Crystal structure of R-phycocyanin and possible energy transfer pathways in the phycobilisome

The crystal structure of R-phycocyanin from Polysiphonia urceolata (R-PC-PU ) at 2.4 Angstrom is reported. The R-PC-PU crystal belongs to space group P 4(3)2(1)2 with cell parameters a = 135.1 Angstrom, c = 210.0 Angstrom, and alpha = beta = gamma = 90 degrees. The structure was determined by molecula r replacement. The crystallographic R-factor of the refined model is 0.189 (R-free = 0.239). Comparison of the microenvironment of chromophore beta 15 5 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 alpha 28 residu es. In the R-PC-PU crystal structure, two (alpha beta)(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 ph ycoerythrin to phycocyanin and from phycocyanin to allophycocyanin are prop osed based on several phycobiliprotein crystal structures.