title: 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 ΔG(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.
authors.affiliation: Department of Plant Biology, University of California, Davis, CA 95616, USA.
authors.forename: Lan-Xin
authors.initials: LX
authors.lastname: Shi
authors.forename: Steven M
authors.initials: SM
authors.lastname: Theg
mesh_terms: D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D004237:Diuron / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D004734:Energy Metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism; D018532:Pisum sativum / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D010456:Peptides, Cyclic / Q000494:pharmacology; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000187:drug effects; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D004791:Enzyme Inhibitors; D010456:Peptides, Cyclic; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; D000255:Adenosine Triphosphate; D004237:Diuron; D000251:Adenosine Triphosphatases; C003384:tentoxin