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© The Rockefeller University Press, 0021-9525/2000//1213 $5.00
The Journal of Cell Biology, Volume 148, Number 6, , 2000 1213-1222

Original Article

Toc64, a New Component of the Protein Translocon of Chloroplasts



Karen Sohrta and Jürgen Solla

a Botanisches Institut, Christian-Albrechts-Universität Kiel, D-24118 Kiel, Germany
Botanisches Institut, Am Botanischen Garten 1-9, Christian-Albrechts-Universität, D-24118 Kiel, Germany.49-431-880422249-431-8804210

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

© 2000 The Rockefeller University Press

Abbreviations used in this paper: DSP, dithiobis[succinimidyl propionate]; EST, expressed sequence tag; OE, oxygen evolving; SSU, Rubisco small subunit; Tic, translocon at the inner membrane of chloroplasts; Toc, translocon at the outer membrane of chloroplasts; Tom, translocon at the outer membrane of mitochondria; TPR, tetratricopeptide repeat.


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