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An investigation of post-translational modification of human papillomavirus E4 and the role of modified E4 proteins during the virus life cycle

Pugh, Alice Georgia (2010)
Ph.D. thesis, University of Birmingham.

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Infection with human papillomaviruses (HPV) is the etiological basis for cervical cancer. The HPV E1^E4 protein is proposed to be a major regulator of the HPV life cycle and the multiple and diverse activities associated with E1^E4 suggest that it is a multi-functional protein. This thesis has sought to address the hypothesis that phosphorylation and proteolysis contribute towards the pleiotrophic functions of the E1^E4 protein, and has investigated the functional significance of these E1^E4 post-translational modifications during the HPV life cycle. This study has uncovered the novel finding that the E1^E4 protein of HPV type 18 (HPV18) exists as a phospho-protein within cells and is a substrate for multiple cellular kinases in vitro. The phospho-acceptor residue for cyclin-dependent kinases (CDK) 1 and 2 has been identified as threonine 23, whilst serine 58 is phosphorylated by protein kinase A (PKA). Furthermore, a cyclin binding motif (\(^{43}\)RRL\(^{45}\)) within the HPV18 E1^E4 protein is required for association with active CDK complexes and this association may influence CDK activity since the activity of CDK2-cyclin A was shown to be reduced in the presence of HPV18 E1^E4. This thesis has revealed that HPV18 E1^E4 is a target for N-terminal proteolysis, and this post-translational modification occurs during the HPV18 replication cycle. Key elements necessary for proteolysis have been mapped to a conserved leucine-rich sequence (LLXLL) present at the N-terminus of the HPV18 E1^E4 protein. Since E1^E4 expression is required for HPV18 genome amplification, N-terminally truncated E4 species may contribute towards its role in the replication cycle. To examine this hypothesis, mutations that attenuate E1^E4 proteolysis were introduced into HPV18 genomes and transfected into human foreskin keratinocytes (HFK). Mutation of the leucine-rich motif prevented efficient proteolysis of the E1^E4 protein during the HPV life cycle and resulted in reduced viral genome amplification within differentiating HFKs suggesting that efficient E1^E4 proteolysis may be required for this E4 function.

Type of Work:Ph.D. thesis.
Supervisor(s):Roberts, Sally and Knight, Gillian
School/Faculty:Colleges (2008 onwards) > College of Medical & Dental Sciences
Department:School of Cancer Sciences
Subjects:R Medicine (General)
RC0254 Neoplasms. Tumors. Oncology (including Cancer)
Institution:University of Birmingham
ID Code:823
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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