Abstract
Photosystem I (PSI) is a large pigment-protein complex mediating light-driven charge separation and generating a highly negative redox potential, which is eventually utilized to produce organic matter. In plants and algae, PSI possesses outer antennae, termed light-harvesting complex I (LHCI), which increase the energy flux to the reaction center. The number of outer antennae for PSI in the green alga Chlamydomonas reinhardtii is known to be larger than that of land plants. However, their exact number and location remain to be elucidated. Here, applying a newly established sample purification procedure, we isolated a highly pure PSI-LHCI supercomplex containing all nine LHCA gene products under state 1 conditions. Single-particle cryo-EM revealed the 3D structure of this supercomplex at 6.9 Å resolution, in which the densities near the PsaF and PsaJ subunits were assigned to two layers of LHCI belts containing eight LHCIs, whereas the densities between the PsaG and PsaH subunits on the opposite side of the LHCI belt were assigned to two extra LHCIs. Using single-particle cryo-EM, we also determined the 2D projection map of the lhca2 mutant, which confirmed the assignment of LHCA2 and LHCA9 to the densities between PsaG and PsaH. Spectroscopic measurements of the PSI-LHCI supercomplex suggested that the bound LHCA2 and LHCA9 proteins have the ability to increase the light-harvesting energy for PSI. We conclude that the PSI in C. reinhardtii has a larger and more distinct outer-antenna organization and higher light-harvesting capability than that in land plants.
Highlights
Photosystem I (PSI) is a large pigment–protein complex mediating light-driven charge separation and generating a highly negative redox potential, which is eventually utilized to produce organic matter
The green band corresponding to the PSI–light-harvesting complex I (LHCI) supercomplex [30] was collected and applied to an Fd affinity column as described under “Experimental procedures.”
After the Fd column purification of the PSI–LHCI supercomplex, -DDM was replaced with nonionic amphipol (NAPol) to further stabilize the supercomplex
Summary
The crystal structure of C. reinhardtii PSI–LHCI supercomplex is not available, the presence of a larger antenna complex than the land plant PSI–LHCI supercomplex has been supported by single-particle EM using negatively stained specimens. These results showed the presence of 6 –14 LHCA subunits in the PSI–LHCI supercomplex [22,23,24,25,26]. It has been reported that the C. reinhardtii PSI– LHCI supercomplex has one to two additional LHCA proteins, possibly located on the PsaK and PsaH subunit side of the PSI core complex [22, 23, 26]. Two of them were located at the side of the PsaG and PsaH subunits of the PSI core complex
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