title: Structural model and spectroscopic characteristics of the FMO antenna protein from the aerobic chlorophototroph, Candidatus Chloracidobacterium thermophilum.
abstract: The Fenna-Matthews-Olson protein (FMO) binds seven or eight bacteriochlorophyll a (BChl a) molecules and is an important model antenna system for understanding pigment-protein interactions and mechanistic aspects of photosynthetic light harvesting. FMO proteins of green sulfur bacteria (Chlorobiales) have been extensively studied using a wide range of spectroscopic and theoretical approaches because of their stability, the spectral resolution of their pigments, their water-soluble nature, and the availability of high-resolution structural data. We obtained new structural and spectroscopic insights by studying the FMO protein from the recently discovered, aerobic phototrophic acidobacterium, Candidatus Chloracidobacterium thermophilum. Native C. thermophilum FMO is a trimer according to both analytical gel filtration and native-electrospray mass spectrometry. Furthermore, the mass of intact FMO trimer is consistent with the presence of 21-24 BChl a in each. Homology modeling of the C. thermophilum FMO was performed by using the structure of the FMO protein from Chlorobaculum tepidum as a template. C. thermophilum FMO differs from C. tepidum FMO in two distinct regions: the baseplate, CsmA-binding region and a region that is proposed to bind the reaction center subunit, PscA. C. thermophilum FMO has two fluorescence emission peaks at room temperature but only one at 77K. Temperature-dependent fluorescence spectroscopy showed that the two room-temperature emission peaks result from two excited-state BChl a populations that have identical fluorescence lifetimes. Modeling of the data suggests that the two populations contain 1-2 BChl and 5-6 BChl a molecules and that thermal equilibrium effects modulate the relative population of the two emitting states.
authors.affiliation: Department of Biology, Washington University in St. Louis, St. Louis MO 63130, USA.
authors.forename: Jianzhong
authors.initials: J
authors.lastname: Wen
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tsukatani
authors.forename: Weidong
authors.initials: W
authors.lastname: Cui
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.forename: Michael L
authors.initials: ML
authors.lastname: Gross
authors.forename: Donald A
authors.initials: DA
authors.lastname: Bryant
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D025541:Bacteriochlorophyll A / Q000378:metabolism; D019414:Chlorobi / Q000737:chemistry / Q000378:metabolism*; D041883:Chlorobium / Q000737:chemistry / Q000378:metabolism*; D017357:Cyclotrons; D005583:Fourier Analysis; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010788:Photosynthesis; D011485:Protein Binding; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry; D016415:Sequence Alignment; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D025541:Bacteriochlorophyll A; C086320:FMO bacteriochlorophyll protein, Bacteria; D045342:Light-Harvesting Protein Complexes; D021122:Protein Subunits