Minisymposium 25: Protein Targeting
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M2502: The targeting of atTic40 to the chloroplast inner membrane is independent of its import into the organelle
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Presenter: |
Li, Ming Contact Presenter |
Authors | Li, Ming (A) (B) Schnell, Danny (A) (B) | | Affiliations: |
(A): University of massachusetts, Department of Biochemistry and Molecular Biology
(B): Program in Molecualr and Celluar Biology
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Chloroplasts contain three independent membrane systems (the thylakoid and double membrane envelope) that segregate and organize a variety of essential metabolic processes. The inner envelope membrane (IM) separates plastid and cytoplasmic metabolism by controlling metabolite and ion transport. Furthermore, it contributes to cellular fatty acid synthesis, and participates in the synthesis of key signaling molecules. Despite the central role of the IM in cellular physiology, little is known about the biogenesis of this membrane. The IM relies on the import and integration of nucleus-encoded proteins. Although most of these proteins possess transit peptides and utilize the Toc-Tic system for their initial import into the organelle, the signals and machinery that direct them to the IM have not been defined. To identify the determinants of protein targeting to the chloroplast IM, we investigated the import characteristics of atTic40, a simple IM protein with a single trans-membrane helix. We found that atTic40 is imported into chloroplasts and processed to an intermediate size (int-atTic40). This intermediate is soluble and localized to the stroma. Pulse-chase experiments demonstrate that int-atTic40 subsequently inserts into the IM and is processed to its mature form. The integration requires ATP and can be reconstituted in vitro using int-atTic40 and isolated inner membrane vesicles. Taken together, our experiments suggest a "post import" mechanism in which IM proteins are first imported into the organelle from the cytoplasm and subsequently inserted into the inner membrane from the stroma. This targeting process is distinct from the " stop-transfer" mechanism observed in mitochondria, and thereby represents a novel eukaryotic protein trafficking pathway.
