CRYSTAL-STRUCTURE OF R-PHYCOERYTHRIN FROM POLYSIPHONIA-URCEOLATA AT 2.8 ANGSTROM RESOLUTION

The structure of R-phycoerythrin (R-PE) from Polysiphonia urceolata wa s 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) g amma. The three-dimensional structure of R-PE was solved by the multip le isomorphous replacement method. After several cycles of model build ing and refinement, the crystallographic X-factor of the final model i s 18.0% with data from 10.0 to 2.8 A resolution. The four phycoerythro bilin chromophores alpha 84, alpha 140a, beta 84 and beta 155 in an (a lpha beta) unit are each covalently bound to a cysteine residue throug h ring A. The phycourobilin chromophore is bound to cysteine beta 50 b y 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 a nd ring C and the four rings form a boat-shaped structure. R-PE contai ns a 34 kDa gamma subunit that is assumed to lie in the central channe l 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