Photosynthesis in plants has long been attracting attention as a system that can use light energy most efficiently on Earth. One of the photosynthetic machineries is called photosystem I (PSI), which is a super molecular complex consisted of pigments and proteins. The PSI is further surrounded by a light harvesting antenna protein complex I (LHCI), which serves to capture light from the sun and transfer it to PSI. Green algae are known to have 9 types of LHCI, while four types of LHCI are bound to PSI in land plants and it could hence collect up to 41% more light energy than land plants. However, it is unclear how the LHCIs transmit light energy to PSI.
   In this study, the collaborative research team succeeded in extracting the complete complex of Chlamydomonas PSI-LHCI by developing a new sample purification method, and reconstructing the 6.9 Å structure by using single particle cryo-electron microscopy (Figure 1). As a result, it was found that 10 molecules (two molecules of LHCA1) of LHCIs surround PSI in Chlamydomonas PSI-LHCI, where eight LHCI molecules divided into two layers are located on one side of PSI, and the remaining two LHCI molecules were bonded to the other side.
   This result has shown the light energy transfer path from LHCI to PSI and clarified the mechanism of the ability of light harvesting in green algae. These findings are expected to be applicable to the development of clean and more efficient photovoltaic cells.

Figure 1 Chlamydomonas PSI―LHCI 6.9 Å cryo-EM map and the fitted atomic models.

Collaborative Researcher

Hisako Kubota-Kawai, Raymond N Burton-Smith, Ryutaro Tokutsu, Jun Minagawa (NIBB)
Chihong Song, Kazuyoshi Murata (NIPS)
 

Funding

KAKENHI, AMED, Collaborative Study Program of the National Institute for Physiological Sciences, ABiS