Ivo A. Hendriks

Site-Specific Identification of SUMO-2 Targets in Cells Reveals an Inverted SUMOylation Motif and a Hydrophobic Cluster SUMOylation Motif

Reversible protein modification by small ubiquitin-like modifiers (SUMOs) is critical for eukaryotic life. Mass spectrometry-based proteomics has proven effective at identifying hundreds of potential SUMO target proteins. However, direct identification of SUMO acceptor lysines in complex samples by mass spectrometry is still very challenging. We have developed a generic method for the identification of SUMO acceptor lysines in target proteins. We have identified 103 SUMO-2 acceptor lysines in endogenous target proteins. Of these acceptor lysines, 76 are situated in the SUMOylation consensus site [VILMFPC]KxE. Interestingly, eight sites fit the inverted SUMOylation consensus motif [ED]xK[VILFP]. In addition, we found direct mass spectrometric evidence for crosstalk between SUMOylation and phosphorylation with a preferred spacer between the SUMOylated lysine and the phosphorylated serine of four residues. In 16 proteins we identified a hydrophobic cluster SUMOylation motif (HCSM). SUMO conjugation of RanGAP1 and ZBTB1 via HCSMs is remarkably efficient.

Uncovering global SUMOylation signaling networks in a site-specific manner

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.

A comprehensive compilation of SUMO proteomics

Small ubiquitin-like modifiers (SUMOs) are essential for the regulation of several cellular processes and are potential therapeutic targets owing to their involvement in diseases such as cancer and Alzheimer disease. In the past decade, we have witnessed a rapid expansion of proteomic approaches for identifying sumoylated proteins, with recent advances in detecting site-specific sumoylation. In this Analysis, we combined all human SUMO proteomics data currently available into one cohesive database. We provide proteomic evidence for sumoylation of 3,617 proteins at 7,327 sumoylation sites, and insight into SUMO group modification by clustering the sumoylated proteins into functional networks. The data support sumoylation being a frequent protein modification (on par with other major protein modifications) with multiple nuclear functions, including in transcription, mRNA processing, DNA replication and the DNA-damage response.

System-wide identification of wild-type SUMO-2 conjugation sites

SUMOylation is a reversible post-translational modification (PTM) regulating all nuclear processes. Identification of SUMOylation sites by mass spectrometry (MS) has been hampered by bulky tryptic fragments, which thus far necessitated the use of mutated SUMO. Here we present a SUMO-specific protease-based methodology which circumvents this problem, dubbed Protease-Reliant Identification of SUMO Modification (PRISM). PRISM allows for detection of SUMOylated proteins as well as identification of specific sites of SUMOylation while using wild-type SUMO. The method is generic and could be widely applied to study lysine PTMs. We employ PRISM in combination with high-resolution MS to identify SUMOylation sites from HeLa cells under standard growth conditions and in response to heat shock. We identified 751 wild-type SUMOylation sites on endogenous proteins, including 200 dynamic SUMO sites in response to heat shock. Thus, we have developed a method capable of quantitatively studying wild-type mammalian SUMO at the site-specific and system-wide level.

SUMO-2 Orchestrates Chromatin Modifiers in Response to DNA Damage

Small ubiquitin-like modifiers play critical roles in the DNA damage response (DDR). To increase our understanding of SUMOylation in the mammalian DDR, we employed a quantitative proteomics approach in order to identify dynamically regulated SUMO-2 conjugates and modification sites upon treatment with the DNA damaging agent methyl methanesulfonate (MMS). We have uncovered a dynamic set of 20 upregulated and 33 downregulated SUMO-2 conjugates, and 755 SUMO-2 sites, of which 362 were dynamic in response to MMS. In contrast to yeast, where a response is centered on homologous recombination, we identified dynamically SUMOylated interaction networks of chromatin modifiers, transcription factors, DNA repair factors, and nuclear body components. SUMOylated chromatin modifiers include JARID1B/KDM5B, JARID1C/KDM5C, p300, CBP, PARP1, SetDB1, and MBD1. Whereas SUMOylated JARID1B was ubiquitylated by the SUMO-targeted ubiquitin ligase RNF4 and degraded by the proteasome in response to DNA damage, JARID1C was SUMOylated and recruited to the chromatin to demethylate histone H3K4.

Site-specific mapping of the human SUMO proteome reveals co-modification with 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.

System-wide Analysis of SUMOylation Dynamics in Response to Replication Stress Reveals Novel Small Ubiquitin-like Modified Target Proteins and Acceptor Lysines Relevant for Genome Stability

Genotoxic agents can cause replication fork stalling in dividing cells because of DNA lesions, eventually leading to replication fork collapse when the damage is not repaired. Small Ubiquitin-like Modifiers (SUMOs) are known to counteract replication stress, nevertheless, only a small number of relevant SUMO target proteins are known. To address this, we have purified and identified SUMO-2 target proteins regulated by replication stress in human cells. The developed methodology enabled single step purification of His10-SUMO-2 conjugates under denaturing conditions with high yield and high purity. Following statistical analysis on five biological replicates, a total of 566 SUMO-2 targets were identified. After 2 h of hydroxyurea treatment, 10 proteins were up-regulated for SUMOylation and two proteins were down-regulated for SUMOylation, whereas after 24 h, 35 proteins were up-regulated for SUMOylation, and 13 proteins were down-regulated for SUMOylation. A site-specific approach was used to map over 1000 SUMO-2 acceptor lysines in target proteins. The methodology is generic and is widely applicable in the ubiquitin field. A large subset of these identified proteins function in one network that consists of interacting replication factors, transcriptional regulators, DNA damage response factors including MDC1, ATR-interacting protein ATRIP, the Bloom syndrome protein and the BLM-binding partner RMI1, the crossover junction endonuclease EME1, BRCA1, and CHAF1A. Furthermore, centromeric proteins and signal transducers were dynamically regulated by SUMOylation upon replication stress. Our results uncover a comprehensive network of SUMO target proteins dealing with replication damage and provide a framework for detailed understanding of the role of SUMOylation to counteract replication stress. Ultimately, our study reveals how a post-translational modification is able to orchestrate a large variety of different proteins to integrate different nuclear processes with the aim of dealing with the induced DNA damage.

c-Myc is targeted to the proteasome for degradation in a SUMOylation-dependent manner, regulated by PIAS1, SENP7 and RNF4

c-Myc is the most frequently overexpressed oncogene in tumors, including breast cancer, colon cancer and lung cancer. Post-translational modifications comprising phosphorylation, acetylation and ubiquitylation regulate the activity of c-Myc. Recently, it was shown that c-Myc-driven tumors are strongly dependent on the SUMO pathway. Currently, the relevant SUMO target proteins in this pathway are unknown. Here we show that c-Myc is a target protein for SUMOylation, and that SUMOylated c-Myc is subsequently ubiquitylated and degraded by the proteasome. SUMO chains appeared to be dispensable for this process, polymerization-deficient SUMO mutants supported proteolysis of SUMOylated c-Myc. These results indicate that multiple SUMO monomers conjugated to c-Myc could be sufficient to direct SUMOylated c-Myc to the ubiquitin-proteasome pathway. Knocking down the SUMO-targeted ubiquitin ligase RNF4 enhanced the levels of SUMOylated c-Myc, indicating that RNF4 could recognize a multi-SUMOylated protein as a substrate in addition to poly-SUMOylated proteins. Knocking down the SUMO E3 ligase PIAS1 resulted in reduced c-Myc SUMOylation and increased c-Myc transcriptional activity, indicating that PIAS1 mediates c-Myc SUMOylation. Increased SUMOylation of c-Myc was noted upon knockdown of the SUMO protease SENP7, indicating that it also could regulate a multi-SUMOylated protein in addition to poly-SUMOylated proteins. C-Myc lacks KxE-type SUMOylation consensus motifs. We used mass spectrometry to identify 10 SUMO acceptor lysines: K52, K148, K157, K317, K323, K326, K389, K392, K398 and K430. Intriguingly, mutating all 10 SUMO acceptor lysines did not reduce c-Myc SUMOylation, suggesting that SUMO acceptor lysines in c-Myc act promiscuously. Our results provide novel insight into the complexity of c-Myc post-translational regulation.

Ubiquitin-specific Protease 11 (USP11) Deubiquitinates Hybrid Small Ubiquitin-like Modifier (SUMO)-Ubiquitin Chains to Counteract RING Finger Protein 4 (RNF4).

Background: RNF4 is a ubiquitin ligase targeted to SUMOylated proteins. Results: USP11 co-purified with RNF4 and can remove ubiquitin polymers attached to SUMO chains. Conclusion: USP11 is a ubiquitin protease with the ability to counteract RNF4 in the DNA damage response. Significance: Identification of a ubiquitin protease to balance the activity of a SUMO-targeted ubiquitin ligase. Ring finger protein 4 (RNF4) is a SUMO-targeted ubiquitin E3 ligase with a pivotal function in the DNA damage response (DDR). SUMO interaction motifs (SIMs) in the N-terminal part of RNF4 tightly bind to SUMO polymers, and RNF4 can ubiquitinate these polymers in vitro. Using a proteomic approach, we identified the deubiquitinating enzyme ubiquitin-specific protease 11 (USP11), a known DDR-component, as a functional interactor of RNF4. USP11 can deubiquitinate hybrid SUMO-ubiquitin chains to counteract RNF4. SUMO-enriched nuclear bodies are stabilized by USP11, which functions downstream of RNF4 as a counterbalancing factor. In response to DNA damage induced by methyl methanesulfonate, USP11 could counteract RNF4 to inhibit the dissolution of nuclear bodies. Thus, we provide novel insight into cross-talk between ubiquitin and SUMO and uncover USP11 and RNF4 as a balanced SUMO-targeted ubiquitin ligase/protease pair with a role in the DDR.

A high-yield double-purification proteomics strategy for the identification of SUMO sites

The small ubiquitin-like modifier (SUMO) is a protein modifier that is post-translationally coupled to thousands of lysines in more than a thousand proteins. An understanding of which lysines are modified by SUMO is critical in unraveling its function as a master regulator of all nuclear processes, as well as its involvement in diseases such as cancer. Here we describe a protocol for the lysine-deficient (K0) method for efficient identification of SUMOylated lysines by mass spectrometry (MS). To our knowledge, the K0 method is the only currently available method that can routinely identify >1,000 SUMO sites in mammalian cells under standard growth conditions. The K0 strategy relies on introducing a His10-tagged SUMO wherein all lysines have been substituted to arginines. Lysine deficiency renders the SUMO immune to digestion by the endoproteinase Lys-C, which in turn allows for stringent and high-yield tandem purification through the His10 tag. In addition, the His10-tagged SUMO also contains a C-terminal Q87R mutation, which accommodates generation of SUMO-site peptides with a QQTGG mass remnant after digestion with trypsin. This remnant possesses a unique mass signature and readily generates diagnostic ions in the fragment ion scans, which increases SUMO-site identification confidence. The K0 method can be applied in any mammalian cell line or in any model system that allows for integration of the K0-SUMO construct. From the moment of cell lysis, the K0 method takes ∼7 d to perform.