Effects of stability on the biological function of p53.

The core domain of the tumor suppressor p53 has low thermodynamic stability, and many oncogenic mutations cause it to denature rapidly at body temperature. We made a series of core domain mutants that are significantly less or more stable than wild type to investigate effects of stability on the transcriptional activity and levels of native full-length p53 in H1299 mammalian cells. The levels of transcriptionally inactive native protein with inactivating mutations in the N-terminal transactivation domain correlated strongly with stability. The levels of transcriptionally active proteins, however, depended on both their stability and the transcriptional activity that leads to the feedback loop of proteolytic degradation via transcription of E3 ligases. A very highly stabilized quadruple mutant and an even more stable hexamutant were more active than wild-type p53 in terms of Bax transcription and apoptotic activity, and reached higher levels than wild type in cells. The increased activity did not result from increased overall stability but was due to a single known suppressor mutation, N239Y. It is possible that the low intrinsic stability of p53 is a means of keeping its level low in the cell by spontaneous denaturation, by a route additional to that of proteolytic degradation via E3 ligase pathways. Denatured p53 does accumulate in cells, and there are pathways for the proteolysis of denatured proteins.