Investigation on the Thermodynamic Dissociation Kinetics of Photosystem II Supercomplexes To Determine the Binding Strengths of Light-Harvesting Complexes

The photosystem II (PSII) supercomplex splits water utilizing light energy and is composed of a core dimer complex surrounded by light-harvesting complexes (LHCs). In green algae, the major LHCs which are LHCII trimers have thus far been categorized into strongly, moderately, or loosely binding LHCII trimers based on their predicted binding to core complexes. However, the binding energies have been indirectly predicted based on the presence or absence of LHCII trimers in the PSII supercomplex under electron microscopy and have not been determined experimentally. In this study, we investigated the binding of LHCII trimers by analyzing thermodynamic dissociation kinetics using isolated PSII supercomplexes. We identified two activation energies for dissociation of LHCII trimers: 54 ± 19 and 134 ± 8 kJ/mol. This result indicated the types of intermolecular interactions between LHCII trimers and core complexes

Chloroplast-mediated regulation of CO2-concentrating mechanism by Ca2+-binding protein CAS in the green alga Chlamydomonas reinhardtii.

「光合成のターボエンジン」CO2濃縮機構が葉緑体を介して制御される仕組みを新たに発見. 京都大学プレスリリース. 2016-10-28.Aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii, induce a CO2-concentrating mechanism (CCM) to maintain photosynthetic activity in CO2-limiting conditions by sensing environmental CO2 and light availability. Previously, a novel high-CO2–requiring mutant, H82, defective in the induction of the CCM, was isolated. A homolog of calcium (Ca2+)-binding protein CAS, originally found in Arabidopsis thaliana, was disrupted in H82 cells. Although Arabidopsis CAS is reported to be associated with stomatal closure or immune responses via a chloroplast-mediated retrograde signal, the relationship between a Ca2+ signal and the CCM associated with the function of CAS in an aquatic environment is still unclear. In this study, the introduction of an intact CAS gene into H82 cells restored photosynthetic affinity for inorganic carbon, and RNA-seq analyses revealed that CAS could function in maintaining the expression levels of nuclear-encoded CO2-limiting–inducible genes, including the HCO3– transporters high-light activated 3 (HLA3) and low-CO2–inducible gene A (LCIA). CAS changed its localization from dispersed across the thylakoid membrane in high-CO2 conditions or in the dark to being associated with tubule-like structures in the pyrenoid in CO2-limiting conditions, along with a significant increase of the fluorescent signals of the Ca2+ indicator in the pyrenoid. Chlamydomonas CAS had Ca2+-binding activity, and the perturbation of intracellular Ca2+ homeostasis by a Ca2+-chelator or calmodulin antagonist impaired the accumulation of HLA3 and LCIA. These results suggest that Chlamydomonas CAS is a Ca2+-mediated regulator of CCM-related genes via a retrograde signal from the pyrenoid in the chloroplast to the nucleus