Cryo-EM Structural Analyses of Chlorophyll B-Enriched PSI-LHC and PSII-LHC Supercomplexes of the Siphonous Green Alga Bryopsis Corticulans.

The light-harvesting complexes of photosystem I (PSI) and PSII (LHCI and LHCII) in Bryopsis corticulans (B. corticulans) are homologous to those in Chlamydomonas reinhardtii and land plants but exhibit a distinct chlorophyll (Chl) and carotenoid composition. Here, we report cryo-electron microscopy structures of the PSI-LHCI10-LHCII9 supercomplex, comprising three LHCII trimers, and the C2S2M2N2-type PSII-LHCII supercomplex from B. corticulans. In the PSI supercomplex, ten LHCI subunits assemble into two belts and one heterodimer, coordinating a total of 86 Chl a and 65 Chl b molecules (Chl a/b ratio of 1.3, compared with 3.4 in C. reinhardtii), as well as 18 siphonaxanthin, 2 siphonein, and 13 α-carotene molecules. Of the three LHCII trimers bound to the PSI-LHCI supercomplex, two are anchored to the PSI core primarily via phosphorylated subunits, whereas the third, non-phosphorylated trimer is stabilized through interactions with Lhca-d and the adjacent LHCII trimer. In the C2S2M2N2-type PSII-LHCII supercomplex, the N-LHCII is positioned closer to the PSII core than in C. reinhardtii, likely owing to loss of the linker motif in the N-terminal region of B. corticulans CP29. Structure-based energy transfer analysis suggests that this spatial rearrangement enhances the efficiency of excitation energy transfer from N-LHCII to the PSII core. Collectively, these findings reveal structural adaptations that underlie the acclimation strategies of siphonous green algae inhabiting intertidal environments.