The structural basis of far-red light absorbance by allophycocyanins
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ORIGINAL ARTICLE
The structural basis of far‑red light absorbance by allophycocyanins Nathan Soulier1 · Donald A. Bryant1,2,3 Received: 28 July 2020 / Accepted: 2 October 2020 © Springer Nature B.V. 2020
Abstract Phycobilisomes (PBS), the major light-harvesting antenna in cyanobacteria, are supramolecular complexes of colorless linkers and heterodimeric, pigment-binding phycobiliproteins. Phycocyanin and phycoerythrin commonly comprise peripheral rods, and a multi-cylindrical core is principally assembled from allophycocyanin (AP). Each AP subunit binds one phycocyanobilin (PCB) chromophore, a linear tetrapyrrole that predominantly absorbs in the orange-red region of the visible spectrum (600– 700 nm). AP facilitates excitation energy transfer from PBS peripheral rods or from directly absorbed red light to accessory chlorophylls in the photosystems. Paralogous forms of AP that bind PCB and are capable of absorbing far-red light (FRL; 700–800 nm) have recently been identified in organisms performing two types of photoacclimation: FRL photoacclimation (FaRLiP) and low-light photoacclimation (LoLiP). The FRL-absorbing AP (FRL-AP) from the thermophilic LoLiP strain Synechococcus sp. A1463 was chosen as a platform for site-specific mutagenesis to probe the structural differences between APs that absorb in the visible region and FRL-APs and to identify residues essential for the FRL absorbance phenotype. Conversely, red light-absorbing allophycocyanin-B (AP-B; ~ 670 nm) from the same organism was used as a platform for creating a FRL-AP. We demonstrate that the protein environment immediately surrounding pyrrole ring A of PCB on the alpha subunit is mostly responsible for the FRL absorbance of FRL-APs. We also show that interactions between PCBs bound to alpha and beta subunits of adjacent protomers in trimeric AP complexes are responsible for a large bathochromic shift of about ~ 20 nm and notable sharpening of the long-wavelength absorbance band. Keywords Photosynthesis · Phycobiliproteins · Phycobilisomes · Far-red light photoacclimation · Low-light photoacclimation · Site-directed mutagenesis · Phycocyanobilin
Introduction Phycobiliproteins (PBPs) are brilliantly colored proteins that assemble into elaborate, supramolecular light-harvesting antenna complexes, phycobilisomes (PBS), in cyanobacteria Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11120-020-00787-y) contains supplementary material, which is available to authorized users. * Donald A. Bryant [email protected] 1
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA
2
Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, USA
3
S‑002 Frear Laboratory, Dept. of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA
and red algae (Bryant et al. 1979; Glazer 1989; Sidler 1994; Bryant and Canniffe 2018). PBPs obtain their colors from covalently bound, linea
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