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Updated: Jul 10, 2026
06:29
Studying Protein Import into Chloroplasts Using Protoplasts
Published on: December 10, 2018
Protein targeting in "secondary" or "complex" chloroplasts
Balbir K Chaal1, Beverley R Green
1Botany Department, University of British Columbia, Vancouver, Canada.
Methods in Molecular Biology (Clifton, N.J.)
|October 24, 2007
Summary
Algae with chlorophyll c gained chloroplasts via secondary endosymbiosis, resulting in four membranes. Researchers study how proteins cross these inner chloroplast membranes using in vitro import assays.
Area of Science:
- * Cell Biology
- * Photosynthesis Research
- * Marine Biology
Background:
- * Algae with chlorophyll c, including diatoms, possess chloroplasts with four bounding membranes, acquired through secondary endosymbiosis with a red alga.
- * The outermost membrane, the chloroplast endoplasmic reticulum (ER), is continuous with the host's rough ER and can bear cytoplasmic ribosomes.
- * Nuclear-encoded chloroplast proteins utilize an N-terminal ER targeting sequence for transit across the outer membrane, but mechanisms for inner membrane translocation remain unclear.
Purpose of the Study:
- * To investigate the mechanisms of protein import across the inner chloroplast envelope membranes.
- * To understand how nuclear-encoded proteins are targeted to chloroplasts after initial ER translocation.
Main Methods:
- * In vitro translation of protein constructs lacking the ER signal sequence.
- * Assaying the import of these constructs into isolated pea chloroplasts.
- * Utilizing modified proteins to study translocation across inner envelope membranes.
Main Results:
- * Demonstrated the necessity of specific targeting sequences for protein import into chloroplasts.
- * Provided insights into the pathways proteins take across the chloroplast envelope.
- * Identified key steps in the post-ER translocation of chloroplast proteins.
Conclusions:
- * Protein import into chloroplasts involves complex translocation steps across multiple membranes.
- * Further research is needed to fully elucidate the translocon machinery of the inner chloroplast envelope.
- * Understanding these pathways is crucial for comprehending chloroplast biogenesis and function in secondary endosymbiotic algae.
Related Concept Videos
01:11Protein 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.
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.
01:18Protein 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...
01:22Protein Transport to the Thylakoids
Thylakoids are membrane-bound sac-like structures within the chloroplast that serve as sites for photosynthesis. Thylakoid lumen contains many electron transport proteins and is enclosed by a thylakoid membrane rich in the light-harvesting complex. Proteins targeted to the thylakoids are transported as precursors and are sorted by the general TOC/TIC import pathway. Once the precursor reaches the stroma, stromal processing peptidases remove their transit signal and expose thylakoid signal...
01:24Protein 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...
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...
01:32Photosystems
Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
Functioning of Photosystems
Photosystems contain many pigment molecules, such as chlorophylls and carotenoids, arranged in a particular organization across two domains — the antenna complex and the reaction center. The main aim of the pigment molecules...
Functioning of Photosystems
Photosystems contain many pigment molecules, such as chlorophylls and carotenoids, arranged in a particular organization across two domains — the antenna complex and the reaction center. The main aim of the pigment molecules...
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Eukaryotic cells have different membrane-bound organelles with distinct protein requirements. The process by which proteins are targeted to a specific organelle is called protein sorting.
Protein sorting can be of two types: signal-based sorting and vesicle-based trafficking. In signal-based sorting, specific amino acid sequences called sorting signals target proteins to the proper location inside the cell either via gated transport or by protein translocation. In gated transport, folded...
Protein sorting can be of two types: signal-based sorting and vesicle-based trafficking. In signal-based sorting, specific amino acid sequences called sorting signals target proteins to the proper location inside the cell either via gated transport or by protein translocation. In gated transport, folded...