The tumor suppressor protein p53 is a phosphoprotein and has growth and transformation suppression functions. Phosphorylation of wild-type p53 is known to modulate its function. To investigate the role of phosphorylation in modulating the functions of mutant p53, we constructed a series of phosphorylation site mutants based on mutant p53 Ala143 (p53-143) and p53 His175 (p53-175). When transfected into p53-negative Saos-2 cells, parental mutant p53-143 and p53-175 abolished both growth suppression and induction of apoptosis. However, DNA-activated protein kinase (DNA-PK) or cyclin-dependent kinase (cdks) phosphorylation site double mutants partially restored the growth suppression and induction of apoptosis and recovered the p53-specific DNA binding activity. We also observed a difference in sensitivity to calpain from parental mutants p53-175 and p53-175/15 or p53-175/315. These results suggest that the lack of phosphorylation at either the DNA-PK or cdks site in p53 mutants partially restores the wild-type functions by altering their conformation.
Copyright 1999 Academic Press.