A new chloroplast protein import intermediate reveals distinct translocation machineries in the two envelope membranes: energetics and mechanistic implications.
Chloroplast protein import presents a complex membrane traversal problem: precursor proteins must cross two envelope membranes to reach the stromal compartment. This work characterizes a new chloroplast protein import intermediate which has completely traversed the outer envelope membrane but has not yet reached the stroma. The existence of this intermediate demonstrates that distinct protein transport machineries are present in both envelope membranes, and that they are able to operate independently of one another under certain conditions. Energetic characterization of this pathway led to the identification of three independent energy-requiring steps: binding of the precursor to the outer envelope membrane, outer membrane transport, and inner membrane transport. Localization of the sites of energy utilization for each of these steps, as well as their respective nucleotide specificities, suggest that three different ATPases mediate chloroplast envelope transport.
Scott
S V
SV
Division of Biological Sciences, University of California, Davis 95616, USA.
Theg
S M
SM
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D001692:Biological Transport; D002451:Cell Compartmentation; D002458:Cell Fractionation; D002736:Chloroplasts / Q000201:enzymology / Q000378:metabolism*; D010450:Endopeptidases / Q000378:metabolism; D004734:Energy Metabolism; D007425:Intracellular Membranes / Q000201:enzymology / Q000378:metabolism*; D008565:Membrane Proteins; D008954:Models, Biological; D009711:Nucleotides / Q000378:metabolism; D018532:Pisum sativum; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011498:Protein Precursors / Q000378:metabolism*; D012697:Serine Endopeptidases
D008565:Membrane Proteins; D009711:Nucleotides; D045322:Photosynthetic Reaction Center Complex Proteins; D011498:Protein Precursors; D000255:Adenosine Triphosphate; D010450:Endopeptidases; D012697:Serine Endopeptidases; C026515:type I signal peptidase; D000251:Adenosine Triphosphatases