title: Degradation of phycobilisomes in Synechocystis sp. PCC6803: evidence for essential formation of an NblA1/NblA2 heterodimer and its codegradation by A Clp protease complex.
abstract: When cyanobacteria acclimate to nitrogen deficiency, they degrade their large (3-5-MDa), light-harvesting complexes, the phycobilisomes. This massive, yet specific, intracellular degradation of the pigmented phycobiliproteins causes a color change of cyanobacterial cultures from blue-green to yellow-green, a process referred to as chlorosis or bleaching. Phycobilisome degradation is induced by expression of the nblA gene, which encodes a protein of ~7 kDa. NblA most likely acts as an adaptor protein that guides a Clp protease to the phycobiliproteins, thereby initiating the degradation process. Most cyanobacteria and red algae possess just one nblA-homologous gene. As an exception, the widely used "model organism" Synechocystis sp. PCC6803 expresses two such genes, nblA16803 and nblA26803, both of whose products are required for phycobilisome degradation. Here, we demonstrate that the two NblA proteins heterodimerize in vitro and in vivo using pull-down assays and a Förster energy-transfer approach, respectively. We further show that the NblA proteins form a ternary complex with ClpC (the HSP100 chaperone partner of Clp proteases) and phycobiliproteins in vitro. This complex is susceptible to ATP-dependent degradation by a Clp protease, a finding that supports a proposed mechanism of the degradation process. Expression of the single nblA gene encoded by the genome of the N2-fixing, filamentous cyanobacterium Nostoc sp. PCC7120 in the nblA1/nblA2 mutant of Synechocystis sp. PCC6803 induced phycobilisome degradation, suggesting that the function of the NblA heterodimer of Synechocystis sp. PCC6803 is combined in the homodimeric protein of Nostoc sp. PCC7120.
authors.affiliation: Institut für Biologie, Humboldt Universität zu Berlin, Chausseestrasse 117, D-10115 Berlin, Germany.
authors.forename: Antje
authors.initials: A
authors.lastname: Baier
authors.affiliation: Institut für Biologie, Humboldt Universität zu Berlin, Chausseestrasse 117, D-10115 Berlin, Germany.
authors.forename: Wiebke
authors.initials: W
authors.lastname: Winkler
authors.affiliation: Group of Molecular Biophysics, Humboldt-Universität zu Berlin, Invalidenstrasse 42, D-10115 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Korte
authors.affiliation: Institut für Biologie, Humboldt Universität zu Berlin, Chausseestrasse 117, D-10115 Berlin, Germany. Electronic address: wolfgang.lockau@biologie.hu-berlin.de.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lockau
authors.affiliation: Institut für Biologie, Humboldt Universität zu Berlin, Chausseestrasse 117, D-10115 Berlin, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Karradt
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D019281:Dimerization; D049071:Endopeptidase Clp / Q000378:metabolism*; D031541:Fluorescence Resonance Energy Transfer; D045524:Phycobilisomes / Q000378:metabolism*; D059748:Proteolysis; D046939:Synechocystis / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045524:Phycobilisomes; D049071:Endopeptidase Clp
keywords: Cyanobacteria; Fluorescence Resonance Energy Transfer (FRET); NblA; Nitrogen Starvation; Phycobilisome Degradation; Protease; Protein Degradation; Protein Synthesis; Synechocystis