| Description |
Photosynthesis is the most basic energy metabolism process on earth. This process provides energy sources for various life activities in the biosphere by converting light energy into chemical energy, and provides oxygen for the respiration of animals, plants and microorganisms. The products of photosynthesis provide energy and material basis for the sustainable development of human society. Chloroplasts rely on the translocon complexes in the outer and inner membranes (TOC and TIC complexes, respectively) to import thousands of different nuclear-encoded proteins from the cytosol. We have solved the cryo-EM structure of the TOC-TIC supercomplex from Chlamydomonas reinhardtii, elucidating the main functions of the TOC complex (Toc75, Toc90, and Toc34) and the TIC complex (Tic214, Tic20, Tic100, and Tic56). The subunit composition, localization of three chloroplast translocon-associated proteins (Ctap3, Ctap4, and Ctap5) and three newly identified small inner membrane proteins (Simp1-3) in the complex have been assigned in the supercomplex. Tic214, the largest protein in the complex, crosses the inner membrane, intermembrane space, and outer membrane, linking the TOC complex with TIC proteins. An inositol hexaphosphate molecule is located at the Tic214-Toc90 interface and stabilize their assembly. Four lipid molecules are located within or above the inner membrane funnel formed by Tic214, Tic20, Simp1, and Ctap5. Multiple potential pathways discovered in the TOC-TIC supercomplex may support the transport of different substrate precursor proteins into the chloroplast. In green algae and plants, state transitions act as short-term photosynthetic acclimation processes in modulating the light-harvesting capabilities and excitation levels of photosystems I and II (PSI and PSII, respectively). During this process, a portion of light-harvesting complex II (LHCII) is phosphorylated, dissociates from PSII and associates with PSI to form the PSI-LHCI-LHCII supercomplex. We have solved the high-resolution structure of PSI-LHCI-LHCII from Chlamydomonas reinhardtii, revealing the mechanism of assembly between the PSI-LHCI complex and two phosphorylated LHCII trimers comprising four LhcbM proteins. Two specific isoforms of LhcbM (i.e., LhcbM1 and LhcbM5) directly interact with the PSI core through their phosphorylated amino-terminal regions. Furthermore, biochemical and functional studies of the mutant strains lacking either LhcbM1 or LhcbM5 revealed that only LhcbM5 is dispensable for the supercomplex formation. The results reveal specific interactions and potential excitation energy transfer pathways between the green algal PSI and two phosphorylated LHCII trimers. |
