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Title: Characterising the signalling mechanism of the mTOR-dependent phosphatase
Author: Seymour, Lyndsey A.
ISNI:       0000 0004 2752 0188
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2011
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The mechanistic Target of Rapamycin Complex 1 (mTORC1) complex is central in the regulation of many crucial cellular processes including translation, transcription, proliferation and autophagy. Deregulation of the complex is evident in a number of diseases including Tuberous Sclerosis, Alzheimer's Disease and cancer. Whilst the signalling events leading to activation of mTORC1 are well understood, the inhibitory phosphatase activity that prevents aberrant signalling has received comparatively little attention. In yeast, phosphatases are an integral part of TORC1 signalling. Poor nitrogen supply leads to activation of the phosphatases Pph21/22 and Sit4 and subsequent dephosphorylation of TORC1 substrates. Under these conditions, the phosphatase negative regulatory protein Tap42 is sequestered by Tip41. In good nitrogen supply, TORC1 phosphorylatesTip41 leading to release of Tap42 and subsequent inhibition of Pph21/22 and Sit4. This allows the accumulation of phosphorylated TORC1 substrates. This thesis investigated the role of Tip41 in mTORC1 signalling. Purification of Tip41 identified direct interaction with PP2Ac (human Pph21/22). As overexpression of Tip41 resulted in inhibition of mTORC1 signalling, Tip41 is proposed as a bona fide positive regulatory subunit of PP2Ac. Further investigation indicated that hypophosphorylated PP2A-rjp4i may directly oppose Rheb-mediated activation of mTORC1 thus promoting Raptor degradation. In addition, a specific nuclear isoform of Tip41 was identified, which may specifically regulate the transcription factor HIF1. Studies using the adenoviral protein E40RF4 also identified the PP2ABa complex in regulation of mTORC1 signalling. The data in this thesis show that PP2ABa acts downstream of the TSC 1/2 complex to inhibit mTORC1. Results also indicate that PP2ABa may be negatively regulated by ubiquitin-mediated proteasomal degradation of Ba in an mTORC1-specific manner. Therefore PP2ABa may be subject to an mTORC1 feedback mechanism that is required for activation of downstream substrates. These data indicate that phosphatase activity is critical in regulation of mTORC1, reflecting the mechanism in yeast.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available