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BIS104 Note6

BIS104 Note6 - Lecture 6 Biol Sci 104 Protein and lipid...

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Lecture 6 1 Biol Sci 104 Winter 2010 Protein and lipid trafficking III IX. Transport of proteins into mitochondria and chloroplasts A. Are complex organelles with subcompartments. e.g. mitochondria have internal matrix space and an intermembrane space, with an inner and outer membrane. Chloroplasts have a third subcompartments -the thylakoid space surrounded by thylakoid membrane. B. Although both mitochondria and chloroplasts have their own DNA genomes and transcribe and translate some of their own proteins (most of which end up in the inner mitochondrial membrane and the thylakoid membrane), the vast majority of the chloroplast and mitochondrial proteins are actually encoded by the nucleus, and are translated on cytoplasmic polysomes. C. Translocation into the mitochondrial matrix is mediated by a 20 - 80 amino acid signal sequence ( amphipathetic α -helix ), which is recognized by mitochondrial membrane receptors, and is cleaved off after entry of protein into mitochondria. This process requires energy, which is supplied both by the electrochemical proton gradient across the mitochondrial inner membrane and by hydrolysis of ATP 1. occurs in three steps- assoc iat ion of the prote in w ith a receptor ( TOM ) in the outer mitochondrial membrane (requires ATP), followed by contact of the TOM complex with inner mitochondrial membrane receptor ( TIM ) and transport into matrix. 2. Proteins are imported in unfolded forms. These precursor proteins are then threaded through TOM/TIM, finally refolded in mitochondrial matrix by separate mitochondrial chaperone Hsp 70 (requires ATP); this refolding also helps drive the import of the polypeptide chain (pulling vs trapping). The membrane potential established by the electro transport chain also helps drive protein import into the mitochondrial matrix. ( The following sections are FYI only) D. Targeting into the mitochondrial inner membrane and the intermembrane space requires two
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Lecture 6 2 1. However, chloroplasts do have an extra membrane compartment, the thylakoid. Proteins targeted to thylakoids have an extra hydrophobic thylakoid targeting sequence. 2. Thus, thylakoid-destined protein first associates through N-terminal signal sequence with
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BIS104 Note6 - Lecture 6 Biol Sci 104 Protein and lipid...

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