The conformational changes converting BAX from an inert cytosolic monomer into the homo-oligomers that permeabilize the mitochondrial outer membrane (MOM) are crucial steps toward apoptosis. Here, we have explored the potential role of the BAX α1−α2 loop in this process by three mutagenic approaches: replacing loop segments with cognate loop regions from closely related proteins, alanine scanning and analysis of BAX α1−α2 loop missense mutations observed in tumours. Responsiveness to a death signal, such as tBID, was reduced by mutations in the N-terminal but not C-terminal half of the loop. N-terminal loop variants, which were enriched in tumours, impaired MOM integration by allosterically reducing exposure of the BAX α9 transmembrane anchor. Most C-terminal loop variants reduced BAX stability, leading to increased BAX apoptotic function in some variants. Thus, our systematic mutagenesis suggests that the two halves of the α1-α2 loop have distinct functions. We show that the N-terminal half of the loop (its first nine residues) comprises an important allosteric regulator of BAX activation by setting the proportion of MOM-integrated BAX following a death signal. The enrichment of N-terminal loop mutations in tumours indicates that they may promote tumour cell survival and underscore the loop as a target for therapeutic manipulation of BAX function.

将 BAX 从惰性胞质单体转化为可透化线粒体外膜 (MOM) 的同型寡聚体的构象变化是细胞凋亡的关键步骤。在这里，我们通过三种诱变方法探索了 BAX α1-α2 环在此过程中的潜在作用：用来自密切相关蛋白质的同源环区域替换环片段，丙氨酸扫描和分析在肿瘤中观察到的 BAX α1-α2 环错义突变. 对死亡信号（例如 tBID）的响应会因环路 N 端而非 C 端一半的突变而降低。在肿瘤中富集的 N 末端环变体通过变构减少 BAX α9 跨膜锚的暴露来损害 MOM 整合。大多数 C 端环变体降低了 BAX 稳定性，导致某些变体中 BAX 凋亡功能增加。因此，我们的系统诱变表明 α1-α2 环的两半具有不同的功能。我们通过在死亡信号后设置 MOM 整合 BAX 的比例，表明环的 N 端一半（其前九个残基）包含 BAX 激活的重要变构调节剂。肿瘤中 N 末端环突变的富集表明它们可能促进肿瘤细胞存活并强调该环是 BAX 功能治疗性操作的靶标。我们通过在死亡信号后设置 MOM 整合 BAX 的比例，表明环的 N 端一半（其前九个残基）包含 BAX 激活的重要变构调节剂。肿瘤中 N 末端环突变的富集表明它们可能促进肿瘤细胞存活并强调该环是 BAX 功能治疗性操作的靶标。我们通过在死亡信号后设置 MOM 整合 BAX 的比例，表明环的 N 端一半（其前九个残基）包含 BAX 激活的重要变构调节剂。肿瘤中 N 末端环突变的富集表明它们可能促进肿瘤细胞存活并强调该环是 BAX 功能治疗性操作的靶标。