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J. Cell Biol.,
Volume 143, Number 4, November 16, 1998 991-1002
Department of Biological Sciences, Rutgers, The State University of New Jersey, Newark, New Jersey 07102
Two components of the chloroplast envelope, Tic20 and Tic22, were previously identified as
candidates for components of the general protein import machinery by their ability to covalently cross-link
to nuclear-encoded preproteins trapped at an intermediate stage in import across the envelope (Kouranov,
A., and D.J. Schnell. 1997. J. Cell Biol. 139:1677-1685).
We have determined the primary structures of Tic20
and Tic22 and investigated their localization and association within the chloroplast envelope. Tic20 is a 20-kD integral membrane component of the inner envelope
membrane. In contrast, Tic22 is a 22-kD protein that is
located in the intermembrane space between the outer
and inner envelope membranes and is peripherally associated with the outer face of the inner membrane.
Tic20, Tic22, and a third inner membrane import component, Tic110, associate with import components of
the outer envelope membrane. Preprotein import intermediates quantitatively associate with this outer/inner membrane supercomplex, providing evidence that the
complex corresponds to envelope contact sites that mediate direct transport of preproteins from the cytoplasm to the stromal compartment. On the basis of these results, we propose that Tic20 and Tic22 are core
components of the protein translocon of the inner envelope membrane of chloroplasts.
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