Regulation and effects of IRF-1 and p53 ubiquitination
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Protein ubiquitination is a key regulator of both protein stability and activity, and is involved in the regulation of a vast variety of cellular pathways. The ubiquitination system therefore provides an exciting target for drug development aiming to regulate the function of specific proteins. Our understanding of ubiquitin signalling is far from complete; and if we are to exploit this system for the benefit of human health, it is important to gain a better understanding of this complex posttranslational modification system as well as the effect of ubiquitination on the target protein. The E3 ligases MDM2 and CHIP were implicated in the control of the two transcriptional activators (TAs) IRF-1 and p53, that normally function to maintain health at the cellular and organismal level. Research carried out as part of my PhD has focused on gaining a mechanistic understanding of the ubiquitination process in particular the relationship between the E3 ligase and its substrate. Broadly, the mechanisms of E3 ligase regulation have been linked to substrate specificity and then to the physiological outcome of site-specific ubiquitination of the DNA binding domain of the TAs IRF-1 and p53. More specifically I have; (i) identified a mechanism by which the E3 ligase activity of the CHIP U-box can be allosterically regulated by ligand binding to its TPR domain. (ii) Residues on IRF-1 that are targeted by MDM2 and CHIP have been mapped, revealing that both ligases modify sites exclusively in IRF-1's DNA binding domain (DBD). Furthermore, I showed that, in its DNA bound conformation, IRF-1 is neither bound nor ubiquitinated by the ligases, suggesting a mechanism by which IRF-1 ubiquitination and possibly degradation can be regulated through its DNA binding state. And lastly, (iii) I have shown that both IRF-1 and p53, which have ubiquitin acceptor lysines in their DBD, bind DNA more stably when ubiquitinated. Modelling suggests that interactions between a positively charged surface area of ubiquitin and the negatively charged DNA can stabilises the TA-ubiquitin complex. DBD ubiquitination sites are required for full transactivation potential of both TAs, supporting a role of ubiquitin in their activation. p53 is ubiquitinated in response to activation by IR or Nutlin-3 and these ubiquitinated forms of p53 are localised in the cell nucleus associated with chromatin and do not lead to protein degradation. Taken together, the data imply that p53 and IRF-1 DNA binding ability, and thereby activity, can be modulated by ubiquitin modification.