1, Title: Evolution and mechanism of chloroplast protein import
Speaker: Prof. Masato Nakai, Institute for Protein Research, Osaka University, JAPAN
Date: August 24th, 2018(Friday)
Host PI: Prof. Chanhong Kim
Venue: PSC Auditorium
Virtually all chloroplasts in extant photosynthetic eukaryotes derive from a single endosymbiotic event that likely occurred more than a billion years ago between a host eukaryotic cell and a cyanobacterium-like ancestor. Many endosymbiont genes were subsequently transferred to the host nuclear genome, concomitant with the establishment of a system for protein transport through the chloroplast double-membrane envelope. Presently, 2000–3000 different nucleus-encoded chloroplast proteins must be imported into the chloroplast following their synthesis in the cytosol. The TOC (translocon at the outer envelope membrane of chloroplasts) and TIC (translocon at the inner envelope membrane of chloroplasts) complexes are protein translocation machineries at the outer and inner envelope membranes, respectively, that facilitate this chloroplast protein import with the aid of a TIC-associated ATP-driven import motor. All the essential components of this protein import system seemed to have been identified through biochemical analyses and subsequent genetic studies that initiated in the late 1990’s. However, in 2013, my group reported a completely novel 1-megadalton inner-envelope membrane protein complex as an actual TIC complex. And now we have further identified a completely novel 2-megadalton ATP-driven import motor complex associated with this TIC complex. Those findings lead to an extensively revised view of the evolution and molecular mechanisms of chloroplast protein import.
2, Title: Structure, assembly and physiological function of the chloroplast NDH-PSI supercomplex
Speaker: Prof. Toshiharu Shikanai, Graduate School od Science, Kyoto University
In photosynthesis, coupled with electron transport from water to NADP+, proton motive force is formed across the thylakoid membrane. But this linear electron transport does not satisfy the ATP/NADPH ratio required by the Calvin-Benson cycle. In angiosperms, additional ATP is supplied by two pathways of PSI cyclic electron transport, depending on PGR5/PGRL1 proteins and the chloroplast NDH complex. Chloroplast NDH mediates ferredoxin-dependent plastoquinone reduction, coupled with proton pumping across the thylakoid membrane. The complex consists of 29 subunits and further forms the NDH-PSI supercomplex with two copies of the PSI supercomplexes (PSI-LHCIs). Lhca5 and Lhca6 are linker proteins mediating the supercomplex formation and substitute for Lhca4 and Lhca2 in each copy of the PSI-LHCIs, respectively. In the copy of PSI-LHCIs including Lhca6, Lhca6 forms a heterodimer with Lhca3. During the assembly of the NDH-PSI supercomplex, subcomplex B of the chloroplast NDH complex stabilizes this unusual heterodimer and this assembly intermediate primes the NDH-PSI supercomplex formation. The NDH-PSI supercomplex formation is necessary for stabilizing the NDH complex especially under high light conditions. Because of this reason, it was necessary to evolve two linker proteins and also modify the assembly order. I will also discuss on the physiological function of the NDH complex in Arabidopsis.
The presentation will be delivered in English.