Autophagy, the process by which proteins and organelles are sequestered in autophagosomal vesicles and delivered to the lysosome/vacuole for degradation, provides a primary route for turnover of stable and defective cellular proteins. Defects in this system are linked with numerous human diseases. Although conserved protein kinase, lipid kinase and ubiquitin-like protein conjugation subnetworks controlling autophagosome formation and cargo recruitment have been defined, our understanding of the global organization of this system is limited. Here we report a proteomic analysis of the autophagy interaction network in human cells under conditions of ongoing (basal) autophagy, revealing a network of 751 interactions among 409 candidate interacting proteins with extensive connectivity among subnetworks. Many new autophagy interaction network components have roles in vesicle trafficking, protein or lipid phosphorylation and protein ubiquitination, and affect autophagosome number or flux when depleted by RNA interference. The six ATG8 orthologues in humans (MAP1LC3/GABARAP proteins) interact with a cohort of 67 proteins, with extensive binding partner overlap between family members, and frequent involvement of a conserved surface on ATG8 proteins known to interact with LC3-interacting regions in partner proteins. These studies provide a global view of the mammalian autophagy interaction landscape and a resource for mechanistic analysis of this critical protein homeostasis pathway.

The ubiquitin-proteasome pathway (UPP) regulates synaptic function, but little is known about specific UPP targets and mechanisms in mammalian synapses. We report here that the SCF(beta-TRCP) complex, a multisubunit E3 ubiquitin ligase, targets the postsynaptic spine-associated Rap GTPase activating protein (SPAR) for degradation in neurons. SPAR degradation by SCF(beta-TRCP) depended on the activity-inducible protein kinase Polo-like kinase 2 (Plk2). In the presence of Plk2, SPAR physically associated with the SCF(beta-TRCP) complex through a canonical phosphodegron. In hippocampal neurons, disruption of the SCF(beta-TRCP) complex by overexpression of dominant interfering beta-TRCP or Cul1 constructs prevented Plk2-dependent degradation of SPAR. Our results identify a specific E3 ubiquitin ligase that mediates degradation of a key postsynaptic regulator of synaptic morphology and function.

Autophagy, the process by which proteins and organelles are sequestered in double-membrane structures called autophagosomes and delivered to lysosomes for degradation, is critical in diseases such as cancer and neurodegeneration. Much of our understanding of this process has emerged from analysis of bulk cytoplasmic autophagy, but our understanding of how specific cargo, including organelles, proteins or intracellular pathogens, are targeted for selective autophagy is limited. Here we use quantitative proteomics to identify a cohort of novel and known autophagosome-enriched proteins in human cells, including cargo receptors. Like known cargo receptors, nuclear receptor coactivator 4 (NCOA4) was highly enriched in autophagosomes, and associated with ATG8 proteins that recruit cargo-receptor complexes into autophagosomes. Unbiased identification of NCOA4-associated proteins revealed ferritin heavy and light chains, components of an iron-filled cage structure that protects cells from reactive iron species but is degraded via autophagy to release iron through an unknown mechanism. We found that delivery of ferritin to lysosomes required NCOA4, and an inability of NCOA4-deficient cells to degrade ferritin led to decreased bioavailable intracellular iron. This work identifies NCOA4 as a selective cargo receptor for autophagic turnover of ferritin (ferritinophagy), which is critical for iron homeostasis, and provides a resource for further dissection of autophagosomal cargo-receptor connectivity.

USP14 is a major regulator of the proteasome and one of three proteasome-associated deubiquitinating enzymes. Its effects on protein turnover are substrate-specific, for unknown reasons. We report that USP14 shows a marked preference for ubiquitin-cyclin B conjugates that carry more than one ubiquitin modification or chain. This specificity is conserved from yeast to humans and is independent of chain linkage type. USP14 has been thought to cleave single ubiquitin groups from the distal tip of a chain, but we find that it removes chains from cyclin B en bloc, proceeding until a single chain remains. The suppression of degradation by USP14's catalytic activity reflects its capacity to act on a millisecond time scale, before the proteasome can initiate degradation of the substrate. In addition, single-molecule studies showed that the dwell time of ubiquitin conjugates at the proteasome was reduced by USP14-dependent deubiquitination. In summary, the specificity of the proteasome can be regulated by rapid ubiquitin chain removal, which resolves substrates based on a novel aspect of ubiquitin conjugate architecture.

Previously, we identified QIL1 as a subunit of mitochondrial contact site (MICOS) complex and demonstrated a role for QIL1 in MICOS assembly, mitochondrial respiration, and cristae formation critical for mitochondrial architecture (Guarani et al., 2015). Here, we identify QIL1 null alleles in two siblings displaying multiple clinical symptoms of early-onset fatal mitochondrial encephalopathy with liver disease, including defects in respiratory chain function in patient muscle. QIL1 absence in patients' fibroblasts was associated with MICOS disassembly, abnormal cristae, mild cytochrome c oxidase defect, and sensitivity to glucose withdrawal. QIL1 expression rescued cristae defects, and promoted re-accumulation of MICOS subunits to facilitate MICOS assembly. MICOS assembly and cristae morphology were not efficiently rescued by over-expression of other MICOS subunits in patient fibroblasts. Taken together, these data provide the first evidence of altered MICOS assembly linked with a human mitochondrial disease and confirm a central role for QIL1 in stable MICOS complex formation.

Mitochondrial sirtuins, SIRT3-5, are NAD+-dependent deacylases and ADP-ribosyltransferases that are critical for stress responses. However, a comprehensive understanding of sirtuin targets, regulation of sirtuin activity, and the relationships between sirtuins remains a key challenge in mitochondrial physiology. Here, we employ systematic interaction proteomics to elucidate the mitochondrial sirtuin protein interaction landscape. This work reveals sirtuin interactions with numerous functional modules within mitochondria, identifies candidate sirtuin substrates, and uncovers a fundamental role for sequestration of SIRT3 by ATP synthase in mitochondrial homeostasis. In healthy mitochondria, a pool of SIRT3 binds ATP synthase, but upon matrix pH reduction with concomitant loss of mitochondrial membrane potential, SIRT3 dissociates. This release correlates with rapid deacetylation of matrix proteins, and SIRT3 is required for recovery of membrane potential. In vitro reconstitution experiments, as well as analysis of CRISPR/Cas9-engineered cells, indicate that pH-dependent SIRT3 release requires H135 in the ATP5O subunit of ATP synthase. Our SIRT3-5 interaction network provides a framework for discovering novel biological functions regulated by mitochondrial sirtuins.

System-wide quantitative analysis of ubiquitylomes has proven to be a valuable tool for elucidating targets and mechanisms of the ubiquitin-driven signaling systems, as well as gaining insights into neurodegenerative diseases and cancer. Current mass spectrometry methods for ubiquitylome detection require large amounts of starting material and rely on stochastic data collection to increase replicate analyses. We describe a method compatible with cell line and tissue samples for large-scale quantification of 5,000-9,000 ubiquitylation forms across ten samples simultaneously. Using this method, we reveal site-specific ubiquitylation in mammalian brain and liver tissues, as well as in cancer cells undergoing proteasome inhibition. To demonstrate the power of the approach for signal-dependent ubiquitylation, we examined protein and ubiquitylation dynamics for mitochondria undergoing PARKIN- and PINK1-dependent mitophagy. This analysis revealed the largest collection of PARKIN- and PINK1-dependent ubiquitylation targets to date in a single experiment, and it also revealed a subset of proteins recruited to the mitochondria during mitophagy.

Modular cullin-RING E3 ubiquitin ligases (CRLs) use substrate binding adaptor proteins to specify target ubiquitylation. Many of the ~200 human CRL adaptor proteins remain poorly studied due to a shortage of efficient methods to identify biologically relevant substrates. Here, we report the development of parallel adaptor capture (PAC) proteomics and its use to systematically identify candidate targets for the leucine-rich repeat family of F-box proteins (FBXLs) that function with SKP1-CUL1-F-box protein (SCF) E3s. In validation experiments, we identify the unstudied F-box protein FBXL17 as a regulator of the NFR2 oxidative stress pathway. We demonstrate that FBXL17 controls the transcription of the NRF2 target HMOX1 via turnover of the transcriptional repressor BACH1 in the absence or presence of extrinsic oxidative stress. This work identifies a role for SCF(FBXL17) in controlling the threshold for NRF2-dependent gene activation and provides a framework for elucidating the functions of CRL adaptor proteins.

Autophagy is a conserved process that delivers components of the cytoplasm to lysosomes for degradation. The E1 and E2 enzymes encoded by Atg7 and Atg3 are thought to be essential for autophagy involving the ubiquitin-like protein Atg8. Here, we describe an Atg7- and Atg3-independent autophagy pathway that facilitates programmed reduction of cell size during intestine cell death. Although multiple components of the core autophagy pathways, including Atg8, are required for autophagy and cells to shrink in the midgut of the intestine, loss of either Atg7 or Atg3 function does not influence these cellular processes. Rather, Uba1, the E1 enzyme used in ubiquitylation, is required for autophagy and reduction of cell size. Our data reveal that distinct autophagy programs are used by different cells within an animal, and disclose an unappreciated role for ubiquitin activation in autophagy.

Tumor Necrosis Factor (TNF) interacts with two receptors known as TNFR1 and TNFR2. TNFR1 activation may result in either cell proliferation or cell death. TNFR2 activates Nuclear Factor-kappaB (NF-kB) and c-Jun N-terminal kinase (JNK) which lead to transcriptional activation of genes related to cell proliferation and survival. This depends on the binding of TNF Receptor Associated Factor 2 (TRAF2) to the receptor. TNFR2 also induces TRAF2 degradation. In this work we have investigated the structural features of TNFR2 responsible for inducing TRAF2 degradation and have studied the biological consequences of this activity. We show that when TNFR1 and TNFR2 are co-expressed, TRAF2 depletion leads to an enhanced TNFR1 cytotoxicity which correlates with the inhibition of NF-kB. NF-kB activation and TRAF2 degradation depend of different regions of the receptor since TNFR2 mutants at amino acids 343-349 fail to induce TRAF2 degradation and have lost their ability to enhance TNFR1-mediated cell death but are still able to activate NF-kB. Moreover, whereas NF-kB activation requires TRAF2 binding to the receptor, TRAF2 degradation appears independent of TRAF2 binding. Thus, TNFR2 mutants unable to bind TRAF2 are still able to induce its degradation and to enhance TNFR1-mediated cytotoxicity. To test further this receptor crosstalk we have developed a system stably expressing in cells carrying only endogenous TNFR1 the chimeric receptor RANK-TNFR2, formed by the extracellular region of RANK (Receptor activator of NF-kB) and the intracellular region of TNFR2.This has made possible to study independently the signals triggered by TNFR1 and TNFR2. In these cells TNFR1 is selectively activated by soluble TNF (sTNF) while RANK-TNFR2 is selectively activated by RANKL. Treatment of these cells with sTNF and RANKL leads to an enhanced cytotoxicity.

Ribosomes are abundant cellular machines 1,2 that are regulated by assembly, supernumerary subunit turnover and nascent chain quality control mechanisms 1-5 . Moreover, nitrogen starvation in yeast has been reported to promote selective ribosome delivery to the vacuole in an autophagy conjugation system dependent manner, a process called 'ribophagy' 6,7 . However, whether ribophagy in mammals is selective or regulated is unclear. Using Ribo-Keima flux reporters, we find that starvation or mTOR inhibition promotes VPS34-dependent ribophagic flux, which, unlike yeast, is largely independent of ATG8 conjugation and occurs concomitantly with other cytosolic protein autophagic flux reporters 8,9 . Ribophagic flux was not induced upon inhibition of translational elongation or nascent chain uncoupling, but was induced in a comparatively selective manner under proteotoxic stress induced by arsenite 10 or chromosome mis-segregation 11 , dependent upon VPS34 and ATG8 conjugation. Unexpectedly, agents typically used to induce selective autophagy also promoted increased ribosome and cytosolic protein reporter flux, suggesting significant bulk or 'bystander' autophagy during what is often considered selective autophagy 12,13 . These results emphasize the importance of monitoring non-specific cargo flux when assessing selective autophagy pathways.

Mitochondrial stress activates a mitonuclear response to safeguard and repair mitochondrial function and to adapt cellular metabolism to stress. Using a multiomics approach in mammalian cells treated with four types of mitochondrial stressors, we identify activating transcription factor 4 (ATF4) as the main regulator of the stress response. Surprisingly, canonical mitochondrial unfolded protein response genes mediated by ATF5 are not activated. Instead, ATF4 activates the expression of cytoprotective genes, which reprogram cellular metabolism through activation of the integrated stress response (ISR). Mitochondrial stress promotes a local proteostatic response by reducing mitochondrial ribosomal proteins, inhibiting mitochondrial translation, and coupling the activation of the ISR with the attenuation of mitochondrial function. Through a trans-expression quantitative trait locus analysis, we provide genetic evidence supporting a role for Fh1 in the control of Atf4 expression in mammals. Using gene expression data from mice and humans with mitochondrial diseases, we show that the ATF4 pathway is activated in vivo upon mitochondrial stress. Our data illustrate the value of a multiomics approach to characterize complex cellular networks and provide a versatile resource to identify new regulators of mitochondrial-related diseases.

Necroptosis, a cell death pathway mediated by the RIPK1-RIPK3-MLKL signaling cascade downstream of tumor necrosis factor α (TNF-α), has been implicated in many inflammatory diseases. Members of the TAM (Tyro3, Axl, and Mer) family of receptor tyrosine kinases are known for their anti-apoptotic, oncogenic, and anti-inflammatory roles. Here, we identify an unexpected role of TAM kinases as promoters of necroptosis, a pro-inflammatory necrotic cell death. Pharmacologic or genetic targeting of TAM kinases results in a potent inhibition of necroptotic death in various cellular models. We identify phosphorylation of MLKL Tyr376 as a direct point of input from TAM kinases into the necroptosis signaling. The oligomerization of MLKL, but not its membranal translocation or phosphorylation by RIPK3, is controlled by TAM kinases. Importantly, both knockout and inhibition of TAM kinases protect mice from systemic inflammatory response syndrome. In conclusion, this study discovers that immunosuppressant TAM kinases are promoters of pro-inflammatory necroptosis, shedding light on the biological complexity of the regulation of inflammation.

Mammalian cells reorganize their proteomes in response to nutrient stress through translational suppression and degradative mechanisms using the proteasome and autophagy systems1,2. Ribosomes are central targets of this response, as they are responsible for translation and subject to lysosomal turnover during nutrient stress3-5. The abundance of ribosomal (r)-proteins (around 6% of the proteome; 107 copies per cell)6,7 and their high arginine and lysine content has led to the hypothesis that they are selectively used as a source of basic amino acids during nutrient stress through autophagy4,7. However, the relative contributions of translational and degradative mechanisms to the control of r-protein abundance during acute stress responses is poorly understood, as is the extent to which r-proteins are used to generate amino acids when specific building blocks are limited7. Here, we integrate quantitative global translatome and degradome proteomics8 with genetically encoded Ribo-Keima5 and Ribo-Halo reporters to interrogate r-protein homeostasis with and without active autophagy. In conditions of acute nutrient stress, cells strongly suppress the translation of r-proteins, but, notably, r-protein degradation occurs largely through non-autophagic pathways. Simultaneously, the decrease in r-protein abundance is compensated for by a reduced dilution of pre-existing ribosomes and a reduction in cell volume, thereby maintaining the density of ribosomes within single cells. Withdrawal of basic or hydrophobic amino acids induces translational repression without differential induction of ribophagy, indicating that ribophagy is not used to selectively produce basic amino acids during acute nutrient stress. We present a quantitative framework that describes the contributions of biosynthetic and degradative mechanisms to r-protein abundance and proteome remodelling in conditions of nutrient stress.

Translation of aberrant mRNAs induces ribosomal collisions, thereby triggering pathways for mRNA and nascent peptide degradation and ribosomal rescue. Here we use sucrose gradient fractionation combined with quantitative proteomics to systematically identify proteins associated with collided ribosomes. This approach identified Endothelial differentiation-related factor 1 (EDF1) as a novel protein recruited to collided ribosomes during translational distress. Cryo-electron microscopic analyses of EDF1 and its yeast homolog Mbf1 revealed a conserved 40S ribosomal subunit binding site at the mRNA entry channel near the collision interface. EDF1 recruits the translational repressors GIGYF2 and EIF4E2 to collided ribosomes to initiate a negative-feedback loop that prevents new ribosomes from translating defective mRNAs. Further, EDF1 regulates an immediate-early transcriptional response to ribosomal collisions. Our results uncover mechanisms through which EDF1 coordinates multiple responses of the ribosome-mediated quality control pathway and provide novel insights into the intersection of ribosome-mediated quality control with global transcriptional regulation.

The ubiquitin (Ub) kinase-ligase pair PINK1-PRKN mediates the degradation of damaged mitochondria by macroautophagy/autophagy (mitophagy). PINK1 surveils mitochondria and upon stress accumulates on the mitochondrial surface where it phosphorylates serine 65 of Ub to activate PRKN and to drive mitochondrial turnover. While loss of either PINK1 or PRKN is genetically linked to Parkinson disease (PD) and activating the pathway seems to have great therapeutic potential, there is no formal proof that stimulation of mitophagy is always beneficial. Here we used biochemical and cell biological methods to study single nucleotide variants in the activation loop of PINK1 to modulate the enzymatic function of this kinase. Structural modeling and in vitro kinase assays were used to investigate the molecular mechanism of the PINK1 variants. In contrast to the PD-linked PINK1G411S mutation that diminishes Ub kinase activity, we found that the PINK1G411A variant significantly boosted Ub phosphorylation beyond levels of PINK1 wild type. This resulted in augmented PRKN activation, mitophagy rates and increased viability after mitochondrial stress in midbrain-derived, gene-edited neurons. Mechanistically, the G411A variant stabilizes the kinase fold of PINK1 and transforms Ub to adopt the preferred, C-terminally retracted conformation for improved substrate turnover. In summary, we identify a critical role of residue 411 for substrate receptivity that may now be exploited for drug discovery to increase the enzymatic function of PINK1. The genetic substitution of Gly411 to Ala increases mitophagy and may be useful to confirm neuroprotection in vivo and might serve as a critical positive control during therapeutic development.Abbreviations: ATP: adenosine triphosphate; CCCP: carbonyl cyanide m-chlorophenyl hydrazone; Ub-CR: ubiquitin with C-terminally retracted tail; CTD: C-terminal domain (of PINK1); ELISA: enzyme-linked immunosorbent assay; HCI: high-content imaging; IB: immunoblot; IF: immunofluorescence; NPC: neuronal precursor cells; MDS: molecular dynamics simulation; PD: Parkinson disease; p-S65-Ub: ubiquitin phosphorylated at Ser65; RMSF: root mean scare fluctuation; TOMM: translocase of outer mitochondrial membrane; TVLN: ubiquitin with T66V and L67N mutation, mimics Ub-CR; Ub: ubiquitin; WT: wild-type.

Reversible modification of target proteins by ubiquitin and ubiquitin-like proteins (UBLs) is widely used by eukaryotic cells to control protein fate and cell behaviour1. UFM1 is a UBL that predominantly modifies a single lysine residue on a single ribosomal protein, uL24 (also called RPL26), on ribosomes at the cytoplasmic surface of the endoplasmic reticulum (ER)2,3. UFM1 conjugation (UFMylation) facilitates the rescue of 60S ribosomal subunits (60S) that are released after ribosome-associated quality-control-mediated splitting of ribosomes that stall during co-translational translocation of secretory proteins into the ER3,4. Neither the molecular mechanism by which the UFMylation machinery achieves such precise target selection nor how this ribosomal modification promotes 60S rescue is known. Here we show that ribosome UFMylation in vivo occurs on free 60S and we present sequential cryo-electron microscopy snapshots of the heterotrimeric UFM1 E3 ligase (E3(UFM1)) engaging its substrate uL24. E3(UFM1) binds the L1 stalk, empty transfer RNA-binding sites and the peptidyl transferase centre through carboxy-terminal domains of UFL1, which results in uL24 modification more than 150 Å away. After catalysing UFM1 transfer, E3(UFM1) remains stably bound to its product, UFMylated 60S, forming a C-shaped clamp that extends all the way around the 60S from the transfer RNA-binding sites to the polypeptide tunnel exit. Our structural and biochemical analyses suggest a role for E3(UFM1) in post-termination release and recycling of the large ribosomal subunit from the ER membrane.

Cells respond to nutrient stress by trafficking cytosolic contents to lysosomes for degradation via macroautophagy. The endoplasmic reticulum (ER) serves as an initiation site for autophagosomes and is also remodeled in response to nutrient stress through ER-phagy, a form of selective autophagy. Quantitative proteome analysis during nutrient stress identified an unstudied single-pass transmembrane ER protein, TEX264, as an ER-phagy receptor. TEX264 uses an LC3-interacting region (LIR) to traffic into ATG8-positive puncta that often initiate from three-way ER tubule junctions and subsequently fuse with lysosomes. Interaction and proximity biotinylation proteomics identified a cohort of autophagy regulatory proteins and cargo adaptors located near TEX264 in an LIR-dependent manner. Global proteomics and ER-phagy flux analysis revealed the stabilization of a cohort of ER proteins in TEX264-/- cells during nutrient stress. This work reveals TEX264 as an unrecognized ER-phagy receptor that acts independently of other candidate ER-phagy receptors to remodel the ER during nutrient stress.

The Cullin-RING E3 ligase (CRL) family is commonly hijacked by pathogens to redirect the host ubiquitin proteasome machinery to specific targets. During HIV infection, CRL5 is hijacked by HIV Vif to target viral restriction factors of the APOBEC3 family for ubiquitination and degradation. Here, using a quantitative proteomics approach, we identify the E3 ligase ARIH2 as a regulator of CRL5-mediated APOBEC3 degradation. The CUL5Vif/CBFß complex recruits ARIH2 where it acts to transfer ubiquitin directly to the APOBEC3 targets. ARIH2 is essential for CRL5-dependent HIV infectivity in primary CD4+ T cells. Furthermore, we show that ARIH2 cooperates with CRL5 to prime other cellular substrates for polyubiquitination, suggesting this may represent a general mechanism beyond HIV infection and APOBEC3 degradation. Taken together, these data identify ARIH2 as a co-factor in the Vif-hijacked CRL5 complex that contributes to HIV infectivity and demonstrate the operation of the E1-E2-E3/E3-substrate ubiquitination mechanism in a viral infection context.

Most E3 ligases use a RING domain to activate a thioester-linked E2∼ubiquitin-like protein (UBL) intermediate and promote UBL transfer to a remotely bound target protein. Nonetheless, RING E3 mechanisms matching a specific UBL and acceptor lysine remain elusive, including for RBX1, which mediates NEDD8 ligation to cullins and >10% of all ubiquitination. We report the structure of a trapped RING E3-E2∼UBL-target intermediate representing RBX1-UBC12∼NEDD8-CUL1-DCN1, which reveals the mechanism of NEDD8 ligation and how a particular UBL and acceptor lysine are matched by a multifunctional RING E3. Numerous mechanisms specify cullin neddylation while preventing noncognate ubiquitin ligation. Notably, E2-E3-target and RING-E2∼UBL modules are not optimized to function independently, but instead require integration by the UBL and target for maximal reactivity. The UBL and target regulate the catalytic machinery by positioning the RING-E2∼UBL catalytic center, licensing the acceptor lysine, and influencing E2 reactivity, thereby driving their specific coupling by a multifunctional RING E3.