Envelope membrane proteins that interact with chloroplastic precursor proteins.
The post-translational transport of cytoplasmically synthesized precursor proteins into chloroplasts requires proteins in the envelope membranes. To identify some of these proteins, label transfer cross-linking was performed using precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase (prSSU) that was blocked at an early stage of the transport process. Two envelope proteins were identified: an 86-kD protein and a 75-kD protein, both present in the outer membrane. Labeling of both proteins required prSSU and could not be accomplished with SSU lacking a transit peptide. Labeling of the 75-kD protein occurred only when low levels of ATP were present, whereas labeling of the 86-kD protein occurred in the absence of exogenous ATP. Although both labeled proteins were identified as proteins of the outer envelope membrane, the labeled form of the 75-kD protein could only be detected in fractions containing mixed envelope membranes. Based on these observations, we propose that prSSU first binds in an ATP-independent fashion to the 86-kD protein. The energy-requiring step is association with the 75-kD protein and assembly of a translocation contact site between the inner and outer membrane of the chloroplastic envelope.
Perry
S E
SE
Department of Botany, University of Wisconsin, Madison 53706-1381.
Keegstra
K
K
D000255:Adenosine Triphosphate / Q000378:metabolism; D001692:Biological Transport; D002478:Cells, Cultured; D002736:Chloroplasts / Q000378:metabolism*; D003432:Cross-Linking Reagents / Q000494:pharmacology; D015180:Electrophoresis, Gel, Two-Dimensional; D007887:Fabaceae; D007425:Intracellular Membranes / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D010946:Plants, Medicinal; D011485:Protein Binding; D011498:Protein Precursors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D013820:Thermolysin / Q000378:metabolism
D003432:Cross-Linking Reagents; D008565:Membrane Proteins; D010940:Plant Proteins; D011498:Protein Precursors; D000255:Adenosine Triphosphate; D013820:Thermolysin; D012273:Ribulose-Bisphosphate Carboxylase