22 November 2007

Phycobilisomes (PBSs) are huge antennae complexes found in cyanobacteria and red algae.  This complex is built up of pigmented phycobiliproteins (PBPs), arranged as a core surrounded by rods, and a variety of colorless polypeptides known as linkers. Here we present high resolution crystal structures for the PBPs, phycocyanin (PC) and allophycocyanin (APC), from the thermophilic cyanobacterial species, T. vulcanus.  The PC structure presented has a resolution of 1.43Å and an R factor of 21.43% (R free 23.01 %).  The protein was solved as a monomer in the space group R32. Since PC is the best structurally characterised of any of the PBS components, our work is aimed at refining crystallization and data collection conditions in order to obtain very high resolution structures which may elucidate finer structural details.  Numerous structures have been solved in the process and amazingly each of these structures, including those obtained both in salt and polymer precipitants crystallized in the same space group (R32) with a very low RMSD (<0.35).  Comparison of these structures has allowed for identification of consistently observed water molecules with presumable structural significance and, additionally, structural features that are particularly susceptible to influence of the crystallization conditions.

The crystal structure of APC isolated from T. vulcanus has been solved to 2.9Å in the space group P3.  APC arranges as sheets of trimers in the crystal lattice with each of the four monomers in the asymmetric unit constituting one third of a trimeric disk.  Recently a further data set was collected at room temperature, with diffraction to 2.2Å.  Experiments are being carried out to further improve the APC crystal quality and resolution.  Structural comparison and assessment as well as in vitro experiments are being carried out to elucidate the basis for thermostability of the PBS in the thermophilic T. vulcanus.