ALTERNATIVELY SPLICED BNIP3 ISOFORM PROMOTES CELL SURVIVAL BY RECRUITING BCL‐2 TO THE IP3‐RECEPTOR

Abstract

Alternative splicing provides a versatile mechanism by which cells generate proteins with different or even antagonistic properties. We previously identified a novel splice variant of Bnip3 (Bnip3ΔEx3) generated in cardiac myocytes subjected to hypoxia. Like Full‐length Bnip3 (Bnip3FL), Bnip3ΔEx3 forms a complex with the antiapoptotic protein Bcl‐2; whereas, functional analysis of a Bnip3 mutant that lacks a Bcl‐2 interacting domain acts as a more potent inducer of programmed cell death, that can not be inhibited by Bnip3ΔEx3. In silico analysis of Bnip3ΔEx3 reveals an endoplasmic reticulum (ER) targeting domain within its C‐terminus, that is not present in Bnip3FL. We validated that Bnip3ΔEx3 is indeed localized to the ER and demonstrate that Bnip3ΔEx3 mutants that lack this ER targeting domain fail to antagonize cell death induced by Bnip3FL or hypoxia. Furthermore, we demonstrate that Bnip3ΔEx3 physically interacts with the IP3 receptor and serves to recruit Bcl‐2 to the ER; whereas, Bnip3ΔEx3 fails to protect against programmed cell death in the presence of a Bcl‐2 mutant that lacks its IP3 receptor interacting domain. Given the otherwise lethal consequences of de‐regulated Bnip3FL expression in post‐mitotic cells, our findings reveal a novel intrinsic defense mechanism that opposes the lethal consequences mitochondrial‐ER cross‐talk associated with programmed cell death in ventricular myocytes.