The tumour suppressor p53 induces apoptosis mainly by acting as a transcription factor and promoting the expression of several proteins that are involved in apoptosis. Reporting in Science, Doug Green and colleagues have now uncovered a more direct route for p53-mediated apoptosis — similarly to BH3-only proteins, cytosolic p53 can directly activate Bax and trigger apoptosis.

To test whether cytosolic p53 can induce the apoptotic programme, Green and co-workers pretreated p53-expressing cells with an inhibitor of nuclear import. The nuclear accumulation of p53 was blocked as evidenced by the lack of UV-induced p53-dependent gene expression. However, UV-induced apoptosis persisted, even in the absence of nuclear p53. Similarly treated Bax−/− cells were resistant to UV-induced apoptosis, which indicates that Bax is required for the induction of apoptosis in the absence of nuclear p53. The inhibition of nuclear import in cells that expressed a transcriptionally inactive form of p53 also resulted in its cytosolic accumulation and cell death — so the cytosolic localization of p53 is sufficient to induce apoptosis.

To clarify the cytoplasmic effects of p53, the authors microinjected purified p53 — wild-type p53, the transcription-deficient p53 mutant or a p53 mutant that lacked the proline-rich domain — into cells that expressed a cytochrome-c–green-fluorescent-protein fusion protein. Wild-type p53 and the transcription-deficient mutant each induced cytochrome-c release from mitochondria, whereas the mutant that lacked the proline-rich domain did not. Green and colleagues next showed that incubating purified p53 with isolated mitochondria induces cytochrome-c release, but only in the presence of Bax, and that the proline-rich domain of p53 is necessary for Bax activation.

Synthetic liposomes can be permeabilized in the presence of Bid — a BH3-only protein — and Bax. The authors observed similar effects when p53 and Bax were added to the lipid vesicles, the permeabilization of which mimics that of the mitochondrial outer membrane. Bid and another BH3-only protein, Bim, activate Bax by causing its oligomerization. Similarly, wild-type p53, but not the mutant that is defective in the proline-rich domain, induced the formation of Bax oligomers. So, the apoptotic function of cytosolic p53 seems to resemble that of BH3-only proteins.

Some BH3-only proteins, including Bid and Bim, bind to Bcl-2 and Bcl-xL. And the latter block apoptosis by sequestering pro-apoptotic Bcl-2-family proteins such as Bax. Other BH3-only proteins, such as Bad, instead promote apoptosis by releasing Bid and Bim from Bcl-2 and Bcl-xL. p53 also binds to Bcl-2 and Bcl-xL, and Green and co-workers showed that the binding of p53 to Bcl-xL released Bax and Bid from Bcl-xL. Again, this confirms the BH3-only-type function of cytosolic p53.

The authors note that this newly discovered, additional function of p53 in apoptosis provides yet another example of “...an emerging complexity that exists within components of the apoptotic machinery”.