DNA-binding and transactivation activities are essential for TAp63 protein degradation

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 deltaNp63 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 TAp63gamma can act in trans to promote degradation of mutant TAp63gamma defective in DNA binding, and the TA domain deletion mutant of TAp63gamma 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.     

Pin1 modulates p63α protein stability in regulation of cell survival,proliferation and tumor formation

The homolog of p53 gene, p63, encodes multiple p63 protein isoforms. TAp63 proteins contain an N-terminal transactivation domain similar to that of p53 and function as tumor suppressors; whereas ΔNp63 isoforms, which lack the intact N-terminal transactivation domain, are associated with human tumorigenesis. Accumulating evidence demonstrating the important roles of p63 in development and cancer development, the regulation of p63 proteins, however, is not fully understood. In this study, we show that peptidyl-prolyl isomerase Pin1 directly binds to and stabilizes TAp63α and ΔNp63α via inhibiting the proteasomal degradation mediated by E3 ligase WWP1. We further show that Pin1 specifically interacts with T₅₃₈P which is adjacent to the P₅₅₀PxY₅₄₃ motif, and disrupts p63α–WWP1 interaction. In addition, while Pin1 enhances TAp63α-mediated apoptosis, it promotes ΔNp63α-induced cell proliferation. Furthermore, knockdown of Pin1 in FaDu cells inhibits tumor formation in nude mice, which is rescued by simultaneous knockdown of WWP1 or ectopic expression of ΔNp63α. Moreover, overexpression of Pin1 correlates with increased expression of ΔNp63α in human oral squamous cell carcinoma samples. Together, these results suggest that Pin1-mediated modulation of ΔNp63α may have a causative role in tumorigenesis.     

DNA damage in oocytes induces a switch of the quality control factor TAp63α from dimer to tetramer

TAp63α, a homolog of the p53 tumor suppressor, is a quality control factor in the female germline. Remarkably, already undamaged oocytes express high levels of the protein, suggesting that TAp63α's activity is under tight control of an inhibitory mechanism. Biochemical studies have proposed that inhibition requires the C-terminal transactivation inhibitory domain. However, the structural mechanism of TAp63α inhibition remains unknown. Here, we show that TAp63α is kept in an inactive dimeric state. We reveal that relief of inhibition leads to tetramer formation with ～20-fold higher DNA affinity. In vivo, phosphorylation-triggered tetramerization of TAp63α is not reversible by dephosphorylation. Furthermore, we show that a helix in the oligomerization domain of p63 is crucial for tetramer stabilization and competes with the transactivation domain for the same binding site. Our results demonstrate how TAp63α is inhibited by complex domain-domain interactions that provide the basis for regulating quality control in oocytes.     

Structural diversity of p63 and p73 isoforms

AbstractThe p53 protein family is the most studied protein family of all. Sequence analysis and structure determination have revealed a high similarity of crucial domains between p53, p63 and p73. Functional studies, however, have shown a wide variety of different tasks in tumor suppression, quality control and development. Here we review the structure and organization of the individual domains of p63 and p73, the interaction of these domains in the context of full-length proteins and discuss the evolutionary origin of this protein family. Facts

• Distinct physiological roles/functions are performed by specific isoforms.
• The non-divided transactivation domain of p63 has a constitutively high activity while the transactivation domains of p53/p73 are divided into two subdomains that are regulated by phosphorylation.
• Mdm2 binds to all three family members but ubiquitinates only p53.
• TAp63α forms an autoinhibited dimeric state while all other vertebrate p53 family isoforms are constitutively tetrameric.
• The oligomerization domain of p63 and p73 contain an additional helix that is necessary for stabilizing the tetrameric states. During evolution this helix got lost independently in different phylogenetic branches, while the DNA binding domain became destabilized and the transactivation domain split into two subdomains.

Open questions

• Is the autoinhibitory mechanism of mammalian TAp63α conserved in p53 proteins of invertebrates that have the same function of genomic quality control in germ cells?
• What is the physiological function of the p63/p73 SAM domains?
• Do the short isoforms of p63 and p73 have physiological functions?
• What are the roles of the N-terminal elongated TAp63 isoforms, TA* and GTA?

Subject terms: X-ray crystallography, Solution-state NMR     

p63亚型及其与疾病的相关性

p63足p53家族成员的核转录因子,根据N端及C端的不同,已经发现TAp630α、TAp63β、rap63y、ANp630α、△Np63β、△Np63β、△Np63δ、△Np63δ种亚型。p63的表达受到多种转录因子的调控,其mRNA的稳定性由RNPCI调节,蛋白的稳定性主要由HECT家族成员Itch／AIP4、WWPI调节。p63在上皮细胞分化、组织发育过程中起着关键性作用,因此,p63基因突变可以导致外胚层发育不良的相关疾病,同时,p63在肿瘤的形成和转移的过程中具有重要的调控作用。     

Ionizing radiation-induced TAp63α phosphorylation at C-terminal S/TQ motifs requires the N-terminal transactivation (TA) domain

TAp63α, a homolog of p53 and one of six alternatively spliced p63 isoforms, is a critical mediator of the ionizing radiation (IR)-induced DNA damage response in female germ cells and also tumor suppression in somatic cells. The ΔNp63α isoform, lacking the N-terminal transactivation (TA) domain, is associated with oncogenic potential. The mechanism of p63 functional regulation is not well understood. TAp63α is phosphorylated by ionizing radiation (IR)-induced DNA damage and gene transactivation is likely to be involved. Based on information gleaned from studies on p53, we explored the possibility that TAp63α S/TQ sites may be phosphorylated by IR-induced DNA damage. Our findings show a wortmanin-sensitive kinase phosphorylates TAp63α at C-terminal Ser-Gln and Thr-Gln (S/TQ) sites but not N-terminal S/TQ sites. ΔNp63α, lacking the TA domain, and TAp63γ, lacking C-terminal domains, including S/TQ sites, fail to undergo IR-induced phosphorylation. We propose a model for TA domain-dependent C-terminal phosphorylation drawing from previously described self-inactivating intramolecular interaction between N-terminal TA domain and C-terminal Transactivation Inhibitory Domain (TID) of TAp63α. A specific topology adopted only by TAp63α, but not possible for ΔNp63α or TAp63γ, may lead to TAp63α-specific kinase recruitment, phosphorylation and self-inactivation release. TID-lacking TAp63γ, like p53, is constitutively active and thus may forgo phosphorylation-dependent activation. Thus, p53 is regulated by protein stabilization and TAp63α by protein activation but both appear to involve S/TQ phosphorylation. The difference in phosphorylation potential of TAp63α and ΔNp63α may in part help explain why the two similar isoforms have diametrically opposite tumor suppression and oncogene functions, respectively.