Damaged mitochondria are detrimental to cellular homeostasis. One mechanism for removal of damaged mitochondria involves the PINK1-PARKIN pathway, which poly-ubiquitylates damaged mitochondria to ...promote mitophagy. We report that assembly of ubiquitin chains on mitochondria triggers autophagy adaptor recruitment concomitantly with activation of the TBK1 kinase, which physically associates with OPTN, NDP52, and SQSTM1. TBK1 activation in HeLa cells requires OPTN and NDP52 and OPTN ubiquitin chain binding. In addition to the known role of S177 phosphorylation in OPTN on ATG8 recruitment, TBK1-dependent phosphorylation on S473 and S513 promotes ubiquitin chain binding in vitro as well as TBK1 activation, OPTN mitochondrial retention, and efficient mitophagy in vivo. These data reveal a self-reinforcing positive feedback mechanism that coordinates TBK1-dependent autophagy adaptor phosphorylation with the assembly of ubiquitin chains on mitochondria to facilitate efficient mitophagy, and mechanistically links genes mutated in Parkinson’s disease and amyotrophic lateral sclerosis in a common selective autophagy pathway.
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•PINK1-PARKIN pathway promotes TBK1 activation via OPTN and NDP52•Mitochondrial ubiquitylation templates TBK1 activation via OPTN/NDP52 recruitment•TBK1 phosphorylates S473 and S513 in OPTN to enhance UB chain binding in vitro•TBK1 and OPTN/NDP52 promote mitophagy in HeLa cells
Mitochondrial damage promotes PINK1-PARKIN-dependent mitochondrial ubiquitylation and mitophagy. Heo et al. report that mitochondrial damage activates the kinase TBK1, which phosphorylates autophagy adaptors OPTN, NDP52, and SQSTM1, and in the case of OPTN is shown to increase ubiquitin binding and recruitment to damaged mitochondria to promote mitophagy.
Attachment of ubiquitin or ubiquitin-like proteins (known as UBLs) to their targets through multienzyme cascades is a central mechanism to modulate protein functions. This process is initiated by a ...family of mechanistically and structurally related E1 (or activating) enzymes. These activate UBLs through carboxy-terminal adenylation and thiol transfer, and coordinate the use of UBLs in specific downstream pathways by charging cognate E2 (or conjugating) enzymes, which then interact with the downstream ubiquitylation machinery to coordinate the modification of the target. A broad understanding of how E1 enzymes activate UBLs and how they selectively coordinate UBLs with downstream function has come from enzymatic, structural and genetic studies.
The mitochondrial matrix is unique in that it must integrate the folding and assembly of proteins derived from the nuclear and mitochondrial genomes. In Caenorhabditis elegans, the mitochondrial ...unfolded protein response (UPRmt) senses matrix protein misfolding and induces a program of nuclear gene expression, including mitochondrial chaperonins, to promote mitochondrial proteostasis. While misfolded mitochondrial-matrix-localized ornithine transcarbamylase induces chaperonin expression, our understanding of mammalian UPRmt is rudimentary, reflecting a lack of acute triggers for UPRmt activation. This limitation has prevented analysis of the cellular responses to matrix protein misfolding and the effects of UPRmt on mitochondrial translation to control protein folding loads. Here we combine pharmacological inhibitors of matrix-localized HSP90/TRAP1 (ref. 8) or LON protease, which promote chaperonin expression, with global transcriptional and proteomic analysis to reveal an extensive and acute response of human cells to UPRmt. This response encompasses widespread induction of nuclear genes, including matrix-localized proteins involved in folding, pre-RNA processing and translation. Functional studies revealed rapid but reversible translation inhibition in mitochondria occurring concurrently with defects in pre-RNA processing caused by transcriptional repression and LON-dependent turnover of the mitochondrial pre-RNA processing nuclease MRPP3 (ref. 10). This study reveals that acute mitochondrial protein folding stress activates both increased chaperone availability within the matrix and reduced matrix-localized protein synthesis through translational inhibition, and provides a framework for further dissection of mammalian UPRmt.
mTOR complex 1 (mTORC1) regulates cell growth and metabolism in response to multiple environmental cues. Nutrients signal via the Rag guanosine triphosphatases (GTPases) to promote the localization ...of mTORC1 to the lysosomal surface, its site of activation. We identified SAMTOR, a previously uncharacterized protein, which inhibits mTORC1 signaling by interacting with GATOR1, the GTPase activating protein (GAP) for RagA/B. We found that the methyl donor S-adenosylmethionine (SAM) disrupts the SAMTOR-GATOR1 complex by binding directly to SAMTOR with a dissociation constant of approximately 7 μM. In cells, methionine starvation reduces SAM levels below this dissociation constant and promotes the association of SAMTOR with GATOR1, thereby inhibiting mTORC1 signaling in a SAMTOR-dependent fashion. Methionine-induced activation of mTORC1 requires the SAM binding capacity of SAMTOR. Thus, SAMTOR is a SAM sensor that links methionine and one-carbon metabolism to mTORC1 signaling.
Ribosomes are central to the life of a cell, as they translate the genetic code into the amino acid language of proteins. Moreover, ribosomal abundance within the cell is coordinated with protein ...production required for cell function or processes such as cell division. As such, it is not surprising that these elegant machines are both highly regulated at the level of both their output of newly translated proteins but also at the level of ribosomal protein expression, ribosome assembly, and ribosome turnover. In this review, we focus on mechanisms that regulate ribosome abundance through both the ubiquitin–proteasome system and forms of autophagy referred to as “ribophagy.” We discussed mechanisms employed in both yeast and mammalian cells, including the various machineries that are important for recognition and degradation of ribosomal components. In addition, we discussed controversies in the field and how the development of new approaches for examining flux through the proteasomal and autophagic systems in the context of a systematic inventory of ribosomal components is necessary to fully understand how ribosome abundance is controlled under various physiological conditions.
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•Ribosomes are abundance cellular machines and are regulated through multiple degradative pathways.•Multiple ubiquitin ligases have been identified that can ubiquitylate and promote turnover of either excess unassembled ribosomal proteins or ribosomal proteins that are part of mature ribosomes.•In yeast, nutrient stress promotes turnover of ribosomes via “ribophagy” in a process that requires the deubiquitylating enzyme Ubp3p.•In mammalian cells, multiple pathways have been reported to promote ribosomal degradation in response to nutrient and proteotoxic stress.•This review discusses multiple controversies in the field concerning the pathways used for ribophagy.
Parkinson’s disease (PD) is a neurodegenerative disease caused by the loss of dopaminergic neurons in the substantia nigra. PARK2 mutations cause early-onset forms of PD. PARK2 encodes an E3 ...ubiquitin ligase, Parkin, that can selectively translocate to dysfunctional mitochondria to promote their removal by autophagy. However, Parkin knockout (KO) mice do not display signs of neurodegeneration. To assess Parkin function in vivo, we utilized a mouse model that accumulates dysfunctional mitochondria caused by an accelerated generation of mtDNA mutations (Mutator mice). In the absence of Parkin, dopaminergic neurons in Mutator mice degenerated causing an L-DOPA reversible motor deficit. Other neuronal populations were unaffected. Phosphorylated ubiquitin was increased in the brains of Mutator mice, indicating PINK1-Parkin activation. Parkin loss caused mitochondrial dysfunction and affected the pathogenicity but not the levels of mtDNA somatic mutations. A systemic loss of Parkin synergizes with mitochondrial dysfunction causing dopaminergic neuron death modeling PD pathogenic processes.
•Parkin preferentially protects dopaminergic neurons from mitochondrial stress•Phosphorylated-S65 ubiquitin is increased in the brain after mitochondrial stress•Parkin affects the pathogenicity not quantity of somatic mtDNA point mutations•Loss of Parkin exacerbates mitochondrial dysfunction in neurons
Recessive mutations in Parkin contribute to Parkinson’s disease. Pickrell and Huang et al. use a mouse model to discover endogenous Parkin preferentially protects dopaminergic neurons from mitochondrial dysfunction.
Organelle identity depends on protein composition. How mistargeted proteins are selectively recognized and removed from organelles is incompletely understood. Here, we found that the orphan ...P5A-adenosine triphosphatase (ATPase) transporter ATP13A1 (Spf1 in yeast) directly interacted with the transmembrane segment (TM) of mitochondrial tail-anchored proteins. P5A-ATPase activity mediated the extraction of mistargeted proteins from the endoplasmic reticulum (ER). Cryo-electron microscopy structures of
Spf1 revealed a large, membrane-accessible substrate-binding pocket that alternately faced the ER lumen and cytosol and an endogenous substrate resembling an α-helical TM. Our results indicate that the P5A-ATPase could dislocate misinserted hydrophobic helices flanked by short basic segments from the ER. TM dislocation by the P5A-ATPase establishes an additional class of P-type ATPase substrates and may correct mistakes in protein targeting or topogenesis.
Despite the diverse biological pathways known to be regulated by ubiquitylation, global identification of substrates that are targeted for ubiquitylation has remained a challenge. To globally ...characterize the human ubiquitin-modified proteome (ubiquitinome), we utilized a monoclonal antibody that recognizes diglycine (diGly)-containing isopeptides following trypsin digestion. We identify ∼19,000 diGly-modified lysine residues within ∼5000 proteins. Using quantitative proteomics we monitored temporal changes in diGly site abundance in response to both proteasomal and translational inhibition, indicating both a dependence on ongoing translation to observe alterations in site abundance and distinct dynamics of individual modified lysines in response to proteasome inhibition. Further, we demonstrate that quantitative diGly proteomics can be utilized to identify substrates for cullin-RING ubiquitin ligases. Interrogation of the ubiquitinome allows for not only a quantitative assessment of alterations in protein homeostasis fidelity, but also identification of substrates for individual ubiquitin pathway enzymes.
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► α-diGly was used to identify/quantify 19,000 ubiquitylation sites in 5,000 proteins ► Kinetics of ubiquitylation allows for classification of distinct substrate types ► Ubiquitinome formation largely requires ongoing protein synthesis ► diGly proteomics can be used to identify cullin-RING ligase substrates
Ubiquitin (UB)-driven signaling systems permeate biology, and are often integrated with other types of post-translational modifications (PTMs), including phosphorylation. Flux through such pathways ...is dictated by the fractional stoichiometry of distinct modifications and protein assemblies as well as the spatial organization of pathway components. Yet, we rarely understand the dynamics and stoichiometry of rate-limiting intermediates along a reaction trajectory. Here, we review how quantitative proteomic tools and enrichment strategies are being used to quantify UB-dependent signaling systems, and to integrate UB signaling with regulatory phosphorylation events, illustrated with the PINK1/PARKIN pathway. A key feature of ubiquitylation is that the identity of UB chain linkage types can control downstream processes. We also describe how proteomic and enzymological tools can be used to identify and quantify UB chain synthesis and linkage preferences. The emergence of sophisticated quantitative proteomic approaches will set a new standard for elucidating biochemical mechanisms of UB-driven signaling systems.
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