A PAb240+ conformation of wild type p53 binds DNA

It is generally accepted that wild type (growth suppressing) p53 is capable of binding to a consensus DNA sequence and is in a conformation recognizable by antibody PAb246 (for murine p53), but not by antibody PAb240. Conversely, mutant forms of p53 incapable of DNA binding often assume conformations that display the PAb240, but not the PAb246 epitope. Exposure of these two epitopes on p53 is therefore believed to be mutually exclusive. We show that wild type p53 translated in vitro in rabbit reticulocyte lysate (RRL) has a PAb240 epitope that is not always cryptic, even on p53 that is bound sequence-specifically to DNA (presumably as a tetramer). All of the DNA-bound, PAb240+ p53 concurrently displays the PAb246 epitope, and both epitopes can be occupied by antibody while p53 is bound to DNA. This novel 'dual positive' conformation also exists in the absence of DNA and suggests that p53 is not necessarily inactive when the PAb240 epitope is displayed. When the C-terminal 58 amino acids of p53 containing the dimer/tetramerization domains are replaced with a heterologous dimerization domain, the resultant dimeric p53 manifests only the PAb246+/PAb240- conformation while bound to DNA. Thus, the C-terminal 58 amino acids of p53 are required for the PAb246+/PAb240+ phenotype, possibly due to tetramerization. This novel 'dual positive' p53 conformation exists in an excess of wild type p53 that has the PAb246-/PAb240+ 'mutant' conformation, suggesting that the 'mutant' conformation is not dominant negative in and of itself.