title: Structural and functional modularity of the orange carotenoid protein: distinct roles for the N- and C-terminal domains in cyanobacterial photoprotection.
abstract: The orange carotenoid protein (OCP) serves as a sensor of light intensity and an effector of phycobilisome (PB)-associated photoprotection in cyanobacteria. Structurally, the OCP is composed of two distinct domains spanned by a single carotenoid chromophore. Functionally, in response to high light, the OCP converts from a dark-stable orange form, OCP(O), to an active red form, OCP(R). The C-terminal domain of the OCP has been implicated in the dynamic response to light intensity and plays a role in switching off the OCP's photoprotective response through its interaction with the fluorescence recovery protein. The function of the N-terminal domain, which is uniquely found in cyanobacteria, is unclear. To investigate its function, we isolated the N-terminal domain in vitro using limited proteolysis of native OCP. The N-terminal domain retains the carotenoid chromophore; this red carotenoid protein (RCP) has constitutive PB fluorescence quenching activity comparable in magnitude to that of active, full-length OCP(R). A comparison of the spectroscopic properties of the RCP with OCP(R) indicates that critical protein-chromophore interactions within the C-terminal domain are weakened in the OCP(R) form. These results suggest that the C-terminal domain dynamically regulates the photoprotective activity of an otherwise constitutively active carotenoid binding N-terminal domain.
authors.affiliation: Department of Plant and Microbial Biology, University of California, Berkeley, California 94720.
authors.forename: Ryan L
authors.initials: RL
authors.lastname: Leverenz
authors.forename: Denis
authors.initials: D
authors.lastname: Jallet
authors.forename: Ming-De
authors.initials: MD
authors.lastname: Li
authors.forename: Richard A
authors.initials: RA
authors.lastname: Mathies
authors.forename: Diana
authors.initials: D
authors.lastname: Kirilovsky
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Kerfeld
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D002338:Carotenoids / Q000378:metabolism; D000458:Cyanobacteria / Q000378:metabolism*; D004734:Energy Metabolism; D017434:Protein Structure, Tertiary; D059748:Proteolysis
chemical_list: D001426:Bacterial Proteins; D002338:Carotenoids