p73 and p63 Are Homotetramers Capable of Weak Heterotypic Interactions with Each Other but Not with p53

Abstract

Mutations in the p53 tumor suppressor gene are the most frequent genetic alterations found in human cancers. Recent identification of two human homologues of p53 has raised the prospect of functional interactions between family members via a conserved oligomerization domain. Here we report in vitro and in vivo analysis of homo-and hetero-oligomerization of p53 and its homologues, p63 and p73. The oligomerization domains of p63 and p73 can independently fold into stable homotetramers, as previously observed for p53. However, the oligomerization domain of p53 does not associate with that of either p73 or p63, even when p53 is in 15-fold excess. On the other hand, the oligomerization domains of p63 and p73 are able to weakly associate with one another in vitro. In vivo co-transfection assays of the ability of p53 and its homologues to activate reporter genes showed that a DNA-binding mutant of p53 was not able to act in a dominant negative manner over wildtype p73 or p63 but that a p73 mutant could inhibit the activity of wild-type p63. These data suggest that mutant p53 in cancer cells will not interact with endogenous or exogenous p63 or p73 via their respective oligomerization domains. It also establishes that the multiple isoforms of p63 as well as those of p73 are capable of interacting via their common oligomerization domain.Tumor suppressor p53 is a transcriptional regulator of genes involved in control of the cell cycle and/or apoptosis (reviewed in Refs. 1-3). In response to cellular stress, particularly DNA damage, p53 protein levels rise, leading to changes in expression of p53 responsive genes and subsequent cell cycle arrest and/or apoptosis. Growth arrest or cell death prevents damaged DNA from being replicated and suggests a role for p53 in maintaining the integrity of the genome (4). This DNA damage checkpoint activity is central to its role as a tumor suppressor and also of major importance in the response of many cancers to conventional therapies that trigger apoptosis by damaging DNA. Inactivation of p53 through deletion, mutation, or interaction with cellular or viral proteins is a key step in over half of all human cancers (5, 6). Reactivation of the p53 pathway leading to apoptosis in cancer cells containing inactive p53 could greatly improve current modes of treatment (7,8) and could potentially be a treatment modality in itself (9, 10).Recently, two p53 homologues have been identified that can activate some of the same target genes as p53 and can induce apoptosis. The existence of these new proteins adds new complexity to our understanding of the p53 pathway and offers new potential for its reactivation in cancer cells. Human p73 (11, 12) and p63 (13-15) share regions of homology with the activation, DNA-binding, and oligomerization domains of p53. This raises the possibility of physical, and/or genetic interactions between p53 family members (13), as is often the case within other families of transcription factors such as the homeodomain proteins (16) or the Myc superfamily (17). Both p63 a...

Transcriptional activities of p73 splicing variants are regulated by inter-variant association

“…This suggested that at least some p73α forms a complex with p73β. Moreover, it was also reported that the purified recombinant polypeptide of the putative p73 OD (amino acids 345-383) associated with itself, although it did not interact with the p53 OD [19]. As the p73 OD used in that study is present in all p73 variants, it is likely that p73 variants can interact with each other.…”

Mutant p53 subverts p63 control over KLF4 expression in keratinocytes

“…The first is the interplay between p53 and p63/p73: although there is little evidence of heterotetrameric formation of p53-p63 dimers (Davison et al, 1999), several reports have indicated that p63/p73 have an increased affinity for p53 missense mutants (Di Como et al, 1999;Strano et al, 2000;Gaiddon et al, 2001). Specifically, p63 was reported to associate with the p53 mutants of HaCaT cells (Gaiddon et al, 2001).…”

Mechanisms of regulatory diversity within the p53 transcriptional network

“…For example, p63/p73 may bind to REs not efficiently bound by p53 and recruit the latter via protein-protein interactions. However, oligomerization studies discourage this possibility, as p63 and p73 seem capable of heterotypic interactions with each other but not with p53 (Davison et al, 1999). Alternatively, a 'priming' model could be envisioned, in which p63/p73 could bind to a given chromatin-embedded RE not accessible to p53, and then modify said RE in such a way that it would now become 'visible' to p53.…”