Binding of red form of Orange Carotenoid Protein (OCP) to phycobilisome is not sufficient for quenching.

The Orange Carotenoid Protein (OCP) is responsible for photoprotection in many cyanobacteria. Absorption of blue light drives the conversion of the orange, inactive form (OCPO) to the red, active form (OCPR). Concomitantly, the N-terminal domain (NTD) and the C-terminal domain (CTD) of OCP separate, which ultimately leads to the formation of a quenched OCPR-PBS complex. The details of the photoactivation of OCP have been intensely researched. Binding site(s) of OCPR on the PBS core have also been proposed. However, the post-binding events of the OCPR-PBS complex remain unclear. Here, we demonstrate that PBS-bound OCPR is not sufficient as a PBS excitation energy quencher. Using site-directed mutagenesis, we generated a suite of single point mutations at OCP Leucine 51 (L51) of Synechocystis 6803. Steady-state and time-resolved fluorescence analyses demonstrated that all mutant proteins are unable to quench the PBS fluorescence, owing to either failed OCP binding to PBS, or, if bound, an OCP-PBS quenching state failed to form. The SDS-PAGE and Western blot analysis support that the L51A (Alanine) mutant binds to the PBS and therefore belongs to the second category. We hypothesize that upon binding to PBS, OCPR likely reorganizes and adopts a new conformational state (OCP3rd) different than either OCPO or OCPR to allow energy quenching, depending on the cross-talk between OCPR and its PBS core-binding counterpart.

Engineering the photoactive orange carotenoid protein with redox-controllable structural dynamics and photoprotective function.

Photosynthesis requires various photoprotective mechanisms for survival of organisms in high light. In cyanobacteria exposed to high light, the Orange Carotenoid Protein (OCP) is reversibly photoswitched from the orange (OCPO) to the red (OCPR) form, the latter binds to the antenna (phycobilisomes, PBs) and quenches its overexcitation. OCPR accumulation implicates restructuring of a compact dark-adapted OCPO state including detachment of the N-terminal extension (NTE) and separation of protein domains, which is reversed by interaction with the Fluorescence Recovery Protein (FRP). OCP phototransformation supposedly occurs via an intermediate characterized by an OCPR-like absorption spectrum and an OCPO-like protein structure, but the hierarchy of steps remains debatable. Here, we devise and analyze an OCP variant with the NTE trapped on the C-terminal domain (CTD) via an engineered disulfide bridge (OCPCC). NTE trapping preserves OCP photocycling within the compact protein structure but precludes functional interaction with PBs and especially FRP, which is completely restored upon reduction of the disulfide bridge. Non-interacting with the dark-adapted oxidized OCPCC, FRP binds reduced OCPCC nearly as efficiently as OCPO devoid of the NTE, suggesting that the low-affinity FRP binding to OCPO is realized via NTE displacement. The low efficiency of excitation energy transfer in complexes between PBs and oxidized OCPCC indicates that OCPCC binds to PBs in an orientation suboptimal for quenching PBs fluorescence. Our approach supports the presence of the OCPR-like intermediate in the OCP photocycle and shows effective uncoupling of spectral changes from functional OCP photoactivation, enabling redox control of its structural dynamics and function.