The pro-apoptotic protein Bax is instable in many cancer cells but the mechanism of Bax degradation remains unclear. Four different lengths of deductive Bax degradation sensitive (BDS) sequences within BH3-BHI region, BDS-1 (Bax 67-124), BDS-3 (Bax 74-107), BDS-5 (Bax 67-107), and BDS-7 (Bax 74124), were tested for the susceptibility to ubiquitin-dependent degradation. Both BDS-1 and BDS-7 which contain the alpha 5 helix, a putative pore-forming domain of Bax, are sensitive to proteasome-dependent degradation and ubiquitin-conjugation. The Bax alpha 5-deletion mutant (Bax-Delta alpha 5) was stable and also maintained its apoptosis-inducing ability. Deletion of helices alpha 1 and part of alpha 2 (Bax-Delta 1-66) or helices alpha 3 and alpha 4 (Bax-Delta alpha 3,4) did not affect the sensitivity to degradation. However, Bax-Delta 1-66 mutant was not able to induce apoptosis. Thus, we propose that the a5 helix of Bax is sensitive to ubiquitin-dependent degradation. Moreover, Bax mutant retains its pro-apoptosis ability when the alpha 5 helix is deleted. (c) 2008 Elsevier Inc. All rights reserved.