Toc64, a new component of the protein translocon of chloroplasts

A subunit of the preprotein translocon of the outer envelope of chloroplast s (Toc complex) of 64 kD is described, Toc64. Toc64 copurifies on sucrose d ensity gradients with the isolated Toc complex. Furthermore, it can be cros s-linked in intact chloroplasts to a high molecular weight complex containi ng both Toc and Tic subunits and a precursor protein. The 0 Angstrom 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 ex posed at the chloroplast cytosol interface. We propose that Toc64 functions early in preprotein translocation, maybe as a docking protein for cytosoli c cofactors of the protein import into chloroplasts.