title: The Chloroplast SRP Systems of Chaetosphaeridium globosum and Physcomitrella patens as Intermediates in the Evolution of SRP-Dependent Protein Transport in Higher Plants.
abstract: The bacterial signal recognition particle (SRP) mediates the cotranslational targeting of membrane proteins and is a high affinity complex consisting of a SRP54 protein subunit (Ffh) and an SRP RNA. The chloroplast SRP (cpSRP) pathway has adapted throughout evolution to enable the posttranslational targeting of the light harvesting chlorophyll a/b binding proteins (LHCPs) to the thylakoid membrane. In spermatophytes (seed plants), the cpSRP lacks the SRP RNA and is instead formed by a high affinity interaction of the conserved 54-kD subunit (cpSRP54) with the chloroplast-specific cpSRP43 protein. This heterodimeric cpSRP recognizes LHCP and delivers it to the thylakoid membrane. However, in contrast to spermatophytes, plastid SRP RNAs were identified within all streptophyte lineages and in all chlorophyte branches. Furthermore, it was shown that cpSRP43 does not interact with cpSRP54 in chlorophytes (e.g., Chlamydomonas reinhardtii). In this study, we biochemically characterized the cpSRP system of the charophyte Chaetosphaeridium globosum and the bryophyte Physcomitrella patens. Interaction studies demonstrate low affinity binding of cpSRP54 to cpSRP43 (Kd ~10 μM) in Chaetosphaeridium globosum and Physcomitrella patens as well as relatively low affinity binding of cpSRP54 to cpSRP RNA (Kd ~1 μM) in Physcomitrella patens. CpSRP54/cpSRP43 complex formation in charophytes is supported by the finding that specific alterations in the second chromodomain of cpSRP43, that are conserved within charophytes and absent in land plants, do not interfere with cpSRP54 binding. Furthermore, our data show that the elongated apical loop structure of the Physcomitrella patens cpSRP RNA contributes to the low binding affinity between cpSRP54 and the cpSRP RNA.
authors.affiliation: Molecular Biology of Plant Organelles, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Dominik
authors.initials: D
authors.lastname: Ziehe
authors.affiliation: Molecular Biology of Plant Organelles, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Beatrix
authors.initials: B
authors.lastname: Dünschede
authors.affiliation: Molecular Biology of Plant Organelles, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Mira
authors.initials: M
authors.lastname: Zenker
authors.affiliation: Molecular Biology of Plant Organelles, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Silke
authors.initials: S
authors.lastname: Funke
authors.affiliation: Cyanobacterial Membrane Protein Complexes, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Marc M
authors.initials: MM
authors.lastname: Nowaczyk
authors.affiliation: Molecular Biology of Plant Organelles, Ruhr-University Bochum, 44780, Bochum, Germany.
authors.forename: Danja
authors.initials: D
authors.lastname: Schünemann
mesh_terms: D000595:Amino Acid Sequence; D002736:Chloroplasts / Q000378:metabolism*; D046912:Multiprotein Complexes / Q000378:metabolism; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D021381:Protein Transport; D018749:RNA, Plant / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018271:Signal Recognition Particle / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D057949:Streptophyta / Q000378:metabolism*
chemical_list: D046912:Multiprotein Complexes; D018749:RNA, Plant; D018271:Signal Recognition Particle