Direct interaction between mortalin and HIF-1α at the mitochondria inhibits apoptosis by blocking recruitment of Bax.

Hypoxia Inducible Factor 1, a heterodimer of alpha (HIF-1α) and beta (HIF-1β or ARNT) subunits, is a major regulator of the transcriptional response to hypoxia. However, HIF-1α, the oxygen-regulated subunit, also exerts non-transcriptional functions through interaction with proteins other than ARNT. We have previously shown that the subcellular localization and protein interactions of HIF-1α are controlled by ERK-mediated phosphorylation at Ser641/643. When HIF-1α is modified at these sites, it is nuclear, binds to ARNT, interacts with NPM1, and activates transcription of hypoxia-target genes. On the other hand, unmodified HIF-1α is bound by CRM1, exits the nucleus and, via its association with mortalin, is targeted to the mitochondria to form an anti-apoptotic complex. To further characterize the latter function, recombinant fragments of HIF-1α and mortalin were used for in vitro binding assays and immunoprecipitation experiments to map the respective binding sites and show that their interaction is direct and functional. We could also show that embelin, a natural product and known inhibitor of the mortalin-p53 interaction, also disrupts the mortalin-HIF-1α association and, furthermore, removes unmodified HIF-1α from mitochondria. Mitochondrial dissociation of HIF-1α, either by embelin or overexpression of a HIF-1α peptide harboring the mortalin binding site, under stress conditions leads to mitochondrial localization of the pro-apoptotic protein Bax and induction of apoptosis. We suggest that when ERK activity is low under hypoxia, binding of HIF-1α to mortalin inhibits mitochondrial recruitment of Bax and protects cells from apoptotic cell death.