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J. Biol. Chem., Vol. 282, Issue 29, 21404-21414, July 20, 2007

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Functional Similarity between the Chloroplast Translocon Component, Tic40, and the Human Co-chaperone, Hsp70-interacting Protein (Hip)^*

From the Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom

Tic40 is a component of the protein import apparatus of the inner envelope of chloroplasts, but its role in the import mechanism has not been clearly defined. The C terminus of Tic40 shares weak similarity with the C-terminal Sti1 domains of the mammalian Hsp70-interacting protein (Hip) and Hsp70/Hsp90-organizing protein (Hop) co-chaperones. Additionally, Tic40 may possess a tetratricopeptide repeat (TPR) protein-protein interaction domain, another characteristic feature of Hip/Hop co-chaperones. To investigate the functional importance of different parts of the Tic40 protein and to determine whether the homology between Tic40 and co-chaperones is functionally significant, different Tic40 deletion and Tic40:Hip fusion constructs were generated and assessed for complementation activity in the Arabidopsis Tic40 knock-out mutant, tic40. Interestingly, all Tic40 deletion constructs failed to complement tic40, indicating that each part removed is essential for Tic40 function; these included a construct lacking the Sti1-like domain (Sti1), a second lacking a central region, including the putative TPR domain (TPR), and a third lacking the predicted transmembrane anchor region. Moreover, the Sti1 and TPR constructs caused strong dominant-negative, albino phenotypes in tic40 transformants, indicating that the truncated Tic40 proteins interfere with the residual chloroplast protein import that occurs in tic40 plants. Remarkably, the Tic40:Hip fusion constructs showed that the Sti1 domain of human Hip is functionally equivalent to the Sti1-like region of Tic40, strongly suggesting a co-chaperone role for the Tic40 protein. Supporting this notion, yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the in vivo interaction of Tic40 with Tic110, a protein believed to recruit stromal chaperones to protein import sites.

Received for publication, December 18, 2006 , and in revised form, May 25, 2007.

^* This work was supported by a Universities UK Overseas Research Students award (to J. B.), the Royal Society Rosenheim fellowship (to P. J.), and Biotechnology and Biological Sciences Research Council Grants 91/C12976, 91/P12928, 91/C18638, BBS/B/03629, and BB/C006348/1 (to P. J.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental text, Figs. S1-S5, and Refs. 1-6.

¹ To whom correspondence should be addressed. Tel.: 44-116-223-1296; Fax: 44-116-252-3330; E-mail: rpj3{at}le.ac.uk.

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