Covalent modifications of proteins, such as phosphorylation, acetylation and ubiquitylation, play an important role in most cellular processes because they can cause rapid changes in the activities of pre-existing proteins. This type of mechanism for regulating protein function is especially crucial in signal transduction pathways and in cell cycle. The Ubiquitin System is one of the major protein-modification systems required for the highly selective turnover of specific proteins in eukaryotic cells. These ubiquitin-like proteins modulate protein function in the cell through reversible post-translational modification, which has been linked to several cellular regulatory pathways including cell-cycle progression, apoptosis, differentiation, intracellular targeting and responses to stress. One such ubiquitin-like protein is SUMO (Small Ubiquitin-Related Modifier) variously called as PIC1 (Phosphoinositidase-C1); UBL1 (Ubiquitin-like Protein 1); GMP1 (GAP Modifying Protein-1); Smt3p (Suppressor of MIF2 in budding yeast); Pmt3p (in fission yeast) and Sentrin. Similar to ubiquitin, the mechanism of attachment of SUMO to substrate proteins at internal lysine residues is carried out by a battery of activating (E1), conjugating (E2), and ligating (E3) enzymes for covalent attachment (Ref.1). However, unlike ubiquitylation, which targets proteins for proteosome-dependant degradation, sumoylation does not appear to promote protein degradation but rather is involved in mediating protein–protein interactions, subcellular compartmentalization and protein stability by blocking ubiquitylation.
Sumoylation controls the function of substrate proteins by affecting their interaction with cooperative molecules and subcellular localization. Like the ubiquitin conjugation system, SUMO1 is activated in an ATP-dependent manner by the specific activity of E1, which is a heterodimer comprising SAE1 (SUMO-1 Activating Enzyme subunit-1)/AOS1 (in Yeast) and SAE2 (SUMO1 Activating Enzyme subunit-1)/Uba2 (in humans) proteins. Subsequently, SUMO is transferred to E2-conjugating enzyme Ubc9 (Ubiquitin-conjugating enzyme) (Ref.2) and attaches to the epsilon-amino group of lysines in the substrates targeting the consensus sequence. Major substrates for SUMO1 modification include RanGAP1 (Ran GTPase-Activating Protein-1), RanBP1, RanBP2 (Ran-Binding Proteins) implicated in nucleocytoplasmic trafficking, PML (Promyelocytic Leukemia Protein), SP100 found in subnuclear structures known as PODs (PML Oncogenic Domains), Daxx (Fas Death Domain-associated Protein), MKK (Mitogen-Activated Protein Kinase Kinase), the I-KappaB (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells) regulated by IKK, (I-KappaB Kinases), GR (Glucocorticoid Receptor), AR (Androgen Receptors), mammalian p53 tumor suppressor, the c-Jun proto-oncoprotein, c-Myb, HSF (Heat Shock Factor-1&2) and LEF1 (Lymphoid Enhancer Factor-1) (Ref.3). The SUMO1 conjugation of PML enables it to target to NBs (Nuclear Bodies) and their interaction with other proteins. PML acts as a coactivator for p53 and increases acetylation of p53 by the transcriptional coactivator CBP (CREB-Binding Protein). This acetylation of p53 is reversed by the deacetylase SIR (Silent Information Regulator). Sumoylation of p53 leads to a mild enhancement of the transcriptional and apoptotic response. Transcriptional enhancement of the HSF2 by SUMO takes place by targeting the DNA binding domain (Ref.4). PML is involved in non-p53 mediated apoptotic pathways, such as Daxx-mediated apoptosis induced by Fas and TNF (Tumor Necrosis Factor) and regulates the transcriptional repressor activity of Daxx. Two additional mammalian cDNAs encoding closely related proteins similar to SUMO1 have been isolated, and are designated SUMO2/Smt3A and SUMO3/Smt3B. These proteins also conjugate to several target proteins via a mechanism similar to that of SUMO1 conjugation (Ref.5). In yeast, the majority of SUMO conjugation requires the Siz1 and Siz2 proteins. The mammalian proteins to which Siz1 and Siz2 are most closely related are the PIAS (Protein Inhibitor of Activated STAT) proteins, PIAS1 and PIASY which catalyses sumoylation of p53 and LEF1 (Ref.6).
Sumoylation has begun to be recognized as an important posttranslational modification capable of altering stability, gene regulation, subcellular localization, and protein-protein interactions. It has been involved in multiple vital cellular processes such as oncogenesis, cell cycle control, nucleocytoplasmic trafficking, apoptosis, and response to virus infection (Ref.7) but its role in onset or progression of AD (Alzheimer's disease) is still unclear. Another important role of SUMO1 modification is protection of the modified protein from degradation by the proteosome pathway. This function is especially important for proteins that are essential for cell-cycle progression, such as p53, known to be rapidly degraded by the ubiquitin pathway but protected from degradation by SUMO1 conjugation. The disruption of the NBs by HSV (Herpes Simplex virus)-ICPO, CMV (Cytomegalovirus)-IE1 and EBV (Epstein-Barr virus) correlates with the abrogation of sumoylation of PML and SP100 indicating that SUMO modification plays a critical role in nuclear body dynamics (Ref.4). In prolonged hypoxia-induced inflammatory processes, CREB (cAMP Responsive Element Binding Protein) is posttranslationally modified by SUMO1 and its over-expression stabilizes CREB and enhances CREB-dependent reporter gene activity (Ref.8). Thus, sumoylation modifies its substrate's interaction with partner proteins, often leading to visible changes in subcellular compartmentalization. Moreover, the physical and/or genetic association of SUMO or SUMO pathway components, with large multiprotein complexes (nuclear lamina, PMLNB, centromeres/kinetochores, etc.) suggests that sumoylation constitutes an imprint that tells a specific protein complex to stay together or to respond to external stimuli (Ref.4).
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