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© The Rockefeller University Press, 0021-9525/1998//991 $5.00
The Journal of Cell Biology, Volume 143, Number 4, , 1998 991-1002


Regular Articles

Tic20 and Tic22 Are New Components of the Protein Import Apparatus at the Chloroplast Inner Envelope Membrane



Andrei Kouranov, Xuejun Chen, Bruno Fuks, and Danny J. Schnell

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.

Key Words: chloroplast • protein import • membrane translocation • protein targeting



Abbreviations used in this paper: EST, expressed sequence tag; protA, IgG-binding domain of staphylococcal protein A; pS, precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase; Tic, translocon at the inner envelope membrane of chloroplasts; Toc, translocon at the outer envelope membrane of chloroplasts.

A. Kouranov and X. Chen contributed equally to this work.



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