Bax and Bak can localize to the endoplasmic reticulum to initiate apoptosis

TLDR

Bax and Bak are essential for apoptosis, acting at the mitochondrial outer membrane and also localizing to the endoplasmic reticulum where ER stress can trigger their conformational changes and oligomerization. In wild‑type cells, ER‑localized Bax/Bak induce caspase‑12 cleavage, and ER‑targeted Bak alone can trigger apoptosis via ER Ca²⁺ depletion, whereas mitochondria‑targeted Bak preferentially activates caspase‑7 and PARP, demonstrating a parallel ER‑dependent caspase pathway.

Abstract

Bax and Bak play a redundant but essential role in apoptosis initiated by the mitochondrial release of apoptogenic factors. In addition to their presence at the mitochondrial outer membrane, Bax and Bak can also localize to the ER. Agents that initiate ER stress responses can induce conformational changes and oligomerization of Bax on the ER as well as on mitochondria. In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells. In bax-/-bak-/- cells, introduction of Bak mutants selectively targeted to either mitochondria or the ER can induce apoptosis. However, ER-targeted, but not mitochondria-targeted, Bak leads to progressive depletion of ER Ca2+ and induces caspase 12 cleavage. In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak. These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.