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Energetic cost of protein import across the envelope membranes of chloroplasts.

ABSTRACT: Chloroplasts are the organelles of green plants in which light energy is transduced into chemical energy, forming ATP and reduced carbon compounds upon which all life depends. The expenditure of this energy is one of the central issues of cellular metabolism. Chloroplasts contain ~3,000 proteins, among which less than 100 are typically encoded in the plastid genome. The rest are encoded in the nuclear genome, synthesized in the cytosol, and posttranslationally imported into the organelle in an energy-dependent process. We report here a measurement of the amount of ATP hydrolyzed to import a protein across the chloroplast envelope membranes--only the second complete accounting of the cost in Gibbs free energy of protein transport to be undertaken. Using two different precursors prepared by three distinct techniques, we show that the import of a precursor protein into chloroplasts is accompanied by the hydrolysis of ~650 ATP molecules. This translates to a ?G(protein) (transport) of some 27,300 kJ/mol protein imported. We estimate that protein import across the plastid envelope membranes consumes ~0.6% of the total light-saturated energy output of the organelle.

SUBMITTER: Shi LX

PROVIDER: S-EPMC3549074 | biostudies-literature | 2013 Jan

REPOSITORIES: biostudies-literature

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Energetic cost of protein import across the envelope membranes of chloroplasts.

Shi Lan-Xin LX Theg Steven M SM

Proceedings of the National Academy of Sciences of the United States of America 20121231 3

Chloroplasts are the organelles of green plants in which light energy is transduced into chemical energy, forming ATP and reduced carbon compounds upon which all life depends. The expenditure of this energy is one of the central issues of cellular metabolism. Chloroplasts contain ~3,000 proteins, among which less than 100 are typically encoded in the plastid genome. The rest are encoded in the nuclear genome, synthesized in the cytosol, and posttranslationally imported into the organelle in an e ...[more]

PMID: 23277572

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Project description:Dual targeting is a relatively little explored phenomenon by which some proteins are transported to two different subcellular compartments, such as chloroplasts and mitochondria. We previously showed that the R domain and the arm region of the melon necrotic spot virus coat protein could act as a dual-targeting peptide, which shares many relevant features in structure, amino acid composition, and functionality with that of the Thr-tRNA synthetases, the best-studied dual-targeted proteins by far. Here, we show that a viral coat protein hijacks the general pathways for organellar protein import and identifies the coat protein-interacting organellar translocon receptors. Our findings demonstrate for the first time that an exogenous dual-targeted protein uses the general Toc and Tom import systems to reach mitochondria and chloroplasts because it depends on the availability of NbToc75 and NbTom40. Interaction studies, subcellular localization analysis, and viral infection assays in silenced plants revealed the significance of plant receptors, such as the mitochondrion NbOm64 and chloroplast NbToc159A, in this transport. We also show that chloroplast protein translocation impairment could be interlinked with retrograde communication triggering a JA-based response that incidentally impairs viral infection. Future research may profit from our results to better comprehend cellular communication, protein import mechanisms, how preproteins interact with their receptors, and how plant pathogens manipulate host machinery to their ends. IMPORTANCE Many plant proteins and some proteins from plant pathogens are dually targeted to chloroplasts and mitochondria, and are supposed to be transported along the general pathways for organellar protein import, but this issue has not been explored yet. Moreover, organellar translocon receptors exist as families of several members whose functional specialization in different cargos is supposed but not thoroughly studied. This article provides novel insights into such topics showing for the first time that an exogenous protein, the melon necrotic spot virus coat protein, exploits the common Toc/Tom import systems to enter both mitochondria and chloroplasts while identifying the involved specific receptors.

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Energetic cost of protein import across the envelope membranes of chloroplasts.

Publications

Energetic cost of protein import across the envelope membranes of chloroplasts.

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