טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKlartag Merav
SubjectInvestigating the Core Components of the Phycobilisome of
the Thermophilic Cyanobacterium
Termosynechoccus Vulcanus
DepartmentDepartment of Chemistry
Supervisor Professor Noam Adir
Full Thesis textFull thesis text - English Version


Abstract

The phycobilisome (PBS) is the main light harvesting complex found in cyanobacteria and red algae, whose main function is to absorb and transfer the light energy to photosystem II.It is an enormous antenna complex built up by hundreds of polypeptides divided into pigmented proteins, named phycobiliproteins (PBPs), and unpigmented proteins, named linker proteins. 

The PBS exhibits two distinguished domains: (1) the core, composed of the PBP Allophycocyanin (APC) with its relevant linker proteins and (2) the rod, mainly composed of the PBP Phycocyanin (PC) (and from additional PBPs in several species) with its relevant linker proteins. The APC and PC are easily distinguished by their absorption spectra in the range of 550-700nm.

These two domains create the general structure of the PBS which is made up of three core cylinders of APC surrounding with six protruding cylindrical rods.

The main goal in our research project was to determine the structure of the core components of the PBS which include the PBP APC and the protein linkers associated with the APC from the thermophilic cyanobacterium Thermosynechococcus. vulcanus.

Two protein linkers were cloned and sequenced. The small protein linker, ApcC, found inside the cavity of the APC ring, was also overexpressed, purified and put for crystallization, however no crystals were obtained. ApcC, was dissolved in 8M urea lysis buffer (containing NaCl and imidazole) in order to become soluble. Bioinformatic analysis of the ApcC resulted in a prediction of an unfolded protein, partly hydrophilic (hydrophobic only at its edges) containing two α helixes (one much shorter than the other), three β sheets and  three connecting random coils segments.

The structure of APC from T.vulcanus was solved to a resolution of 2.9Å (APC-Tv) and demonstrated a very high similarity to other APC structures solved to date. A comparison between APC-Tv and a mesophilic APC structure revealed small differences that may explain the differences on their thermal stability level. A structural explanation for the formation of the APC monomer, composed from the α and β subunits, was also suggested.


In addition, we investigated the thermal and chemical stability of APC from T.vulcanus. The experiments revealed that APC-Tv exhibited an impressive ability to maintain its quaternary structure at high temperatures, even in the presence of weak denaturing agents (like urea an thiocyanate), although its original absorption spectrum was changed. On the other hand, a mesophilic APC was found to be highly unstable even under dilution.