NBK/BIK antagonizes MCL-1 and BCL-XL and activates BAK-mediated apoptosis in response to protein synthesis inhibition.

Ribonucleases, antibiotics, bacterial toxins, and viruses inhibit protein synthesis, which results in apoptosis in mammalian cells. How the BCL-2 family of proteins regulates apoptosis in response to the shutoff of protein synthesis is not known. Here we demonstrate that an Escherichia coli toxin, MazF, inhibited protein synthesis by cleavage of cellular mRNA and induced apoptosis in mammalian cells. MazF-induced apoptosis required proapoptotic BAK and its upstream regulator, the proapoptotic BH3-only protein NBK/BIK, but not BIM, PUMA, or NOXA. Interestingly, in response to MazF induction, NBK/BIK activated BAK by displacing it from anti-apoptotic proteins MCL-1 and BCL-X(L) that sequester BAK. Furthermore, NBK/BIK- or BAK-deficient cells were resistant to cell death induced by pharmacologic inhibition of translation and by virus-mediated shutoff of protein synthesis. Thus, the BH3-only protein NBK/BIK is the apical regulator of a BAK-dependent apoptotic pathway in response to shutoff of protein synthesis that functions to displace BAK from sequestration by MCL1 and BCL-X(L). Although NBK/BIK is dispensable for development, it is the BH3-only protein targeted for inactivation by viruses, suggesting that it plays a role in pathogen/toxin response through apoptosis activation.