One billion years of p53/p63/p73 evolution.

T he report in this issue of PNAS by Joerger et al. (1) solves the structure of the p73 oligomerization domain that produces a tetrameric protein and compares this structure with p63 (closely related) and p53 (more distantly related). This provides new information about the comparative evolution of these three (p53, p63, and p73) proteins and more. The human genome contains three genes encoding the transcription factors p53, p63, and p73, which are closely related paralogs with some similar and some diverse functions (Fig. 1A). p53 acts in somatic cells in response to stresses (such as DNA damage) that reduce the fidelity of DNA replication and cell division, by initiating apoptosis, senescence or cell cycle arrest, providing a tumor suppressor function (2). Humans heterozygous for a p53 mutation develop a variety of cancers at early ages (3). p63 is one of the major transcription factors required for the development of epithelial cell layers. Humans carrying a mutation in one allele of this gene can have cleft palate, skeletal abnormalities, and skin pathologies (EEC syndrome), but they do not develop cancers at high rates (4, 5). p73 is involved in the development of the central nervous system and the immune system. It also responds to a subset of stress signals observed with p53, initiating apoptosis, and in this sense p73 can back up p53 in response to DNA damage (6). Studies with knock-out mice have indicated that p53, p63, and p73 can also play a role in germ-line surveillance of fidelity (7–9), and p63 and p73 may be involved in aspects of tumor suppression. p53 also is required for efficient implantation of embryos into the uterus in mice (10) and humans (11). It is the germ-line surveillance and implantation functions that result in strong evolutionary selection pressures on this family of genes (12). The p53, p63, and p73 proteins are structurally related. They all have an amino-terminal transcriptional trans-activation domain linked to a DNA-binding domain, which is followed by an oligomerization domain (a tetramer in vertebrates) and a carboxylterminal regulatory region (Fig. 1 A). p63 and p73 have an extra SAM (sterile alpha motif) domain, which confers protein stability. The DNA-binding domains of p53, p63, and p73 have been conserved over long evolutionary time frames, with the three human paralogs sharing 55–87% homology, and the Drosophila gene (dmp53) having 25% homology, whereas the C. elegans gene (Cep-1) has 15% homology at the amino acid level (12). Both invertebrate and vertebrate proteins recognize and bind to the same DNA sequences that regulate some similar genes in these very diverse species, leading to apoptosis after a DNA-damaging event. dmp53 and Cep-1 play a central role in germ-line fidelity in response to a variety of stress signals (DNA damage, starvation, etc.). Most invertebrates harbor a single p63/p73-like gene, with the earliest organism observed during evolution, the sea anemone, expressing this ancestor gene in its germ cells. When exposed to ultraviolet light, which happens when these organisms feed at the surface of the water, the p63/73 ancestor gene initiates apoptosis to protect the germ line from mistakes (12). This means that the structure and the functions of the p53 sister genes have been preserved for about one billion years. The first indication that this ancestor p63/p73 gene duplicated, separating into distinct entities, is in the cartilaginous fish (shark), where a new p53 gene is observed along with a single p63/p73 ancestor gene (12). By the appearance of bony fish (zebra fish) all three genes, p53, p63, and p73, are present and p53 has taken on its new functions ensuring the fidelity of somatic cell division, an adaptation of the p63/ p73 ancestor genes’ role in the germ line (12). Within the higher vertebrates, p63 and p73 have taken on new functions in development of tissues and organs, whereas p53 has become the guardian of the somatic genome and a tumor suppressor. With the advent of employing large numbers of stem cells and tissue regeneration as a strategy for an organism’s growth, development, and maintenance, there is a greater need for stem cell surveillance to prevent cancers from arising. p53 evolved to fill this role. It is