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Single-residue insertion switches the quaternary structure and exciton states of cryptophyte light-harvesting proteins

Significance There is intense interest in determining whether coherent quantum processes play a nontrivial role in biology. This interest was sparked by the discovery of long-lived oscillations in 2D electronic spectra of photosynthetic proteins, including the phycobiliproteins (PBPs) from cryptophyte algae. Using X-ray crystallography, we show that cryptophyte PBPs adopt one of two quaternary structures, open or closed. The key feature of the closed form is the juxtaposition of two central chromophores resulting in excitonic coupling. The switch between forms is ascribed to the insertion of a single amino acid in the open-form proteins. Thus, PBP quaternary structure controls excitonic coupling and the mechanism of light harvesting. Comparing organisms with these two distinct proteins will reveal the role of quantum coherence in photosynthesis. Observation of coherent oscillations in the 2D electronic spectra (2D ES) of photosynthetic proteins has led researchers to ask whether nontrivial quantum phenomena are biologically significant. Coherent oscillations have been reported for the soluble light-harvesting phycobiliprotein (PBP) antenna isolated from cryptophyte algae. To probe the link between spectral properties and protein structure, we determined crystal structures of three PBP light-harvesting complexes isolated from different species. Each PBP is a dimer of αβ subunits in which the structure of the αβ monomer is conserved. However, we discovered two dramatically distinct quaternary conformations, one of which is specific to the genus Hemiselmis. Because of steric effects emerging from the insertion of a single amino acid, the two αβ monomers are rotated by ∼73° to an “open” configuration in contrast to the “closed” configuration of other cryptophyte PBPs. This structural change is significant for the light-harvesting function because it disrupts the strong excitonic coupling between two central chromophores in the closed form. The 2D ES show marked cross-peak oscillations assigned to electronic and vibrational coherences in the closed-form PC645. However, such features appear to be reduced, or perhaps absent, in the open structures. Thus cryptophytes have evolved a structural switch controlled by an amino acid insertion to modulate excitonic interactions and therefore the mechanisms used for light harvesting.