Specific inhibition of Mdm2-mediated neddylation by Tip60

Tip60 is a histone acetyl transferase (HAT) and a cofactor of transcription, but also an interaction partner of the Mdm2 oncoprotein. The functional consequences of this interaction are only partially understood and were further explored in this study. We found that Tip60 is capable of selectively inhibiting the Mdm2-mediated conjugation of Nedd8 to p53, whereas it did not affect p53 ubiquitination. In contrast, the known Mdm2 antagonist p14arf preferentially blocked Ubiquitin conjugation by Mdm2. To identify underlying mechanisms, we studied the intracellular localization of Tip60 and Mdm2. Both proteins relocalized each other to the PML nuclear bodies, but a similar localization pattern was observed even in the absence of PML. Analysis of Tip60 deletion mutants revealed that some mutants, while still interacting with Mdm2, failed to relocalize it and to inhibit Mdm2-mediated neddylation, suggesting that these two phenomena require biochemical activities in addition to the mere interaction between the two proteins. For both activities, the HAT domain of Tip60 was not required. We propose that Tip60 can act as a selective antagonist to Mdm2-mediated neddylation but not ubiquitination. Hence, the two different E3 ligase activities of Mdm2 can be regulated individually.