Single-molecule trapping and spectroscopy reveals photophysical heterogeneity of phycobilisomes quenched by Orange Carotenoid Protein

Allison H. Squires, Peter D. Dahlberg, Haijun Liu, Nikki Cecil M. Magdaong, Robert E. Blankenship and W. E. Moerner ()
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Allison H. Squires: Stanford University
Peter D. Dahlberg: Stanford University
Haijun Liu: Washington University in St. Louis
Nikki Cecil M. Magdaong: Washington University in St. Louis
Robert E. Blankenship: Washington University in St. Louis
W. E. Moerner: Stanford University

Nature Communications, 2019, vol. 10, issue 1, 1-12

Abstract: Abstract The Orange Carotenoid Protein (OCP) is a cytosolic photosensor that is responsible for non-photochemical quenching (NPQ) of the light-harvesting process in most cyanobacteria. Upon photoactivation by blue-green light, OCP binds to the phycobilisome antenna complex, providing an excitonic trap to thermally dissipate excess energy. At present, both the binding site and NPQ mechanism of OCP are unknown. Using an Anti-Brownian ELectrokinetic (ABEL) trap, we isolate single phycobilisomes in free solution, both in the presence and absence of activated OCP, to directly determine the photophysics and heterogeneity of OCP-quenched phycobilisomes. Surprisingly, we observe two distinct OCP-quenched states, with lifetimes 0.09 ns (6% of unquenched brightness) and 0.21 ns (11% brightness). Photon-by-photon Monte Carlo simulations of exciton transfer through the phycobilisome suggest that the observed quenched states are kinetically consistent with either two or one bound OCPs, respectively, underscoring an additional mechanism for excitation control in this key photosynthetic unit.

Date: 2019
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DOI: 10.1038/s41467-019-09084-2

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