Structural modeling of the phycobilisome core and its association with the photosystems.

The phycobilisome (PBS) is a major light-harvesting complex in cyanobacteria and red algae. To obtain the detailed structure of the hemidiscoidal PBS core composed of allophycocyanin (APC) and minor polypeptide components, we analyzed all nine available 3D structures of APCs from different photosynthetic species and found several variants of crystal packing that potentially correspond to PBS core organization. Combination of face-to-face APC trimer crystal packing with back-to-back APC hexamer packing suggests two variants of the tricylindrical PBS core. To choose one of these structures, a computational model of the PBS core complex and photosystem II (PSII) dimer with minimized distance between the terminal PBS emitters and neighboring antenna chlorophylls was built. In the selected model, the distance between two types of pigments does not exceed 37 Å corresponding to the Förster mechanism of energy transfer. We also propose a model of PBS and photosystem I (PSI) monomer interaction showing a possibility of supercomplex formation and direct energy transfer from the PBS to PSI.