Crystal structure of R-phycoerythrin from Polysiphonia urceolata at 2.8 angstrom resolution

The structure of R-phycoerythrin (R-PE) from Polysiphonia urceolata was determined at 2.8 Angstrom resolution. The crystals belong to space group R3 with unit cell dimensions of a = b = 189.8 Angstrom, c = 60.1 Angstrom. The subunit composition of R-PIE is (alpha(2) beta(2))(3) gamma. The three-dimensional structure of R-PE was solved by the multiple isomorphous replacement method. After several cycles of model building and refinement, the crystallographic X-factor of the final model is 18.0% with data from 10.0 to 2.8 A resolution. The four phycoerythrobilin chromophores alpha 84, alpha 140a, beta 84 and beta 155 in an (alpha beta) unit are each covalently bound to a cysteine residue through ring A. The phycourobilin chromophore is bound to cysteine beta 50 by ring A and bound to cysteine beta 61 by ring D. The ring A and ring D of phycourobilin deviate from the conjugate plane formed by ring B and ring C and the four rings form a boat-shaped structure. R-PE contains a 34 kDa gamma subunit that is assumed to lie in the central channel of the molecular disc (alpha(2) beta(2))(3). The energy transfer and relationship between cysteine residues and chromophores are discussed. (C) 1996 Academic Press Limited