| Summary: | 碩士 === 臺灣大學 === 生化科學研究所 === 98 === Downregulation of tumor suppressor PML has been reported in various human tumor cell lines. The decrease of PML protein level in tumor cells was thought to result from post-translational mechanisms, such as aberrant degradation. PML expression and stability has been shown to be regulated by various posttranslational modifications. We recently discovered that targeting PML to proteasome degradation requires three posttranslational modifications at the PML Ser518-Pro519 motif. CDK1/2 can phosphorylate PML at Ser518 and then promote the prolyl cis/trans isomerization by Pin1 and polyubiquitination by KLHL20-Cullin3 ubiquitin ligase. In the first part of this thesis, we try to find deubiquitinating enzymes that may regulate PML stability. However, we need more effort to improve the siRNA knockdown efficiency and to clarify the regulatory mechanism of DUBs on PML protein. In the second part of this thesis, by using functional genomic approach, we identified the small C-terminal domain phosphatase 1 (SCP1) as a specific phosphatase for PML dephosphorylation at Ser518. A catalytically inactive mutant (SCP1 DN) had no effect on PML phosphorylation in vivo and in vitro. Of the other FCP/SCP family members SCP2 and SCP3, but not FCP1, could also dephosphorylate PML at Ser518. Through this dephosphorylation, SCP1 decreased Pin1-KLHL20 mediated PML polyubiquitination and increased PML stability. Taken together, this study implies the complexity of PML regulation by DUBs and reveals an important new substrate for SCPs.
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