Crystal structure of a phycourobilin-containing phycoerythrin at 1.90-Å resolution

The structure of R-phycoerythrin (R-PE) from the red alga Griffithsia monilis was solved at 1.90-Å resolution by molecular replacement, using the atomic coordinates of cyanobacterial phycocyanin from Fremyella diplosiphon as a model. The crystallographic R factor for the final model is 17.5% (R(free) 22.7%) for reflections in the range 100-1.90 Å. The model consists of an (αβ)₂ dimer with an internal noncrystallographic dyad and a fragment of the γ-polypeptide. The α-polypeptide (164 amino acid residues) has two covalently bound phycoerythrobilins at positions α82 and α139. The β- polypeptide (177 residues) has two phycoerythrobilins bound to residues β82 and β158 and one phycourobilin covalently attached to rings A and D at residues β50 and β61, respectively. The electron density of the γ- polypeptide is mostly averaged out by threefold crystallographic symmetry, but a dipeptide (Gly-Tyr) and one single Tyr could be modeled. These two tyrosine residues of the γ-polypeptide are in close proximity to the phycoerythrobilins at position β82 of two symmetry-related β-polypeptides and are related by the same noncrystallographic dyad as the (αβ)₂ dimer. Possible energy transfer pathways are discussed briefly.