Evolution of a light-harvesting protein by addition of new subunits and rearrangement of conserved elements: Crystal structure of a cryptophyte phycoerythrin at 1.63-Å resolution

Cryptophytes are unicellular photosynthetic algae that use a lumenally located light-harvesting system, which is distinct from the phycobilisome structure found in cyanobacteria and red algae. One of the key components of this system is water-soluble phycoerythrin (PE) 545 whose expression is enhanced by low light levels. The crystal structure of the heterodimeric α₁α₂ββ PE 545 from the marine cryptophyte Rhodomonas CS24 has been determined at 1.63-Å resolution. Although the β-chain structure is similar to the α and β chains of other known phycobiliproteins, the overall structure of PE 545 is novel with the α chains forming a simple extended fold with an antiparallel β-ribbon followed by an α-helix. The two doubly linked β50/β61 chromophores (one on each β subunit) are in van der Waals contact, suggesting that exciton-coupling mechanisms may alter their spectral properties. Each α subunit carries a covalently linked 15,16-dihydrobiliverdin chromophore that is likely to be the final energy acceptor. The architecture of the heterodimer suggests that PE 545 may dock to an acceptor protein via a deep cleft and that energy may be transferred via this intermediary protein to the reaction center.