p63 is a master regulator of epithelial development, driving the expansion of progenitor cells in stratified epithelia. Mutations in p63 are linked to squamous cell carcinomas (SCCs) and basal cell carcinomas (BCCs), as well as to ectodermal dysplasia syndromes such as ectrodactyly ectodermal dysplasia-clefting (EEC) and ankyloblepharon-ectodermal dysplasia-clefting (AEC).
Although p63 acts as a nuclear transcription factor, the process by which it enters the nucleus has not been fully understood. Using a combination of imaging, biochemical, structural, and functional approaches, we have comprehensively characterized how p63 is imported into the nucleus by the importin (IMP) α/β1 complex.
Our results reveal that p63 has developed a unique, mutation-tolerant mechanism to ensure efficient nuclear localization, an essential feature for its DNA-binding and transcriptional functions. We identified a canonical bipartite nuclear localization signal (NLS) positioned between the DNA-binding and oligomerization domains. Interestingly, each of the two basic regions within this NLS can independently engage both IMPα binding sites through p63 homodimerization. As a result, unlike most known bipartite NLSs, only the simultaneous mutation of both basic clusters disrupts nuclear import, impairs IMPα interaction, and reduces transcriptional activity.