AgResearch Structural Biology Laboratory, School of Biological Sciences, The University of Auckland, Auckland 1142, New Zealand.
Nature. 2013 Sep 26;501(7468):547-50. doi: 10.1038/nature12465. Epub 2013 Aug 4.
The ABC toxin complexes produced by certain bacteria are of interest owing to their potent insecticidal activity and potential role in human disease. These complexes comprise at least three proteins (A, B and C), which must assemble to be fully toxic. The carboxy-terminal region of the C protein is the main cytotoxic component, and is poorly conserved between different toxin complexes. A general model of action has been proposed, in which the toxin complex binds to the cell surface via the A protein, is endocytosed, and subsequently forms a pH-triggered channel, allowing the translocation of C into the cytoplasm, where it can cause cytoskeletal disruption in both insect and mammalian cells. Toxin complexes have been visualized using single-particle electron microscopy, but no high-resolution structures of the components are available, and the role of the B protein in the mechanism of toxicity remains unknown. Here we report the three-dimensional structure of the complex formed between the B and C proteins, determined to 2.5 Å by X-ray crystallography. These proteins assemble to form an unprecedented, large hollow structure that encapsulates and sequesters the cytotoxic, C-terminal region of the C protein like the shell of an egg. The shell is decorated on one end by a β-propeller domain, which mediates attachment of the B-C heterodimer to the A protein in the native complex. The structure reveals how C auto-proteolyses when folded in complex with B. The C protein is the first example, to our knowledge, of a structure that contains rearrangement hotspot (RHS) repeats, and illustrates a marked structural architecture that is probably conserved across both this widely distributed bacterial protein family and the related eukaryotic tyrosine-aspartate (YD)-repeat-containing protein family, which includes the teneurins. The structure provides the first clues about the function of these protein repeat families, and suggests a generic mechanism for protein encapsulation and delivery.
某些细菌产生的 ABC 毒素复合物因其强大的杀虫活性和在人类疾病中的潜在作用而受到关注。这些复合物至少包含三种蛋白质（A、B 和 C），它们必须组装在一起才能充分发挥毒性。C 蛋白的羧基末端区域是主要的细胞毒性成分，在不同的毒素复合物之间保守性较差。已经提出了一种通用的作用模型，其中毒素复合物通过 A 蛋白与细胞表面结合，被内吞，随后形成一个 pH 触发的通道，允许 C 进入细胞质，在那里它可以在昆虫和哺乳动物细胞中引起细胞骨架的破坏。已经使用单颗粒电子显微镜观察到毒素复合物，但目前还没有这些成分的高分辨率结构，B 蛋白在毒性机制中的作用仍然未知。在这里，我们通过 X 射线晶体学将 B 和 C 蛋白形成的复合物的三维结构确定为 2.5 Å。这些蛋白质组装形成一个前所未有的大型空心结构，将 C 蛋白的细胞毒性、羧基末端区域包裹并隔离起来，就像鸡蛋的壳一样。外壳的一端被一个β-螺旋桨结构域装饰，该结构域介导天然复合物中 B-C 异二聚体与 A 蛋白的附着。该结构揭示了 C 蛋白在与 B 折叠时如何自动蛋白水解。C 蛋白是我们所知的第一个包含重排热点 (RHS) 重复的结构，说明了一种明显的结构架构，可能在这个广泛分布的细菌蛋白家族和相关的真核酪氨酸-天冬氨酸 (YD) 重复蛋白家族中都得到了保守，其中包括 teneurins。该结构为这些蛋白重复家族的功能提供了第一个线索，并提出了一种通用的蛋白包裹和输送机制。
