Explore the vast range of titles available.

Key Points Reversible post-translational protein modification by small ubiquitin-like modifier (SUMO), sumoylation, is a crucial mechanism in the maintenance of genomic integrity, in the regulation of proper gene expression patterns and in numerous signal transduction pathways and is thus essential for cell and tissue homeostasis in all eukaryotes. Numerous protein complexes contain multiple sumoylated proteins, suggesting that sumoylation regulates not only the activity of individual substrates but that of entire functional complexes. Furthermore, in complex signalling pathways, sumoylation frequently targets multiple elements, in some cases antagonizing the activity of one element while promoting that of another within the same pathway. Various biotic and abiotic stresses substantially alter the level of global cellular sumoylation. Importantly, numerous human tumours display marked upregulation of SUMO pathway components. This 'SUMOness' of cancers may indeed be required by tumour cells to maintain the robustness of compromised or otherwise easily misregulated gene expression programmes and signalling pathways. Sumoylation would therefore contribute substantially to cancer cell survival and proliferation in a potentially hostile microenvironment. Although strong inhibition of global cellular sumoylation carries obvious risks for all cells, partial or temporary inhibition may be sufficient to expose certain tumour-specific vulnerabilities (for example, susceptibility to inducers of apoptosis and/or senescence) and therefore could provide a promising approach for future therapeutic intervention in some settings. Sumoylation is an important mechanism in cellular responses to stress, and appears to be upregulated in many cancers. This Review argues that sumoylation protects the stability and functionality of otherwise easily misregulated gene expression programmes and signalling pathways of cancer cells. Post-translational protein modification by small ubiquitin-like modifier (SUMO), termed sumoylation, is an important mechanism in cellular responses to stress and one that appears to be upregulated in many cancers. Here, we examine the role of sumoylation in tumorigenesis as a possibly necessary safeguard that protects the stability and functionality of otherwise easily misregulated gene expression programmes and signalling pathways of cancer cells.

Exosomes are released by most cells to the extracellular environment and are involved in cell-to-cell communication. Exosomes contain specific repertoires of mRNAs, microRNAs (miRNAs) and other non-coding RNAs that can be functionally transferred to recipient cells. However, the mechanisms that control the specific loading of RNA species into exosomes remain unknown. Here we describe sequence motifs present in miRNAs that control their localization into exosomes. The protein heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1) specifically binds exosomal miRNAs through the recognition of these motifs and controls their loading into exosomes. Moreover, hnRNPA2B1 in exosomes is sumoylated, and sumoylation controls the binding of hnRNPA2B1 to miRNAs. The loading of miRNAs into exosomes can be modulated by mutagenesis of the identified motifs or changes in hnRNPA2B1 expression levels. These findings identify hnRNPA2B1 as a key player in miRNA sorting into exosomes and provide potential tools for the packaging of selected regulatory RNAs into exosomes and their use in biomedical applications. Cells secrete micro-RNAs by packaging them into exosomes; however, the mechanisms by which this packaging occurs are unclear. Here, the authors identify a sequence motif that confers exosomal targeting to micro-RNAs and identify a ribonucleoprotein complex that plays a role in this process.

The cellular NLRP3 protein level is crucial for assembly and activation of the NLRP3 inflammasome. Various posttranslational modifications (PTMs), including phosphorylation and ubiquitination, control NLRP3 protein degradation and inflammasome activation; however, the function of small ubiquitin-like modifier (SUMO) modification (called SUMOylation) in controlling NLRP3 stability and subsequent inflammasome activation is unclear. Here, we show that the E3 SUMO ligase tripartite motif-containing protein 28 (TRIM28) is an enhancer of NLRP3 inflammasome activation by facilitating NLRP3 expression. TRIM28 binds NLRP3, promotes SUMO1, SUMO2 and SUMO3 modification of NLRP3, and thereby inhibits NLRP3 ubiquitination and proteasomal degradation. Concordantly, Trim28 deficiency attenuates NLRP3 inflammasome activation both in vitro and in vivo. These data identify a mechanism by which SUMOylation controls the cellular NLRP3 level and inflammasome activation, and reveal correlations and interactions of NLRP3 SUMOylation and ubiquitination during inflammasome activation. Post-translational modifications are important regulators of NLRP3 inflammasome activity. Here the authors show that the E3 ligase TRIM28 can SUMOylate NLRP3, thereby limiting its proteasomal degradation and increasing NLRP3 inflammasome activity.

A comprehensive analysis of the human SUMO proteome, in HeLa and U2OS cell lines and under different conditions, identifies new SUMOylated sites and reveals cross-talk between SUMO and other post-translational modifications, such as phosphorylation. Small ubiquitin-like modifiers (SUMOs) are post-translational modifications (PTMs) that regulate nuclear cellular processes. Here we used an augmented K0–SUMO proteomics strategy to identify 40,765 SUMO acceptor sites and quantify their fractional contribution for 6,747 human proteins. Structural–predictive analyses revealed that lysines residing in disordered regions are preferentially targeted by SUMO, in notable contrast to other widespread lysine modifications. In our data set, we identified 807 SUMOylated peptides that were co-modified by phosphorylation, along with dozens of SUMOylated peptides that were co-modified by ubiquitylation, acetylation and methylation. Notably, 9% of the identified SUMOylome occurred proximal to phosphorylation, and numerous SUMOylation sites were found to be fully dependent on prior phosphorylation events. SUMO-proximal phosphorylation occurred primarily in a proline-directed manner, and inhibition of cyclin-dependent kinases dynamically affected co-modification. Collectively, we present a comprehensive analysis of the SUMOylated proteome, uncovering the structural preferences for SUMO and providing system-wide evidence for a remarkable degree of cross-talk between SUMOylation and other major PTMs.

Metabolic programming and mitochondrial dynamics along with T cell differentiation affect T cell fate and memory development; however, how to control metabolic reprogramming and mitochondrial dynamics in T cell memory development is unclear. Here, we provide evidence that the SUMO protease SENP1 promotes T cell memory development via Sirt3 deSUMOylation. SENP1-Sirt3 signalling augments the deacetylase activity of Sirt3, promoting both OXPHOS and mitochondrial fusion. Mechanistically, SENP1 activates Sirt3 deacetylase activity in T cell mitochondria, leading to reduction of the acetylation of mitochondrial metalloprotease YME1L1. Consequently, deacetylation of YME1L1 suppresses its activity on OPA1 cleavage to facilitate mitochondrial fusion, which results in T cell survival and promotes T cell memory development. We also show that the glycolytic intermediate fructose-1,6-bisphosphate (FBP) as a negative regulator suppresses AMPK-mediated activation of the SENP1-Sirt3 axis and reduces memory development. Moreover, glucose limitation reduces FBP production and activates AMPK during T cell memory development. These data show that glucose limitation activates AMPK and the subsequent SENP1-Sirt3 signalling for T cell memory development. Memory T cells are particularly reliant on fatty acid oxidation as a source of energy. Here the authors show this reliance is controlled by AMPK sensing of glucose deprivation that triggers SENP1-Sirt3 signalling, driving fatty acid oxidation and memory differentiation in T cells via deacetylation of YME1L1 to induce mitochondrial fusion.

Key Points Post-translational modification of proteins by ubiquitin and ubiquitin-like modifiers (UBLs) including SUMO have crucial and widespread roles in promoting cellular responses to DNA double-strand breaks (DSBs). Cascades involving E1 activating enzymes, E2 conjugating enzymes and E3 ligases underlie the conjugation of ubiquitin and UBLs to cellular target proteins. These modifications are recognized and decoded by proteins containing ubiquitin- or UBL-binding domains, and are removed by ubiquitin- or UBL-specific proteases. Chromatin ubiquitylation by RNF8, RNF168 and other E3 ubiquitin ligases gives rise to a complex ubiquitylation landscape at DSB sites that promotes accumulation of a range of important DNA repair factors near the lesions. Multiple regulatory mechanisms control and restrain the activity of these ubiquitin-mediated recruitment programmes. Two major pathways for DSB repair, non-homologous end joining (NHEJ) and homologous recombination, are used by eukaryotic cells. Ubiquitin-dependent signalling processes have a key role in determining DSB repair pathway choice and functionality through the regulation of factors that control DSB end resection, as well as by modification of key NHEJ and homologous recombination components themselves. A further level of complexity in DSB signalling pathways arises from the involvement of SUMO and other UBLs in promoting the functionality of these processes. Crosstalk between and co-regulation by ubiquitin and SUMO occurs at multiple levels within DSB repair responses. Dysfunctions in ubiquitin signalling factors involved in DSB repair are tightly linked to severe disorders and syndromes resulting from genomic instability, demonstrating the physiological importance of these ubiquitin-dependent signalling responses. Mechanistic insights into how ubiquitin- and UBL-dependent processes promote DSB repair offer new therapeutic opportunities for diseases resulting from genomic instability. Signalling by ubiquitin, SUMO and other ubiquitin-like modifiers (UBLs), and crosstalk between these modifications, underlies cellular responses to DNA double-strand breaks (DSBs). Important insights have been gained into the mechanisms by which ubiquitin and UBLs regulate protein interactions at DSB sites to enable accurate repair in mammalian cells, thereby protecting genome integrity. DNA double-strand breaks (DSBs) are highly cytotoxic DNA lesions. The swift recognition and faithful repair of such damage is crucial for the maintenance of genomic stability, as well as for cell and organismal fitness. Signalling by ubiquitin, SUMO and other ubiquitin-like modifiers (UBLs) orchestrates and regulates cellular responses to DSBs at multiple levels, often involving extensive crosstalk between these modifications. Recent findings have revealed compelling insights into the complex mechanisms by which ubiquitin and UBLs regulate protein interactions with DSB sites to promote accurate lesion repair and protection of genome integrity in mammalian cells. These advances offer new therapeutic opportunities for diseases linked to genetic instability.

High-resolution MS identifies >4,300 SUMOylation sites in >1,600 proteins in human cells under standard growth conditions and after proteasome inhibition or heat shock. The data reveal cross-talk between SUMO and other post-translational modifications. SUMOylation is a reversible post-translational modification essential for genome stability. Using high-resolution MS, we have studied global SUMOylation in human cells in a site-specific manner, identifying a total of >4,300 SUMOylation sites in >1,600 proteins. To our knowledge, this is the first time that >1,000 SUMOylation sites have been identified under standard growth conditions. We quantitatively studied SUMOylation dynamics in response to SUMO protease inhibition, proteasome inhibition and heat shock. Many SUMOylated lysines have previously been reported to be ubiquitinated, acetylated or methylated, thus indicating cross-talk between SUMO and other post-translational modifications. We identified 70 phosphorylation and four acetylation events in proximity to SUMOylation sites, and we provide evidence for acetylation-dependent SUMOylation of endogenous histone H3. SUMOylation regulates target proteins involved in all nuclear processes including transcription, DNA repair, chromatin remodeling, precursor-mRNA splicing and ribosome assembly.

Small ubiquitin-like modifier (SUMO) modification regulates numerous cellular processes. Unlike ubiquitin, detection of endogenous SUMOylated proteins is limited by the lack of naturally occurring protease sites in the C-terminal tail of SUMO proteins. Proteome-wide detection of SUMOylation sites on target proteins typically requires ectopic expression of mutant SUMOs with introduced tryptic sites. Here, we report a method for proteome-wide, site-level detection of endogenous SUMOylation that uses α-lytic protease, WaLP. WaLP digestion of SUMOylated proteins generates peptides containing SUMO-remnant diglycyl-lysine (KGG) at the site of SUMO modification. Using previously developed immuno-affinity isolation of KGG-containing peptides followed by mass spectrometry, we identified 1209 unique endogenous SUMO modification sites. We also demonstrate the impact of proteasome inhibition on ubiquitin and SUMO-modified proteomes using parallel quantitation of ubiquitylated and SUMOylated peptides. This methodological advancement enables determination of endogenous SUMOylated proteins under completely native conditions. SUMOylation is post-translational modification implicated in several biological pathways. Here the authors describe an approach for the global profiling of SUMO attachment sites under native conditions that also allows the parallel determination of SUMO and Ub attachments.

Promyelocytic leukemia nuclear bodies (PML-NBs) are PML-based nuclear structures that regulate various cellular processes. SUMOylation, the process of covalently conjugating small ubiquitin-like modifiers (SUMOs), is required for both the formation and the disruption of PML-NBs. However, detailed mechanisms of how SUMOylation regulates these processes remain unknown. Here we report that SUMO5, a novel SUMO variant, mediates the growth and disruption of PML-NBs. PolySUMO5 conjugation of PML at lysine 160 facilitates recruitment of PML-NB components, which enlarges PML-NBs. SUMO5 also increases polySUMO2/3 conjugation of PML, resulting in RNF4-mediated disruption of PML-NBs. The acute promyelocytic leukemia oncoprotein PML-RARα blocks SUMO5 conjugation of PML, causing cytoplasmic displacement of PML and disruption of PML-NBs. Our work not only identifies a new member of the SUMO family but also reveals the mechanistic basis of the PML-NB life cycle in human cells.

The ubiquitin-proteasome axis has been extensively explored at a system-wide level, but the impact of deubiquitinating enzymes (DUBs) on the ubiquitinome remains largely unknown. Here, we compare the contributions of the proteasome and DUBs on the global ubiquitinome, using UbiSite technology, inhibitors and mass spectrometry. We uncover large dynamic ubiquitin signalling networks with substrates and sites preferentially regulated by DUBs or by the proteasome, highlighting the role of DUBs in degradation-independent ubiquitination. DUBs regulate substrates via at least 40,000 unique sites. Regulated networks of ubiquitin substrates are involved in autophagy, apoptosis, genome integrity, telomere integrity, cell cycle progression, mitochondrial function, vesicle transport, signal transduction, transcription, pre-mRNA splicing and many other cellular processes. Moreover, we show that ubiquitin conjugated to SUMO2/3 forms a strong proteasomal degradation signal. Interestingly, PARP1 is hyper-ubiquitinated in response to DUB inhibition, which increases its enzymatic activity. Our study uncovers key regulatory roles of DUBs and provides a resource of endogenous ubiquitination sites to aid the analysis of substrate specific ubiquitin signalling. Deubiquitinases (DUBs) remove ubiquitin from its target proteins. Here, authors compare the regulatory effects of the proteasome and DUBs on the ubiquitinated proteome. They find preferential sets of substrates regulated by DUBs or by the proteasome. Moreover, they find that PARP1 is hyper-ubiquitinated in response to DUB inhibition, which increases its enzymatic activity.