16.18:
Protein Transport to the Inner Chloroplast Membrane
Precursors targeted to the chloroplast inner membrane are first translocated via the TOC complex.
Translocation across the inner membrane through the TIC complex follows two distinct routes of import: the stop-transfer pathway and the re-insertion pathway.
In the stop-transfer pathway, an N-terminal hydrophobic region of the precursor prevents complete translocation of the polypeptide into the stroma. The hydrophobic segment functions as a transmembrane anchor, helping the stalled precursor get spontaneously inserted onto the inner membrane.
In the re-insertion pathway, soluble precursor intermediates with an additional N-terminal re-insertion signal are directed to the stroma through the general TOC/TIC import pathway.
After the transit signal of the soluble intermediates is cleaved by signal peptidases inside the stroma, the TIC stromal chaperone components- Hsp93, TIC40 , and TIC110, bind the processed precursor.
TIC40 coordinates with Hsp93 and TIC110 to help reinsert the precursor into the inner membrane.
16.18:
Protein Transport to the Inner Chloroplast Membrane
Proteins targeted to the inner chloroplast membrane, or plastid proteins, are transported by two general pathways: the stop-transfer and the re-insertion or post-import pathways. Most plastid proteins carry N-terminal transit sequences and internal import sequences targeting it to the specific chloroplast subcompartment. Proteins targeted by the stop-transfer pathway have internal hydrophobic sequences that inhibit their translocation into the stroma. As a result, these precursors are arrested across the TIC complex and are laterally released into the inner membrane. The hydrophobic segment helps anchor these proteins to the inner membrane.
Plastid proteins targeted to the inner membrane by the re-insertion pathway are first translocated to the stroma through the general TOC/TIC import pathway. After the stromal processing peptidases cleave the transit signal, the re-insertion signal is exposed. The re-insertion signal then guides the precursor to Hsp93-TIC40-TIC 110 complex. The precursor is folded and inserted by an unidentified protein, helped by TIC40 and TIC110.
A third pathway translocates plastid proteins that lack the N-terminal transit sequences. HP30-HP30-2 heteromer interacts with HP20 protein to translocate and insert these precursors. Proteins sorted by HP30-HP30-2 pathways include the chloroplast envelope quinone oxidoreductase homologs (ceQORH) and other proteins.
Suggested Reading
- AShih-Chi Hsu et al., Two distinct Omp85 paralogs in the chloroplast outer envelope membrane are essential for embryogenesis in Arabidopsis thaliana. Plant Signaling & Behavior 3:12, 1134-1135; December 2008
- Patrick K. O'Neil et al., The POTRA domains of Toc75 exhibit chaperone-like function to facilitate import into chloroplasts. PNAS, E4868-E-4876, 2017.
- Ute C. Vothknecht et al., Chloroplast membrane transport: Interplay of prokaryotic and eukaryotic traits. Gene 354 (2005) 99 – 109.
- Philip M. Day et al., Evolution of protein transport to the chloroplast envelope membranes. Photosynthesis Research, 2018.
- Hsou-min Li et al. Protein Transport into Chloroplasts, Annu. Rev. Plant Biol. 2010. 61:157–80
- Chi-chou Chiu et al., Tic40 is important for the reinsertion of proteins from the chloroplast stroma into the inner membrane. The Plant Journal (2008) 56, 793–801
- Antonio A. B. Viana et al., Determinants for Stop-transfer and Post-import Pathways for Protein Targeting to the Chloroplast Inner Envelope Membrane. THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 285, NO. 17, pp. 12948 –12960, April 23, 2010
- Claudia Rossig et al., Three proteins mediate import of transit sequence-less precursors into the inner envelope of chloroplasts in Arabidopsis thaliana. 19962–19967, PNAS, December 3, 2013, vol. 110(49).
- Masato Nakai. The TIC complex uncovered: The alternative view on the molecular mechanism of protein translocation across the inner envelope membrane of chloroplasts. Biochimica et Biophysica Acta 1847 (2015) 957–967.