negaIndividualswith Li-Fraumeni syndrome carry inherited tive p53 mutants can inhibit the function of the normalmutations in the p53 tumor suppressor gene and are p53 protein, usually through protein-protein interactionspredisposed to tumor development. To examine the (Milner et al., 1991). The dominant-negative hypothesismechanistic nature of these p53 missense mutations, is strongly supported by the observations that manywe generated mice harboring a G-to-A substitution at mutant p53 proteins have an increased half-life (Finlaynucleotide 515 of p53 (p53_/515A) corresponding to the et al., 1988; Hinds et al., 1990; Slingerland et al., 1993)p53R175H hot spot mutation in human cancers. Al- and that they oligomerize with wild-type p53, inhibitingthough p53_/515Amice display a similar tumor spectrum its function (Farmer et al., 1992; Jeffrey et al., 1995;and survival curve as p53_/_mice, tumors fromp53_/515A Milner et al., 1991; Sturzbecher et al., 1992). The formamicemetastasized with high frequency. Correspond- tion of mixed wild-type and mutant p53 molecules coningly,the embryonic fibroblasts from the p53515A/515A verts wild-type p53 into themutant conformation in vitromutant mice displayed enhanced cell proliferation, (Milner et al., 1991). Gain-of-function mutations, on theDNA synthesis, and transformation potential. The dis- other hand, are those missense mutations in which muruptionof p63 and p73 in p53_/_ cells increased trans- tant p53 has additional functions not seen in wild-typeformation capacity and reinitiated DNA synthesis to p53. For example, the p53R175H mutant, when overexlevelsobserved in p53515A/515A cells. Additionally, p63 pressed in a nontransformed cell line lacking p53, yieldsand p73 were functionally inactivated in p53515A cells. tumors in nude mice, while the parental cell line doesThese results provide in vivo validation for the gain-of- not (Dittmer et al., 1993). Transgenic mice overexpressfunctionproperties of certain p53missensemutations ing the human p53R175H mutation in epithelial cellsand suggest a mechanistic basis for these pheno- exhibit an increased susceptibility to chemical carcinotypes.

To study the importance of phosphorylation for p53transactivation function, we generated mutations ateach of its known phosphorylated serine amino acids.Mutations of murine p53 serine residues individually toeither alanine or glutamic acid at positions 7, 9, 12, 18,37, 312, and 389 resulted in equivalent levels of transcriptionalactivation in standard transient transfectionexperiments. However, when p53 transcriptional activitywas measured in cells that attain G1 arrest uponcontact inhibition, wild-type p53 was inactive, and onlyalteration at serine 389 to glutamic acid resulted in afunctional p53 protein. This Ser 3 Glu mutant also hasan increased ability to bind DNA. Elimination of thephosphorylation site by substitution of an alanineamino acid resulted in loss of transcriptional activity.We also demonstrated that specific phosphorylation ofp53 at serine 389 is induced by cyclin E overexpressionin high-density cells. Our data establish for the first timethat phosphorylation of p53 at serine 389 is important inactivating its function in vivo.