Protein Sumoylation

Modification of substrate protein by SUMO is referred to as sumoylation. Tbe first substrate for sumoylation to be identified was RanGAPl, a GTPase-activating protein. Sumoylation of RanGAPl was found to be essential for nucleocytoplasmic trafficking. Sumoylation RanGAPl enables it to bind to RanBP2, a subunit of tbe nuclear... 

Another protein modification, which occurs through a three-enzyme complex similar to that required for ubiquitin addition, is SUMOylation. SUMO stands for small ubiqutin-like modifier, and when proteins are tagged with SUMO their activites are altered (either positively or negatively, depending on the protein). SUMOylation presents yet another means of fine-tuning existing regulatory systems. 

Denison, C. Rudner, A.D. Gerber, S.A. Bakalarski, C.E. Moazed, D. Gygi, S.P. A proteomic strategy for gaining insights into protein sumoylation in yeast Mol. Cell. Proteomics 2005, 4(3), 246-254. 

AhRR, which significantly enhances the SUMOylation of both proteins. SUMOylated AhRR recruits corepressors ANKRA2, HDAC4, and HDAC5 to form the transcriptional repressor complex. AhRR AhR repressor, ARNT aryl hydrocarbon receptor nuclear translocator... 

The transcriptional activity of NRs is also modulated by various posttranslational modifications of the receptors themselves or of their coregulatory proteins. Phosphorylation, as well as several other types of modification, such as acetylation, SUMOylation, ubiquitinylation, and methylation, has been reported to modulate the functions of NRs, potentially constituting an important cellular integration mechanism. In addition to the modifications of the receptors themselves, such modifications have been reported for their coactivators and corepressors. Therefore, these different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. 

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

Small Ubiquitin-like modifier (SUMO) is a conserved protein that is ubiquitously expressed in eukaryotes and is essential for viability. It serves as a reversible posttranslational modifier by forming an isopeptide bond with lysine residues in many target proteins, in a catalytic process termed SUMOylation. SUMOylation of proteins results in altered inter- or intramolecular interactions of the modified target (Fig. 1). 

SUMOylation. Figure 1 SUMOylation is a reversible and regulated process. Target protein modification by SUMO can be initiated and terminated by different cues. Sumoylation leads to changes in the behavior of the modified protein, for example, different cellular localization, enhanced/reduced activity, or increased stability. These changes are due to alterations either in protein interactions or protein folding. 

The third type of E3 ligases is represented by the polycomb protein 2 (Pc2), which was reported to enhance sumoylation of the substrate CtBP. N- and C-terminal domains in Pc2 that have been implicated in CtBP sumoylation do not resemble known E3 ligases. Like RanBP2, Pc2 expression is restricted to higher eukaryotes. 

Masking of a site Association between two proteins can be disrupted by SUMOylation, if the SUMO attachment site of a target protein overlaps with the... 

Formation of a novel binding site novel complexes may be formed between a SUMOylated protein and an effector protein that contains a SUMO-interacting motif (SIM or SBM). Proteins that contain two binding sites, a SIM and a weak binding motif to protein X, will bind more strongly to this protein if it is SUMOylated (Fig. 3b). Short peptides that contain the hydrophobic core motif [V/I]-X-[V/I]-[V/I] or [V/I-[V/I]]-X-[V/I] can act as a SIM and bind to SUMO. This core is often flanked by acidic amino acids and/or serine residues. 

PTMs are important for the regulation of protein function and the maintenance of cellular hemostasis. There are 300 or more reported PTMs of proteins. PTMs may involve the addition of functional groups such as acetyls in acetylation, hydroxylation, amidation, and oxidation or the addition of peptides or proteins such as ubiquitination, SUMOylation (addition of small ubiquitin-like modifier), and ISGylation (addition of interferon-stimulated gene15). 

Fig. 4.1. Fundamentals of the ubiquitin system. Adapted from Ref [5]. Figure 4.1 shows the fundamentals of the ubiquitin system. (1) Ubiquitin is synthesized in linear chains or as the N-terminal fusion with small ribosomal subunits that are cleaved by de-ubiquitylating enzymes to form the active protein. Ubiquitin is then activated in an ATP-dependent manner by El where a thiolester linkage is formed. It is then transthiolated to the active-site cysteine of an E2. E2s interact with E3s and with substrates and mediate either the indirect (in the case of HECT E3s) or direct transfer of ubiquitin to substrate. A number of factors can affect this process. We know that interactions with Hsp70 can facilitate ubiquitylation in specific instances and competition for lysines on substrates with the processes of acetylation and sumoylation may be inhibitory in certain instances. (2) For efficient proteasomal targeting to occur chains of ubiquitin linked internally through K48 must be formed. This appears to involve multiple...

Shigeno K, Yoshida H, Pan L, Luo JM, Fujisawa S, Naito K, Nakamura S, Shinjo K, Takeshita A, Ohno R, Ohnishi K (2004) Disease-related potential of mutations in transcriptional cofactors CREB-binding protein and p300 in leukemias. Cancer Lett. 213 11—20 Shiio Y, Eisenman RN (2003) Histone sumoylation is associated with transcriptional repression. Proc Natl Acad Sci U S A 100 13225-13230... 

Transcription regulators are also known to be sumoylated. One of tbe early studies of tbis phenomenon showed that promyelocytic leukemia protein (PML) is a substrate for SUMO conjugation. Once SUMO is attached to the PML protein it is directed to a subdomain of tbe nucleus called tbe PML oncogenic domain (POD). It is thought that POD localization of the PML protein allows it to recruit other proteins such as transcription factors. Transcription factors in the POD can activate or inhibit transcription. Another transcription factor known to be sumoylated is Sp3. ° SUMO also has roles in chromatin condensation and interphase chromosome organization. ... 

In the nervous system, so far there is evidence of sumoylation of at least one protein critical for synaptic plasticity. Long found that Drosophila calcium-calmodulin-dependent kinase II (CaMKII) is conjugated... 

Is there a specific consensus site for SUMO attachment in substrate proteins. With studies on just a handful of substrates such as RanGAPl, I/tBa, p53, and c-jun the SUMO-acceptor site was found to be tPKxE (where tp is an aliphatic branched amino acid and x is any amino acid). The identification of a sumoylation consensus site is noteworthy because no such site for ubiquitination is found. Because ubiquitination is vastly more complex with hundreds of ligases, a common consensus sequence for ubiquitination may not exist. It must be pointed out, however, that the recent identification of the TEK box motif for APC suggests that consensus motifs for ubiquitin-acceptor sites might be found at least for some ligases. 

SUMO attachment can be removed by SUMO-specific isopeptidases just as UCHs and UBPs remove ubiquitin from substrate proteins. Thus, sumoylation is also a reversible modification. SUMO-specific isopeptidases possess a Cys residue in their catalytic core similar to UCHs and UBPs. Two SUMO-specific isopeptidases in yeast are called Ulpl and Ulp2. In humans, six Ulp homologues have been identified and are given the name sentrin-specific proteases (SENPs) because SUMO was previously known as sentrin . 

The N-terminal tails of histone proteins are rich in arginine and lysine residues and undergo various types of posttranslational modifications. There are small modifications such as acetylation, methylation, phosphorylation but also the attachment of larger peptide groups such as ubiquitinylation and sumoylation [1]. This has an impact on chromatin structure and subsequently on gene transcription and the epigenetic maintenance of altered transcription after cell division [2],... 

Van Gelder Yes, PIAS3 suppresses STAT3 in vivo and in vitro. It basically functions as a sumoyl hgase. It interacts with a large number of proteins that require sumoylation for their function. 

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