Investigation of Phycobilisome Subunit Interaction Interfaces by Coupled Cross-linking and Mass Spectrometry [Bioenergetics]

October 8th, 2014 by Tal, O., Trabelcy, B., Gerchman, Y., Adir, N.

The Phycobilisome (PBS) is an extremely large light harvesting complex, common in cyanobacteria and red-algae, composed of rods and core substructures. These substructures are assembled from chromophore-bearing Phycocyanin and Allophycocyanin subunits, non-pigmented linker proteins and in some cases additional subunits. To-date, despite the determination of crystal structures of isolated PBS components, critical questions regarding the interaction and energy flow between rods and core are still unresolved. Additionally, the arrangement of minor PBS components located inside the core cylinders is unknown. Different models of the general architecture of the PBS have been proposed, based on low-resolution images from electron microscopy or high-resolution crystal structures of isolated components. This work presents a model of the assembly of the rods onto the core arrangement and for the positions of inner core components, based on cross-linking and mass spectrometry analysis of isolated, functional intact Thermosynecococcus vulcanus PBS as well as functional cross-linked adducts. The experimental results were utilized to predict potential docking interactions of different protein pairs. Combining modeling and cross-linking results we identify specific interactions within the PBS sub-components that enable us to suggest possible functional interactions between the chromophores of the rods and the core, and improve our understanding of the assembly, structure and function of PBS.