|Ph.D Student||Marx Ailie|
|Subject||Elucidation of Phycobilisome Functionalities Using High|
Resolution Structures of Phycobiliproteins
|Department||Department of Chemistry||Supervisor||Professor Noam Adir|
|Full Thesis text|
Several new X-ray crystal
structures of the phycobilisome core component allophycocyanin and rod
component phycocyanin have been solved and used to provide a structure based
i) the specific absorption properties of the different phycobiliproteins;
ii) features of non-hexamer forming phycocyanin in crystal;
iii) mechanisms by which misassembly between highly similar building blocks is prevented;
iv) the effect of linker complexation with allophycocyanin and phycocyanin and
v) the relevance of these isolated component structures to the entire phycobilisome complex.
Analysis of the structures of phycocyanin and allophycocyanin from T. vulcanus explained the bathochromic shift observed upon allophycocyanin, but not phycocyanin, trimerization. Whilst the overall structures, including the positioning and structure of the chemically identical cofactors, are highly similar, specificity was found in the protein environment which encloses the α PCB cofactor upon trimerization. The tight hydrophobic nature of this surrounding in allophycocyanin apparently facilitates overlap of the excited states of the two cofactors which become closer upon trimerization, whereas the shallower more polar environment in phycocyanin does not.
Unlike all but one of the tens of phycocyanin structures in the PDB the Synechocystis sp PCC 6803 structure does not form hexamers in crystal. This structure has higher and more variable temperature factors however this does not appear to translate into specific structural variations which could explain the anomaly. The formation of a non-hexamer forming phycocyanin lattice was mimicked by crystallization in high concentrations of urea. Phycocyanin isolated from S. elongatus PCC 7942 is the first phycocyanin crystal structure shown to have N terminal cleavage of the methionine residue, an important feature of the allophycocyanin α subunit. In phycocyanin this cleavage is not necessary however it is apparently not deleterious either.
The structure of allophycocyanin from S. elongatus PCC 7942 is apparently complexed with the small core linker although the linker could not be identified in the electron density map because of high symmetry. Allophycocyanin in this structure, unlike the previous linker complexed structure, forms hexamers in crystal. Due to the flatter nature of the linker associated trimers the hexamers are formed such that a cavity is created in the outer face of the disc. It is now possible to hypothesis as to how a linker may mediate the interaction between rods and cores by “linking” although other results suggest that there may be different interaction modes between the rods and the core.