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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 correct sorting of chloroplast proteins. Newly synthesized precursors targeted to the stroma bind cytosolic chaperones Hsp90 and Hsp70-Hsp90 Organizing Protein (HOP). Hsp90 and HOP use energy from ATP hydrolysis to keep the precursor in an unfolded state. A cleavable 13 to 146 amino acids long transit peptide sequence at the N-terminal and a non-cleavable internal signal sequence help precursors translocate to the correct chloroplast subcompartment. The transit peptide is recognized by two homologous GTPases, TOC159 and TOC34. TOC159 is the initial receptor for the transit peptide, while TOC34 facilitates GTP hydrolysis to help the precursor enter through the TOC channel.

The precursor is then transported across the inner membrane through the TIC complex. Sufficient ATP concentration coupled with proper temperature facilitates the translocation of the precursor across the outer and inner chloroplast membrane into the stroma. TIC40 also stimulates Hsp93 to utilize the energy of ATP hydrolysis and pull the precursor completely into the stroma. Stromal processing peptidases cleave the transit signal as the processed precursor is released into the stroma and folded by Cpn60 to form the active protein. Hsp93 undergoes ADP to ATP exchange and helps in consecutive cycles of import.

Tags
Protein TransportStromaChloroplastsTriple Membrane StructuresOuter MembraneInner MembraneThylakoid MembraneMetabolite TransportersMembrane TransloconsEnzymesSortingTranslocatingChloroplast FunctionTOC ComplexTIC ComplexPrecursorsCytosolic ChaperonesHsp90Hsp70 Hsp90 Organizing Protein HOPATP HydrolysisUnfolded StateTransit Peptide SequenceN terminalInternal Signal SequenceChloroplast SubcompartmentGTPases TOC159 And TOC34TOC ChannelInner Membrane Transport Complex TIC ComplexATP ConcentrationTemperature RegulationHsp93

来自章节 16:

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16.16 : Protein Transport to the Stroma

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16.1 : 蛋白质分选和运输概述

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16.2 : 信号序列和分选受体

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16.3 : 核蛋白分选

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16.4 : 核定位信号和导入

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16.5 : 核能出口

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16.6 : 核传输的方向性

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16.7 : 核蛋白分选的调控

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16.8 : 线粒体蛋白质分选

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16.9 : 线粒体前体蛋白

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16.10 : 蛋白质易位到线粒体中

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16.11 : 能源驱动易位

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16.12 : 孔蛋白的结构

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16.13 : 线粒体外膜中的孔蛋白插入

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16.14 : 蛋白质转运到线粒体内膜

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