PDF(Pubmed)
Abstract：
Regulation of apoptosis is tightly linked with the targeting of numerous Bcl-2 proteins to the mitochondrial outer membrane (MOM), where their activation or inhibition dictates cell death or survival. According to the traditional view of apoptotic regulation, BH3-effector proteins are indispensable for the cytosol-to-MOM targeting and activation of proapoptotic and antiapoptotic members of the Bcl-2 protein family. This view is challenged by recent studies showing that these processes can occur in cells lacking BH3 effectors by as yet to be determined mechanism(s). Here, we exploit a model membrane system that recapitulates key features of MOM to demonstrate that the proapoptotic Bcl-2 protein BAX and antiapoptotic Bcl-xL have an inherent ability to interact with membranes in the absence of BH3 effectors, but only in the presence of cellular concentrations of Mg2+/Ca2+ Under these conditions, BAX and Bcl-xL are selectively targeted to membranes, refolded, and activated in the presence of anionic lipids especially the mitochondrial-specific lipid cardiolipin. These results provide a mechanistic explanation for the mitochondrial targeting and activation of Bcl-2 proteins in cells lacking BH3 effectors. At cytosolic Mg2+ levels, the BH3-independent activation of BAX could provide localized amplification of apoptotic signaling at regions enriched in cardiolipin (e.g., contact sites between MOM and mitochondrial inner membrane). Increases in MOM cardiolipin, as well as cytosolic [Ca2+] during apoptosis could further contribute to its MOM targeting and activity. Meanwhile, the BH3-independent targeting and activation of Bcl-xL to the MOM is expected to counter the action of proapoptotic BAX, thereby preventing premature commitment to apoptosis.
摘要：
凋亡的调节与许多Bcl-2蛋白靶向线粒体外膜（MOM）紧密相关，它们的激活或抑制决定了细胞死亡或存活。根据细胞凋亡调节的传统观点,BH3效应蛋白对于Bcl-2蛋白家族的促凋亡和抗凋亡成员的胞质溶胶到MOM靶向和激活是必不可少的。这种观点受到最近研究的挑战,这些研究表明这些过程可以通过尚未确定的机制在缺乏BH3效应子的细胞中发生。这里,我们利用一个模型膜系统，概括了MOM的关键特征，以证明促凋亡Bcl-2蛋白BAX和抗凋亡Bcl-xL在不存在BH3效应物的情况下具有与膜相互作用的固有能力。但只有在细胞浓度的Mg2+/Ca2+存在下，BAX和Bcl-xL选择性靶向膜，重新折叠，并在阴离子脂质，尤其是线粒体特异性脂质心磷脂的存在下被激活。这些结果为缺乏BH3效应子的细胞中Bcl-2蛋白的线粒体靶向和激活提供了机制解释。在胞质Mg2+水平，不依赖BH3的BAX激活可以在富含心磷脂的区域提供凋亡信号的局部放大（例如，MOM和线粒体内膜之间的接触部位)。MOM心磷脂的增加，以及细胞凋亡过程中的胞浆[Ca2]可能进一步促进其MOM靶向和活性。同时,BH3非依赖性靶向和激活Bcl-xL到MOM有望对抗促凋亡BAX的作用，从而防止过早承诺凋亡。
