Structural basis for molecular assembly of fucoxanthin chlorophyll a/c-binding proteins in a diatom photosystem I supercomplex

Photosynthetic organisms display considerable diversity in light-harvesting complexes (LHCs). LHCs are attached to photosystem I (PSI), contributing to the formation of the PSI-LHCI supercomplex. The number of LHCIs and their protein and pigment compositions have been found to differ greatly among the PSI-LHCI structures. However, it remains unclear how LHCIs recognize their specific binding sites in the PSI core. In this study, we elucidated the cryo-electron microscopic structure of a PSI supercomplex incorporating fucoxanthin chlorophyll a / c -binding proteins (FCPs), designated as PSI-FCPI, isolated from the diatom Thalassiosira pseudonana CCMP1335. The structural analysis of PSI-FCPI revealed a composition of five FCPI subunits associated with a PSI monomer, specifically identified as RedCAP, Lhcr3, Lhcq10, Lhcf10, and Lhcq8. Through structural and sequence analyses, we identified distinct protein-protein interactions at the interfaces between FCPI and PSI subunits, as well as among FCPI subunits themselves. Comparative structural analyses of PSI-FCPI supercomplexes and phylogenetic analysis of FCPs across T. pseudonana and the diatom Chaetoceros gracilis highlight the evolutionary conservation of protein motifs crucial for the selective binding of individual FCPI subunits. These findings significantly advance our understanding of the molecular mechanisms governing the assembly and selective binding of FCPIs in diatoms.