6-mediated degradation, we observed that ” multiple point mutagenesis of the strech 99107 was sufficient to prevent p53 degradation in the presence of HPV16 E6 both in vitro and in vivo. In the structure of p53 core domain Y103 and Y107 are in contact with L264 and L265 situated in the linker between S9 and S10 segments. The single point mutation of Y107 to glycine protected p53 from HPV16 E6-mediated degradation, probably due to the alteration of the fold of the core domain. Indeed, mutant Y107G failed to transactivate the p21 promoter and it did not bind to the Pab 1620 antibody that selectively recognises the folded conformation of p53, whereas it bound to the Pab 240 antibody directed against a buried epitope on b-strand S7, which becomes accessible upon Neuromedin N site unfolding of this region. On the other hand, replacement of Y103 by glycine was found to protect from the E6-mediated degradation, and although it impacted on the conformation of the resulting protein, it did not disrupt its ability to fold. The replacement of amino acid L264, reported to be implicated 
in the ubiquitination of p53 by MDM2, to alanine had no effect on p53 degradation in the presence of HPV16 E6 or MDM2. However, replacement of L265 to alanine protected p53 from HPV16 E6-mediated degradation in vitro. Interestingly, when transiently expressed in H1299 p53-null cells, p53-L265A mutant retained its susceptibility to MDM2-mediated degradation but resisted to HPV16 E6-mediated degradation and displayed an impaired transcriptional activation capacity. Further investigation using Pab 240 and Pab 1620 showed that p53L265A had a “mutant”conformation. Both CD and NMR results showed that the p53core-L265A had a mainly unfolded conformation. Taken together these results show that a p53 mutant possessing an unfolded core domain remains capable of interacting with and be degraded by MDM2. However, we noticed that p53L265A, together with p53-Y103G and p53-Y107G, were less October 2011 | Volume 6 | Issue 10 | e25981 p53 Degradation Mediated by HPV E6 efficiently degraded by MDM2 as compared to p53 wild-type. This may be explained by a decreased binding of MDM2 to its putative target site within the core domain, which may influence the ubiquitination level of p53. Alternatively, the unfolding or conformational alteration of the core domain due to these mutations might provoke some aggregation of the mutants in vivo, thereby decreasing the efficiency of their recognition and degradation by MDM2. p53 is subjected to an array of post-translational modifications that regulate its stability and consequently half-life that in turn influences the expression of p53 target genes. In particular phosphorylation generally results in its stabilization. It has been reported that the phosphorylation status on T155 is important for the E6-mediated degradation of p53 in HeLa cells. We observed that the replacement of T155 by aspartate protected p53 from HPV16 E6-mediated degradation but not when T155 was substituted by alanine or valine, and that all mutant proteins were still degraded by MDM2. When transfected in H1299 cells, p53-T155A and p53-T155V transactivated the p21 promoter but not p53-T155D. These data suggested a possible effect of the p53-T155D mutation on p53 fold. Indeed, while p53-T155A and, to some extent p53-T155V, were October 2011 | Volume 6 | Issue 10 | e25981 p53 Degradation Mediated by HPV E6 recognised by the Pab 1620 as the wild-type core domain, p53T155D mutant was recognised by