Wu G, Chai J, Suber T L, Wu J W, Du C, Wang X, Shi Y
Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
Nature. 2000;408(6815):1008-12. doi: 10.1038/35050012.
Apoptosis is an essential process in the development and homeostasis of all metazoans. The inhibitor-of-apoptosis (IAP) proteins suppress cell death by inhibiting the activity of caspases; this inhibition is performed by the zinc-binding BIR domains of the IAP proteins. The mitochondrial protein Smac/DIABLO promotes apoptosis by eliminating the inhibitory effect of IAPs through physical interactions. Amino-terminal sequences in Smac/DIABLO are required for this function, as mutation of the very first amino acid leads to loss of interaction with IAPs and concomitant loss of Smac/DIABLO function. Here we report the high-resolution crystal structure of Smac/DIABLO complexed with the third BIR domain (BIR3) of XIAP. Our results show that the N-terminal four residues (Ala-Val-Pro-Ile) in Smac/DIABLO recognize a surface groove on BIR3, with the first residue Ala binding a hydrophobic pocket and making five hydrogen bonds to neighbouring residues on BIR3. These observations provide a structural explanation for the roles of the Smac N terminus as well as the conserved N-terminal sequences in the Drosophila proteins Hid/Grim/Reaper. In conjunction with other observations, our results reveal how Smac may relieve IAP inhibition of caspase-9 activity. In addition to explaining a number of biological observations, our structural analysis identifies potential targets for drug screening.
细胞凋亡是所有后生动物发育和体内平衡过程中的一个重要过程。凋亡抑制蛋白（IAP）通过抑制半胱天冬酶的活性来抑制细胞死亡；这种抑制作用是由IAP蛋白的锌结合BIR结构域执行的。线粒体蛋白Smac/DIABLO通过与IAP进行物理相互作用消除其抑制作用，从而促进细胞凋亡。Smac/DIABLO的氨基末端序列对于此功能是必需的，因为第一个氨基酸的突变会导致与IAP相互作用的丧失以及Smac/DIABLO功能的随之丧失。在此，我们报道了与XIAP的第三个BIR结构域（BIR3）复合的Smac/DIABLO的高分辨率晶体结构。我们的结果表明，Smac/DIABLO中的N端四个残基（丙氨酸-缬氨酸-脯氨酸-异亮氨酸）识别BIR3上的一个表面凹槽，第一个残基丙氨酸结合一个疏水口袋并与BIR3上的相邻残基形成五个氢键。这些观察结果为Smac N端以及果蝇蛋白Hid/Grim/Reaper中保守的N端序列的作用提供了结构上的解释。结合其他观察结果，我们的结果揭示了Smac如何缓解IAP对caspase-9活性的抑制。除了解释一些生物学观察结果外，我们的结构分析还确定了药物筛选的潜在靶点。
