title: Crystal structure of NblA from Anabaena sp. PCC 7120, a small protein playing a key role in phycobilisome degradation.
abstract: Cyanobacterial light-harvesting complexes, the phycobilisomes, are proteolytically degraded when the organisms are starved for combined nitrogen, a process referred to as chlorosis or bleaching. Gene nblA, present in all phycobilisome-containing organisms, encodes a protein of about 7 kDa that plays a key role in phycobilisome degradation. The mode of action of NblA in this degradation process is poorly understood. Here we presented the 1.8-A crystal structure of NblA from Anabaena sp. PCC 7120. In the crystal, NblA is present as a four-helix bundle formed by dimers, the basic structural units. By using pull-down assays with immobilized NblA and peptide scanning, we showed that NblA specifically binds to the alpha-subunits of phycocyanin and phycoerythrocyanin, the main building blocks of the phycobilisome rod structure. By site-directed mutagenesis, we identified amino acid residues in NblA that are involved in phycobilisome binding. The results provided evidence that NblA is directly involved in phycobilisome degradation, and the results allowed us to present a model that gives insight into the interaction of this small protein with the phycobilisomes.
authors.affiliation: Crystallography Group, Max-Delbrück Center for Molecular Medicine, D-13125 Berlin.
authors.forename: Ralf
authors.initials: R
authors.lastname: Bienert
authors.forename: Kerstin
authors.initials: K
authors.lastname: Baier
authors.forename: Rudolf
authors.initials: R
authors.lastname: Volkmer
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lockau
authors.forename: Udo
authors.initials: U
authors.lastname: Heinemann
mesh_terms: D000595:Amino Acid Sequence; D017033:Anabaena / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000378:metabolism*; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D015964:Gene Expression Regulation, Bacterial; D005982:Glutathione Transferase / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000737:chemistry; D052980:Phycobilins; D045524:Phycobilisomes / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology*; D010798:Phycocyanin / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D010455:Peptides; D052980:Phycobilins; D045524:Phycobilisomes; C426123:nblA protein, Synechocystis; C038397:phycoerythrocyanin; D010798:Phycocyanin; D005982:Glutathione Transferase