A new 30-kDa ubiquitin-related SUMO-1 hydrolase from bovine brain

Functional analysis and intracellular localization of p53 modified by SUMO-1

p53 tumor suppressor is a subject of several post-translational modifications, including phosphorylation, ubiquitination and acetylation, which regulate p53 function. A new covalent modification of p53 at lysine 386 by SUMO-1 was recently identified. To elucidate the function of sumoylated p53, we compared the properties of wild type p53 and sumoylation-deficient p53 mutant, K386R. No differences were found between wild type p53 and K386R mutant of p53 in transactivation or growth suppression assays. Moreover, overexpression of SUMO-1 has no effect on p53-regulated transcription. Biochemical fractionation showed that sumoylated p53 is localized in the nucleus and is tightly bound to chromatin structures. p53 and SUMO-1 co-localized in PML nuclear bodies in 293 cells and the nucleoli in MCF7 and HT1080 cells. However, sumoylation-deficient p53 mutant showed a similar pattern of intranuclear localization, suggesting that SUMO-1 does not target p53 to subnuclear structures. These data indicate that SUMO-1 modification of p53 at lysine 386 may not be essential for p53's cellular localization, transcriptional activation, or growth regulation.

SUMO-1 conjugation in vivo requires both a consensus modification motif and nuclear targeting

SUMO-1 is a small ubiquitin-related modifier that is covalently linked to many cellular protein targets. Proteins modified by SUMO-1 and the SUMO-1-activating and -conjugating enzymes are located predominantly in the nucleus. Here we define a transferable sequence containing the PsiKXE motif, where Psi represents a large hydrophobic amino acid, that confers the ability to be SUMO-1-modified on proteins to which it is linked. Whereas addition of short sequences from p53 and IkappaBalpha, containing the PsiKXE motif, to a carrier protein is sufficient for modification in vitro, modification in vivo requires the additional presence of a nuclear localization signal. Thus, protein substrates must be targeted to the nucleus to undergo SUMO-1 conjugation.

SUMO--nonclassical ubiquitin

SUMO (small ubiquitin-related modifier) is the best-characterized member of a growing family of ubiquitin-related proteins. It resembles ubiquitin in its structure, its ability to be ligated to other proteins, as well as in the mechanism of ligation. However, in contrast to ubiquitination-often the first step on a one-way road to protein degradation-SUMOlation does not seem to mark proteins for degradation. In fact, SUMO may even function as an antagonist of ubiquitin in the degradation of selected proteins. While most SUMO targets are still at large, available data provide compelling evidence for a role of SUMO in the regulation of protein-protein interactions and/or subcellular localization.

African swine fever virus protease, a new viral member of the SUMO-1-specific protease family

African swine fever virus (ASFV) is a complex DNA virus that employs polyprotein processing at Gly-Gly-Xaa sites as a strategy to produce several major core components of the viral particle. The virus gene S273R encodes a 31-kDa protein that contains a "core domain" with the conserved catalytic residues characteristic of SUMO-1-specific proteases and the adenovirus protease. Using a COS cell expression system, it was found that protein pS273R is capable of cleaving the viral polyproteins pp62 and pp220 in a specific way giving rise to the same intermediates and mature products as those produced in ASFV-infected cells. Furthermore, protein pS273R, like adenovirus protease and SUMO-1-specific enzymes, is a cysteine protease, because its activity is abolished by mutation of the predicted catalytic histidine and cysteine residues and is inhibited by sulfhydryl-blocking reagents. Protein pS273R is expressed late after infection and is localized in the cytoplasmic viral factories, where it is found associated with virus precursors and mature virions. In the virions, the protein is present in the core shell, a domain where the products of the viral polyproteins are also located. The identification of the ASFV protease will allow a better understanding of the role of polyprotein processing in virus assembly and may contribute to our knowledge of the emerging family of SUMO-1-specific proteases.

Covalent modification of the androgen receptor by small ubiquitin-like modifier 1 (SUMO-1)

Modification by SUMO-1 is proposed to play a role in protein targeting and/or stability. The SUMO-1-conjugating enzyme Ubc9 interacts with androgen receptor (AR), a ligand-activated transcription factor belonging to the steroid receptor superfamily. We show here that AR is covalently modified by SUMO-1 (sumoylated) in an androgen-enhanced fashion and identify the principal acceptor site in the N-terminal domain of AR. Substitutions of sumoylated Lys residues enhanced transcriptional activity of AR without influencing its transrepressing activity. Interestingly, the same Lys residues form the cores of the recently described transcriptional synergy control motifs in AR [Iñiguez-Lluhi, J. A. & Pearce, D. (2000) Mol. Cell. Biol. 20, 6040-6050]. These motifs, which match perfectly with the sumoylation consensus sequence, are also present in the N-terminal domains of glucocorticoid, mineralocorticoid, and progesterone receptor. Taken together, our data suggest that reversible sumoylation is a mechanism for regulation of steroid receptor function.

Covalent modification of p73alpha by SUMO-1. Two-hybrid screening with p73 identifies novel SUMO-1-interacting proteins and a SUMO-1 interaction motif

Two-hybrid screening in yeast with p73alpha isolated SUMO-1 (small ubiquitin-like modifier 1), the enzyme responsible for its conjugation, Ubc-9, and a number of novel SUMO-1-interacting proteins, including thymine DNA glycosylase, PM-Scl75, PIASx, PKY, and CHD3/ZFH. A subset of these proteins contain a common motif, hhXSXS/Taaa, where h is a hydrophobic amino acid and a is an acidic amino acid, that is shown to interact with SUMO-1 in the two-hybrid system. We show here that p73alpha, but not p73beta, can be covalently modified by SUMO-1. The major SUMO-1-modified residue in p73alpha is the C-terminal lysine (Lys(627)). The sequence surrounding this lysine conforms to a consensus SUMO-1 modification site b(X)XXhKXE, where b is a basic amino acid. SUMO-1-modified p73 is more rapidly degraded by the proteasome than unmodified p73, although SUMO-1 modification is not required for p73 degradation. SUMO-1 modification does not affect the transcriptional activity of p73alpha on an RGC-luciferase reporter gene in SK-N-AS cells. Instead, SUMO-1 modification may alter the subcellular localization of p73, because SUMO-1-modified p73 is preferentially found in detergent-insoluble fractions. Alternatively, it may modulate the interaction of p73 with other proteins that are substrates for SUMO-1 modification or which interact with SUMO-1, such as those identified here.

A novel mammalian Smt3-specific isopeptidase 1 (SMT3IP1) localized in the nucleolus at interphase

A novel Smt3-specific isopeptidase, SMT3IP1, was cloned using a yeast two-hybrid screen with Smt3b as bait. The clone, named SMT3IP1 (Smt3-specific isopeptidase 1), which bound to Smt3b but not SUMO-1 in the two-hybrid system, was distantly related to budding yeast Saccharomyces cerevisiae Ulp1, human SENP1 or human SUSP1. The catalytic domains in the C-terminal region were very similar, but the N-terminal region was quite different to other enzymes. The cysteine, histidine and asparatic acid residues in the catalytic domains were conserved. SMT3IP1 expressed by the baculovirus-expression system had the ability to cleave SUMO-1 or Smt3b from SUMO-1/RanGAP1 or Smt3b/RanGAP1 conjugates, respectively, and the activity was a little stronger towards the Smt3b conjugate than towards the SUMO-1 conjugate. Furthermore, the enzyme bound more strongly to Smt3a and Smt3b than to SUMO-1 in vitro. The enzyme did not cleave Nedd8 from Nedd8/cullin-1. Nor did it cleave ubiquitin from ubiquitinated p53. SMT3IP1 was localized almost exclusively at the nucleolus during interphase. The N-terminal sequence was responsible for the nucleolar localization of this enzyme. Whether SMT3IP1 functions in the nucleolus or just stays there before it functions in the nucleus, as shown in the case of CDC14 phosphatase, remains to be elucidated.

Alphaherpesvirus proteins related to herpes simplex virus type 1 ICP0 affect cellular structures and proteins

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP0 interacts with several cellular proteins and induces the proteasome-dependent degradation of others during infection. In this study we show that ICP0 is required for the proteasome-dependent degradation of the ND10 protein Sp100 and, as with the other target proteins, the ICP0 RING finger domain is essential. Further, comparison of the kinetics and ICP0 domain requirements for the degradation of PMI and Sp100 suggests that a common mechanism is involved. Homologues of ICP0 are encoded by other members of the alphaherpesvirus family. These proteins show strong sequence homology to ICP0 within the RING finger domain but limited similarity elsewhere. Using transfection assays, we have shown that all the ICP0 homologues that we tested have significant effects on the immunofluorescence staining character of at least one of the proteins destabilized by ICP0, and by using a recombinant virus, we found that the equine herpesvirus ICP0 homologue induced the proteasome-dependent degradation of endogenous CENP-C and modified forms of PML and Sp100. However, in contrast to ICP0, the homologue proteins had no effect on the distribution of the ubiquitin-specific protease USP7 within the cell, consistent with their lack of a USP7 binding domain. We also found that ICP0 by itself could induce the abrogation of SUMO-1 conjugation and then the proteasome-dependent degradation of unmodified exogenous PML in transfected cells, thus demonstrating that other HSV-1 proteins are not required. Surprisingly, the ICP0 homologues were unable to cause these effects. Overall, these data suggest that the members of the ICP0 family of proteins may act via a similar mechanism or pathway involving their RING finger domain but that their intrinsic activities and effects on endogenous and exogenous proteins differ in detail.

The reversible modification regulates the membrane-binding state of Apg8/Aut7 essential for autophagy and the cytoplasm to vacuole targeting pathway

Autophagy and the Cvt pathway are examples of nonclassical vesicular transport from the cytoplasm to the vacuole via double-membrane vesicles. Apg8/Aut7, which plays an important role in the formation of such vesicles, tends to bind to membranes in spite of its hydrophilic nature. We show here that the nature of the association of Apg8 with membranes changes depending on a series of modifications of the protein itself. First, the carboxy-terminal Arg residue of newly synthesized Apg8 is removed by Apg4/Aut2, a novel cysteine protease, and a Gly residue becomes the carboxy-terminal residue of the protein that is now designated Apg8FG. Subsequently, Apg8FG forms a conjugate with an unidentified molecule "X" and thereby binds tightly to membranes. This modification requires the carboxy-terminal Gly residue of Apg8FG and Apg7, a ubiquitin E1-like enzyme. Finally, the adduct Apg8FG-X is reversed to soluble or loosely membrane-bound Apg8FG by cleavage by Apg4. The mode of action of Apg4, which cleaves both newly synthesized Apg8 and modified Apg8FG, resembles that of deubiquitinating enzymes. A reaction similar to ubiquitination is probably involved in the second modification. The reversible modification of Apg8 appears to be coupled to the membrane dynamics of autophagy and the Cvt pathway.

SUMO-1 conjugation to human DNA topoisomerase II isozymes

Topoisomerase I-mediated DNA damage induced by camptothecin has been shown to induce rapid small ubiquitin-related modifier (SUMO)-1 conjugation to topoisomerase I. In the current study, we show that topoisomerase II-mediated DNA damage induced by teniposide (VM-26) results in the formation of high molecular weight conjugates of both topoisomerase IIalpha and IIbeta isozymes in HeLa cells. Immunological characterization of these conjugates suggests that both topoisomerase IIalpha and IIbeta isozymes are conjugated to SUMO-1. The involvement of SUMO-1/UBC9 in the modification of topoisomerase II isozymes is also supported by the demonstration of physical interaction between topoisomerase II and SUMO-1/UBC9. Surprisingly, ICRF-193, which does not induce topoisomerase II-mediated DNA damage but traps topoisomerase II into a circular clamp conformation, is also shown to induce similar SUMO-1 conjugation to topoisomerase II isozymes. In addition, we show that both oxidative and heat shock stresses, which can cause protein damage, rapidly increase nuclear SUMO-1 conjugates. These studies raise the question on whether SUMO-1 conjugation to topoisomerases is an indirect result of a DNA damage response or a direct result because of protein conformational changes.

A new SUMO-1-specific protease, SUSP1, that is highly expressed in reproductive organs

A full-length cDNA encoding a SUMO-1-specific protease, named SUSP1, was identified and cloned for the first time from the human brain. Nucleotide sequence analysis of the cDNA containing an open reading frame of 3336 base pairs revealed that the protease consists of 1112 amino acids with a calculated molecular mass of 126,116 Da. Like yeast Ulp1, SUSP1 is a cysteine protease containing the well conserved His/Asp/Cys catalytic triad. SUSP1 expressed in Escherichia coli cells efficiently released SUMO-1 from SUMO-1. beta-galactosidase fusion but not from other ubiquitin-like protein fusions, including Smt3.beta-galactosidase, suggesting its role in the generation of matured SUMO-1 specifically from its precursors. Interestingly, reproductive organs, such as testis, ovary, and prostate, contained much higher amounts of SUSP1 mRNA than colon and peripheral blood leukocyte, whereas other tissues, such as heart and spleen, had little or none. In addition, confocal microscopy using green fluorescent protein.SUSP1 fusion showed that SUSP1 is exclusively localized to the cytoplasm of NIH3T3 and HeLa cells. These results suggest that SUSP1 may play a role in the regulation of SUMO-1-mediated cellular processes particularly related to reproduction.

Ubiquitin-like proteins: new wines in new bottles

Ubiquitin is a small polypeptide that covalently modifies other cellular proteins and targets them to the proteasome for degradation. In recent years, ubiquitin-dependent proteolysis has been demonstrated to play a critical role in the regulation of many cellular processes, such as cell cycle progression, cell signaling, and immune recognition. The recent discovery of three new ubiquitin-like proteins, NEDD8, Sentrin/SUMO, and Apg12, has further broadened the horizon of this type of post-translational protein modification. This review will focus on the biology and biochemistry of the Sentrin/SUMO and NEDD8 modification pathways, which are clearly distinct from the ubiquitination pathway and have unique biological functions.

Functional heterogeneity of small ubiquitin-related protein modifiers SUMO-1 versus SUMO-2/3

Post-translational modification marked by the covalent attachment of the ubiquitin-like protein SUMO-1/SMT3C has been implicated in a wide variety of cellular processes. Recently, two cDNAs encoding proteins related to SUMO-1 have been identified in human and mouse. The functions and regulation of these proteins, known as SUMO-2/SMT3A and SUMO-3/SMT3B, remain largely uncharacterized. We describe herein quantitative and qualitative distinctions between SUMO-1 and SUMO-2/3 in vertebrate cells. Much of this was accomplished through the application of an antibody that recognizes SUMO-2 and -3, but not SUMO-1. This antibody detected multiple SUMO-2/3-modified proteins and revealed that, together, SUMO-2 and -3 constitute a greater percentage of total cellular protein modification than does SUMO-1. Intriguingly, we found that there was a large pool of free, non-conjugated SUMO-2/3 and that the conjugation of SUMO-2/3 to high molecular mass proteins was induced when the cells were subjected to protein-damaging stimuli such as acute temperature fluctuation. In addition, we demonstrated that SUMO-2/3 conjugated poorly, if at all, to a major SUMO-1 substrate, the Ran GTPase-activating protein RanGAP1. Together, these results support the concept of important distinctions between the SUMO-2/3 and SUMO-1 conjugation pathways and suggest a role for SUMO-2/3 in the cellular responses to environmental stress.