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Structure of a TOC-TIC supercomplex spanning two chloroplast envelope membranes
Zeyu Jin1, Li Wan1, Yuqi Zhang1
1Key Laboratory of Structural Biology of Zhejiang Province, School of Life Sciences, Westlake University, Hangzhou, Zhejiang 310024, China; Westlake Laboratory of Life Sciences and Biomedicine, Hangzhou, Zhejiang 310024, China; Institute of Biology, Westlake Institute for Advanced Study, Hangzhou, Zhejiang 310024, China.
Cell
|November 22, 2022
Summary
This study reveals the structure of the TOC-TIC supercomplex, crucial for protein import into chloroplasts. It identifies key components and their arrangement, clarifying the translocation pathway across the chloroplast envelope membranes.
Area of Science:
- Chloroplast biology
- Protein translocation
- Structural biology
Background:
- The Toc-TIC supercomplex mediates protein import into chloroplasts.
- Its precise molecular composition and assembly remain largely unknown.
Purpose of the Study:
- To determine the high-resolution structure of the TOC-TIC supercomplex.
- To elucidate the molecular identities and assembly of TOC and TIC components.
- To understand the preprotein translocation mechanism.
Main Methods:
- Cryoelectron microscopy (cryo-EM) of the TOC-TIC supercomplex from Chlamydomonas.
- Component identification and structural analysis.
Main Results:
- A cryo-EM structure of the 14-component TOC-TIC supercomplex was determined.
- The TOC pore is a hybrid β-barrel (Toc120/Toc75); the TIC path involves Tic20/YlmG transmembrane helices.
- A structural bridge connects TOC and TIC via an intermembrane space scaffold.
Conclusions:
- The study clarifies the composition and assembly of the chloroplast TOC-TIC translocon.
- It reveals a novel translocation pathway through TIC, challenging previous models.
- Provides structural basis for understanding protein import and evolutionary aspects of this machinery.
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