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     

p73 and p63: Estranged relatives?

Comment on: Holembowski L, et al. Cell Cycle 2011; 10:680-9.     

From p63 to p53 across p73

Most genes are members of a family. It is generally believed that a gene family derives from an ancestral gene by duplication and divergence. The tumor suppressor p53 was a striking exception to this established rule. However, two new p53 homologs, p63 and p73, have recently been described [1, 2, 3, 4, 5 and 6]. At the sequence level, p63 and p73 are more similar to each other than each is to p53, suggesting the possibility that the ancestral gene is a gene resembling p63/p73, while p53 is phylogenetically younger [1 and 2].

The complexity of the family has also been enriched by the alternatively spliced forms of p63 and p73, which give rise to a complex network of proteins involved in the control of cell proliferation, apoptosis and development [1, 2, 4, 7, 8 and 9].

In this review we will mainly focus on similarities and differences as well as relationships among p63, p73 and p53.     

p63 and p73: roles in development and tumor formation

The tumor suppressor p53 is critically important in the cellular damage response and is the founding member of a family of proteins. All three genes regulate cell cycle and apoptosis after DNA damage. However, despite a remarkable structural and partly functional similarity among p53, p63, and p73, mouse knockout studies revealed an unexpected functional diversity among them. p63 and p73 knockouts exhibit severe developmental abnormalities but no increased cancer susceptibility, whereas this picture is reversed for p53 knockouts. Neither p63 nor p73 is the target of inactivating mutations in human cancers. Genomic organization is more complex in p63 and p73, largely the result of an alternative internal promoter generating NH2-terminally deleted dominant-negative proteins that engage in inhibitory circuits within the family. Deregulated dominant-negative p73 isoforms might play an active oncogenic role in some human cancers. Moreover, COOH-terminal extensions specific for p63 and p73 enable further unique protein-protein interactions with regulatory pathways involved in development, differentiation, proliferation, and damage response. Thus, p53 family proteins take on functions within a wide biological spectrum stretching from development (p63 and p73), DNA damage response via apoptosis and cell cycle arrest (p53, TAp63, and TAp73), chemosensitivity of tumors (p53 and TAp73), and immortalization and oncogenesis (DeltaNp73).     

p63 and p73: Life and Death in Squamous Cell Carcinoma

The p53 family member p63 plays an essential role in the developing epithelium, and overexpression of the ΔNp63a isoform is frequently observed in human squamous cell carcinomas (SCCs). These findings have suggested that ΔNp63a might function as an oncogene within squamous epithelial cells. Nevertheless, the mechanism by which ΔNp63a might promote tumorigenesis remains poorly understood, and data from mouse models implies that the p63 locus might in fact function as a tumor suppressor in these same tissues. A recent study using RNA interference in human SCC-derived cell lines shows that ΔNp63a mediates an essential survival function in human SCC cells by virtue of its ability to suppress the pro-apoptotic function of the related p53 family member p73. These findings support an oncogenic role for ΔNp63a and they demonstrate the existence of critical physical and functional interactions between endogenous p53 family members in human cancer. Specific chemotherapeutic agents and future targeted approaches may be able to exploit this pathway to therapeutic advantage.     

p53 Family members p63 and p73 are SAM domain-containing proteins.

Homologs of the tumor suppressor p53, called p63 and p73, have been identified. The p63 and p73 family members possess a domain structure similar to p53, but contain variable C-terminal extensions. We find that some of the C-terminal extensions contain Sterile Alpha Motif (SAM) domains. SAM domains are protein modules that are involved in protein-protein interactions. Consistent with this role, the C-terminal SAM domains of the p63 and p73 may regulate function by recruiting other protein effectors.     

p73 and p63: Why Do We Still Need Them?

When p73 and p63 were initially described as homologues of the tumor suppressor p53, the three family members seemed almost exchangeable, raising the question why all three were retained during evolution. It later turned out that the corresponding genes, TP63 and TP73, appear phylogenetically older than TP53, and that their targeted deletion causes severe developmental defects, in contrast to a deletion of TP53. Hence, p63 and p73 are responsible for biological effects that cannot be elicited by p53 alone. Here, we provide an overview of properties ascribed to p63 and p73 that distinguish them from p53. Differences occur at the following levels: i) protein structure, especially with regard to the aminoterminal transactivation domains and the carboxyterminal portions unique to p63 and p73; ii) regulation, affecting mRNA levels, posttranslational modifications and interaction with other cellular proteins; iii) activities, resulting in the regulation of gene expression, the programming of development, and the emergence of tumors. We speculate that, during the course of evolution, p63 and p73 have first pursued a broader range of activities, whereas p53 later specialized on genome maintenance.     

p63/p73 in the control of cell cycle and cell death

The p53 family apparently derives from a common ancient ancestor that dates back over a billion years, whose function was protecting the germ line from DNA damage. p63 and p73 would maintain this function through evolution while acquiring novel roles in controlling proliferation and differentiation of various tissues. p53 on the other hand would appear in early vertebrates to protect somatic cells from DNA damage with similar mechanism used by its siblings to protect germ line cells. For the predominant role played by p53 mutations in cancer this was the first family member to be identified and soon became one of the most studied genes. Its siblings were identified almost 20 years later and interestingly enough their ancestral function as guardians of the germ-line was one of the last to be identified. In this review we shortly summarize the current knowledge on the structure and function of p63 and p73.     

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.