16.16:
Protein Transport to the Stroma
Most chloroplast proteins are synthesized in the cytosol and translocated as unfolded precursors.
These precursors contain a cleavable transit signal at the N-terminal and an internal import signal that targets the protein to different subcompartments.
Cytosolic chaperones interact with the transit signal and direct the precursor to the chloroplast outer membrane.
Multimeric complexes embedded in the chloroplast membranes called the translocase of the outer chloroplast membrane or TOC complex, and the translocase of the inner chloroplast membrane or TIC complex mediate protein transport to the internal subcompartments.
GTP-bound TOC receptors recognize and bind the transit signal. GTP hydrolysis allows the precursor to enter and pass through the TOC channel and move into the intermembrane space.
Next, the TIC complex allows the precursor to translocate into the stroma.
Stromal Hsp70 uses energy from ATP hydrolysis to pull the emerging peptide out of the TIC complex.
Once the complete peptide is released into the stroma, stromal processing peptidases cleave the transit signal releasing the active protein.
16.16:
Protein Transport to the Stroma
Chloroplasts are triple membrane structures with an outer membrane, an inner membrane, and a thylakoid membrane, each containing distinct metabolite transporters, membrane translocons, and enzymes. Appropriate sorting and translocating these proteins to their correct membrane systems is essential for chloroplast function.
Protein complexes called the translocon of the outer chloroplast membrane or TOC complex, and the translocon of the inner chloroplast membrane or TIC complex mediate the correct sorting of chloroplast proteins. Newly synthesized precursors targeted to the stroma bind cytosolic chaperones Hsp90 and Hsp70-Hsp90 Organizing Protein (HOP). Hsp90 and HOP use energy from ATP hydrolysis to keep the precursor in an unfolded state. A cleavable 13 to 146 amino acids long transit peptide sequence at the N-terminal and a non-cleavable internal signal sequence help precursors translocate to the correct chloroplast subcompartment. The transit peptide is recognized by two homologous GTPases, TOC159 and TOC34. TOC159 is the initial receptor for the transit peptide, while TOC34 facilitates GTP hydrolysis to help the precursor enter through the TOC channel.
The precursor is then transported across the inner membrane through the TIC complex. Sufficient ATP concentration coupled with proper temperature facilitates the translocation of the precursor across the outer and inner chloroplast membrane into the stroma. TIC40 also stimulates Hsp93 to utilize the energy of ATP hydrolysis and pull the precursor completely into the stroma. Stromal processing peptidases cleave the transit signal as the processed precursor is released into the stroma and folded by Cpn60 to form the active protein. Hsp93 undergoes ADP to ATP exchange and helps in consecutive cycles of import.
Suggested Reading
- Alberts, Bruce, et al. Molecular Biology of the Cell. 6th ed. Garland Science, 2017. Pp 664-666
- Lodish, Harvey, et al. Molecular Cell Biology. 8th ed. W.H. Freeman and Company, 2016. Pp 617-618.
- Hsou-min Li, et al. Protein Transport into Chloroplasts, Annu. Rev. Plant Biol. 2010. 61:157–80.
- Simon M. Thomson, et al. Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system, Biochemical Society Transactions (2020) 48 71–82
- Mireille C. Perret, et al. Chloroplast Protein Translocation, The Molecular Biology of Chloroplasts and Mitochondria in Chlamydomonas, pp. 219–231.
- Xiumei Xu, et al. Protein sorting within the chloroplast, Trends in Cell Biology, 1642, Month 2020.