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Original Article |
Correspondence to: Jürgen Soll, Botanisches Institut, Am Botanischen Garten 1-9, Christian-Albrechts-Universität, D-24118 Kiel, Germany. Tel:49-431-8804210 Fax:49-431-8804222 E-mail:jsoll{at}bot.uni-kiel.de.
A subunit of the preprotein translocon of the outer envelope of chloroplasts (Toc complex) of 64 kD is described, Toc64. Toc64 copurifies on sucrose density gradients with the isolated Toc complex. Furthermore, it can be cross-linked in intact chloroplasts to a high molecular weight complex containing both Toc and Tic subunits and a precursor protein. The 0 Å cross-linker CuCl2 yields the reversible formation of disulfide bridge(s) between Toc64 and the established Toc complex subunits in purified outer envelope membranes. Toc64 contains three tetratricopeptide repeat motifs that are exposed at the chloroplast cytosol interface. We propose that Toc64 functions early in preprotein translocation, maybe as a docking protein for cytosolic cofactors of the protein import into chloroplasts.
Key Words: protein import, Toc complex, chloroplasts, outer envelope, tetratricopeptide repeat motif
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