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

Abstract

Light-harvesting complexes (LHCs) in photosynthetic organisms are attached to photosystem I (PSI), forming a PSI-LHCI supercomplex; however, it is unknown how LHCIs recognize their specific binding sites in the PSI core. Here we determine a cryo-electron microscopic structure of a PSI supercomplex containing fucoxanthin chlorophyll a/c-binding proteins (FCPs), namely PSI-FCPI, isolated from the diatom Thalassiosira pseudonana CCMP1335. The PSI-FCPI structure shows five FCPI subunits together with a PSI monomer. The five FCPIs are identified as RedCAP, Lhcr3, Lhcq10, Lhcf10, and Lhcq8. Structural and sequence analyses reveal specific protein-protein interactions at the binding sites between FCPI and PSI subunits and among FCPI subunits. Structural comparisons of PSI-FCPI supercomplexes and phylogenetic analysis of FCPs between T. pseudonana and the diatom Chaetoceros gracilis unveil the conservation of protein motifs involved in selective bindings of individual FCPI subunits. These findings provide molecular insights into the assembly and selective binding mechanisms of FCPIs.