OCP\u2013FRP protein complex topologies suggest a mechanism for controlling high light tolerance in cyanobacteria

Nikolai N Sluchanko,Evgeny A Shirshin,Marcus Moldenhauer,Thomas Friedrich,Eugene G Maksimov,Yury B Slonimskiy

doi:10.1038/s41467-018-06195-0

Abstract

In cyanobacteria, high light photoactivates the orange carotenoid protein (OCP) that binds to antennae complexes, dissipating energy and preventing the destruction of the photosynthetic apparatus. At low light, OCP is efficiently deactivated by a poorly understood action of the dimeric fluorescence recovery protein (FRP). Here, we engineer FRP variants with defined oligomeric states and scrutinize their functional interaction with OCP. Complemented by disulfide trapping and chemical crosslinking, structural analysis in solution reveals the topology of metastable complexes of OCP and the FRP scaffold with different stoichiometries. Unable to tightly bind monomeric FRP, photoactivated OCP recruits dimeric FRP, which subsequently monomerizes giving 1:1 complexes. This could be facilitated by a transient OCP–2FRP–OCP complex formed via the two FRP head domains, significantly improving FRP efficiency at elevated OCP levels. By identifying key molecular interfaces, our findings may inspire the design of optically triggered systems transducing light signals into protein–protein interactions.

Highlights

In cyanobacteria, high light photoactivates the orange carotenoid protein (OCP) that binds to antennae complexes, dissipating energy and preventing the destruction of the photosynthetic apparatus
The dimeric state of the prototypical Synechocystis fluorescence recovery protein (FRP) and two of its homologs from Anabaena and Arthrospira was shown by size-exclusion chromatography (SEC)[24,25] and the common dimeric conformation in solution was established by SAXS25, permitting manipulations of the oligomeric state (Fig. 1a)
To avoid the necessity of dealing with the high conformational flexibility of photoactivated OCP analogs with separated domains, we focused on the analysis of the FRP complex with the compact ΔNTEO having the exposed FRP binding site on the CTD30, which represents an intermediate of the OCP compaction process associated with the alignment of OCP domains, immediately preceding FRP detachment and termination of its action cycle

Summary

Introduction

High light photoactivates the orange carotenoid protein (OCP) that binds to antennae complexes, dissipating energy and preventing the destruction of the photosynthetic apparatus. OCPR is thought to quench PBs fluorescence by directly interacting with the PBs core[8,19,20,21,22] This photoactivated OCP form is metastable but can be mimicked by mutation of the conserved Tyr/Trp residues coordinating the ketocarotenoid, which leads to destabilization of the compact protein structure and separation of the domains, such as in OCPW288A 23,24 and OCPY201A/W288A (hereafter, OCPAA)[15,25] variants. Despite the efforts of several laboratories, the whole chain of molecular events associated with the OCP-mediated photoprotection mechanism remains poorly understood, mainly due to the remarkable metastability of the photoactivated OCPR state and the dynamic and transient nature of its complexes with PBs and FRP22. This suggested that the FRP mechanism is rather universal across cyanobacterial species;[25] the intermediates of the OCP–FRP interaction and the topology of their complexes remained largely unknown

Methods

Results

Conclusion