THE CARBOXY-TERMINAL SERINE-392 PHOSPHORYLATION SITE OF HUMAN P53 IS NOT REQUIRED FOR WILD-TYPE ACTIVITIES

Wild-type p53 functions in the G1 DNA damage checkpoint pathway by act ivating gene transcription and preventing cell cycle progression. Othe rs reported that mutation of the serine 386 codon in mouse p53 abolish ed its ability to suppress growth. Serine 386 of murine p53 and the ho mologous residue of human p53, serine 392, are phosphorylated in vivo and can be phosphorylated in vitro by casein kinase II (CKII). We cons tructed mutants that changed serine 392 of human p53 to alanine (p53-S 392A) or aspartic acid (p53S392D); cotransfection of both these mutant s with a reporter gene carrying a p53-responsive element into the p53- null Saos-2 cell line activated transcription as well as did wild-type p53. Furthermore, both mutants blocked cell cycle progression after t ransient transfection in these cells. A stable derivative of the T98G human glioblastoma cell line was established that expressed p53-S392A in response to dexamethasone. Overexpression of this mutant activated transcription of the endogenous waf1 (also called cip1) and mdm2 genes to the same extent as wild-type p53 and also produced growth arrest. Finally, p53-S392A and p53-S392D suppressed foci formation by activate d ras and adenovirus EIA oncogenes as efficiently as did wild-type p53 . Thus, unlike mutants that altered the serine 15 phosphorylation site , elimination of the serine 392 phosphorylation site had no discernibl e effect on p53 function. We conclude that neither phosphorylation nor RNA attachment to serine 392 are required for human p53's ability to suppress cell growth or to activate transcription in vivo.