16.17:
Protein Transport to the Outer Chloroplast Membrane
Chloroplast protein precursors targeted to the outer membrane carry non-cleavable transit signals at their N-terminal end. Precursor proteins dock onto the TOC complex and translocate across the outer membrane.
The TOC complex interacts with the TIC complex of the inner membrane and the TIC stromal components, forming a TOC-TIC super complex that helps the proteins move inwards.
As the precursor enters the TIC complex, a polyglycine stretch close to the transit signal stalls further translocation, arresting the precursor at the TOC-TIC super complex.
Plastidic type 1 signal peptidases cleave the polyglycine stretch while an N-terminal domain of the TOC complex, called the polypeptide transport-associated domain or POTRA domain, functions as a chaperone preventing precursor aggregation.
The processed precursor is then transferred to an insertion pore complex called the outer membrane protein for integration into the chloroplast outer membrane.
16.17:
Protein Transport to the Outer Chloroplast Membrane
Chloroplast outer membrane proteins encoded by the nucleus are synthesized in the cytosol. Soon after synthesis, they bind cytosolic factors such as 14-3-3 protein and the Hsp70 chaperones that keep these precursors in an unfolded state until their translocation.
Two models describe the mechanism of precursor recognition and entry across the outer membrane through the TOC complex. Model 1 suggests the newly synthesized precursor binds to the TOC receptor 159 and forms a complex. TOC159-precursor complex docks onto the outer chloroplast membrane via another TOC receptor, TOC34, stimulating GTP hydrolysis of TOC34 and TOC159. This allows TOC159 to associate with TOC central channel and facilitate precursor translocation across the outer membrane.
Model 2 suggests that the transit signal of the precursor protein is phosphorylated at its C-terminal by an unknown kinase. TOC34 receptor functions as the initial receptor and binds to the phosphorylated transit signal, undergoing GTP hydrolysis. TOC34 dephosphorylates the transit peptide itself before transferring it to the TOC159 GTPase. GTP hydrolysis by TOC159 further promotes peptide translocation through the TOC complex into the intermembrane space.
As the precursors reach the TIC complex on the inner membrane, a stretch of polyglycine residues close to the transit signal, blocks further translocation. As a result, the precursor gets stalled across the TIC complex and is not translocated into the stroma. Plastid type 1 peptidases present in the intermembrane space cleave the polyglycine signal. The processed precursor is transferred to an insertion protein called outer envelope protein (OEP) or outer membrane protein (OMP) that folds and inserts the processed proteins into the outer membrane of the chloroplast.
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.