Minisymposium 18: Protein Targeting

Abs # 32002: Tic40, a membrane-anchored co-chaperone homologue in the chloroplast protein translocon

Presenter:	Li, Hsou-min , mbhmli@ccvax.sinica.edu.tw
Authors	Li, Hsou-min  (A)   Ming-Lun, Chou  (A)
Affiliations:	(A): Institute of Molecular Biology, Academia Sinica, Taipei, TAIWAN

Most proteins in chloroplasts are encoded by the nuclear genome, synthesized in the cytosol as higher molecular weight precursor with N-terminal extension called transit peptides. Chloroplast import of precursor proteins requires specific interactions between transit peptides and translocon complexes located at the outer and inner envelope membranes. We have investigated the function of a previously identified but poorly characterized inner membrane translocon component Tic40. Proteolytic analysis indicated that Tic40 spanned the inner membrane with its N terminus, and the C-terminal hydrophilic domain was located in the stroma. An Arabidopsis null mutant of the atTic40 gene was isolated. Mutant chloroplasts were normal in allowing binding of precursor proteins. However, during subsequent translocation across the inner membrane, fewer precursors were translocated and more precursors were released from the mutant chloroplasts. Cross-linking experiments demonstrated that Tic40 was part of the translocon complex and functioned at the same stage of import as Tic110 and Hsp93, a member of the Hsp100-family molecular chaperones. Results from tertiary structure prediction and immunological studies indicated that the C-terminal portion of Tic40 contains a TPR domain followed by a domain with sequence similarity to co-chaperones Sti1p/Hop and Hip. From these data, we propose that Tic40 functions as a co-chaperone in the stromal chaperone complex that facilitates protein translocation across the inner membrane. Progress on mapping the interacting domains among Tic40, Tic110 and Hsp93 will be reported.