A double point mutation at residues Ile14 and Val15 of Bcl-2 uncovers a role for the BH4 domain in both protein stability and function

B-cell lymphoma 2 (Bcl-2) protein is the archetype apoptosis suppressor protein. The N-terminal Bcl-2-homology 4 (BH4) domain of Bcl-2 is required for the antiapoptotic function of this protein at the mitochondria and endoplasmic reticulum (ER). The involvement of the BH4 domain in Bcl-2′s antiapoptotic functions has been proposed based on Gly-based substitutions of the Ile14/Val15 amino acids, two hydrophobic residues located in the center of Bcl-2′s BH4 domain. Following this strategy, we recently showed that a BH4-domain-derived peptide in which Ile14 and Val15 have been replaced by Gly residues, was unable to dampen proapoptotic Ca²⁺-release events from the ER. Here, we investigated the impact of these mutations on the overall structure, stability, and function of full-length Bcl-2 as a regulator of Ca²⁺ signaling and cell death. Our results indicate that full-length Bcl-2 Ile14Gly/Val15Gly, in contrast to wild-type Bcl-2, (a) displayed severely reduced structural stability and a shortened protein half-life; (b) failed to interact with Bcl-2-associated X protein (BAX), to inhibit the inositol 1,4,5-trisphosphate receptor (IP₃R) and to protect against Ca²⁺-mediated apoptosis. We conclude that the hydrophobic face of Bcl-2′s BH4 domain (Ile14, Val15) is an important structural regulatory element by affecting protein stability and turnover, thereby likely reducing Bcl-2′s ability to modulate the function of its targets, like IP₃R and BAX. Therefore, Bcl-2 structure/function studies require pre-emptive and reliable determination of protein stability upon introduction of point mutations at the level of the BH4 domain.