Poster: Protein Targeting & Vesicular Trafficking

Abs # 1178: The role of Tic110 in protein import at the inner envelope of chloroplasts

Presenter:	Inaba, Takehito , tinaba@biochem.umass.edu
Authors	Inaba, Takehito  (A)   Li, Ming  (A)   Schnell, Danny J (A)
Affiliations:	(A): Dpt. of Biochem. and Mol. Biol., University of Massachusetts, Amherst

Most of chloroplastic proteins are synthesized in the cytosol and imported into chloroplasts. After recognition by receptors at the chloroplast surface, the preproteins are translocated into organelle by multimeric translocons at the outer and inner envelope, designated Toc and Tic complexes, respectively. Although the activities and functions of the Toc components have been extensively investigated, the nature of each Tic components remains to be established. Tic110 was the first Tic component identified and represents a major component of active Tic complexes. It is an integral inner envelope membrane protein consisting of two predicted transmembrane helices and a large hydrophilic carboxyl-terminal region. Two hypotheses have been proposed for Tic110 function. The first model proposes that the large hydrophilic domain of Tic110 serves as a scaffold for the docking of soluble stromal chaperones that assist in the translocation and folding of imported proteins. The second model suggests that Tic110 is a b-barrel membrane protein that functions as the protein conducting channel of the Tic translocon. To distinguish these two models, we have investigated the structure, topology and function of Arabidopsis Tic110, atTic110. We demonstrate that the bulk of Tic110 lacking only the amino-terminal transmembrane domains is expressed in vivo as a soluble protein. Circular dichroism analysis reveals that this region mainly consists of a-helices. This domain selectively interacts with chloroplast preproteins. These data in conjugation with previous studies that demonstrate the ability of Tic110 to bind stromal chaperones support the hypothesis that Tic110 acts as a scaffold to coordinate the stromal events of protein translocation into chloroplasts.