Crystal structure of a light-harvesting protein C-phycocyanin from Spirulina platensis

The crystal structure of C-phycocyanin, a light-harvesting phycobiliprotein from cyanobacteria (blue-green algae) Spirulina platensis has been solved by molecular replacement technique. The crystals belong to space group P2(1 ) with cell parameters a 107.20, b = 115.40, c = 183.04 Angstrom; beta = 90 .2 degrees. The structure has been refined to a crystallographic R factor o f 19.2% (R-free = 23.9%) using the X-ray diffraction data extending up to 2 .2 Angstrom resolution. The asymmetric unit of the crystal cell consists of two (alpha beta)(6)-hexamers, each hexamer being the functional unit in th e native antenna rod of cyanobacteria. The molecular structure resembles th at of other reported C-phycocyanins. However, the unique form of aggregatio n of two (alpha beta)(6)-hexamers in the crystal asymmetric unit, suggests additional pathways of energy transfer in lateral direction between the adj acent hexamers involving beta 155 phycocyanobilin chromophores. (C) 2001 Ac ademic Press.