title: The structure of allophycocyanin B from Synechocystis PCC 6803 reveals the structural basis for the extreme redshift of the terminal emitter in phycobilisomes.
abstract: Allophycocyanin B (AP-B) is one of the two terminal emitters in phycobilisomes, the unique light-harvesting complexes of cyanobacteria and red algae. Its low excitation-energy level and the correspondingly redshifted absorption and fluorescence emission play an important role in funnelling excitation energy from the hundreds of chromophores of the extramembraneous phycobilisome to the reaction centres within the photosynthetic membrane. In the absence of crystal structures of these low-abundance terminal emitters, the molecular basis for the extreme redshift and directional energy transfer is largely unknown. Here, the crystal structure of trimeric AP-B [(ApcD/ApcB)3] from Synechocystis sp. PCC 6803 at 1.75 Å resolution is reported. In the crystal lattice, eight trimers of AP-B form a porous, spherical, 48-subunit assembly of 193 Å in diameter with an internal cavity of 1.1 × 10(6) Å(3). While the overall structure of trimeric AP-B is similar to those reported for many other phycobiliprotein trimers, the chromophore pocket of the α-subunit, ApcD, has more bulky residues that tightly pack the phycocyanobilin (PCB). Ring D of the chromophores is further stabilized by close interactions with ApcB from the adjacent monomer. The combined contributions from both subunits render the conjugated rings B, C and D of the PCB in ApcD almost perfectly coplanar. Together with mutagenesis data, it is proposed that the enhanced planarity effectively extends the conjugation system of PCB and leads to the redshifted absorption (λmax = 669 nm) and fluorescence emission (679 nm) of the ApcD chromophore in AP-B, thereby enabling highly efficient energy transfer from the phycobilisome core to the reaction centres.
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Pan Pan
authors.initials: PP
authors.lastname: Peng
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Liang Liang
authors.initials: LL
authors.lastname: Dong
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Ya Fang
authors.initials: YF
authors.lastname: Sun
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Xiao Li
authors.initials: XL
authors.lastname: Zeng
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Wen Long
authors.initials: WL
authors.lastname: Ding
authors.affiliation: Department Biologie I, Universität München, Menzinger Strasse 67, D-80638 München, Germany.
authors.forename: Hugo
authors.initials: H
authors.lastname: Scheer
authors.affiliation: Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, IL 60637, USA.
authors.forename: Xiaojing
authors.initials: X
authors.lastname: Yang
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, People's Republic of China.
authors.forename: Kai Hong
authors.initials: KH
authors.lastname: Zhao
mesh_terms: D001665:Binding Sites; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000235:genetics; D005453:Fluorescence; D008958:Models, Molecular; D045524:Phycobilisomes / Q000737:chemistry; D010798:Phycocyanin / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D011487:Protein Conformation; D046939:Synechocystis / Q000737:chemistry* / Q000235:genetics
chemical_list: D045524:Phycobilisomes; C504557:allophycocyanin B; D010798:Phycocyanin
keywords: allophycocyanin B; antenna; cyanobacteria; energy transfer; light harvesting; photosynthesis; phycocyanobilin