Hydrophobic Residues near the Bilin Chromophore-Binding Pocket Modulate Spectral Tuning of Insert-Cys Subfamily Cyanobacteriochromes

• Published in: Scientific reports 2017-01, Vol.7 (1), p.40576-40576, Article 40576
• Main Authors: Cho, Sung Mi, Jeoung, Sae Chae, Song, Ji-Young, Song, Ji-Joon, Park, Youn-Il
• Format: Article
• Language: English
• Subjects: 631/326; 631/449/2686; 631/45/612; 82/83; Absorption; Amino Acid Motifs; Amino Acid Sequence; Amino Acids - chemistry; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Circular Dichroism; Conserved Sequence; Cyanobacteria; Cysteine - chemistry; E coli; Humanities and Social Sciences; Hydrophobic and Hydrophilic Interactions; Hydrophobicity; Light; multidisciplinary; Photoreceptors; Photoreceptors, Microbial - chemistry; Photoreceptors, Microbial - metabolism; Phycocyanobilin; Proteins; Residues; Science; Science (multidisciplinary); Ultraviolet radiation
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep40576

Cyanobacteriochromes (CBCRs) are a subfamily of phytochrome photoreceptors found exclusively in photosynthetic cyanobacteria. Four CBCRs containing a second Cys in the insert region (insert-Cys) have been identified from the nonheterocystous cyanobacterium Microcoleus B353 (Mbr3854g4 and Mbl3738g2) and the nitrogen fixing, heterocystous cyanobacterium Nostoc punctiforme (NpF2164g3 and NpR1597g2). These insert-Cys CBCRs can sense light in the near-UV to orange range, but key residues responsible for tuning their colour sensitivity have not been reported. In the present study, near-UV/Green (UG) photosensors Mbr3854g4 (UG1) and Mbl3738g2 (UG2) were chosen for further spectroscopic analysis of their spectral sensitivity and tuning. Consistent with most dual-Cys CBCRs, both UGs formed a second thioether linkage to the phycocyanobilin (PCB) chromophore via the insert-Cys. This bond is subject to breakage and relinkage during forward and reverse photoconversions. Variations in residues equivalent to Phe that are in close contact with the PCB chromophore D-ring in canonical red/green CBCRs are responsible for tuning the light absorption peaks of both dark and photoproducts. This is the first time these key residues that govern light absorption in insert-Cys family CBCRs have been identified and characterised.