Department of Structural Biology, Saarland University, Center of Human and Molecular Biology (ZHMB), Faculty of Medicine, Building 60, 66421 Homburg, Germany.
Molecular Genetics of Eukaryotes, University of Kaiserslautern, Erwin-Schrödinger-Strasse 70, 67663 Kaiserslautern, Germany.
Acta Crystallogr D Struct Biol. 2022 Oct 1;78(Pt 10):1259-1272. doi: 10.1107/S2059798322009068. Epub 2022 Sep 27.
The folding of newly synthesized polypeptides requires the coordinated action of molecular chaperones. Prokaryotic cells and the chloroplasts of plant cells possess the ribosome-associated chaperone trigger factor, which binds nascent polypeptides at their exit stage from the ribosomal tunnel. The structure of bacterial trigger factor has been well characterized and it has a dragon-shaped conformation, with flexible domains responsible for ribosome binding, peptidyl-prolyl cis-trans isomerization (PPIase) activity and substrate protein binding. Chloroplast trigger-factor sequences have diversified from those of their bacterial orthologs and their molecular mechanism in plant organelles has been little investigated to date. Here, the crystal structure of the plastidic trigger factor from the green alga Chlamydomonas reinhardtii is presented at 2.6 Å resolution. Due to the high intramolecular flexibility of the protein, diffraction to this resolution was only achieved using a protein that lacked the N-terminal ribosome-binding domain. The eukaryotic trigger factor from C. reinhardtii exhibits a comparable dragon-shaped conformation to its bacterial counterpart. However, the C-terminal chaperone domain displays distinct charge distributions, with altered positioning of the helical arms and a specifically altered charge distribution along the surface responsible for substrate binding. While the PPIase domain shows a highly conserved structure compared with other PPIases, its rather weak activity and an unusual orientation towards the C-terminal domain points to specific adaptations of eukaryotic trigger factor for function in chloroplasts.
新合成多肽的折叠需要分子伴侣的协调作用。原核细胞和植物细胞的叶绿体拥有核糖体相关伴侣触发因子，它在新生多肽从核糖体隧道出口处结合。细菌触发因子的结构已经得到很好的描述，它具有龙形构象，具有负责核糖体结合、肽基脯氨酰顺反异构化（PPIase）活性和底物蛋白结合的柔性结构域。叶绿体触发因子序列已经从其细菌同源物多样化，并且迄今为止，它们在植物细胞器中的分子机制尚未得到充分研究。这里，呈现了来自绿藻衣藻的质体触发因子的晶体结构，分辨率为 2.6 Å。由于蛋白质的分子内柔性很高，仅使用缺乏 N 端核糖体结合结构域的蛋白质才能达到该分辨率。来自 C. reinhardtii 的真核触发因子表现出与其细菌对应物相似的龙形构象。然而，C 端伴侣结构域显示出不同的电荷分布，螺旋臂的位置发生改变，并且负责底物结合的表面的电荷分布也发生了特定改变。尽管 PPIase 结构域与其他 PPIases 相比具有高度保守的结构，但与其较弱的活性以及朝向 C 端结构域的异常取向相关，表明真核触发因子在叶绿体中具有特定的功能适应性。
