Minisymposium 23: Protein targeting

Abs # 44001: An essential role of atTic110 in protein translocation into chloroplasts in Arabidopsis

Presenter:	Inaba, Takehito , tinaba@nsm.umass.edu
Authors	Inaba, Takehito  (A)   Li, Ming  (A)   Ewers, Carolin  (A)   Alvarez-Huerta, Mayte  (B)   Kessler, Felix  (B)   Schnell, Danny J (A)
Affiliations:	(A): Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst (B): Institut de Botanique, Laboratoire de Physiologie Vegetale, Universite de Neuchatel

Chloroplast biogenesis is dependent upon the import of ~3000 different nucleus-encoded proteins. The majority of these proteins are synthesized as preproteins carrying an amino-terminal transit peptide that serves as the necessary and sufficient signal for targeting to the organelle. The transit peptide is recognized by receptor components of the translocon at the outer envelope membrane of chloroplasts (Toc), and a GTP-regulated switch initiates translocation through the protein-conducting channel of the Toc complex. At this stage, the Toc complex associates with the translocon at the inner envelope membrane (Tic) and this Toc-Tic supercomplex mediates the direct transport of the preproteins from the cytoplasm into the chloroplast stroma. In addition to components that mediate and potentially regulate the translocation of preproteins across the inner membrane (e.g. Tic20, Tic22, Tic55 and Tic62), the Tic complex also must mediate the processing and folding of preproteins prior to their release into the stroma. We proposed that Tic110 functions as a key component of the Tic complex by serving as a scaffold for coordinating the stromal events in protein import. Tic110 has been shown to associate with preproteins at the late stages of import and bind to molecular chaperones that may be required to provide the translocation driving force and mediate protein folding. As such, Tic110 in conjunction with a second Tic component, Tic40, are envisioned to participate in the formation of a protected environment for the maturation of newly imported proteins. Tic110 consists of two transmembrane helices at its extreme amino-terminus and the stromal domain that is largely hydrophilic. To further address the nature of this protein, our presentation will focus on the in vivo analysis of Arabidopsis Tic110, atTic110. Genetic analysis shows that atTic110, has essential roles for early embryogenesis as well as vegetative growth. In addition, we will also present data showing that separable domains on atTic110 have distinctive but essential functions in vivo. In conjunction with biochemical data, our in vivo data strongly support the idea that atTic110 has a central role in coordinating the stromal events of protein translocation into chloroplasts, a function that is essential for normal plastid development.