Cyanobacteria possess a family of one-helix high light-inducible proteins (Hlips) that are homologous to light-harvesting antenna of plants and algae. An Hlip protein, high light-inducible protein D (HliD) purified as a small complex with the Ycf39 protein is evaluated using resonance Raman spectroscopy. We show that the HliD binds two different β-carotenes, each present in two non-equivalent binding pockets with different conformations, having their (0,0) absorption maxima at 489 and 522 nm, respectively. Both populations of β-carotene molecules were in all-trans configuration and the absorption position of the farthest blue-shifted β-carotene was attributed entirely to the polarizability of the environment in its binding pocket. In contrast, the absorption maximum of the red-shifted β-carotene was attributed to two different factors: the polarizability of the environment in its binding pocket and, more importantly, to the conformation of its β-rings. This second β-carotene has highly twisted β-rings adopting a flat conformation, which implies that the effective conjugation length N is extended up to 10.5 modifying the energetic levels. This increase in N will also result in a lower S1 energy state, which may provide a permanent energy dissipation channel. Analysis of the carbonyl stretching region for chlorophyll a excitations indicates that the HliD binds six chlorophyll a molecules in five non-equivalent binding sites, with at least one chlorophyll a presenting a slight distortion to its macrocycle. The binding modes and conformations of HliD-bound pigments are discussed with respect to the known structures of LHCII and CP29.

From the Institute for Integrative Biology of the Cell CEA CNRS Univ Paris Sud Université Paris Saclay F 91198 Gif sur Yvette cedex France

the Centre Algatech Institute of Microbiology Academy of Sciences of the Czech Republic Třeboň 379 81 Czech Republic and

the Institute of Physics and Biophysics Faculty of Science University of South Bohemia České Budějovice 370 01 Czech Republic

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