Structure of C-phycocyanin from Spirulina platensis at 2.2 angstrom resolution: a novel monoclinic crystal form for phycobiliproteins in phycobilisomes

The crystal structure of C-phycocyanin from the cyanobacterium S. platensis has been determined at 2.2 Angstrom resolution. The crystals belong to the monoclinic crystal form, which has not been previously reported for phycob iliprotein structures. The structure was solved using the molecular-replace ment method with a final R value of 18.9% (R-free = 23.7%) after model buil ding and refinement. In the crystals used for the study, the C-phycocyanin hexamers formed by face-to-face association of two trimers are arranged in layers rather than in columns. Three different kinds of packing between adj acent hexamers in the layer were compared. The tight packing of two adjacen t hexamers formed by four trimers in the asymmetric unit brings beta 155 PC B chromophores close together, so it is possible that lateral energy transf er takes place through the beta 155-beta 155 route.