Isoform-specific cofactor recruitment through the intrinsically disordered N-terminus of p63 underlies differential transcriptional activities

The transcription factor p63 is critical for epithelial development and implicated in tumorigenesis. However, our understanding of the role of p63 in development and disease has been complicated by its diverse isoforms. As a member of the p53 family member of genes, TP63 encodes for numerous isoforms, including the N-terminal variants TAp63 and {Delta}Np63, which are generated through alternative promoter usage. TAp63 and {Delta}Np63 share various structural domains, including the DNA-binding domain, and primarily differ in their N-terminus which consists of intrinsically disordered regions (IDRs). The isoforms are known to have different functions, including tumor suppression in the case of TAp63 and pro-tumor formation for {Delta}Np63, but how the N-terminus contributes to isoform-specific gene regulatory effects has yet to be elucidated. Using both genomic and TurboID proximity-labeling proteomic approaches, we show that the N-terminus mediates differential interactions with cofactors that have direct effects on isoform function, specifically the regulation of apoptosis. We find that the N-terminus of TAp63 interacts with more transcriptional machinery, leading to stronger transcriptional activity by TAp63 than {Delta}Np63. However, {Delta}Np63 maintains interactions with coactivators, suggesting it can retain some transactivation capabilities. Strikingly, the N-terminus of TAp63 displays enriched interactions with chromatin modifiers, including the histone acetyltransferase KAT2A, that result in TAp63-specific binding at inaccessible sites. We find that an IDR-mediated interaction with KAT2A is involved in regulation of apoptosis by TAp63. Collectively, our results suggest a model in which TAp63 and {Delta}Np63 broadly share genomic occupancy, but differential interactions with cofactors contribute to isoform-specific regulation by TAp63 and {Delta}Np63.