Cardiac function and disease: emerging role of SUMO conjugation

Abstract

Small ubiquitin-related modifiers, or SUMOs, have emerged as versatile regulators of many biological functions that do so by covalent attachment to a variety of substrates via enzymatic reactions. SUMO conjugation has also been shown to be involved in a number of human pathogenic processes. More recent advances in the SUMO field have indicated a potential role for SUMO conjugation pathway in cardiogenesis. This advanced review will describe the basic features of the SUMO conjugation pathway and will summarize the most recent studies implicating the influence of the sumoylation pathway in cardiac function under both physiological and pathological conditions.

Introduction

Ubiquitin and ubiquitin-like proteins (Ubls) are a superfamily of evolutionarily conserved small proteins that can be utilized to covalently modify substrates and subsequently regulate the function of target proteins. Since the discovery of ubiquitin, a number of Ubls have been identified (Table 1). While similarity of the primary sequence between Ubls varies considerably, they do share highly related tertiary structure. Also, similar enzymatic cascade applies to the conjugation of Ubls to their targets, however, unique enzymes are involved in each particular conjugation reaction. Ubls are involved in regulation of a variety of cellular functions (Table 1). This review will feature the properties of SUMO conjugation pathway and highlight its potential implication in the cardiac function and diseases.

Table 1.

Ubiquitin and ubiquitin-like proteins (Ubls) and their functions

Ubls	Functions	References
Ubiquitin	Proteasome-associated degradation and non-proteasomal activity	111, 112
SUMO (Small ubiquitin-like modifier)	Transcriptional regulation, cell cycle, protein stability, DNA damage and repair, chromatin remodeling	4, 54
ISG15 (Interferon-stimulated gene-15)	Regulation of immune response and cell proliferation	113, 114
Fub1 (Fau ubiquitin-Like protein)	Immunosuppression and phagocytosis,	115, 116
FAT10 (F-adjacent transcript-10 or human leukocyte antigen F associated)	Proteasomal degradation and apoptosis	117–119
Nedd8 (Neural precursor cell-expressed developmentally down-regulated-8)	Transcriptional regulation, protein-protein interaction, regulation of ubiquitin E3s’ activity	120
Ufm1 (Ubiquitin fold modifier-1)	Unclear	121
Apg8 and Apg12 (Autophagy-8 and -12)	Autophagy	122
Urm1 (Ubiquitin-related modifier-1)	Sulphur carrier and tRNA modification	123, 124
UBL5 (Ubiquitin-like protein-5)	Mitochondrial stress response	125
MUB (Membrane-anchored UBL)	Unclear	126

Fundamental features of SUMO conjugation pathway

SUMO conjugation pathway

SUMOs (Small ubiquitin-related modifiers), a member of Ubls superfamily, can be covalently conjugated to the target proteins via an ATP-dependent enzymatic cascade that requires heterodimeric activating enzyme (E1), SAE1 and SAE2, and conjugation enzyme (E2), Ubc9, which is the lone E2 so far identified in mammalian cells ^{1, 2}. The initially synthesized SUMO precursors are inactive until the C-terminal extensions are cleaved by sentrin-specific proteases (SENPs), which act as isopeptidases to expose the C-terminal di-glycine motif for subsequent conjugation ³. SENPs also act as de-sumoylation enzymes (see below). The presence of E3 ligases, represented by PIAS (Protein Inhibitors of Activated STAT) proteins, stimulates the efficiency of SUMO conjugation and promotes the formation of poly-SUMO chain ^{4, 5} (Figure 1). The SUMO targeted lysine residues are usually embedded in the SUMO targeting consensus sequence, identified as ΨKXE where Ψ stands for a large hydrophobic amino acid and X represents any residue ⁶, although lysine residues localized in non-consensus sequences may also be SUMO acceptor sites ^{4, 7}, particularly in the presence of SUMO E3 ligases.

Figure 1. Illustration of SUMO conjugation pathway.

E1, SAE1/SAE2; E2, Ubc9. E3, PIAS, etc (see text or table 2 for details). xxxx, C-terminal extension after di-glycine motif.

SUMO proteins

Since the discovery of the first SUMO protein, SUMO-1, in 1996, three other SUMO isoforms, SUMO-2, -3 and -4, have been identified in higher vertebrates ^8–10. SUMO-1 shares ~50% homology with SUMO-2 and -3, but the active forms of SUMO-2 and -3 exhibit highly similar identity (~ 97%). SUMO-4 has ~ 87% homology with SUMO-2 at protein level (Table 2). Tissue distribution studies reveal that SUMO-3 expression is more tissue-restricted than SUMO-1 and -2 ¹¹, and that SUMO-4 is mainly present in kidney ¹⁰. Among these four isoforms, SUMO-1, -2 and -3 are conjugatable, and SUMO-4 is defective in conjugation due to the presence of proline-90 residue, which impedes the cleavage of C-terminal tail of SUMO-4 ¹². Thus, although overexpressed, constitutively-active SUMO-4 is conjugatable, the physiological relevance of SUMO-4 conjugation is unclear. Generally, more free SUMO-2/3 is observed under physiological conditions and these two isoforms are more reactive to external stimuli ⁹, although the level of SUMO-1 can also change in response to stress such as hypoxia ¹³. SUMO paralogs have overlapped targets but also exhibit substrate specificity. Also, the subcellular distribution of SUMO isoforms varies to some extent ^{14, 15}. These findings support the notion that SUMO paralogs play differential roles in regulating specific cellular and tissue functions, although the degree of overlapping vs. distinct functions these SUMO isoforms is not fully understood.

Table 2. Alignment of amino acid sequences of human SUMO isoforms.

The accession number of each listed human SUMO isoform is shown below: SUMO-1: NM_001005781; SUMO-2: NM_006937; SUMO-3: NM_006936; SUMO-4: NM_001002255. The vertical red line indicates the location of cleavage in SUMO-1, -2 and -3 by SENPs to expose di-glycine residues for conjugation. The native SUMO-4 cannot be cleaved due to the presence of pro-90 (see text for details).

SUMO E3 ligases

SUMO E3 ligases are dispensable in the execution of the SUMO conjugation reaction in vitro; however, they can stimulate the efficiency of SUMO conjugation in vivo, i.e., promoting the formation of poly-SUMO chain and/or activating non-principal sumoylation site(s). A number of SUMO E3 ligases has been identified in vertebrates, such as RanBP2 ¹⁶, polycomb 2 ¹⁷, the PIAS family ¹⁸, TOPORS ^{19, 20}, TRAF7 ²¹, mitochondrial-anchored protein ligase (MAPL) ²², and RHES ²³ (Table 3), of which the PIAS family proteins are the most extensively studied and have the largest repertoire of substrates to date. The PIAS family contains five isoforms, PIASxα, PIASxβ, PIAS1, PIAS3, and PIASy ^{18, 24}, which differ in their substrate specificity. Although in most sumoylation reactions the E3 ligase activity of PIAS proteins is associated with their RING domain, this activity is also present in other region(s), as evidenced by YY1 sumoylation by PIASy ²⁵. SUMO E3 ligases such as TOPORS, MAPL and TRAF7 also harbor ubiquitination E3 ligase activity ^{19, 22, 26}. There are also two non-RING domain-containing SUMO E3 ligases, the polycomb protein Pc2 and RanBP2; the former promotes sumoylation of CtBP for transcriptional repression ¹⁷, whereas the latter stimulates sumoylation of RanGAP1, HDAC4 and topoisomerase IIα (TopoIIα) ^{16, 27, 28}.

Table 3.

Summary of SUMO E3 ligases.

SUMO E3 ligases	Targets	References
RanBP2	RanGAP1, HDAC4, TopoIIα, etc	16, 27, 28
Pc2	CtBP	17
PIAS	YY1, GATA4, etc	25, 56
TOPOR	P53, Sin3A	19, 20
TRAF7	c-Myb	21
MAPL	Drp1	22
Rhes	mHtt (mutant huntingtin)	127

Sentrin-specific proteases (SENPs)

The SUMO conjugation pathway is a dynamic and reversible process in which SUMO conjugated proteins can be freed or de-sumoylated by a class of isopeptidase named SENPs ^{3, 29}. Six members of the SENP family related to de-sumoylation activity have been identified in humans (SENP1, SENP2, SENP3, SENP5, SENP6, SENP7), and they exhibit different subcellular localizations, activities and specificities for both SUMO isoforms and substrates ³⁰. Basically, SENP1 and SENP2 exhibit de-conjugation activity against all SUMO paralogs³¹, but SENP3, 5, 6 and 7 mainly de-conjugate SUMO-2/3 ^32–35. Knockout of either SENP1 or SENP2 causes early embryonic lethality ^36–38, indicative of indispensability of these two SENPs for normal mouse embryogenesis.

The consequences after sumoylation

The functional consequences of sumoylation are quite diverse and substrate/context specific. Generally speaking, SUMO modification may increase, decrease or exert no detectable effects on the activity of a given substrate. The fates of sumoylated substrates are listed below and summarized in Figure 2.

Figure 2. The functional consequences after sumoylation.

SUMO modification may result in the following consequences: changes in subcellular localization, alteration of protein-DNA binding, alteration of protein-protein interaction, stabilization, and promoting proteasomal degradation. See text for details.

1. Changes in subcellular localization. While RanGAP1, the first SUMO target identified, stays exclusively in cytosol in its free form, it translocates to nuclear pore complex (NPC) once SUMO-conjugated ³⁹. Mutation of the SUMO acceptor site in CtBP1, Lys 428 to Arg (K428R), shuttles CtBP1 from the nucleus to the cytoplasm ⁴⁰.

2. Alteration of protein-DNA binding affinity. Heat shock transcription factors (HSF) play a critical role in modulating heat shock protein expression in response to external stress. HSF2 is a substrate for sumoylation, which enhances the DNA binding of HSF2 ⁴¹. In sharp contrast, SUMO-conjugated hepatoma-derived growth factor (HDGF) exhibits no DNA binding activity in comparison to un-conjugated HDGF ⁴².

3. Alteration of protein-protein interaction. Sumoylation of the aryl hydrocarbon receptor repressor (AhRR) enhances its inhibitory effect on AhR activity, at least partially due to potentiated interaction of sumoylated AhRR with its co-repressors ⁴³. In addition to SUMO’s covalent conjugation ability, SUMO also mediates protein-protein interaction via a SUMO interacting motif (SIM) ⁴⁴. Thus, SUMO modification creates an additional platform for substrates to physically interact with binding partners, as in the case of Elg1 favorably interacting with SUMO-conjugated PCNA ⁴⁵.

4. Stabilization of target. IkappaBalpha, which inhibits the activity NF-kappaB, is SUMO modified on Lys 21 (K21), which is also a ubiquitination site ⁴⁶. Therefore, SUMO conjugation to K21 stabilizes IkppaBalpha by directly antagonizing its ubiquitination, and, consequently, suppresses the activity of NF-kappaB ⁴⁶. SUMO modification also prevents ubiquitination of N4BP1, promoting its accumulation at promyelocytic leukemia (PML) nuclear bodies (NBs) ⁴⁷. Interestingly, the presence of SUMO acceptor site K51 prevents the ubiquitination of Nkx2.5, however, the ubiquitination of the Nkx2.5 K51R mutant does not increase its turnover ⁴⁸.

5. Promoting proteasomal degradation. The long standing view that sumoylation does not cause degradation of substrates has been challenged recently by the finding in yeast that SUMO-specific ubiquitination E3 ligases target SUMO-conjugated substrates for degradation ⁴⁹. In vertebrates, SUMO conjugation to glucocorticoid receptor (GR) increases its turnover, which is inhibited by the proteasome inhibitor MG132 ⁵⁰. SUMO modification of hypoxia-inducible factor (HIF)1α provides a signal for the recruitment of ubiquitination E3 ligase during hypoxia, thus destabilizing HIF1α by inducing proteasome-mediated degradation³⁶. However, the functional consequence of HIF1α sumoylation remains controversial ^51–53.

Since SUMO targets a broad spectrum of substrates that are involved in many cellular processes, SUMO regulates a variety of biological activities (see reviews ^{4, 54}).

Potential role of SUMO conjugation pathway in cardiac physiology and pathophysiology

SUMO modifies a multitude of transcription factors such as Nkx2.5 ⁴⁸, GATA4 ^{55, 56}, SRF ⁵⁷, myocardin ⁵⁸, and prox1 ^{59, 60}, which are essential for normal cardiac development, thus pointing to a potential role of SUMO pathway in cardiac structural morphogenesis. Readers are recommended to consult two recent reviews for further details ^{54, 61}. The present review will focus on the newly identified SUMO substrates whose importance in regulating cardiac function under both physiological and pathological conditions has been demonstrated.

Peroxisome proliferator-activated receptor gamma-coactivator-1α (PGC-1α) is an inducible factor critical for mitochondrial function, metabolism and energy homeostasis⁶², and is implicated in cardiac muscle disorders and heart failure ⁶³. A threshold level of activity of PGC-1α is required for normal myocardial function; either increase or decrease in the activity of PGC1α causes cardiac dysfunction ^{64, 65}. Sumoylation of PGC-1α on conserved lysine residue VK₁₈₃TE, which is located in its activation domain, diminishes its transcriptional activity. Correspondingly, disruption of PCG-1α sumoylation potentiates its function ^{65, 66}. Although K₁₈₃ also serves as an acetylation site of total 13 acetylation sites in PGC-1α, it appears that there is no functional interplay between sumoylation and acetylation on PGC-1α ⁶⁶. In addition, no direct crosstalk between sumoylation and phosphorylation or ubiquitination on PCG-1α is observed ⁶⁶. It is noteworthy that the transcriptional activity of PGC-1α is regulated by its cofactors, one of which is the pyrimidine tract-binding protein-associated splicing factor (PSF) and is also a SUMO target ⁶⁷. SUMO modification promotes the binding of PSF to PGC-1α and subsequently suppresses PGC-1α’s transcriptional activity; conversely, a sumoylation-defective mutant of PSF exhibits diminished binding affinity to PGC-1α, enhancing the transcriptional synergy between PGC-1α and DJ-1, which is another cofactor and SUMO substrate that regulates PGC-1α function ⁶⁷.

The nuclear receptor peroxisome proliferator-activator receptor (PPAR) family contains three members, PPARα, β and γ. PPAR proteins form heterodimers with retinoid X receptor (RXR), bind to the consensus sequence TGACCTnTGACCT and play a key role in the maintenance of cellular energy balance ^{68, 69}. Genetically modified mice with altered expression of PPARs exhibit dysregulated metabolism and/or cardiomyopathy ^70–73, indicating the importance of these factors to cardiac function. PPARγ is the first member in this family that was identified as a SUMO target on IK₁₀₇VE ^{74, 75}. The K107R mutant exhibits more potency to induce transdifferentiation of NIH3T3 cells into adipocytes compared with the wild type ⁷⁵, indicating the inhibitory effect of sumoylation on the transcriptional activity of PPARγ. Intriguingly, although the SUMO E3 ligase PIASxβ stimulates the sumoylation of PPARγ, the presence of PIASxβ potentiates the transcriptional activity of PPARγ ⁷⁴. As with PPARγ, sumoylation suppresses the activity of PPARα ^{76, 77}. The lysine harbored in PK₃₈₅FD is the principal SUMO site in murine PPARα, and mutation of lysine 385 to arginine is sufficient to abolish the inhibitory effect ⁷⁶. Interestingly, K385 is not the major sumoylation site in human PPARα; the primary SUMO attachment site in human PPARα is LK₁₈₅AE ⁷⁷, although both of these two lysine residues are conserved among various species. These findings indicate that the SUMO acceptor site in a given substrate may be species-dependent. Although sumoylation of human PPARα is substantially decreased upon ligand stimulation ⁷⁷, mouse PPARα sumoylation is increased ⁷⁶, suggesting that this sumoylation associated effect may be cell type dependent. Sumoylation suppresses the activity of human PPARα mainly via enhanced physical association with the nuclear corepressor, NCoR ⁷⁷. Despite the presence of the consensus SUMO target sequence in PPARβ identified by bioinformatics analysis, direct evidence that it is a SUMO substrate is currently lacking.

The estrogen receptor related receptor α (ERRα) is a nuclear orphan receptor that governs target gene expressions by directly binding to its cognate DNA sequence TNAAGGTCA or the estrogen response element ^{78, 79}, as well as by physically interacting with cofactors. ERRα is rich in tissues with high energy demand, including the heart. Knockout of ERRα in mice compromises the capacity of the heart to respond to cardiac overload stress, and is accompanied with decreased transcripts of a number of metabolism-associated genes ⁸⁰, suggestive of its important role in maintaining cardiac function in response to external stimuli. ERRα was shown to be a SUMO target on two lysine residues, IK₁₄AE and VK₄₀₃LE ⁸¹. Phosphorylation on Ser19 of ERRα inhibits the transcriptional activity of ERRα via increased sumoylation on K₁₄ but not on K₄₀₃ ⁸¹, indicating potential differences in the function of alternative SUMO acceptor sites. The SUMO sites IK₁₄AE and VK₄₀₃LE are conserved between the mouse and human ERRα proteins, but are not conserved in the other two ERR family members, ERRβ and γ. However, bioinformatics analysis reveals the presence of a consensus SUMO target sequence in both ERRβ and γ. Whether SUMO can modify ERRβ and γ awaits exploration.

The mitochondrion is the major source of energy supply in cells, and mitochondrial malfunction plays a key role in a number of cell-associated pathogenic processes such as apoptosis and in muscle disorders. More recent studies have demonstrated that dynamin-related protein 1 (Drp1), which plays a central role in mitochondrial fission and function under both physiological and pathological circumstances, is targeted by all SUMO paralogs ^82–84, although the exact SUMO conjugation sites have not been clearly identified. SUMO conjugation elevates the activity of Drp1, at least partially, via stabilization ⁸². Particularly, the conjugation of SUMO to Drp1 is elevated in the process of induced apoptosis ⁸⁴, pointing to a potential role of Drp1 sumoylation in mitochondria-mediated cell death. Importantly, SENP5, an isopeptidase in the SUMO conjugation pathway, regulates mitochondrial morphology and function, at least partially, via reversal of covalent linkage of SUMO to Drp1 ⁸⁵.

In addition to the above mentioned SUMO substrates, SUMO modification targets a number of cardiac ion channels such as valtage-gated potassium (Kv) channels Kv2.1, Kv1.5, K2P1 and transient receptor potential cation channel, subfamiyly M, member 4 gene (TRPM4) ^86–89 (see recent reviews ^{54, 90}), among which sumoylation of TRPM4 is of particular interest because it is potentially involved in human familial cardiac conduction blocks (see below).

The first evidence that the SUMO conjugation pathway is involved in heart pathophysiology is the observation of elevated SUMO-1 expression in hypoxic heart, which is correlated with increased SUMO-1 conjugation to HIF1α, a factor that is typically induced under hypoxic conditions ¹³. However, whether there is any increase in sumoylation of other cardiac SUMO substrates under hypoxia remains unknown.

Recently, the extracellular signal-regulated kinase 5 (Erk5), a protein that plays a critical anti-inflammatory role when released by activated endothelial cells ⁹¹, was identified as a target of SUMO at two lysine residues, lys6 and lys22 ⁹². SUMO modification of Erk5 can be induced by external stimuli such as H₂O₂ and can inhibit the anti-inflammatory effects stimulated by free Erk5, implicating SUMO conjugation in ischemia/perfusion-induced injury. Also, the level of SUMO-conjugated Erk5 is increased in the aortas of diabetic mice ⁹², indicative of the involvement of elevated SUMO activity in metabolism-associated cardiovascular disorders.

Another cardiovascular pathophysiology-associated factor, nuclear enzyme poly(ADP-ribose) polymerase 1 (PARP1), is a SUMO substrate that can be conjugated on two major lysine residues, 203 and 486 ^{93, 94}. PARP1 is the founding member of the PARP super family and is associated with damage to heart function; when PARP1 is activated under pathophysiological conditions such as ischemia and reperfusion, it contributes to cell death ⁹⁵. Moreover, SUMO-2 conjugation to PARP1 is induced in response to heat shock ⁹³. The poly-SUMO modified PARP1 is also a target for proteasome-associated degradation initiated by ubiquitination E3 ligase RNF4 ⁹³. Thus, the status of the sumoylation of PARP1 may serve as a therapeutic intervention for the potential treatment of myocardial injuries. However, how important the SUMO attached PARP1 is in the context of heart injury remains to be revealed.

Potential role of SUMO conjugation pathway in human cardiac disease

While the implication of the SUMO conjugation pathway in a number of pathogenic processes such as palatogenesis, cancer development and neurodegenerative diseases has been well established ^96–99, the evidence directly linking cardiovascular diseases to SUMO modification is just emerging. Lamin A protein is a critical component of nuclear structure lamina that serves as an important scaffold for regulation of nuclear structure and function. Mutations in the lamin A protein is associated with a variety of human diseases, including familial cardiomyopathy ¹⁰⁰. Lamin A is a SUMO substrate with a primary sumoylation site, K201, localized in the consensus sequence MKEE ¹⁰¹; two naturally occurring missense mutations, E203G and E203K, that cause familial dilated cardiomyopathy, abolish sumoylation and are associated with similar molecular phenotypes to the SUMO site mutant K201R ^{101, 102}. Fibroblasts obtained from a human patient bearing the E203K mutation also show aberrant subcellular distribution ¹⁰¹, providing compelling evidence directly linking a sumoylation-defective substrate to human cardiomyopathy.

It is well documented that the accumulation of polyglutamine repeats is involved in neurodegenerative diseases. Recent work reveals the presence of pre-amyloid oligomer (PAO) in many human failing heart samples ¹⁰³. Expressing polyglutamine repeats of 83 in murine cardiomyocytes leads to cardiomyopathy and heart failure, which are associated with the toxicity of the accumulated misfolded amyloidogenic proteins in heart ¹⁰⁴. SUMO modifies a number of polyglutamine repeat proteins, such as Huntingtin ¹⁰⁵, ataxin-1¹⁰⁶, androgen receptor ¹⁰⁷, and amyloid precursor protein ¹⁰⁸, thus affecting the activities of these proteins. Also, the diseased regions of brain sections from patients afflicted with neurodegenerative disorders exhibit strong staining against SUMO-1 ¹⁰⁹, indicating the involvement of the SUMO pathway in amyloidotic diseases. Despite these observations, there is currently no direct evidence that SUMO conjugation is involved in oligomer-associated cardiomyopathy, but this topic warrants further investigation.

In addition, comparing gene profiling data obtained from human non-failing and failing hearts (the latter caused by dilated or ischemic-induced cardiomyopathy) reveals significant changes in some SUMO pathway components ⁶³. Whether up- and/or down-regulation of each of these factors is the consequence or part of the cause of cardiomyopathy leading to heart failure requires further exploration, and whether changes in these SUMO pathway components are beneficial or detrimental to heart function remains an open question; however, the above findings support the notion that dysregulation of SUMO pathway underlies human cardiac pathophysiology.

More recently, an implication of sumoylation in the human familial cardiac conduction block via targeting TRPM4 has been reported ^{89, 110}. TRPM4 is a Ca²⁺-activated nonselective cation channel that regulates the transient inward current modulated by free Ca²⁺. In two separate studies, all four natural occurring missense mutations in TRPM4, TRPM4^Glu7Lys, TRPM4^Arg164Trp, TRPM4^Ala432Thr, TRPM4^Gly844Asp, cause human familial conduction blocks via increased current density ^{89, 110}. While it is clear that the increased sumoylation and decreased degradation of TRPM4^Glu7Lys is involved in the pathogenesis ⁸⁹, whether the sumoylation of the other three mutants contributes to the elevated expression of these mutated proteins remains to be further confirmed because no altered sumoylation of these mutants was detected compared with that of wild type TRPM4, although SUMO E2 Ubc9 increases the current density of TRPM4^Ala432Thr and TRPM4^{Gly844Asp 110}. Thus, the regulation of TRPM4 by sumoylation and its pathogenic role in the initiation of heart conduction disease merit further exploration.

Concluding remarks

Since the discovery of the first SUMO protein, the SUMO conjugation pathway has gained increasing interest due to its diverse roles in a variety of cellular events. Recent advances in SUMO studies turn our attention to its contribution to cardiac development and function ^{54, 61}. Still, certain critical evidence is missing in terms of its role in human heart diseases. For instance, is there any change of the level of SUMO conjugated substrates (such as PGC-1a, PPAR, etc) in heart under pathological conditions vs. that under physiological status? If so, is that change important for the development of cardiac diseases such as cardiac muscle disorders? With the rapid progress in the SUMO field, i.e., the identification of more cardiac-associated SUMO targets and the generation and analysis of more SUMO pathway related knockout and/or transgenic murine models, the importance of the SUMO conjugation pathway in cardiac function and diseases will be clarified.

Table 4.

Major SUMO targets important for cardiac function under both physiological and pathophysiological conditions.

SUMO Substrates	Primary SUMO Acceptor Sites	Activity After Sumoylation	References
PGC-1α	VK183TE	Repression	66
PPARα	PK385FD (murine) LK185AE (human)	Repression	76, 77
PPARγ	IK107VE	Repression	74, 75
ERRα	IK14AE, VK403LE	Repression	81
Drp1	Non-identified	Activation	82
Erk5	LK6EE, VK22AE	Repression	92
PARP1	VK486AE, VK203SE	Repression	93, 94
TRPM4	Non-identified	Activation	89