The lipid phosphatase PTEN is most well-known for its role in dephosphorylating PIP₃ to negatively regulate the Akt pathway. This pathway is important in tumour suppression, and has more recently been shown to be involved in synaptic plasticity. PTEN is mutated in various cancers and dysregulated in Alzheimer’s Disease, suggesting it may be a useful therapeutic target.
PTEN activity is controlled by a range of post-translational modifications (PTMs) including SUMOylation, which has been reported to regulate the localisation, function, stability and coupling to the Akt pathway.
The initial aim of this study was to assess if, and how, PTEN SUMOylation affects its reported functions. To achieve this, I optimised protocols to detect SUMOylation and characterise a novel “PTEN-3KR” mutant, in which all three known SUMO acceptor lysine residues are substituted to arginine.
Unexpectedly, PTEN-3KR displayed both enhanced SUMOylation and ubiquitination. While surprising, this provided an interesting tool to investigate the consequences of these increased levels of PTEN modification. I found that PTEN-3KR is unable to supress the Akt pathway, but its turnover and ability to dimerize were unaffected. I also investigated how PTEN-3KR impacted on AMPAR trafficking and retromer function. PTEN-WT, but not PTEN- 3KR, overexpression upregulated total protein levels of retromer components SNX27 and VPS26. Compared to PTEN-WT, PTEN-3KR also bound less to SNX27 and decreased surface expression of a prototypic retromer cargo GluT1.
These results confirm the importance of SUMOylation and/or ubiquitination in the regulation of PTEN activity. Moreover, they demonstrate a novel role for SUMOylation of PTEN in regulating the retromer complex and protein trafficking.
|Date of Award||23 Mar 2021|
|Supervisor||Michael C Ashby (Supervisor) & Jeremy M Henley (Supervisor)|