The structure of PSI- LHCI from Cyanidium caldarium provides evolutionary insights into conservation and diversity of red- lineage LHCs

Light- harvesting complexes (LHCs) are diversified among photosynthetic organisms, and the structure of the photosystem I-LHC (PSI-LHCI) supercomplex has been shown to be variable depending on the species of organisms. However, the structural and evolutionary correlations of red- lineage LHCs are unknown. Here, we determined a 1.92- & Aring; resolution cryoelectron microscopic structure of a PSI-LHCI supercomplex isolated from the red alga Cyanidium caldarium RK - 1 (NIES- 2137), which is an important taxon in the Cyanidiophyceae. We subsequently investigated the correlations of PSI-LHCIs from different organisms through structural comparisons and phylogenetic analysis. The PSI-LHCI structure obtained shows five LHCI subunits surrounding a PSI- monomer core. The five LHCIs are composed of two Lhcr1s, two Lhcr2s, and one Lhcr3. Phylogenetic analysis of LHCs bound to PSI in the red- lineage algae showed clear orthology of LHCs between C. caldarium and Cyanidioschyzon merolae, whereas no orthologous relationships were found between C. caldarium Lhcr1-3 and LHCs in other red- lineage PSI-LHCI structures. These findings provide evolutionary insights into conservation and diversity of red- lineage LHCs associated with PSI.