|M.Sc Student||Harris Dvir|
|Subject||Determination of the Precise Interaction between the|
Phycobilisome (PBS)Light Harvesting Antenna and
the Orange Carotenoid Protein (OCP)
|Department||Department of Chemistry||Supervisor||Professor Noam Adir|
|Full Thesis text|
The Phycobilisome (PBS) is a 4-7 MDa light harvesting antenna complex containing hundreds of chromophores bound to a multi-subunit self-assembled phycobiliproteins (PBP). Under conditions of extreme light irradiation, over-excitation of the reaction centers could lead to destruction of the reaction centers (PI - photoinhibition). Different organisms have developed different mechanisms to prevent PI.
One mechanism to avoid such over-excitation in some cyanobacteria is non-photochemical quenching (NPQ) induced in the presence of the 35kDa Orange Carotenoid Protein (OCP). The presence of this protein bound to the PBS has the effect of almost total energy dissipation; however the exact mechanism is yet to be fully determined. One of the major problems in all previously proposed models is that it is not clear how the OCP can approach the PBS core chromophores, a prerequisite for efficient NPQ.
Using the general cross-linking reagent glutaraldehyde (GA) which interacts mostly with the amines of lysines, arginines and the N-termini, we were able to obtain intersubunit cross-linked peptides between the OCP and isolated PBS complex, which were then identified using LC/MS-MS.
This has enabled us to construct two possible models of the site of OCP binding. The models are based on previous reported results from mutagenesis studies, as well as our cross-linking/MS results which constrain the possible arrangements of subunits in the PBS-OCP complex. We suggest that the loop domain of OCP is within crosslinking distance of ApcB in the exterior surface of the PBS basal core cylinder, and that the small core linker protein ApcC, changes position from its original residence in the middle of the cylinder edge into the cylinder cavity, thus enabling the N-terminal domain, which binds the carotenoid chromophore of the OCP, to be within 5-10 angstroms from a central core chromophore, enabling the efficient quenching of the entire light harvesting process. The major change on position of the ApcC protein was confirmed by performing the same cross-linking MS analysis in the absence or presence of the OCP.