DNA-Binding and Transactivation Activities Are Essential for TAp63 Protein Degradation

Abstract

The p53-related p63 gene encodes six isoforms with differing N and C termini. TAp63 isoforms possess a transactivation domain at the N terminus and are able to transactivate a set of genes, including some targets downstream of p53. Accumulating evidence indicates that TAp63 plays an important role in regulation of cell proliferation, differentiation, and apoptosis, whereas transactivation-inert ΔNp63 functions to inhibit p63 and other p53 family members. Mutations in the p63 gene that abolish p63 DNA-binding and transactivation activities cause human diseases, including ectrodactyly ectodermal dysplasia and facial clefting (EEC) syndrome. In this study, we show that mutant p63 proteins with a single amino acid substitution found in EEC syndrome are DNA binding deficient, transactivation inert, and highly stable. We demonstrate that TAp63 protein expression is tightly controlled by its specific DNA-binding and transactivation activities and that p63 is degraded in a proteasome-dependent, MDM2-independent pathway. In addition, the N-terminal transactivation domain of p63 is indispensable for its protein degradation. Furthermore, the wild-type TAp63γ can act in trans to promote degradation of mutant TAp63γ defective in DNA binding, and the TA domain deletion mutant of TAp63γ inhibits transactivation activity and stabilizes the wild-type TAp63 protein. Taken together, these data suggest a feedback loop for p63 regulation, analogous to the p53-MDM2 feedback loop.

ACKNOWLEDGMENTS

We thank Guillermina Lozano for wild-type and p53^−/− MDM2^−/− MEF and Qiang Yu for U2-OS-Tet cells. We thank Gail Sonenshein, Yujun Zhang, Michael Sherman, and Duc Tran for technical assistance on EMSA, construction of chimeric proteins, and immunofluorescence analysis.

This work was supported by Public Health Service grant GM-70017 (to Z.-X.J.X.) from the National Institute of General Medical Sciences.