Structure Determination of a novel Phycoerythrin C from a hemihedral twinned crystal

Department of Biophysics, All India Institute of Medical Sciences, New Delhi* PG Department of Biosciences, Sardar Patel University, Vallabh Vidyanagar -388 120, Gujarat, INDIA**
â—‹Punit Kaur* Badrish R. Soni** A. S. Ethayathulla* Imtaiyaz Hassan* Savita Yadav* Datta Madamwar** Tej P. Singh*

Phycobiliproteins belong to a fluorescent family of light harvesting proteins which are components of the photosynthetic mechanism in Cyanobacterium. Phycoerythrin C, a phycobiliprotein, isolated from the blue green algae absorbs light and is the first protein involved in the transfer of energy to the photoactive reactions localized in the thylakoid membrane. They are composed of two structural subunits, alpha and beta which aggregate to form trimers, tetramers or hexamers with an additional gamma-subunit. All the subunits have different numbers of chromophores which are classified according to their spectral differences as phycocyanobilin (PCB), phycoerythrin (PEB) and phycourobilin (PUB). Phycobiliproteins have earlier been reported as dimers, tetramers or hexamers. This is the first crystal structure where the phycoerythrin C has crystallized as a single alpha-subunit. The crystal diffracted to 2.5Å resolution and could unambiguously be indexed in both orthorhombic and monoclinic space groups. The diffraction data indicated the presence of twinning in monoclinic space group P21 with unit cell dimensions a = 57.3Å, b = 83.7Å, c = 62.5Å and beta = 90.2o. The structure was solved by molecular replacement with two molecules of the alpha-subunit in the asymmetric unit. The twinning fraction was estimated to be 0.49. The structure refined to Rcryst = 23.0 and Rfree = 28.1. The alpha-subunits in the present phycoerythrin are shorter at the N-terminus by 31 residues and have 132 amino acid residues unlike other phycoerythrins which have 164 residues. The structure predominantly comprises of alpha-helices with the chromophore (phycourobilin) units.