The alpha-5 helix of Bax is sensitive to ubiquitin-dependent degradation

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–BH1 region, BDS-1 (Bax 67–124), BDS-3 (Bax 74–107), BDS-5 (Bax 67–107), and BDS-7 (Bax 74–124), were tested for the susceptibility to ubiquitin-dependent degradation. Both BDS-1 and BDS-7 which contain the α5 helix, a putative pore-forming domain of Bax, are sensitive to proteasome-dependent degradation and ubiquitin-conjugation. The Bax α5-deletion mutant (Bax-Δα5) was stable and also maintained its apoptosis-inducing ability. Deletion of helices α1 and part of α2 (Bax-Δ1–66) or helices α3 and α4 (Bax-Δα3,4) did not affect the sensitivity to degradation. However, Bax-Δ1–66 mutant was not able to induce apoptosis. Thus, we propose that the α5 helix of Bax is sensitive to ubiquitin-dependent degradation. Moreover, Bax mutant retains its pro-apoptosis ability when the α5 helix is deleted.