Crystal Structure of R-Phycocyanin and Possible Energy Transfer Pathways in the Phycobilisome

AbstractThe crystal structure of R-phycocyanin from Polysiphonia urceolata (R-PC-PU) at 2.4Å is reported. The R-PC-PU crystal belongs to space group P43212 with cell parameters a=135.1Å, c=210.0Å, and α=β=γ=90°. The structure was determined by molecular replacement. The crystallographic R-factor of the refined model is 0.189 (Rfree=0.239). Comparison of the microenvironment of chromophore β155 in R-PC-PU and in C-PC from Fremyolla diphosiphon (C-PC-FD) reveals that their spectral differences may be caused by their different α28 residues. In the R-PC-PU crystal structure, two (αβ)3 trimers assemble face to face to form a hexamer, and two such hexamers assemble in two novel side-to-side arrangements. Possible models for the energy transfer from phycoerythrin to phycocyanin and from phycocyanin to allophycocyanin are proposed based on several phycobiliprotein crystal structures