Asymmetrical core structure in phycobilisomes of the cyanobacterium Synechocystis 6701

The light-harvesting complex of cyanobacteria and red algae, the phycobilisome, has two structural domains, the core and the rods. Both contain biliproteins and linker peptides. The core contains the site of attachment to the thylakoid membrane and the energy transfer link between the phycobilisome and chlorophyll. There are also six rod-binding sites in the membrane-distal periphery of the core. The structure of phycobilisomes in the cyanobacterium Synechococcus 6301 was studied by Glazer, who proposed a model for the internal organization of the bicylindrical core. In the construction of that model, it was necessary to make arbitrary decisions between two possible locations for one of the trimeric protein complexes within a core cylinder and between two possible orientations of the basal core cylinders relative to one another. We isolated the tricylindrical cores from an ultraviolet-light-induced mutant of the cyanobacterium Synechocystis 6701 and obtained, by partial dissociation, a unique core substructure that maintained some contacts between the two basal cylinders. From its structure and spectral properties, we conclude that this particle is a central core substructure that resulted from dissociation of the two layers of peripheral trimers in the intact core. The compositions of this particle and the dissociated trimers were inconsistent with the proposed location of one of the trimers in the 6301 core model, but supported the placement of that trimer in the alternative position within the basal core cylinder. Rod-binding sites within the central core substructure were studied by partial dissociation of the short-rod phycobilisomes from another mutant of 6701. This dissociation generated particles that were interpreted as being central core substructures with the two basal rods attached. The appearance of these particles in the electron microscope suggested that both basal rods would be localized towards the same side of the intact core. Such an asymmetrical arrangement of basal rods is supported by previously published edge-views of intact cores with basal rods from strain 6701. These observations suggest a parallel arrangement of the basal cylinders with respect to each other, creating an asymmetrical core. A phycobilisome model was constructed that incorporated core asymmetry. This model predicts the energy transfer pathways from the basal and upper rods to specific trimers in the core.