Structural basis for assembly and function of a diatom photosystem I-light-harvesting supercomplex

Abstract

Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis. One of the major LHCs are fucoxanthin chlorophyll a/c-binding proteins (FCPs) present in diatoms, a group of organisms having important contribution to the global carbon cycle. Here, we report a 2.40-Å resolution structure of the diatom photosystem I (PSI)-FCPI supercomplex by cryo-electron microscopy. The supercomplex is composed of 16 different FCPI subunits surrounding a monomeric PSI core. Each FCPI subunit showed different protein structures with different pigment contents and binding sites, and they form a complicated pigment-protein network together with the PSI core to harvest and transfer the light energy efficiently. In addition, two unique, previously unidentified subunits were found in the PSI core. The structure provides numerous insights into not only the light-harvesting strategy in diatom PSI-FCPI but also evolutionary dynamics of light harvesters among oxyphototrophs.

Structural basis for energy transfer in a huge diatom PSI-FCPI supercomplex

“…It should be noted that, although the assignment of Chls and Cars in FCPI subunits in the present study is mostly consistent with those of the previous study 39 , there are some differences (Supplementary Table 3). Because of the current resolution, it is natural that there may be some different assignments of Fx and Ddx in the two different structures, giving the small differences of the structures of the two types of Cars.…”

“…1a, b, d). This is eight more than the recently reported PSI-LHCI supercomplex structure (FCPI-1/2/12/17/18/19/20/21) 39 . The reason for this discrepancy may be due to the detachment of Supplementary Fig.…”

“…7). FCPI-11 in the current structure has two Chls more than the corresponding FCPI-1 in the structure of Nagao et al 39 , and FCPI-14, FCPI-16 of the current structure have one more Chl, respectively, than the corresponding FCPI-11, FCPI-13 in the structure of Nagao et al 39 . These Chls are located at the periphery region and thus have no significant effects on the energy transfer pathways ( Supplementary Fig.…”

“…Comparing with the PSI-FCPI structures with 16 antennae 39 , fewer Chls c are identified in our FCPI antennae. This is because of the slightly higher resolution of the present structure, where the phytol tail of Chl a was more clearly visible, so they were assigned as Chl a, whereas in the previous study 39 , the Chls without the phytol tail are all assigned as Chl c. The total number of Chl a and c in our FCPIs are 232 and 34, giving rise to a Chl a/c ratio of 6.82, which is much higher than the ratio of 2.37 in the 16 FCPI antennae 39 . The average number of Chl c assigned in our FCPIs structure is 1.4 per FCPI, indicating weaker absorptions of FCPI than the dimeric and tetrameric FCPII at 465 nm 8,21 .…”

Structure of the far-red light utilizing photosystem I of Acaryochloris

“…To date, several structures of type I reaction centers have been determined from higher plants 15,16 , green algae 17,18 , red algae 19 , diatoms 20 , cyanobacteria 1,21,22 and an anoxygenic bacterium 23 . All structures have Chl a in the reaction center with the exception of the one from the anoxygenic bacterium which has BChl.…”