title: Energetics of protein transport across biological membranes. a study of the thylakoid DeltapH-dependent/cpTat pathway.
abstract: Among the pathways for protein translocation across biological membranes, the DeltapH-dependent/Tat system is unusual in its sole reliance upon the transmembrane pH gradient to drive protein transport. The free energy cost of protein translocation via the chloro-plast DeltapH-dependent/Tat pathway was measured by conducting in vitro transport assays with isolated thylakoids while concurrently monitoring energetic parameters. These experiments revealed a substrate-specific energetic barrier to cpTat-mediated transport as well as direct utilization of protons from the gradient, consistent with a H+/protein antiporter mechanism. The magnitude of proton flux was assayed by four independent approaches and averaged 7.9 x 10(4) protons released from the gradient per transported protein. This corresponds to a DeltaG transport of 6.9 x 10(5) kJ.mol protein translocated(-1), representing the utilization of an energetic equivalent of 10(4) molecules of ATP. At this cost, we estimate that the DeltapH-dependent/cpTat pathway utilizes approximately 3% of the total energy output of the chloroplast.
authors.affiliation: Section of Plant Biology, Division of Biological Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA.
authors.forename: Nathan N
authors.initials: NN
authors.lastname: Alder
authors.forename: Steven M
authors.initials: SM
authors.lastname: Theg
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D029681:Arabidopsis Proteins; D002352:Carrier Proteins / Q000378:metabolism*; D060365:Chloroplast Proteins; D004734:Energy Metabolism; D006863:Hydrogen-Ion Concentration; D008027:Light; D018532:Pisum sativum; D011485:Protein Binding; D021381:Protein Transport; D011506:Proteins / Q000378:metabolism*; D011522:Protons; D013379:Substrate Specificity; D013816:Thermodynamics; D020524:Thylakoids / Q000378:metabolism*; D003313:Zea mays
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D060365:Chloroplast Proteins; D011506:Proteins; D011522:Protons; C435728:TATC protein, Arabidopsis; D000255:Adenosine Triphosphate