Despite the burden mis-localized mitochondrial precursors place on cells, our understanding of the systems that dispose of these proteins is incomplete. Here, we cataloged the location and steady-state abundance of mitochondrial precursor proteins during mitochondrial impairment in S. cerevisiae. We found that a number of non-imported mitochondrial proteins localize to the nucleus, where they are subjected to proteasome-dependent degradation through a process we term nuclear-associated mitoprotein degradation (mitoNUC). Recognition and destruction of mitochondrial precursors by the mitoNUC pathway requires the presence of an N-terminal mitochondrial targeting sequence (MTS) and is mediated by combined action of the E3 ubiquitin ligases San1, Ubr1, and Doa10. Impaired breakdown of precursors leads to alternative sequestration in nuclear-associated foci. These results identify the nucleus as an important destination for the disposal of non-imported mitochondrial precursors.

Genome-wide regulations of the pre-initiation complex formation and elongating RNA polymerase II by an E3 ubiquitin ligase, San1.

2021 ◽  
Author(s):  
Priyanka Barman ◽  
Rwik Sen ◽  
Amala Kaja ◽  
Jannatul Ferdoush ◽  
Shalini Guha ◽  
...  
Keyword(s):  
Quality Control ◽  
Rna Polymerase ◽  
Rna Polymerase Ii ◽  
Ubiquitin Ligase ◽  
Nuclear Protein ◽  
Protein Quality ◽  
Initiation Complex ◽  
Pol Ii ◽  
Intrinsically Disordered ◽  
Genome Wide

San1 ubiquitin ligase is involved in nuclear protein quality control via its interaction with intrinsically disordered proteins for ubiquitylation and proteasomal degradation. Since several transcription/chromatin regulatory factors contain intrinsically disordered domains and can be inhibitory to transcription when in excess, San1 might be involved in transcription regulation. To address this, we analyzed the role of San1 in genome-wide association of TBP [that nucleates pre-initiation complex (PIC) formation for transcription initiation] and RNA polymerase II (Pol II). Our results reveal the roles of San1 in regulating TBP recruitment to the promoters and Pol II association with the coding sequences, and hence PIC formation and coordination of elongating Pol II, respectively. Consistently, transcription is altered in the absence of San1. Such transcriptional alteration is associated with impaired ubiquitylation and proteasomal degradation of Spt16 and gene association of Paf1, but not the incorporation of centromeric histone, Cse4, into the active genes in Δsan1 . Collectively, our results demonstrate distinct functions of a nuclear protein quality control factor in regulating the genome-wide PIC formation and elongating Pol II (and hence transcription), thus unraveling new gene regulatory mechanisms.

scholarly journals The GET pathway safeguards against non-imported mitochondrial protein stress

2020 ◽  
Author(s):  
Tianyao Xiao ◽  
Viplendra P.S. Shakya ◽  
Adam L. Hughes
Keyword(s):  
Endoplasmic Reticulum ◽  
Mitochondrial Protein ◽  
Transmembrane Proteins ◽  
Mitochondrial Proteins ◽  
Mitochondrial Import ◽  
Cellular Toxicity ◽  
Precursor Proteins ◽  
Dependent Protein ◽  
Er Targeting ◽  
Get Complex

SUMMARYDeficiencies in mitochondrial import cause the toxic accumulation of non-imported mitochondrial precursor proteins. Numerous fates for non-imported mitochondrial precursors have been identified, including proteasomal destruction, deposition into protein aggregates, and mis-targeting to other organelles. Amongst organelles, the endoplasmic reticulum (ER) has emerged as a key destination for non-imported mitochondrial proteins, but how ER-targeting of these proteins is achieved remains unclear. Here, we show that the guided entry of tail-anchored proteins (GET) complex is required for ER-targeting of endogenous mitochondrial multi-transmembrane proteins. Without a functional GET pathway, non-imported mitochondrial proteins destined for the ER are alternatively sequestered into Hsp42-dependent protein foci. The ER targeting of non-imported mitochondrial proteins by the GET complex prevents cellular toxicity and facilitates re-import of mitochondrial proteins from the ER via the recently identified ER-SURF pathway. Overall, this study outlines an important and unconventional role for the GET complex in mitigating stress associated with non-imported mitochondrial proteins.

scholarly journals The intermembrane space protein Mix23 is a novel stress-induced mitochondrial import factor

2020 ◽  
Vol 295 (43) ◽  
pp. 14686-14697 ◽  
Author(s):  
Eva Zöller ◽  
Janina Laborenz ◽  
Lena Krämer ◽  
Felix Boos ◽  
Markus Räschle ◽  
...  
Keyword(s):  
Protein Import ◽  
Mitochondrial Protein ◽  
Mitochondrial Proteins ◽  
Temperature Sensitive ◽  
Intermembrane Space ◽  
Mitochondrial Import ◽  
Induced Stress ◽  
Evolutionarily Conserved ◽  
Precursor Proteins ◽  
Biogenesis Of Mitochondria

The biogenesis of mitochondria requires the import of hundreds of precursor proteins. These proteins are transported post-translationally with the help of chaperones, meaning that the overproduction of mitochondrial proteins or the limited availability of chaperones can lead to the accumulation of cytosolic precursor proteins. This imposes a severe challenge to cytosolic proteostasis and triggers a specific transcription program called the mitoprotein-induced stress response, which activates the proteasome system. This coincides with the repression of mitochondrial proteins, including many proteins of the intermembrane space. In contrast, herein we report that the so-far-uncharacterized intermembrane space protein Mix23 is considerably up-regulated when mitochondrial import is perturbed. Mix23 is evolutionarily conserved and a homolog of the human protein CCDC58. We found that, like the subunits of the proteasome, Mix23 is under control of the transcription factor Rpn4. It is imported into mitochondria by the mitochondrial disulfide relay. Mix23 is critical for the efficient import of proteins into the mitochondrial matrix, particularly if the function of the translocase of the inner membrane 23 is compromised such as in temperature-sensitive mutants of Tim17. Our observations identify Mix23 as a novel regulator or stabilizer of the mitochondrial protein import machinery that is specifically up-regulated upon mitoprotein-induced stress conditions.

Mitochondria-Associated Proteostasis

2020 ◽  
Vol 49 (1) ◽  
pp. 41-67 ◽  
Author(s):  
Linhao Ruan ◽  
Yuhao Wang ◽  
Xi Zhang ◽  
Alexis Tomaszewski ◽  
Joshua T. McNamara ◽  
...  
Keyword(s):  
Quality Control ◽  
Cardiovascular Diseases ◽  
Protein Quality ◽  
Therapeutic Potential ◽  
Nuclear Genome ◽  
Mitochondrial Proteins ◽  
Protein Homeostasis ◽  
Control Mechanisms ◽  
Mitochondrial Proteome ◽  
Functional States

Mitochondria are essential organelles in eukaryotes. Most mitochondrial proteins are encoded by the nuclear genome and translated in the cytosol. Nuclear-encoded mitochondrial proteins need to be imported, processed, folded, and assembled into their functional states. To maintain protein homeostasis (proteostasis), mitochondria are equipped with a distinct set of quality control machineries. Deficiencies in such systems lead to mitochondrial dysfunction, which is a hallmark of aging and many human diseases, such as neurodegenerative diseases, cardiovascular diseases, and cancer. In this review, we discuss the unique challenges and solutions of proteostasis in mitochondria. The import machinery coordinates with mitochondrial proteases and chaperones to maintain the mitochondrial proteome. Moreover, mitochondrial proteostasis depends on cytosolic protein quality control mechanisms during crises. In turn, mitochondria facilitate cytosolic proteostasis. Increasing evidence suggests that enhancing mitochondrial proteostasis may hold therapeutic potential to protect against protein aggregation–associated cellular defects.

scholarly journals USP5 enhances SGTA mediated protein quality control

2021 ◽  
Author(s):  
Yvonne Nyathi ◽  
Jake Alfie Hill
Keyword(s):  
Quality Control ◽  
Steady State ◽  
Type Ii Diabetes ◽  
Protein Quality ◽  
Proteasomal Degradation ◽  
Aggregate Formation ◽  
Deubiquitinating Enzymes ◽  
Physiological Processes ◽  
Hydrophobic Amino Acids ◽  
Steady State Levels

Mislocalised membrane proteins (MLPs) present a risk to the cell due to exposed hydrophobic amino acids which cause MLPs to aggregate. Previous studies identified SGTA as a key component of the machinery that regulates the quality control of MLPs. Overexpression of SGTA promotes deubiqutination of MLPs resulting in their accumulation in cytosolic inclusions, suggesting SGTA acts in collaboration with deubiquitinating enzymes (DUBs) to exert these effects.  However, the DUBs that play a role in this process have not been identified.  In this study we have identified the ubiquitin specific peptidase 5 (USP5) as a DUB important in regulating the quality control of MLPs. We show that USP5 is in complex with SGTA, and this association is increased in the presence of an MLP. Overexpression of SGTA results in an increase in steady-state levels of MLPs suggesting a delay in proteasomal degradation of substrates. However, our results show that this effect is strongly dependent on the presence of USP5.  We find that in the absence of USP5, the ability of SGTA to increase the steady state levels of MLPs is compromised. Moreover, knockdown of USP5 results in a reduction in the steady state levels of MLPs, while overexpression of USP5 increases the steady state levels. Our findings suggest that the interaction of SGTA with USP5 enables specific MLPs to escape proteasomal degradation allowing selective modulation of MLP quality control. These findings progress our understanding of aggregate formation, a hallmark in a range of neurodegenerative diseases and type II diabetes, as well as physiological processes of aggregate clearance.

scholarly journals Coordinated Translocation of Presequence-Containing Precursor Proteins Across Two Mitochondrial Membranes: Knowns and Unknowns of How TOM and TIM23 Complexes Cooperate With Each Other

2022 ◽  
Vol 12 ◽  
Author(s):  
Marcel G. Genge ◽  
Dejana Mokranjac
Keyword(s):  
Protein Interactions ◽  
Nuclear Genome ◽  
Hydrophobic Surface ◽  
Mitochondrial Proteins ◽  
Intermembrane Space ◽  
Mitochondrial Targeting ◽  
Mitochondrial Membranes ◽  
Inner Mitochondrial Membrane ◽  
Precursor Proteins ◽  
Targeting Signals

The vast majority of mitochondrial proteins are encoded in the nuclear genome and synthesized on cytosolic ribosomes as precursor proteins with specific mitochondrial targeting signals. Mitochondrial targeting signals are very diverse, however, about 70% of mitochondrial proteins carry cleavable, N-terminal extensions called presequences. These amphipathic helices with one positively charged and one hydrophobic surface target proteins to the mitochondrial matrix with the help of the TOM and TIM23 complexes in the outer and inner membranes, respectively. Translocation of proteins across the two mitochondrial membranes does not take place independently of each other. Rather, in the intermembrane space, where the two complexes meet, components of the TOM and TIM23 complexes form an intricate network of protein–protein interactions that mediates initially transfer of presequences and then of the entire precursor proteins from the outer to the inner mitochondrial membrane. In this Mini Review, we summarize our current understanding of how the TOM and TIM23 complexes cooperate with each other and highlight some of the future challenges and unresolved questions in the field.

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