Crystal Structure of Allophycocyanin from Marine Cyanobacterium Phormidium sp A09DM

Isolated phycobilisome (PBS) sub-assemblies have been widely subjected to X-ray crystallography analysis to obtain greater insights into the structure-function relationship of this light harvesting complex. Allophycocyanin (APC) is the phycobiliprotein always found in the PBS core complex. Phycocyanobilin (PCB) chromophores, covalently bound to conserved Cys residues of alpha- and beta-subunits of APC, are responsible for solar energy absorption from phycocyanin and for transfer to photosynthetic apparatus. In the known APC structures, heterodimers of alpha- and beta-subunits (known as alpha beta monomers) assemble as trimer or hexamer. We here for the first time report the crystal structure of APC isolated from a marine cyanobacterium (Phormidium sp. A09DM). The crystal structure has been refined against all the observed data to the resolution of 2.51 angstrom to R-work (R-free) of 0.158 (0.229) with good stereochemistry of the atomic model. The Phormidium protein exists as a trimer of alpha beta monomers in solution and in crystal lattice. The overall tertiary structures of alpha- and beta-subunits, and trimeric quaternary fold of the Phormidium protein resemble the other known APC structures. Also, configuration and conformation of the two covalently bound PCB chromophores in the marine APC are same as those observed in fresh water cyanobacteria and marine red algae. More hydrophobic residues, however, constitute the environment of the chromophore bound to alpha-subunit of the Phormidium protein, owing mainly to amino acid substitutions in the marine protein.