title: Top-Down Mass Spectrometry Analysis of Membrane-Bound Light-Harvesting Complex 2 from Rhodobacter sphaeroides.
abstract: We report a top-down proteomic analysis of the membrane-bound peripheral light-harvesting complex LH2 isolated from the purple photosynthetic bacterium Rhodobacter sphaeroides. The LH2 complex is coded for by the puc operon. The Rb. sphaeroides genome contains two puc operons, designated puc1BAC and puc2BA. Although previous work has shown consistently that the LH2 β polypeptide coded by the puc2B gene was assembled into LH2 complexes, there are contradictory reports as to whether the Puc2A polypeptides are incorporated into LH2 complexes. Furthermore, post-translational modifications of this protein offer the prospect that it could coordinate bacteriochlorophyll a (Bchl a) by a modified N-terminal residue. Here, we describe the components of the LH2 complex on the basis of electron-capture dissociation fragmentation to confirm the identity and sequence of the protein's subunits. We found that both gene products of the β polypeptides are expressed and assembled in the mature LH2 complex, but only the Puc1A-encoded polypeptide α is observed here. The methionine of the Puc2B-encoded polypeptide is missing, and a carboxyl group is attached to the threonine at the N-terminus. Surprisingly, one amino acid encoded as an isoleucine in both the puc2B gene and the mRNA is found as valine in the mature LH2 complex, suggesting an unexpected and unusual post-translational modification or a specific tRNA recoding of this one amino acid.
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Yue
authors.initials: Y
authors.lastname: Lu
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Weidong
authors.initials: W
authors.lastname: Cui
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Rafael
authors.initials: R
authors.lastname: Saer
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Haijun
authors.initials: H
authors.lastname: Liu
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Gross
authors.affiliation: Department of Chemistry, ‡Department of Biology, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D025541:Bacteriochlorophyll A / Q000737:chemistry*; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D013058:Mass Spectrometry; D009876:Operon; D012242:Rhodobacter sphaeroides / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D025541:Bacteriochlorophyll A; D045342:Light-Harvesting Protein Complexes