臨時生物科学セミナー（第１６１回）

　　日　時：平成２０年　１月２１日（月）　１７：００～１８：００

場　所：理学部２号館　第二講義室（２２３号室）

講演題目：Plastid protein import and its role in organelle biogenesis

講演者名：Dr. Danny J. Schnell

University of Massachusetts, Amherst

講演概要

Plastid biogenesis is dependent upon the coordinated expression and import of more than 3000 nucleus-encoded proteins during plant development. The majority of plastid proteins are synthesized with cleavable transit peptides, and their import is mediate by the TOC-TIC translocon system. The translocon at the outer envelope membrane (TOC) recognizes the transit peptides of plastid preproteins and initiates import by transferring the preprotein to a protein-conducting channel. The TOC complex associates with the translocon at the inner envelope membrane (TIC) to facilitate simultaneous transport of proteins across the double membrane of the envelope. Cytoplasmic preproteins are recognized by a TOC receptor system consisting of two membrane-bound GTPases, Toc159 and Toc34. The intrinsic GTPase activity of the receptors is a key regulator of the import process, but the precise function of nucleotide binding and hydrolysis are unknown. We have used a combination of in vitro and in vivo studies to define the roles of the TOC GTPases in the import reaction. We have shown that the GTP-bound form of atToc159 promotes preprotein binding, suggesting that its GTPase activity regulates the initial docking of preproteins to the translocon and likely plays a key role in selective preprotein targeting. Subsequent GTP binding and hydrolysis at Toc34 are required for preprotein dissociation from the receptors and transfer to the TOC channel, thereby initiating the protein import reaction. Our preliminary studies suggest that the molecular basis of the GTP-dependent switch is to regulate the association and dissociation of Toc159 and Toc34 within the translocon. As such, GTP hydrolysis appears to function as a molecular switch that controls access to the translocon channel.

　東京大学理学部生物学科植物学コース