Architecture of chloroplast TOC-TIC translocon supercomplex

Area of Science:

• Plant Biology
• Cell Biology
• Structural Biology

Background:

• Chloroplasts import nuclear-encoded proteins via outer and inner envelope membrane translocon complexes (TOC and TIC).
• Previous studies suggested TOC and TIC complexes form larger supercomplexes, but their architecture and protein translocation mechanisms remained unclear.

Purpose of the Study:

• To determine the cryo-electron microscopy structure of the TOC-TIC supercomplex from Chlamydomonas reinhardtii.
• To elucidate the architecture of TOC-TIC supercomplexes and the mechanism of preprotein translocation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the TOC-TIC supercomplex.
• Identification and localization of major TOC and TIC complex subunits, associated proteins, and newly identified inner-membrane proteins within the supercomplex.

Main Results:

• The structure reveals the arrangement of TOC complex subunits (Toc75, Toc90, Toc34) and TIC complex subunits (Tic214, Tic20, Tic100, Tic56).
• Chloroplast translocon-associated proteins (Ctap3-5) and small inner-membrane proteins (Simp1-3) were located within the supercomplex.
• Tic214 spans all membranes, connecting TOC and TIC complexes, stabilized by inositol hexaphosphate.
• Lipid molecules form an inner-membrane funnel, potentially aiding preprotein translocation.

Conclusions:

• The determined structure provides unprecedented insight into the architecture of the chloroplast TOC-TIC supercomplex.
• The findings reveal the roles of Tic214 and lipids in mediating protein translocation across chloroplast membranes.
• Multiple translocation pathways within the supercomplex may accommodate diverse substrate preproteins.