A 12 Å carotenoid translocation in a photoswitch associated with cyanobacterial photoprotection

Abstract

Pigment-protein and pigment-pigment interactions are of fundamental importance to the light-harvesting and photoprotective functions essential to oxygenic photosynthesis. The orange carotenoid protein (OCP) functions as both a sensor of light and effector of photoprotective energy dissipation in cyanobacteria. We report the atomic-resolution structure of an active form of the OCP consisting of the N-terminal domain and a single noncovalently bound carotenoid pigment. The crystal structure, combined with additional solution-state structural data, reveals that OCP photoactivation is accompanied by a 12 angstrom translocation of the pigment within the protein and a reconfiguration of carotenoid-protein interactions. Our results identify the origin of the photochromic changes in the OCP triggered by light and reveal the structural determinants required for interaction with the light-harvesting antenna during photoprotection.

Fluorescent Labeling Preserving OCP Photoactivity Reveals Its Reorganization during the Photocycle

“…Simultaneously, a 2.2 Å decrease of the distance between the TMR dimer in the NTD and ECN was observed (Fig. 5), which is in line with the data considering that ECN in the red form stays in the NTD and even penetrates deeper into the protein core (15,18). It should be noted that the Förster theory considers donor and acceptor as point-like electromagnetic oscillators, i.e., objects without size and shape.…”

“…Although the structure of the red form of OCP is still unknown, recent efforts provided valuable information about photoinduced rearrangements of OCP (15)(16)(17)(18)(19)(20). It appeared that the red carotenoid protein (RCP), which is a spectral and functional analog of OCP R , could be obtained by partial proteolysis of OCP upon removal of its C-terminal domain (CTD) (15).…”

“…On the basis of the SAXS data (17), it was proposed that the CTD and NTD become completely separated upon photoconversion. Simultaneously, it was assumed that ECN not only stays in the NTD during photoconversion, but may also be translocated within the protein, as indicated by the 12-Å shift in chromophore position between the x-ray crystal structure of OCP O and that of RCP (18,27,28), which could be an important step for the formation of the active quenching state. Thus, the OCP structure is significantly rearranged upon the photoconversion, and protein-chromophore interactions play an important role in spectral tuning of this photoactive protein.…”

The terminal phycobilisome emitter, L CM : A light-harvesting pigment with a phytochrome chromophore

“…A variation in the kink could then modulate the distance between the PCB and the nearby chlorophyll of PSII, and, thereby, energy transfer. Importantly, it could also modulate the interaction with the cyanobacterial quenching protein, OCP, where the carotenoid chromophore undergoes a translocation of 12 Å upon activation (30). In combination, these modifications would provide a structural model for modulating the flow of excitation between productive (to PSII) and photoprotective (to activated OCP) (discussed below).…”

“…Energy transfer from L CM to the carotenoid is, however, more demanding (33). Dexter (electron exchange) transfer can be excluded, because the 4-keto-carotenoid is buried within activated OCP (30). The intense absorption of the carotenoid, however, makes energy transfer possible from L CM by a Förster process.…”