Seventh Rebeiz Foundation for Basic Research Paper Award

The Rebeiz Foundation for Basic Research (RFFBR) Paper Award for 2012 has been awarded to Dr. Masato Nakai and his colleagues: for their paper entitled " Uncovering the protein translocon at the chloroplast inner envelope membrane. The paper appeared in Science, 339: as a February 1 Report.

In selecting this paper for the Seventh RFFBR Paper Award, the Board of directors of the Rebeiz Foundation felt that the described work that will greatly benefit other chloroplast researchers, and enhance our knowledge of protein translocation into the chloroplast.

A Brief Summary of the Paper is Given Below
Almost all chloroplasts in today’s photosynthetic eukaryotes likely derived from one primary endosymbiosis of a cyanobacterium-like prokaryote, which is thought to have occurred more than a billion years ago. This was followed by massive transfer of genes from the endosymbiont to the host’s nuclear genome, accompanied with the evolution of protein transport system that allows these nuclear-encoded proteins back into the endosymbiont. Today’s chloroplasts import more than 2000 different nuclear-encoded proteins synthesized outside the chloroplast, across the double envelope membranes surrounding this organelle, to fulfill their complex physiological roles.

While many proteins had been proposed to be involved in preprotein translocation across the chloroplast envelope membranes, the identity of the protein translocon at the inner envelope, termed TIC, had long been a matter of debate; two proteins, Tic20 and Tic110, had been proposed to be central to protein translocation across the inner envelope membrane. Previously the authors identified a central component of TIC (i.e., Tic20), and, in this study, using transgenic Arabidopsis plants expressing a tagged-form of Tic20, they succeeded in purifying the entire translocon of approximately 1-megadalton in size, which was composed of Tic20 and three previously uncharacterized essential proteins, namely Tic56, Tic100, and Tic214. On the other hand, Tic110 was not part of the isolated translocon. The authors showed that the purified complex formed membrane channels in the lipid bilayers, where chloroplast preproteins specifically interacted with the channel pore. More compelling evidence for the direct involvement of this complex as a general protein translocation machinery at the inner envelope membrane was obtained by demonstrating that all the components of the 1-megadalton TIC complex were indeed found stoichiometrically associated with different translocating preproteins, and, importantly, together with well-established components of the TOC protein translocation machinery at the chloroplast outer envelope membrane.

Surprisingly, one of the TIC complex components, namely Tic214, is encoded by a previously enigmatic long open reading frame ycf1 in the chloroplast genome. While encoded in the chloroplast genome, the gene appears not to be of direct cyanobacterial origin but likely evolved largely after the primary endosymbiotic event. Further phylogenetic analysis revealed that, the complete TIC214/100/56/20 complex seems to have been established when land plants evolved. Thus, during evolution, the translocon at the chloroplast inner envelope has changed dramatically through modifications of both the nuclear and chloroplast genomes.

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A Photograph of Masato Nakai and his Research group are displayed below

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