Sumoylation of MDM2 and SUSP4
The p53 tumor suppressor protein plays a key role in the control of cell growth and differentiation and in the maintenance of genome integrity. p53 is kept labile under normal conditions, but in response to stresses, such as DNA damage, it is accumulated in the nucleus for transcriptional activation of genes involved in cell cycle arrest, DNA repair, and/or apoptosis. Therefore, tight regulation of p53 level is essential for the control of p53-mediated cellular processes. One of the main regulators of p53 is the ubiquitin ligase Mdm2 that targets p53 for ubiquitination and subsequent degradation by the proteasome. Mdm2 is also self-ubiquitinated and degraded by the proteasome. At the same time, the Mdm2 gene is a transcriptional target of p53, thereby forming a negative feedback loop. In the p53-Mdm2 feedback loop, it is imperative that p53 be relieved from inhibition by Mdm2 for its activation in response to stress. Multiple mechanisms have been shown to interrupt this loop, including post-translational modification of p53 and Mmd2, interaction with specific modulators, and changes in their subcellular localization. However, it remained unclear how Mdm2 function is switched from p53 ubiquitination to self-ubiquitination under stress conditions.
p53 is modified by SUMO-1, but the functional relevance of its sumoylation remains controversial. It has been reported that overexpression of SUMO-1 or Ubc9 induces an increase in p53-dependent transcription. However, it has also been reported that sumoylation of p53 has no effects on its transcriptional activity and coexpression of PIAS1 and SUMO-1 rather represses p53 activity. Mdm2 is also modified by SUMO-1. However, neither is known about the functional relevance of Mdm2 sumoylation.
We demonstrate that a new SUMO-specific protease SUSP4 removes SUMO-1 from Mdm2 and p53. Desumoylation of Mdm2 by SUSP4 enhanced its self-ubiquitination and degradation by the proteasome, leading to p53 stabilization. Consistently, SUSP4 promoted p53 transcriptional activity, and reversed the inhibitory effect of Mdm2 on p53 activity. While stable expression of SUSP4 inhibited cell growth, shRNA-mediated knock down of SUSP4 promoted it. These results also suggest that reversible SUMO modification of Mdm2 can be a potential mechanism for switching between p53 ubiquitination and Mdm2 self-ubiquitination.
Sumoylation/Ubiquitination of PolyQ proteins.
Mammals have poly-CAG-repeated genes in their genome. CAG repeats make hairpin structures and cause genomic unstability. The hairpin structure interrupts elongation of DNA polymerase and often causes unusual expansion of the repeats. CAG codes glutamine, and the proteins with expanded polyQ repeats have abnormal folding. Abnormal function of polyQ proteins lose their own function or gain malfuncion which lead cell death. The known disease-related polyQ proteins are huntingtin (huntington's disease : HD), Ataxin family proteins (Spinocelebellar ataxia : SCA), Atrophin-1 (Denaturobral pallidoluysian atrophy : DRPLA), and Androgen receptor (Spinal and bulbar muscular atrophy : SBMA).
Misfolded polyQ aggregates must be ubiquitinated and degraded via proteasome, however, too much aggregates overwhelm the capacity of normal ubiquitin-proteasome system (UPS). Recently, it is known that sumoylation of polyQ proteins is more important for progress of disease than ubiquitination. In huntingtin case, the modification residues are the same for ubiquitin and sumo. In Drosophila huntingtin model, sumo knockout makes slow progress of disease, however, ubiquitin knockout makes slightly rapid progress of disease. We guess that deubiquitinating enzyme and desumoylating enzyme play important role in huntington's disease. We hope to find the enzymes and study about function in polyQ disease.