Chloroplasts protein import into

Chen, X., and Schnell, D. (1999). Protein import into chloroplasts. Trends Cell Biol. 9, 222-227. 

Protein import into chloroplasts follows similar mechanisms to those in mitochondria but the signals used must be different since mitochondria and chloroplasts are present together in some plant cells and yet proteins become targeted to the correct destination. 

The mechanism involved in the posttranslational transport of proteins into chloro-plasts has so far received only limited attention. However, the import of plastocyanin into the thylakoid lumen has been determined to require two import signals near the N-terminal of the newly synthesized protein. Assuming that chloroplast protein import resembles the import process for mitochondria, suggest a reasonable hypothesis to explain how plastocyanin (a lumen protein associated with the inner surface of the thylakoid membrane) is transported and processed. What enzymatic activities and transport structures do you expect are involved in this process ... 

In both mitochondria and chloroplasts, protein import requires energy and occurs at points where the outer and inner organelle membranes are in close contact. Because mitochondria and chloroplasts contain multiple membranes and membrane-limited spaces, sorting of many proteins to their correct location often requires the sequential action of two targeting sequences and two membrane-bound translocation systems one to direct the protein into the organelle, and the other to direct It Into the correct organellar compartment or membrane. As we will see, the mechanisms for sorting various proteins to mitochondria and chloroplasts are related to some of the mechanisms discussed previously. 

The available evidence suggests that chloroplast stromal proteins, like mitochondrial matrix proteins, are imported In the unfolded state. Import into the stroma depends on ATP hydrolysis catalyzed by a stromal Hsc70 chaperone whose function is similar to Hsc70 in the mitochondrial matrix and BiP in the ER lumen. Unlike mitochondria, chloroplasts cannot generate an electrochemical gradient (proton-motive force) across their inner membrane. Thus protein Import Into the chloroplast stroma appears to be powered solely by ATP hydrolysis. 

Protein Import into the chloroplast stroma occurs through inner-membrane and outer-membrane translocation channels that are analogous in function to mitochondrial channels but composed of proteins unrelated in sequence to the corresponding mitochondrial proteins. 

The peroxisome Import machinery, unlike most systems that mediate protein Import into the ER, mitochondria, and chloroplast, can translocate folded proteins across the... 

Lysed chloroplasts from norflurazon-treated pea plants, containing only 5% oftheirnormal carotenoid contents, stably inserted less than 10% of Lhcbl into their thylakoid membranes compared to control experiments with non-treated plants (Dahlin and timko, 1994). This indicates either that the low amounts of carotenoids (and/or Chi b) in these chloroplasts were insufficient to stabilize newly inserted Lhcbl, or that the lack of pigments inhibited the insertion of Lhcbl into the thylakoid. Carotenoid deficiency due to norfluorazon treatment appears also to impair protein import into isolated chloroplasts. 

Protein Import Into Chloroplasts of Chlamydomonas reinhardtii 823... 

It was earlier hypothesized that the AAC had been invented either by chlamydiae with subsequent LGT to rickettsiae or by rickettsiae, and was physically but not functionally transferred from the latter to the genome of an amitochondriate host. Similarly to Coxl, also containing 12 transmembrane domains incompatible with import, rickettsial AAC could not be imported into mitochondria, but was later recruited to chloroplasts in a plant lineage (Emelyanov 2001b). Therefore, unrelated mitochondrial AAC - a member of the mitochondrial carrier family proteins containing six transmembrane... 

Most of the proteins of chloroplasts and mitochondria are synthesized in the cytoplasm and post-translationally imported into the organelle [29,34], One can divide this process into four steps (1) synthesis of the individual polypeptides as precursors on free ribosomes in the cytoplasm (2) binding of the precursors to specific receptors on the organellar surface (3) transmembrane movement and (4) processing and sorting into the correct compartment. 

Most mitochondrial and chloroplast proteins are encoded by nuclear genes, synthesized on cytosolic ribosomes, and Imported post-translatlonally into the organelles. 

Cytosolic chaperones maintain the precursors of mitochondrial and chloroplast proteins in an unfolded state. Only unfolded proteins can be Imported into the organelles. Translocation occurs at sites where the outer and inner membranes of the organelles are close together. 

By antibody screening of a Agtll cDNA expression library prepared from spinach poly (A+)-RNA cDNA clones encoding the entire precursor protein of CP24 were isolated. N-terminal amino acid sequence of the 20 kDa polypeptide of CP24 matched the corresponding sequence deduced from the nucleotide sequence. The nuclear encoded precursor protein is composed of 261 amino acids while the mature protein contains 210 amino acids (Table 1). When the isolated cDNA was translated vitro the product was imported into chloroplasts at high efficiency and processed correctly to its mature size (Fig. 1). 

Chemical energy in the form of ATP hydrolysis is required to keep the protein in the translocation competent state and possibly also during the translocation step. Import into mitochondria also requires an electrical potential across the inner mitochondrial membrane in contrast, chloroplast import has no such requirement . 

Most proteins that are imported into mitochondria and chloroplasts are made as pre-proteins with amino-terminal extensions that contain the targeting information. For mitochondrial proteins, the term "targeting... 

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