title: Orange carotenoid protein burrows into the phycobilisome to provide photoprotection.
abstract: In cyanobacteria, photoprotection from overexcitation of photochemical centers can be obtained by excitation energy dissipation at the level of the phycobilisome (PBS), the cyanobacterial antenna, induced by the orange carotenoid protein (OCP). A single photoactivated OCP bound to the core of the PBS affords almost total energy dissipation. The precise mechanism of OCP energy dissipation is yet to be fully determined, and one question is how the carotenoid can approach any core phycocyanobilin chromophore at a distance that can promote efficient energy quenching. We have performed intersubunit cross-linking using glutaraldehyde of the OCP and PBS followed by liquid chromatography coupled to tandem mass spectrometry (LC/MS-MS) to identify cross-linked residues. The only residues of the OCP that cross-link with the PBS are situated in the linker region, between the N- and C-terminal domains and a single C-terminal residue. These links have enabled us to construct a model of the site of OCP binding that differs from previous models. We suggest that the N-terminal domain of the OCP burrows tightly into the PBS while leaving the OCP C-terminal domain on the exterior of the complex. Further analysis shows that the position of the small core linker protein ApcC is shifted within the cylinder cavity, serving to stabilize the interaction between the OCP and the PBS. This is confirmed by a ΔApcC mutant. Penetration of the N-terminal domain can bring the OCP carotenoid to within 5-10 Å of core chromophores; however, alteration of the core structure may be the actual source of energy dissipation.
authors.affiliation: Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel;
authors.forename: Dvir
authors.initials: D
authors.lastname: Harris
authors.affiliation: Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel;
authors.forename: Ofir
authors.initials: O
authors.lastname: Tal
authors.affiliation: Institute for Integrative Biology of the Cell (I2BC), Commissariat à l'Energie Atomique (CEA), CNRS, Université Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette, France; CEA, Institut de Biologie et Technologies de Saclay (iBiTec-S), 91191 Gif-sur-Yvette, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Jallet
authors.affiliation: Institute for Integrative Biology of the Cell (I2BC), Commissariat à l'Energie Atomique (CEA), CNRS, Université Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette, France; CEA, Institut de Biologie et Technologies de Saclay (iBiTec-S), 91191 Gif-sur-Yvette, France.
authors.forename: Adjélé
authors.initials: A
authors.lastname: Wilson
authors.affiliation: Institute for Integrative Biology of the Cell (I2BC), Commissariat à l'Energie Atomique (CEA), CNRS, Université Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette, France; CEA, Institut de Biologie et Technologies de Saclay (iBiTec-S), 91191 Gif-sur-Yvette, France diana.kirilovsky@cea.fr nadir@tx.technion.ac.il.
authors.forename: Diana
authors.initials: D
authors.lastname: Kirilovsky
authors.affiliation: Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel; diana.kirilovsky@cea.fr nadir@tx.technion.ac.il.
authors.forename: Noam
authors.initials: N
authors.lastname: Adir
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000502:physiology; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004735:Energy Transfer; D005976:Glutaral / Q000494:pharmacology; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D052980:Phycobilins / Q000528:radiation effects; D045524:Phycobilisomes / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D010798:Phycocyanin / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D011487:Protein Conformation / Q000528:radiation effects; D021122:Protein Subunits; D011836:Radiation Tolerance; D013050:Spectrometry, Fluorescence; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D053719:Tandem Mass Spectrometry
chemical_list: D001426:Bacterial Proteins; D003432:Cross-Linking Reagents; D052980:Phycobilins; D045524:Phycobilisomes; D021122:Protein Subunits; C002497:allophycocyanin; C109473:orange carotenoid protein, Synechocystis; D010798:Phycocyanin; C011069:phycocyanobilin; D005976:Glutaral
keywords: cross-linking; cyanobacteria; light harvesting; nonphotochemical quenching; photosynthesis