Structural and functional modularity of the orange carotenoid protein: distinct roles for the N- and C-terminal domains in cyanobacterial photoprotection.

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.