| The tumor suppressor p53 is found mutated in over half of all human cancers. Following a wide range of cellular stresses, wild-type p53 becomes heavily post-translationally modified and activated to facilitate the transcription of genes involved in senescence, DNA repair, cell-cycle arrest, and apoptosis. Its role as a transcription factor is underscored by the large majority of tumor-derived mutations found within the core domain that impair its ability to recognize target gene promoters. As of yet, it is unclear exactly how p53 discernes between its wide repetoir of activities; a combination of the effects of modifying kinases, interacting proteins, and conformational changes are thought to direct the ability of p53 to recognize various DNA sequences and impart its effects upon the cell.; One region of the core domain that is conformationally flexible is the L1 loop (residues 112-124). This span of amino acids is rarely found associated with cancer, often arises in yeast screens for mutants with unique properties, and contains a residue that makes a direct DNA base contact. Our mutational analysis of the L1 loop found a number of mutants that imparted both enhanced DNA affinity and altered sequence specificity in vitro. Further characterization of these mutations in vivo demonstrated an important, yet highly modular role for the L1 loop in p53's recogniztion of varying DNA sequences, target gene transactivation, and apoptotic signalling.; Outside of the core domain, the effects of modification on the C-terminus of p53 remains a contentious subject. We mutated various combinations of lysines to either block all modification (arginine) or to mimic acetylation (glutamine) and examined their effects. Surprisingly, arginine mutation of the two lysines in the tetramerization domain did not affect DNA binding or transactivation, yet impaired p53's apoptotic ability. Glutamine substitution, on the other hand, drastically hindered transcription and arrest, yet still permitted apoptosis. These findings point to the possibility of novel modifications within the tetramerization domain that can dictate the choice between p53's cell cycle arrest or apoptotic functions. |
