Stages of OCP-FRP Interactions in the Regulation of Photoprotection in Cyanobacteria, Part 2: Small-Angle Neutron Scattering with Partial Deuteration

We used small-angle neutron scattering partially coupled with size-exclusion chromatography to unravel the solution structures of two variants of the Orange Carotenoid Protein (OCP) lacking the N terminal extension (OCP-Delta NTE) and its complex formation with the Fluorescence Recovery Protein (FRP). The dark-adapted, orange form OCP-Delta NTEO is fully photoswitchable and preferentially binds the pigment echinenone. Its complex with FRP consists of a monomeric OCP component, which closely resembles the compact structure expected for the OCP ground state, OCPO. In contrast, the pink form OCP-Delta NTEP, preferentially binding the pigment canthaxanthin, is mostly nonswitchable. The pink OCP form appears to occur as a dimer and is characterized by a separation of the N-and C-terminal domains, with the canthaxanthin embedded only into the N-terminal domain. Therefore, OCP-Delta NTEP can be viewed as a prototypical model system for the active, spectrally red-shifted state of OCP, OCPR. The dimeric structure of OCP-Delta NTEP is retained in its complex with FRP. Small-angle neutron scattering using partially deuterated OCP-FRP complexes reveals that FRP undergoes significant structural changes upon complex formation with OCP. The observed structures are assigned to individual intermediates of the OCP photocycle in the presence of FRP.

Automated summary

Small-angle neutron scattering combined with size-exclusion chromatography was used to study the solution structures of two variants of the Orange Carotenoid Protein (OCP) and its complex with the Fluorescence Recovery Protein (FRP). The OCP-Delta NTEO variant is fully photoswitchable and preferentially binds echinenone, while the OCP-Delta NTEP variant is mostly nonswitchable and preferentially binds canthaxanthin. The structures of the OCP-FRP complexes reveal significant structural changes in FRP upon complex formation.