The tumor suppressor p53 is a key regulator of apoptosis induced by various cellular stresses. p53 can induce apoptosis by two mechanisms. First, p53 acts as a transcription factor inducing and repressing pro-apoptotic and anti-apoptotic targets genes, respectively. Second, p53 is able to translocate to the mitochondria, where it interacts with BCL-2 family members to induce membrane permeabilization and cytochrome c release. p53 transcriptional activity is regulated by a set of post-translational modifications that have been well documented. However, how these modifications impact the direct mitochondrial pathway of death remain poorly understood. In this study, we focused on the role of serine 392 phosphorylation in the control of p53-dependent apoptosis. We used CRISPR/Cas9 genome editing to substitute serine 392 by a non-phosphorylatable alanine in HCT-116 colon carcinoma cells. The S392A mutant displayed normal transcriptional activity following genotoxic stress, but markedly impaired ability to localize to mitochondria. The decreased mitochondrial localization of the S392A mutant correlated with a lower ability to induce apoptosis. Confirmatory observations were made following enforced expression of the S392A p53 mutant or a phospho-mimetic S392E mutant in H1299 lung carcinoma cells. Our observations support the premise that serine 392 phosphorylation of p53 influences its mitochondrial translocation and transcription-independent apoptotic function.

Following a genotoxic stress, the tumor suppressor p53 translocates to mitochondria to take part in direct induction of apoptosis, via interaction with BCL-2 family members such as BAK and BAX. We determined the kinetics of the mitochondrial translocation of p53 in HCT-116 and PA-1 cells exposed to different genotoxic stresses (doxorubicin, camptothecin, UVB). This analysis revealed an early escalation in the amount of mitochondrial p53, followed by a peak amount and a decrease of mitochondrial p53 at later time points. We show that the serine 20 phosphorylated form of p53 is present at the mitochondria and that the decrease of p53 mitochondrial level during late apoptosis correlates with a decrease of Ser-20 phosphorylation. Moreover, the S20A p53 mutant translocates well to mitochondria after a genotoxic stress but its mitochondrial localization is very low during late apoptosis when compared to wt p53. The S20A mutant also appears to be compromised for interaction with BAK. We propose here that the level of serine 20 phosphorylation is influential on p53 mitochondrial localization during late apoptosis. Additionally, we report the presence of a new '45 kDa caspase-cleaved fragment of p53 in the cytosolic and mitochondrial fractions of apoptotic cells.

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