近日，美国加州大学伯克利分校Eunyong Park团队揭示TIM23复合物导入线粒体蛋白的结构基础。该项研究成果于2023年6月21日在线发表在《自然》杂志上。

研究人员确定了来自酿酒酵母的核心TIM23复合物（异源Tim17-Tim23-Tim44）的冷冻电镜结构。与普遍的模型相反，Tim23和Tim17本身并没有形成一个充满水的通道，而是有独立的、暴露于脂质的凹腔，这些凹腔朝向相反的方向。结构和生化分析表明，Tim17的空腔而非Tim23，形成了蛋白质的易位路径，而Tim23可能具有结构上的作用。结果进一步表明，在底物多肽的易位过程中，非必需的亚单位Mgr2封住了Tim17空腔的侧向开口，进而促进易位过程。研究人员为TIM23介导的蛋白质输入和分拣机制提出了一个新的模型，这是线粒体生物生成的一个核心途径。

据悉，线粒体几乎将其所有的约1000-2000种组成蛋白从细胞膜上跨过其双膜包膜导入。遗传和生化研究表明，保守的蛋白质易位酶，即TIM23复合物，介导了含有前序列的蛋白质（前蛋白）进入线粒体基质和内膜。在TIM23复合物的大约10个不同的亚单位中，重要的多通膜蛋白Tim23与演化相关的蛋白Tim17一起，长期以来被推测为形成了一个蛋白传导通道。然而，由于缺乏结构信息，这些亚单位在膜上形成一个易位路径并促成输入过程的机制仍不清楚。

附：英文原文

Title: Structural basis of mitochondrial protein import by the TIM23 complex

Author: Sim, Sue Im, Chen, Yuanyuan, Lynch, Diane L., Gumbart, James C., Park, Eunyong

Issue&Volume: 2023-06-21

Abstract: Mitochondria import nearly all of their approximately 1,000–2,000 constituent proteins from the cytosol across their double-membrane envelope1,2,3,4,5. Genetic and biochemical studies have shown that the conserved protein translocase, termed the TIM23 complex, mediates import of presequence-containing proteins (preproteins) into the mitochondrial matrix and inner membrane. Among about ten different subunits of the TIM23 complex, the essential multipass membrane protein Tim23, together with the evolutionarily related protein Tim17, has long been postulated to form a protein-conducting channel6,7,8,9,10,11. However, the mechanism by which these subunits form a translocation path in the membrane and enable the import process remains unclear due to a lack of structural information. Here we determined the cryo-electron microscopy structure of the core TIM23 complex (heterotrimeric Tim17–Tim23–Tim44) from Saccharomyces cerevisiae. Contrary to the prevailing model, Tim23 and Tim17 themselves do not form a water-filled channel, but instead have separate, lipid-exposed concave cavities that face in opposite directions. Our structural and biochemical analyses show that the cavity of Tim17, but not Tim23, forms the protein translocation path, whereas Tim23 probably has a structural role. The results further suggest that, during translocation of substrate polypeptides, the nonessential subunit Mgr2 seals the lateral opening of the Tim17 cavity to facilitate the translocation process. We propose a new model for the TIM23-mediated protein import and sorting mechanism, a central pathway in mitochondrial biogenesis.

DOI: 10.1038/s41586-023-06239-6

Source: https://www.nature.com/articles/s41586-023-06239-6