TI - PHOSphorylation of p53 at Ser46 is essential for counteracting Zfra-mediated apoptosis . AB - Similarly , overexpressed p53 and Zfra nullified each other's effect in inducing death of COS7 cells ( Figure 11 ) . Zfra (S8G) mutant lost its apoptotic function ( Figures 2 and 11 ) , and had no effect in counteracting p53-mediated cell death ( Figure 11 ) . Mutant p53 (S46G) had a reduced activity in causing cell death , and interestingly this mutant also inhibited Zfra's apoptotic function ( Fig 11 ) . The extent of protein expression was more than 70% , as determined by fluorescence microscopy .