p53-Ubl fusions as models of ubiquitination, sumoylation and neddylation of p53.

The tumour suppressor p53 has been shown to be modified at its C-terminus with ubiquitin and the ubiquitin-like proteins SUMO and NEDD8. Whereas monoubiquitination of p53 is strongly associated with nuclear export, the effects of sumoylation and neddylation remain unclear. In this study we have generated p53-Ub, p53-SUMO-1 and p53-NEDD8 fusion proteins as models for the effect of these modifications on the localization and function of p53. As shown before, the ubiquitin fusion clearly drives nuclear export of p53 and we now find that this is also partially the case for a SUMO-1 fusion, which does not localise to PML bodies. In contrast a NEDD8 fusion has little effect on nuclear export, and mutating NEDD8 to more closely resemble ubiquitin did not restore nuclear export. Despite their differing subcellular localization, we find that both p53-ubiquitin and p53-NEDD8 retain similar transcriptional activity and both induce apoptosis at a similar level to unfused p53. The p53-ubiquitin fusion protein is potentially a good model for studying the role of p53 outside the nucleus. However, in our experiments we find that the export of p53 from the nucleus is not sufficient to activate its cytoplasmic apoptotic function which may depend on the ability to deubiquitinate cytoplasmic p53.