Deciphering the Quenching Mechanism in Cyanobacteria’ Photosynthesis

Dvir Harris, Chemistry, Technion, Haifa, Israel
Prof. Diana Kirilovsky, Institut de Biologie et Technologies de Saclay, Commissariat a` l’Energie Atomique, Gif Sur Yvette, France
Prof. Noam Adir, Chemistry, Technion, Haifa, Israel

Enter the body of your aThe major light harvesting antenna found in cyanobacteria is the Phycobilisome (PBS), a 6 MDa complex containing hundreds of chromophores bound to a multi-subunit phycobiliprotein (PBP) assembly. It has been established that the PBS is assembled from two main sub-structures called rods (containing phycocyanin (PC) and in some organisms additional PBPs) which surround cylinders that make up the core (containing allophycocyanin, APC). Energy is transferred from the distal absorbing rods, to core and finally to one of two terminal emitting subspecies of APC, ApcD or ApcE. In case of extreme light irradiation conditions, where over excitation of the reaction center by the PBS could lead to photoinhibition. One mechanism to avoid such over-excitation in some cyanobacteria is non-photochemical quenching, involving the 35kDa orange carotenoid protein (OCP). The presence of this protein has the effect of almost total energy dissipation yet the exact mechanism is yet to be fully determined. One of the most important issues is to pinpoint the exact OCP binding site on the PBS, since the mechanism of energy dissipation should include a tight interaction between PBS chromophores and the OCP. By using a cross-linking/MS-MS method, our aim was to identify the interacting residues and build a model of the OCP-PBS complex. Using the more general cross-linker glutaraldehyde (GA) which interacts mostly with the amines of lysines and arginines (and the N-termini) that are found close enough in space, we were able to obtain enough intersubunit cross-linked peptides which were then identified using LC/MS-MS. This has enabled us to construct a semi-empirical docking-and-refining process, with distance constraints taking under consideration according to the LC/MS-MS results. The data obtained correspond with published literature [1-2] and shed a brighter light upon similar work being done short time ago [3].

Acknowledgements: this work is supported by the Israel Science Foundation (1576/12).