4LM6: Light harvesting complex PC612 from the cryptophyte Hemiselmis virescens M1635

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 alphabeta subunits in which the structure of the alphabeta 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 alphabeta monomers are rotated by approximately 73 degrees 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.
PDB ID: 4LM6Download
MMDB ID: 120774
PDB Deposition Date: 2013/7/10
Updated in MMDB: 2017/11
Experimental Method:
x-ray diffraction
Resolution: 1.7  Å
Source Organism:
Similar Structures:
Biological Unit for 4LM6: tetrameric; determined by author and by software (PISA)
Molecular Components in 4LM6
Label Count Molecule
Proteins (4 molecules)
Cryptophyte Phycocyanin Alpha Chain
Molecule annotation
Cryptophyte Phycocyanin Beta Chain
Molecule annotation
Chemicals (8 molecules)
* Click molecule labels to explore molecular sequence information.

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