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Most organelle proteins are synthesized in the cytoplasm from nuclear-encoded mRNAs.

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These proteins must be imported into the organelle.

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Special sequences, called signal sequences, target the protein to its proper organelle.

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Organelles contain protein translocator complexes that are required for this transport.

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Key players in this process are protein, a signal sequence, chaperonins, ATP, protein

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translocator complexes, and signal peptidase.

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Proteins destined for import into an organelle, such as a mitochondria or chloroplast, contain

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a signal sequence.

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This sequence acts as a targeting mechanism to ensure the protein is delivered to the proper organelle.

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In addition, chaperonin proteins aid in the import process.

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They become associated with a protein while it is still in the cytoplasm.

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This association requires energy from ATP.

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Chaperonins aid in unfolding the protein so it can travel through the organelle membrane.

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Here we see two chaperonins bound to the protein that will enter the mitochondria.

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Protein translocator complexes are embedded in the mitochondrial membrane.

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These are multiprotein complexes required for protein import.

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The protein being transported first attaches to the complex on the cytosolic side.

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The protein then moves into the mitochondria.

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As it enters the organelle, it is again bound by a chaperonin to prevent premature folding.

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Once the protein has fully entered the mitochondria,

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the first chaperonin is released and another class of chaperonins bind.

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Then a complex called the signal peptidase removes the signal sequence.

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Lastly, the protein is folded into its final shape

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and is ready to perform its proper function in the organelle.