c-Jun, P53, RanGAP1, Pc2, SAE1, MDM2, TOP2, UBE2E3, GCR-alpha, PIAS1, DAXX, c-Myb, ATP, SP3, E2I, NF-kB, PML, SENP1, HSF2, SP100, SUMO-1, Ubiquitin, SAE1/2, SAE2, RanBP2, HSF1, FasR(CD95), PIAS2, NFKBIA
Putative SUMO-1 pathway
Sumoylation is a multi-step protein modification reaction. It implicates Small ubiquitin-like modifier (SUMO) proteins, such as SMT3 suppressor of mif two 3 homolog 1 ( SUMO-1 ). These proteins get attached covalently to lysine residues of substrate/target proteins. As a result, in contrast to ubiquitination that targets proteins for degradation, activities of the sumoylated proteins get modulated to affect a number of biological functions, including control of gene expression, maintenance of genome integrity, intracellular transport and protein stability , .
Attachment of SUMO-1 to substrate proteins is carried out by enzymatic cascade involving SUMO-activating enzyme (E1), SUMO-conjugating enzyme (E2) and SUMO protein ligase (E3). A group of proteases known as SENPs are involved in both the maturation of SUMO precursors (endopeptidase cleavage) and deconjugation of the targets (isopeptidase cleavage).
SUMO-1 is processed by SUMO1/sentrin specific peptidase 1 ( SENP1 ) (endopeptidase cleavage) before being activated. Processed SUMO-1, in the ATP-dependent manner, is covalently linked to the SUMO E1-activating enzyme complex ( SAE1/2 ) composed of two catalytically active subunits, SUMO1 activating enzyme subunit 1 ( SAE1 ) and Ubiquitin-like modifier activating enzyme 2 ( SAE2 ). SUMO-1 is then transferred to the SUMO E2-conjugating enzymes, such as Ubiquitin-conjugating enzyme E2I ( E2I ) and Ubiquitin-conjugating enzyme E2E 3 ( UBE2E3 ), that mediate target protein modification by SUMO E3 ligases, such as RAN binding protein 2 ( RanBP2 ), Chromobox homolog 4 ( Pc2 ) and specific E3-like ligases PIAS1 and PIAS2 ( Protein inhibitors of activated STAT, 1 and 2) , , , , . Cleavage of the SUMO-1 from the target protein is mediated by SENP1 peptidase (isopeptidase cleavage) .
Mdm2 p53 binding protein homolog ( MDM2 ) is an ubiquitin ligase (E3) that acts on Tumor protein p53 ( p53 ). It attaches Ubiquitin to p53 leading to proteasomal degradation of the latter . E3 ligase RanBP2 is a nuclear pore protein and E3 ligases PIAS1 and PIAS2 are localized within the nucleus. MDM2 is sumoylated during nuclear translocation by RanBP2, and then sumoylated again in the nucleus by PIAS1 and PIAS2 .
PIAS1 and PIAS2 also promote sumoylation of several transcription factors, such as p53, c-Jun and SP3. This modification modulates their transcriptional activity, e.g., SUMO-1 modification silences SP3 activity , .
Sumoylation is involved in both the direct regulation of p53 protein stability and function via direct modification of p53, and indirect modulation of the stability of MDM2. Although, the functional consequence of direct SUMO-1 modification of p53 is under debate, it is generally believed that sumoylation represses activity of this transcription factor. The indirect process has to do with, the turnover rate of p53 being related to E3 ubiquitin ligase activity of MDM2, the latter itself being a target of sumoylation. SUMO-1 -modified MDM2 cannot be ubiquitinated as efficiently as the free MDM2. Thereby, SUMO-1 -modified MDM2 exhibits reduced self-ubiquitination which leads to an accumulation of MDM2. Since p53 is a target of MDM2 E3 ubiquitin ligase activity, the p53 levels stay low in the presence of SUMO-1 -modified MDM2 , .
Activation of Nuclear factor NF-kappa-B ( NF-kB ) is achieved by ubiquitination and proteasome-mediated degradation of inhibitory I-kappa-B proteins ( NFKBIA or NFKBIB). The latter inactivate NF-kB by trapping it in the cytoplasm. NFKBIA, conjugated to SUMO-1, is resistant to ubiquitin-induced degradation. Thus, NFKBIA sumoylation inhibits signal-induced activation of NF-kB -dependent transcription .
The sumoylation of TNF receptor superfamily member 6 ( FasR(CD95) ), v-Myb myeloblastosis viral oncogene homolog ( c-Myb ), Promyelocytic leukemia protein ( PML ), Heat shock transcription factor 1 ( HSF1 ), Heat shock transcription factor 2 ( HSF2 ), Glucocorticoid receptor ( GCR-alpha ), Nuclear antigen SP100 ( SP100 ), Death-domain associated protein ( DAXX ), and DNA topoisomerase II ( TOP2 ) regulates subcellular localization, stability and functional activity of these proteins , , , , , , , , , , , , , , , , .