Phylogeny and Function of the Invertebrate p53 Superfamily

The origin of the p53 superfamily predates animal evolution and first appears in unicellular Flagellates. Invertebrate p53 superfamily members appear to have a p63-like domain structure, which seems to be evolutionarily ancient. The radiation into p53, p63, and p73 proteins is a vertebrate invention. In invertebrate models amenable to genetic analysis p53 superfamily members mainly act in apoptosis regulation in response to genotoxic agents and do not have overt developmental functions. We summarize the literature on cnidarian and mollusc p53 superfamily members and focus on the function and regulation of Drosophila melanogaster and Caenorhabditis elegans p53 superfamily members in triggering apoptosis. Furthermore, we examine the emerging evidence showing that invertebrate p53 superfamily proteins also have functions unrelated to apoptosis, such as DNA repair, cell cycle checkpoint responses, compensatory proliferation, aging, autophagy, and innate immunity.The vertebrate p53 family of proteins consists of three members, p53, p63, and p73. p53 has received considerable attention because of the fact that it is mutated in approximately 50% of all human cancers and plays an important role in protecting cells against DNA damage and cellular stressors. p63 and p73 on the other hand, seem to be less involved in tumorigenesis but play important roles in epithelial development and neurogenesis, respectively. p53 related sequences also exist in invertebrate species. We review the functional data on invertebrate p53 superfamily proteins, largely focusing on the model organisms, Caenorhabditis elegans and Drosophila melanogaster. Invertebrate p53 superfamily members act in apoptosis regulation in response to genotoxic agents and the deletion of invertebrate p53 superfamily proteins does not lead to overall developmental defects. Nevertheless, there is emerging evidence that invertebrate p53-like proteins also have functions unrelated to apoptosis.There has been a debate whether invertebrate p53 superfamily proteins are phylogenetically more related to vertebrate p53 or p63. Taking advantage of recent genome sequencing projects, we analyze the phylogenetic relationships of the p53 superfamily from vertebrates and invertebrates. Consistent with previous reports, our phylogenetic analysis supports the conclusion that a p63-like domain structure is evolutionarily more ancient. It thus appears that a protein with a p63-like domain structure originally evolved, possibly to mediate apoptosis of damaged cells. In vertebrates, this earlier role of p53-like proteins is largely performed by p53. However, it appears that p63 has maintained the evolutionary ancient role of apoptosis in the female germline (Suh et al. 2006)