title: Single-residue insertion switches the quaternary structure and exciton states of cryptophyte light-harvesting proteins.
abstract: 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.
authors: School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia;
authors: Stephen J
authors: SJ
authors: Harrop
authors: School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia;
authors: Krystyna E
authors: KE
authors: Wilk
authors: Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6;
authors: Rayomond
authors: R
authors: Dinshaw
authors: Department of Chemical Sciences, University of Padua, 35131 Padua, Italy;
authors: Elisabetta
authors: E
authors: Collini
authors: Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6;
authors: Tihana
authors: T
authors: Mirkovic
authors: Department of Botany, University of British Columbia, Vancouver, BC, Canada V6T 1Z4;
authors: Chang Ying
authors: CY
authors: Teng
authors: Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6;
authors: Daniel G
authors: DG
authors: Oblinsky
authors: Department of Botany, University of British Columbia, Vancouver, BC, Canada V6T 1Z4;
authors: Beverley R
authors: BR
authors: Green
authors: Botanical Institute, Cologne Biocenter, University of Cologne, 50674 Cologne, Germany;
authors: Kerstin
authors: K
authors: Hoef-Emden
authors: Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia; and.
authors: Roger G
authors: RG
authors: Hiller
authors: Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6;
authors: Gregory D
authors: GD
authors: Scholes
authors: School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia;Centre for Applied Medical Research, St Vincent's Hospital, Sydney, NSW 2010, Australia p.curmi@unsw.edu.au.
authors: Paul M G
authors: PM
authors: Curmi
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D044785:Cryptophyta / Q000235:genetics*; D018360:Crystallography, X-Ray; D019281:Dimerization; D019143:Evolution, Molecular; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional / Q000235:genetics*; D052979:Phycobiliproteins / Q000737:chemistry / Q000235:genetics*; D011487:Protein Conformation; D017422:Sequence Analysis, DNA; D013057:Spectrum Analysis
chemical_list: D052979:Phycobiliproteins
keywords: X-ray crystallography; excitonic switching; protein evolution; quantum coherence