TA*p63 and GTAp63 achieve tighter transcriptional regulation in quality control by converting an inhibitory element into an additional transactivation domain

The p53 homolog p63 plays important roles in development of epithelial tissues and quality control in germ cells. These two functions are executed by two distinct isoforms of p63. They are created by different promotors resulting in isoforms having either an N-terminal transactivation domain (TAp63) or a truncated form (ΔNp63). In addition to these two N-terminal isoforms a third one with an even longer N-terminus, named TA*p63, has been found. A fourth N-terminal isoform, GTAp63, that closely resembles TA*p63 was discovered in male germ cells where it is involved in genetic quality control. Here, we characterize TA*p63α and GTAp63α and show that their N-terminal extensions stabilize the closed and only dimeric conformation adopted by the shorter TAp63α protein. Both proteins can be activated by the two kinases Chk2 and CK1 resulting in the open tetrameric state. In this conformation, the N-terminal extension acts as an additional transactivation domain enhancing transcriptional activity. Through this mechanism, the difference in transcriptional activity between the repressed and the active state of the protein gets enhanced relative to TAp63α. Finally, we show by mass spectrometry that TA*p63α is expressed in the breast cancer cell line Sum159 at the protein level together with mutant p53. Upon doxorubicin treatment, TA*p63α gets activated, providing a potential new tool to fight cancer.