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© The Rockefeller University Press, 0021-9525/1997//983 $5.00
The Journal of Cell Biology, Volume 136, Number 5, , 1997 983-994

Article

Identification of Protein Transport Complexes in the Chloroplastic Envelope Membranes via Chemical Cross-Linking



Mitsuru Akita, Erik Nielsen, and Kenneth Keegstra

MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824-1312

Transport of cytoplasmically synthesized proteins into chloroplasts uses an import machinery present in the envelope membranes. To identify the components of this machinery and to begin to examine how these components interact during transport, chemical cross-linking was performed on intact chloroplasts containing precursor proteins trapped at a particular stage of transport by ATP limitation. Large crosslinked complexes were observed using three different reversible homobifunctional cross-linkers. Three outer envelope membrane proteins (OEP86, OEP75, and OEP34) and one inner envelope membrane protein (IEP110), previously reported to be involved in protein import, were identified as components of these complexes. In addition to these membrane proteins, a stromal member of the hsp100 family, ClpC, was also present in the complexes. We propose that ClpC functions as a molecular chaperone, cooperating with other components to accomplish the transport of precursor proteins into chloroplasts. We also propose that each envelope membrane contains distinct translocation complexes and that a portion of these interact to form contact sites even in the absence of precursor proteins.

Abbreviations used in this paper: DSP, dithiobissuccinimidylpropionate; DST, disuccinimidyltartarate; DTSSP, dithiobissulfosuccinimidylpropionate; IEP and OEP, inner and outer envelope membrane proteins; LDS, lithiumdodecylsulfate; LS, the large subunit of ribulose 1,5-bisphosphate carboxylase; prSS, precursor to the small subunit of ribulose 1,5-bisphosphate carboxylase; Rubisco, ribulose 1,5-bisphosphate carboxylase; S78, stromal hsp70; SS, mature small subunit.

Address all correspondence to Kenneth Keegstra, MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312. Tel.: (517) 353-7874. Fax: (517) 353-9168. E-mail: Keegstra{at}msu.edu


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