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-angstrom resolution

Cryptophytes are unicellular photosynthetic algae that use a lumenally loca ted light-harvesting system, which is distinct from the phycobilisome struc ture found in cyanobacteria and red algae, One of the key components of thi s system is water-soluble phycoerythrin (PE) 545 whose expression is enhanc ed by low light levels. The crystal structure of the heterodimeric alpha(1) alpha(2)beta beta PE 545 from the marine cryptophyte Rhodomonas CS24 has be en determined at 1.63-Angstrom resolution, Although the beta-chain structur e is similar to the alpha and beta chains of other known phycobiliproteins, the overall structure of PE 5-15 is novel with the alpha chains forming a simple extended fold with an antiparallel beta-ribbon followed by an alpha- helix, The two doubly linked beta 50/beta 61 chromophores tone on each beta subunit) are in van der Waals contact, suggesting that exciton-coupling me chanisms may alter their spectral properties. Each alpha subunit carries a covalently linked 15,16-dihydrobiliverdin chromophore that is likely to be the final energy acceptor. The architecture of the heterodimer suggests tha t PE 545 may dock to an acceptor protein via a deep cleft and that energy m ay be transferred via this intermediary protein to the reaction center.