Department of Pharmacology and Toxicology, School of Medicine and Dentistry, University of Alabama at Birmingham, Birmingham, Alabama, United States of America.
PLoS One. 2010 Oct 1;5(10):e13094. doi: 10.1371/journal.pone.0013094.
The four residues at the amino-terminus of mature Smac/DIABLO are an IAP binding motif (IBM). Upon exit from mitochondria, mature Smac interacts with inhibitor of apoptosis proteins (IAPs), abrogating caspase inhibition. We used the ubiquitin fusion model to express mature Smac in the cytosol. Transiently expressed mature Smac56-239 (called Smac56) and Smac60-239 (called Smac60), which lacks the IBM, interacted with X-linked inhibitor of apoptosis protein (XIAP). However, stable expression produced wild type Smac56 that failed to homodimerize, interact with XIAP, and potentiate caspase activation. Cytosolic Smac60 retained these functions. Cytosolic Smac56 apparently becomes posttranslationally modified at the dimer interface region, which obliterated the epitope for a monoclonal antibody. Cytosolic Smacδ, which has the IBM but lacks amino acids 62-105, homodimerized and weakly interacted with XIAP, but failed to potentiate apoptosis. These findings suggest that the IBM of Smac is a recognition point for a posttranslational modification(s) that blocks homodimerization and IAP interaction, and that amino acids 62-105 are required for the proapoptotic function of Smac.
成熟 Smac/DIABLO 氨基末端的四个残基是一个 IAP 结合基序（IBM）。成熟 Smac 从线粒体逸出后，与凋亡蛋白抑制剂（IAPs）相互作用，解除半胱天冬酶的抑制作用。我们使用泛素融合模型在细胞质中表达成熟 Smac。瞬时表达的成熟 Smac56-239（称为 Smac56）和 Smac60-239（称为 Smac60），缺乏 IBM，与 X 连锁凋亡蛋白抑制剂（XIAP）相互作用。然而，稳定表达产生的野生型 Smac56 不能形成同源二聚体，与 XIAP 相互作用，并增强半胱天冬酶的激活。细胞质 Smac60 保留了这些功能。细胞质 Smac56 显然在二聚体界面区域发生了翻译后修饰，消除了单克隆抗体的表位。具有 IBM 但缺乏氨基酸 62-105 的细胞质 Smacδ 形成同源二聚体，并与 XIAP 弱相互作用，但不能增强凋亡。这些发现表明 Smac 的 IBM 是一种翻译后修饰的识别点，该修饰阻止同源二聚体和 IAP 相互作用，并且氨基酸 62-105 是 Smac 促凋亡功能所必需的。
