Regulation and Function of SUMO Modification

Abstract

Small ubiquitin-like modifier (SUMO) 1The abbreviations used are: SUMO, small ubiquitin-like modifier; E1, ubiquitin-activating enzyme; E2, ubiquitin carrier protein; E3, ubiquitin-protein isopeptide ligase; PIAS, protein inhibitors of activated STATs; STAT, signal transducer and activator of transcription; PML, promyelocytic leukemia; NPC, nuclear pore complex; TGF-β, transforming growth factor β; NB, nuclear body; AR, androgen receptor; GR, glucocorticoid receptor; PR, progesterone receptor; C/EBP, CCAAT enhancer-binding protein; SREBP, sterol regulatory element-binding protein; SRF, serum response factor; HDAC, histone deacetylase; PCNA, proliferating cell nuclear antigen; EBV, Epstein-Barr virus; HSF, heat shock factor; TCF, T-cell factor; LEF, lymphoid enhancer factor. is a protein of 97 amino acids that is structurally similar to ubiquitin and has been called by other names including Smt3p, Pmt2p, PIC-1, GMP1, Ubl1, and Sentrin (1Seeler J.S. Dejean A. Oncogene. 2001; 20: 7243-7249Crossref PubMed Scopus (136) Google Scholar). Like ubiquitin, SUMO has been found to be covalently attached to certain lysine residues of specific target proteins (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar). In contrast to ubiquitination, however, sumoylation does not promote the degradation of proteins but instead alters a number of different functional parameters of proteins, depending on the protein substrate in question. These parameters include but are not limited to properties such as subcellular localization, protein partnering, and DNA-binding and/or transactivation functions of transcription factors (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 4Schwartz D.C. Hochstrasser M. Trends Biochem. Sci. 2003; 28: 321-328Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). The contrast between the functional effects of ubiquitination and sumoylation is most striking in the case of IκB, where sumoylation stabilizes the protein by modifying the same residue that is ubiquitinated, thereby directly competing with that pathway (5Hay R.T. Vuillard L. Desterro J.M. Rodriguez M.S. Philos. Trans. R. Soc. Lond. B Biol. Sci. 1999; 354: 1601-1609Crossref PubMed Scopus (82) Google Scholar). This review will focus on the regulation of SUMO modification and its role in controlling the functional properties of proteins. The reader is also referred to other excellent reviews on this topic (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 4Schwartz D.C. Hochstrasser M. Trends Biochem. Sci. 2003; 28: 321-328Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 7Seeler J.S. Dejean A. Nat. Rev. Mol. Cell. Biol. 2003; 4: 690-699Crossref PubMed Scopus (578) Google Scholar, 8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). Three different ubiquitous SUMO-related proteins have been identified in mammalian cells, SUMO-1, SUMO-2, and SUMO-3, with SUMO-2 and SUMO-3 having greater sequence relatedness with each other than with SUMO-1 (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 4Schwartz D.C. Hochstrasser M. Trends Biochem. Sci. 2003; 28: 321-328Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). Recently a tissue-specific SUMO-4 has been identified in human kidney with homology to SUMO-2/3, which raises the possibility that some SUMO proteins could have tissue-dependent functions (9Bohren K.M. Nadkarni V. Song J.H. Gabbay K.H. Owerbach D. J. Biol. Chem. 2004; 279: 27233-27238Abstract Full Text Full Text PDF PubMed Scopus (286) Google Scholar). SUMO modification occurs on the lysine in the consensus sequence ψKXE (where ψ represents a hydrophobic amino acid, and X represents any amino acid) (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar). The mechanism involved in maturation and transfer of SUMO to target substrates is very similar to that seen with ubiquitination and other ubiquitin-like proteins (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 4Schwartz D.C. Hochstrasser M. Trends Biochem. Sci. 2003; 28: 321-328Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). This process involves four enzymatic steps: maturation, activation, conjugation, and ligation (Fig. 1). In the first step the SUMO protein is cleaved by SUMO-specific carboxyl-terminal hydrolase to produce a carboxyl-terminal diglycine motif. This process of maturation is identical with all three mammalian SUMO forms. After maturation, SUMO proteins are able to be utilized for conjugation to proteins. The SUMO-activating (E1) enzyme is a heterodimer consisting of Aos1 and Uba2 (also known as SAE1/SAE2 or Sua1/hUba2 in humans). Activation of SUMO by the E1 is an ATP-dependent process and results in the formation of a thioester bond between SUMO and the Uba2 subunit of the E1-activating enzyme. Activation is followed by transfer of SUMO from the E1 enzyme to a conserved cysteine in the conjugating (E2) enzyme, Ubc9. This single E2 enzyme identified so far for the sumoylation pathway contrasts with the multiple E2 enzymes involved in attaching ubiquitin to proteins (4Schwartz D.C. Hochstrasser M. Trends Biochem. Sci. 2003; 28: 321-328Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, 10Pickart C.M. Annu. Rev. Biochem. 2001; 70: 503-533Crossref PubMed Scopus (2922) Google Scholar). The final step of sumoylation involves ligation of SUMO to the target protein. Until recently there was speculation as to whether SUMO ligation to target proteins involved E3 ligase-like proteins such as are required for ubiquitination. However, it is now clear that such E3 ligases do exist for the SUMO-1 modification pathway and that they play important roles in modulating the efficiency of SUMO attachment to target proteins (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar). As with the ubiquitin system, SUMO E3 proteins are defined by three characteristics: binding to the substrate protein either directly or indirectly, binding to the E2 conjugation enzyme, and the ability to stimulate transfer of the modifier to the substrate or to another modifier in the case of modifier chain formation. Three different general types of SUMO E3 ligases have been described (11Johnson E.S. Gupta A.A. Cell. 2001; 106: 735-744Abstract Full Text Full Text PDF PubMed Scopus (528) Google Scholar, 12Sachdev S. Bruhn L. Sieber H. Pichler A. Melchior F. Grosschedl R. Genes Dev. 2001; 15: 3088-3103Crossref PubMed Scopus (464) Google Scholar, 13Takahashi K. Taira T. Niki T. Seino C. Iguchi-Ariga S.M. Ariga H. J. Biol. Chem. 2001; 276: 37556-37563Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar, 14Kotaja N. Karvonen U. Janne O.A. Palvimo J.J. Mol. Cell. Biol. 2002; 22: 5222-5234Crossref PubMed Scopus (355) Google Scholar, 15Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (641) Google Scholar, 16Kagey M.H. Melhuish T.A. Wotton D. Cell. 2003; 113: 127-137Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar). The first E3 group comprises the PIAS family of proteins. In yeast only two E3 proteins have been identified (Siz1 and Siz2) which have sequence similarity to mammalian PIAS proteins, of which at least five members have SUMO E3 activity (11Johnson E.S. Gupta A.A. Cell. 2001; 106: 735-744Abstract Full Text Full Text PDF PubMed Scopus (528) Google Scholar, 12Sachdev S. Bruhn L. Sieber H. Pichler A. Melchior F. Grosschedl R. Genes Dev. 2001; 15: 3088-3103Crossref PubMed Scopus (464) Google Scholar, 14Kotaja N. Karvonen U. Janne O.A. Palvimo J.J. Mol. Cell. Biol. 2002; 22: 5222-5234Crossref PubMed Scopus (355) Google Scholar, 17Takahashi Y. Toh-e A. Kikuchi Y. Gene (Amst.). 2001; 275: 223-231Crossref PubMed Scopus (107) Google Scholar). These proteins share a common RING finger-like structure and bind directly to the Ubc9 E2 enzyme and some SUMO protein targets. This RING finger motif has also been identified in some of the ubiquitin E3 ligases (18Hershko A. Ciechanover A. Annu. Rev. Biochem. 1998; 67: 425-479Crossref PubMed Scopus (6907) Google Scholar). A second type of SUMO E3 protein found in mammalian systems is RanBP2, which is part of the nuclear pore complex (15Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (641) Google Scholar). RanBP2 differs from the PIAS proteins in that it does not have a RING finger domain or homology to ubiquitin E3 proteins. However, it interacts with Ubc9 although not the sumoylation target protein (15Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (641) Google Scholar). The final E3 protein type (Pc2) belongs to the Poly-comb protein family and stimulates sumoylation of C terminus binding protein (16Kagey M.H. Melhuish T.A. Wotton D. Cell. 2003; 113: 127-137Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar). In some cases sumoylation can exist in the form of polymeric chains because the SUMO-1 paralogs SUMO-2 and SUMO-3 have internal SUMO modification consensus sites that allow the formation of polymeric SUMO chains on modified proteins (19Tatham M.H. Jaffray E. Vaughan O.A. Desterro J.M. Botting C.H. Naismith J.H. Hay R.T. J. Biol. Chem. 2001; 276: 35368-35374Abstract Full Text Full Text PDF PubMed Scopus (643) Google Scholar). Other post-translational modifications can regulate the SUMO modification of a protein. For example, phosphorylation negatively regulates the sumoylation of several substrate proteins, including c-Jun, promyelocytic leukemia (PML), and IκBα, (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar). Phosphorylation can also act positively, as in the case of the transcription factor HSF1 where sumoylation is stimulated by phosphorylation of serine residues near the SUMO modification site (20Hilgarth R.S. Hong Y. Park-Sarge O.K. Sarge K.D. Biochem. Biophys. Res. Commun. 2003; 303: 196-200Crossref PubMed Scopus (38) Google Scholar, 21Hietakangas V. Ahlskog J.K. Jakobsson A.M. Hellesuo M. Sahlberg N.M. Holmberg C.I. Mikhailov A. Palvimo J.J. Pirkkala L. Sistonen L. Mol. Cell. Biol. 2003; 23: 2953-2968Crossref PubMed Scopus (249) Google Scholar). As with other post-translational modifications, SUMO groups can be removed from proteins in a reaction catalyzed by SUMO-specific proteases (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar). Some of these proteases have dual functionality in that they both process SUMO to its mature diglycine form and also cleave the isopeptide bond between SUMO and its target proteins (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar). In yeast, two SUMO proteases have been identified, Ulpl and Ulp2/Smt4, both of which are specific for SUMO and display compartmentalization, with Ulp1 being present at the nuclear pore complex and Upl2/Smt4 being present in the nucleoplasm. In mammals, several SUMO proteases have been confirmed with the possibility of many more being present due to alternative splice variants (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar). As with yeast, many of the mammalian SUMO proteases are localized to different cellular compartments, which may function to regulate the balance of protein sumoylation in these compartments. Depending on the target protein, sumoylation can occur in the cytoplasm or nucleus, and this modification is involved in regulating the subcellular localization of a number of substrate proteins. RanGAP1 was the first identified SUMO substrate and plays an important role in the regulation of transport of ribonucleoproteins and proteins across the NPC (22Matunis M.J. Coutavas E. Blobel G. J. Cell Biol. 1996; 135: 1457-1470Crossref PubMed Scopus (957) Google Scholar, 23Mahajan R. Gerace L. Melchior F. J. Cell Biol. 1998; 140: 259-270Crossref PubMed Scopus (239) Google Scholar). Unmodified RanGAP1 resides predominantly in the cytoplasm and upon conjugation with SUMO associates with the cytoplasmic fibers of the NPC (22Matunis M.J. Coutavas E. Blobel G. J. Cell Biol. 1996; 135: 1457-1470Crossref PubMed Scopus (957) Google Scholar, 24Mahajan R. Delphin C. Guan T. Gerace L. Melchior F. Cell. 1997; 88: 97-107Abstract Full Text Full Text PDF PubMed Scopus (1006) Google Scholar). SUMO modification directs RanGAP1 to the NPC by an interaction with RanBP2/Nup358, possibly mediated by sumoylation-induced formation of a binding interface for interaction of these two proteins (25Matunis M.J. Wu J. Blobel G. J. Cell Biol. 1998; 140: 499-509Crossref PubMed Scopus (379) Google Scholar). Analysis of nuclear localization signal mutants of a protein called Smad4, a factor with a major role in the TGF-β signal transduction pathway, indicates that nuclear import of this protein is required for it to be sumoylated (26Lin X. Liang M. Liang Y.Y. Brunicardi F.C. Feng X.H. J. Biol. Chem. 2003; 278: 31043-31048Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar). Smad4 moves to the nucleus in response to TGF-β stimulation, and immunofluorescence analysis of TGF-β-induced cells that were SUMO-1 transfected demonstrated an increase in the nuclear localization of Smad4 (26Lin X. Liang M. Liang Y.Y. Brunicardi F.C. Feng X.H. J. Biol. Chem. 2003; 278: 31043-31048Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar). Nuclei contain a number of distinct bodies that are defined, at least in part, by the proteins contained in them. For example, the PML and Sp100 proteins are major components of PML nuclear bodies (PML NBs), also called ND10. Sumoylation has been found to be required for the subcellular localization of some, but not all, proteins found in bodies such as ND10. For example, the sumoylated forms of PML and Sp100 are found exclusively in the nucleus (27Sternsdorf T. Jensen K. Will H. J. Cell Biol. 1997; 139: 1621-1634Crossref PubMed Scopus (291) Google Scholar). SUMO conjugation was determined to be essential for PML protein localization in ND10, whereas the targeting and accumulation of Sp100 in these bodies was not sumoylation-dependent (27Sternsdorf T. Jensen K. Will H. J. Cell Biol. 1997; 139: 1621-1634Crossref PubMed Scopus (291) Google Scholar, 28Sternsdorf T. Jensen K. Reich B. Will H. J. Biol. Chem. 1999; 274: 12555-12566Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar). This appears to be true for other SUMO substrates as well (p53, LEF1, Daxx, and SRF1) which will localize to ND10 even after mutation of their target lysine (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar). Topors, a DNA topoisomerase I-binding protein, interacts with both Ubc9 and SUMO-1 leading to the formation of Topors nuclear speckles with close association to ND10, although sumoylation-deficient mutants were still able to localize to the nuclear speckles (29Weger S. Hammer E. Engstler M. Exp. Cell Res. 2003; 290: 13-27Crossref PubMed Scopus (41) Google Scholar). SUMO modification also targets cellular localization of tumor suppressors TEL and Smad4, which in the case of TEL, a suspected tumor suppressor, leads to localization of this phospho-protein in TEL bodies, a cell cycle-dependent nuclear structure (30Chakrabarti S.R. Sood R. Nandi S. Nucifora G. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 13281-13285Crossref PubMed Scopus (100) Google Scholar). Sumoylation appears to also be important for localization of the transcription factor HSF1 to nuclear bodies (31Hong Y. Rogers R. Matunis M.J. Mayhew C.N. Goodson M.L. Park-Sarge O.K. Sarge K.D. J. Biol. Chem. 2001; 276: 40263-40267Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar). An exciting development in understanding both the subcellular sites of sumoylation and the role of this modification in regulating subcellular localization of proteins has been the discovery that components of the sumoylation machinery are localized at the nuclear pore complex (32Zhang H. Saitoh H. Matunis M.J. Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (229) Google Scholar, 33Pichler A. Melchior F. Traffic. 2002; 3: 381-387Crossref PubMed Scopus (156) Google Scholar). This localization suggests that sumoylation of at least some proteins occurring as they enter the nucleus could be involved in nuclear import itself or perhaps retention of these proteins in the nucleus. For example, Nup358, a nuclear pore protein demonstrated to have SUMO E3 ligase activity, localizes predominantly to the cytoplasmic filaments of the NPC and regulates targeting of RanGAP1 to the NPC (2Melchior F. Schergaut M. Pichler A. Trends Biochem. Sci. 2003; 28: 612-618Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 25Matunis M.J. Wu J. Blobel G. J. Cell Biol. 1998; 140: 499-509Crossref PubMed Scopus (379) Google Scholar). Other SUMO enzymes, Ubc9 and SENP2, have also been found to localize at the nuclear pore. Ubc9, the E2 conjugating enzyme for SUMO, localizes to the cytoplasmic and nucleoplasmic faces of the NPC as visualized by immunogold analysis of the nuclear envelope (32Zhang H. Saitoh H. Matunis M.J. Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (229) Google Scholar). Ubc9 interacts with Nup358 as well as RanGAP1/SUMO-1, and a model has been proposed in which these three proteins interact to form a stable trimeric complex (15Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (641) Google Scholar, 32Zhang H. Saitoh H. Matunis M.J. Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (229) Google Scholar). This model is strengthened by the observation that RanGAP1 is protected from SENP2 (SUMO protease) degradation when found in this complex (15Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (641) Google Scholar, 32Zhang H. Saitoh H. Matunis M.J. Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (229) Google Scholar). SENP2/Axam itself associates with the nucleoplasmic face of the NPC via its NH2-terminal domain (32Zhang H. Saitoh H. Matunis M.J. Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (229) Google Scholar, 34Hang J. Dasso M. J. Biol. Chem. 2002; 277: 19961-19966Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar), and loss of this domain results in relocalization of this enyzme and increased capacity for deconjugation of substrates (34Hang J. Dasso M. J. Biol. Chem. 2002; 277: 19961-19966Abstract Full Text Full Text PDF PubMed Scopus (194) Google Scholar). SENP2 also associates with Nup153, a component of the nuclear basket in humans. Ulp1, the yeast homologue of the vertebrate SUMO isopeptidase SENP2, is required for progression through the cell cycle and also localizes to the NPC via the NH2-terminal domain, which appears to be necessary for localization as well as enzymatic specificity (35Li S.J. Hochstrasser M. J. Cell Biol. 2003; 160: 1069-1081Crossref PubMed Scopus (165) Google Scholar). Further establishing the connection between sumoylation and nuclear import are data showing that yeast Ulp1 and Ulp2 mutants, deficient in SUMO conjugation, display an impairment of classical nuclear localization signal-mediated protein import (36Stade K. Vogel F. Schwienhorst I. Meusser B. Volkwein C. Nentwig B. Dohmen R.J. Sommer T. J. Biol. Chem. 2002; 277: 49554-49561Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). Sumoylation is most often implicated in promoting localization of proteins to the nucleus and in some cases to nuclear bodies. However, there is evidence that SUMO modification could also function to regulate nuclear export of some substrates. For example, nuclear sumoylation of Dictyostelium Mek1 is responsible for its movement to the cytoplasm (37Sobko A. Ma H. Firtel R.A. Dev. Cell. 2002; 2: 745-756Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar), and mutation of lysine 99 of the TEL protein leads to increased levels of this protein in the nucleus, suggesting a possible role for sumoylation in its nuclear export (38Wood L.D. Irvin B.J. Nucifora G. Luce K.S. Hiebert S.W. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 3257-3262Crossref PubMed Scopus (100) Google Scholar). In addition, sumoylation of heterogeneous nuclear ribonucleoproteins M and C has been proposed to function as a regulator of conformational changes that may influence nucleocytoplasmic transport of these protein complexes (39Vassileva M.T. Matunis M.J. Mol. Cell. Biol. 2004; 24: 3623-3632Crossref PubMed Scopus (86) Google Scholar). The sumoylation of transcription activators, repressors, coactivators, corepressors, and components of PML NBs is involved in the regulation of gene expression (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). The activities of many transcription factors are regulated by association with PML NBs, and assembly of PML NBs requires sumoylation of the PML protein (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 7Seeler J.S. Dejean A. Nat. Rev. Mol. Cell. Biol. 2003; 4: 690-699Crossref PubMed Scopus (578) Google Scholar). Thus, alteration of PML sumoylation has broad effects on transcription (1Seeler J.S. Dejean A. Oncogene. 2001; 20: 7243-7249Crossref PubMed Scopus (136) Google Scholar, 7Seeler J.S. Dejean A. Nat. Rev. Mol. Cell. Biol. 2003; 4: 690-699Crossref PubMed Scopus (578) Google Scholar). For example, sumoylation of PML recruits corepressor Daxx to PML NBs, thereby relieving Daxx-mediated repression of these genes. Similarly, sumoylation of PML directs p53 to PML NBs and could then trigger some modification, such as acetylation and sumoylation, which stimulates the transcriptional activity of p53. Also, sumoylation of PML recruits another sumoylated nuclear body-associated protein, Sp100. Sumoylation of other transcription factors has also been found to regulate their localization, including Drosophila Dorsal, Bicoid, p73α, and Pdx1 (1Seeler J.S. Dejean A. Oncogene. 2001; 20: 7243-7249Crossref PubMed Scopus (136) Google Scholar). Similar to what is observed for PML, corepressor HIPK2 and repressor TEL and TEL-AML1 localize to nuclear dots in a SUMO-dependent manner. Whether sumoylation alters the repressive function of these transcription factors is unclear. The sumoylation of transcription factors has been reported to have different effects on their activities in various pathways including those involving cytokines, WNT, steroid hormone, and AP-1 (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). In most cases, SUMO modification plays a negative role in transcription regulation. The transcription factors that are inhibited by SUMO modification include STAT1, catenin-TCF/LEF, c-Jun, Ah receptor nuclear translocator (ARNT), CEBPα, c-Myb, Sp3, IRF-1, SREBPs, SRF, Elk, AP1, AP2, androgen receptor (AR), glucocorticoid receptor (GR), and progesterone receptor (PR), as well as huntingtin (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 40Schmidt D. Muller S. Cell Mol. Life Sci. 2003; 60: 2561-2574Crossref PubMed Scopus (223) Google Scholar). The ψKXE sumoylation site motifs of some factors such as GR, Sp3, c-Myb, C/EBP, and the SREBPs are located within an inhibitory or negative regulatory domain or the so-called “synergy control” motifs that can transrepress transcriptional activity. Mutation of sumoylation sites in transcription factors has been found to increase their transcriptional activity, for example, transcription factors Elk-1, Sp-3, SREBPs, STAT-1, SRF, c-Myb, C/EBPs, AR, p300, c-Jun, GR, and peroxisome proliferator-activated receptor γ that may reflect a role for SUMO-1 modification as a negative regulator of transactivation domains (3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 41Ohshima T. Koga H. Shimotohno K. J. Biol. Chem. 2004; 279: 29551-29557Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Consistent with this idea, overexpression of free SUMO-1 can suppress AP2 and AP2-mediated transcription (8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). Direct evidence for repression of transcriptional activity by sumoylation is that fusion of SUMO to GAL4 drastically reduces its activity in reporter gene assays (42Ross S. Best J.L. Zon L.I. Gill G. Mol. Cell. 2002; 10: 831-842Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar). Furthermore, SUMO is also able to inhibit transcription in trans as demonstrated by SUMO-dependent trans-repression of the VP-16 activation domain (43Yang S.H. Jaffray E. Hay R.T. Sharrocks A.D. Mol. Cell. 2003; 12: 63-74Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). The effects of SUMO on transcriptional activity may be complicated by the finding that a number of transcription co-factors, such as GRIP1, SRC-1, and histone deacetylases (HDAC) 1 and 4, are also sumoylated (1Seeler J.S. Dejean A. Oncogene. 2001; 20: 7243-7249Crossref PubMed Scopus (136) Google Scholar, 3Johnson E.S. Annu. Rev. Biochem. 2004; 73: 355-382Crossref PubMed Scopus (1385) Google Scholar, 6Muller S. Ledl A. Schmidt D. Oncogene. 2004; 23: 1998-2008Crossref PubMed Scopus (243) Google Scholar, 8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). Sumoylation might be involved in modulating the functions of proteins as co-activators (GRIP1, SRC-1) or co-repressors (HDAC1, HDAC4) but is not essential (8Verger A. Perdomo J. Crossley M. EMBO Rep. 2003; 4: 137-142Crossref PubMed Scopus (375) Google Scholar). Several observations reveal that the PIAS/SUMO system may modulate the assembly of coactivator or corepressor complexes that regulate transcription (40Schmidt D. Muller S. Cell Mol. Life Sci. 2003; 60: 2561-2574Crossref PubMed Scopus (223) Google Scholar). Other findings indicate further links between the SUMO system and class I and class II HDACs that mediate transcription repression. For example, sumoylation of p300 can mediate repression of gene activity by recruitment of the corepressor HDAC6 (44Girdwood D. Bumpass D. Vaughan O.A. Thain A. Anderson L.A. Snowden A.W. Garcia-Wilson E. Perkins N.D. Hay R.T. Mol. Cell. 2003; 11: 1043-1054Abstract Full Text Full Text PDF PubMed Scopus (383) Google Scholar). The AR, which interacts with the corepressor SMRT, is part of a larger HDAC1-containing complex (45Dotzlaw H. Moehren U. Mink S. Cato A.C. Iniguez Lluhi J.A. Baniahmad A. Mol. Endocrinol. 2002; 16: 661-673Crossref PubMed Scopus (125) Google Scholar). Mutation of the sumoylation site in AR abrogates SMRT binding, suggesting that sumoylation is required for the association of SMRT and class 1 HDACs. Similar data show that histone H4 sumoylation mediates transcriptional repression through recruitment of HDAC1 and HP1 (46Shiio Y. Eisenman R.N. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 13225-13230Crossref PubMed Scopus (523) Google Scholar). However, sumoylation of methyl-transferase 3a (Dnmt3a) disrupts its ability to interact with HDAC1/2, which abolishes its capacity to repress transcription (47Ling Y. Sankpal U.T. Robertson A.K. McNally J.G. Karpova T. Robertson K.D. Nucleic Acids Res. 2004; 32: 598-610Crossref PubMed Scopus (111) Google Scholar). Regarding other possible mechanisms by which sumoylation could mediate effects on transcription factors/co-factors, examples where sumoylation has been found to regulate ubiquitination of proteins such as NFκB inhibitor IκBα, PCNA, Smad4, and Mdm2, either directly by competition for t