Solution Structures of Two Different FRP-OCP Complexes as Revealed via SEC-SANS.

Area of Science:

• Biophysics
• Photosynthesis research
• Structural biology

Background:

• Photosynthetic organisms use non-photochemical quenching to dissipate excess light energy.
• Cyanobacteria employ the orange carotenoid protein (OCP) as a sensor and quencher, undergoing conformational changes to an active red state (OCP^R) under high light.
• The fluorescence recovery protein (FRP) aids OCP's return to its dark-adapted state, but molecular details of their interaction remain unclear.

Purpose of the Study:

• To elucidate the solution structures of FRP-OCP complexes.
• To investigate the interaction between FRP and a truncated OCP mutant (^∆NTEOCP^O).
• To characterize intermediate states in the OCP photocycle facilitated by FRP.

Main Methods:

• Utilized small-angle neutron scattering combined with size exclusion chromatography (SEC-SANS).
• Analyzed solution structures of FRP-^∆NTEOCP^O complexes.
• Performed ab initio low-resolution shape reconstructions and compared them to homology models.

Main Results:

• Identified stable 2:2 and novel 2:1 FRP-^∆NTEOCP^O complexes in solution.
• Provided low-resolution structural models for both complex types.
• Observed evidence suggesting these complexes are transient intermediates in the OCP photocycle.

Conclusions:

• The study reveals previously uncharacterized intermediate states of FRP-OCP complexes.
• SEC-SANS is effective for determining structures of transient protein complexes in polydisperse solutions.
• Findings contribute to understanding photoprotection mechanisms in cyanobacteria.