ISOLATION, CRYSTALLIZATION, CRYSTAL-STRUCTURE ANALYSIS AND REFINEMENT OF B-PHYCOERYTHRIN FROM THE RED ALGA PORPHYRIDIUM-SORDIDUM AT 2.2 ANGSTROM RESOLUTION

The light-harvesting pigment-protein complex B-phycoerythrin from the red alga Porphyridium sordidum has been isolated and crystallized. B-Phycoerythrin consists of three different subunits forming an (αβ)6γ aggregate. The three-dimensional structure of the (αβ)6 hexamer was solved by Patterson search techniques using the molecular model of C-phycocyanin from Fremyella diplosiphon. The asymmetric unit of the crystal cell (space group P3, with a = b = 111·2 A ̊, c = 59·9 A ̊, α = β = 90 °, γ = 120 °) contains two (αβ) monomers related by a local dyad. Three asymmetric units are arranged around the crystallographic 3-fold axis building an (αβ)6 hexamer, as in C-phycocyanin. The crystal structure has been refined by energy-restrained crystallographic refinement and model building. The conventional R-factor of the final model was 18.9% with data to 2.2 Å resolution. The molecular structures of the α and β-subunits resemble those of C-phycocyanin. Major changes in comparison to phycocyanin are caused by deletion or insertion of segments involved in protein-chromophore interactions. The singly linked phycoerythrobilin chromophores α-84, α-140a, β-84 and β-155 are each covalently bound to a cysteine by ring A. The doubly linked chromophore β-50/β-61 is attached at cysteine β-50 through ring A and at cysteine β-61 through ring D. B-Phycoerythrin contains additionally a 30 kDa γ-subunit, which is presumably located in the central cavity of the hexamer. It is disordered, as a consequence of crystal and local symmetry averaging. © 1992.