Isolation, crystallization, crystal structure analysis and refinement of allophycocyanin from the cyanobacterium Spirulina platensis at 2.3 A resolution.

The phycobiliprotein allophycocyanin from the cyanobacterium Spirulina platensis has been isolated and crystallized. The crystals belong to space group P6(3)22 with cell constants a = b = 101.9 A, c = 130.6 A, alpha = beta = 90 degrees, gamma = 120 degrees, with one (alpha beta) monomer in the asymmetric unit. The three-dimensional structure of the (alpha beta) monomer was solved by multiple isomorphous replacement. The crystal structure has been refined in a cyclic manner by energy-restrained crystallographic refinement and model building. The conventional crystallographic R-factor of the final model is 19.6% with data from 8.0 to 2.3 A. The molecular structure of the subunits resembles other solved phycobiliprotein structures. In comparison to C-phycocyanin and b-phycoerythrin the major differences arise from deletions and insertions of segments involved in the protein-chromophore interactions. The stereochemistry of the alpha 84 and beta 84 chiral atoms are C(2)-R, C(3)-R and C(31)-R. The configuration (C(4)-Z, C(10)-Z and C(15)-Z) and the conformation (C(5)-anti, C(9)-syn and C(14)-anti) are equal for both chromophores.