Structural Research of Photosystem+Light-Harvesting Complex Supercomplex
During oxygen-producing photosynthesis, photosystems II (PSII) and I (PSI) operate in tandem for light-driven electron transport. Both photosystems contain a core and a peripheral antenna system. In plants, the peripheral antenna consists of the light-harvesting complex (LHC). LHCI and LHCII are connected to the PSI and PSII cores, respectively, to form the PS-LHC complex. Upon preferential PSII excitation, part of LHCII is phosphorylated and binds to PSI. The structural analysis of the PS-LHC complex is essential for revealing the energy transfer pathway between the antenna and the PSI core.
Structural analysis of PSI-LHCI supercomplexes
A total of three types of PSI-LHCI have been reported, two of which are supercomplexes with 10 Lhca proteins (PSI-10Lhca), while the other lacks the heterodimeric Lhca2/Lhca9 proteins (PSI-8Lhca). PSI-10Lhca and PSI-8Lhca structures are reconstructed by 3D cryo-electron microscopy (cryo-EM) at 2.9 Å and 3.1 Å resolution. The structures show that the four LHCI proteins (Lhca1-4) form two heterodimers, Lhca1/Lhca4 and Lhca2/Lhca3, which further form a heterotetramer that binds to the PSI core at the PsaF/PsaJ site, and the termini are stabilized by binding to PsaG and PsaK, respectively.
Progress in the structural research of PSI-LHCII supercomplexes
In plants, the transition from state 1 to state 2 is characterized by the formation of the characteristic PSI-LHCII supercomplex. The researchers analyzed the typical structure of PSI-LHCII by single-particle negative-staining electron microscopy (EM). where a single LHCII trimer binds to the PSI core on the PsaK side and attaches to the Lhca subunit. It was found that the transmembrane regions of PSI-LHCI and LHCII are not coplanar but form an angle of about 10°.
Figure 1. The maize PSI-LHCI-LHCII supercomplex. (Pan X, et al., 2018)
| Protein | Organism | Method | Resolution | PDB Entry ID |
| PSI-LHCI | Physcomitrium patens | Cryo-EM single particle analysis | 3.76 Å | 7KU5 |
| PSI-LHCR | Cyanidioschyzon merolae strain 10D | Cryo-EM single particle analysis | 3.63 Å | 5ZGB |
| M-LHCII and CP24 complexes in the stacked C2S2M2-type PSII-LHCII supercomplex | Pisum sativum | Cryo-EM single particle analysis | 3.6 Å | 5XNN |
| PSI-isiA supercomplex | Thermostichus vulcanus | Cryo-EM single particle analysis | 2.74 Å | 6K33 |
| PSII-FCP supercomplex | Chaetoceros gracilis | Cryo-EM single particle analysis | 3.02 Å | 6JLU |
| Photosystem I IsiA super-complex | Synechocystis sp. PCC 6803 | Cryo-EM single particle analysis | 3.48 Å | 6NWA |
| Photosystem I-IsiA supercomplex | Synechococcus elongatus PCC 7942 = FACHB-805 | Cryo-EM single particle analysis | 2.9 Å | 6KIG |
| Photosystem I-light harvesting complex I supercomplex | Pisum sativum | X-ray diffraction | 2.393 Å | 7DKZ |
| Photosystem I-LHCI super complex | Chlamydomonas reinhardtii | X-ray diffraction | 3.4 Å | 8H2U |
| Photosystem I-IsiA-flavodoxin supercomplex | Synechococcus elongatus PCC 7942 = FACHB-805 | Cryo-EM single particle analysis | 3.3 Å | 6KIF |
| Photosystem I supercomplex with light-harvesting complex I | Chlamydomonas reinhardtii | Cryo-EM single particle analysis | 2.9 Å | 6JO5 |
| C2S2M2N2-type PSII-LHCII | Chlamydomonas reinhardtii | Cryo-EM single particle analysis | 3.73 Å | 6KAF |
| Photosystem I supercomplex with light-harvesting complexes I and II | Zea mays | Cryo-EM single particle analysis | 3.3 Å | 5ZJI |
| State transition supercomplex PSI-LHCI-LHCII | Chlamydomonas reinhardtii | Cryo-EM single particle analysis | 2.84 Å | 7DZ7 |
| Photosystem I-LHCI-LHCII | Chlamydomonas reinhardtii | Cryo-EM single particle analysis | 3.42 Å | 7D0J |
| Photosystem I-LHCI-Lhca5 supercomplex | Hordeum vulgare subsp. spontaneum | Cryo-EM single particle analysis | 3.4 Å | 7EW6 |
| Photosystem I-LHCI-Lhca6 supercomplex | Hordeum vulgare | X-ray diffraction | 3.88 Å | 7EWK |
Table 1. Structural research of photosystem+light-harvesting complex supercomplex.
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References
- Pan X, et al. Structure of the maize photosystem I supercomplex with light-harvesting complexes I and II. Science. 2018.360(6393):1109-1113.
- Crepin A, et al. Isolation and characterization of a large photosystem I-light-harvesting complex II supercomplex with an additional Lhca1-a4 dimer in Arabidopsis. Plant J. 2020.102(2):398-409.
- Yadav KN, et al. Supercomplexes of plant photosystem I with cytochrome b6f, light-harvesting complex II and NDH. Biochim Biophys Acta Bioenerg. 2017.1858(1):12-20.
