A dimer-monomer transition captured by the crystal structures of cyanobacterial apo flavodoxin.

Shuwen Liu, Yuanyuan Chen, Tianming Du, Wencong Zhao, Xuejing Liu, Heng Zhang, Qing Yuan, Liang Gao, Yuhui Dong, Xueyun Gao, Yong Gong, Peng Cao

Biochemical and biophysical research communications 2023 Jan 08

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In cyanobacteria and algae (but not plants), flavodoxin (Fld) replaces ferredoxin (Fd) under stress conditions to transfer electrons from photosystem I (PSI) to ferredoxin-NADP+ reductase (FNR) during photosynthesis. Fld constitutes a small electron carrier noncovalently bound to flavin mononucleotide (FMN), and also an ideal model for revealing the protein/flavin-binding mechanism because of its relative simplicity compared to other flavoproteins. Here, we report two crystal structures of apo-Fld from Synechococcus sp. PCC 7942, one dimeric structure of 2.09 Å and one monomeric structure of 1.84 Å resolution. Analytical ultracentrifugation showed that in solution, apo-Fld exists both as monomers and dimers. Our dimer structure contains two ligand-binding pockets separated by a distance of 45 Å, much longer than the previous structures of FMN-bound dimers. These results suggested a potential dimer-monomer transition mechanism of cyanobacterial apo-Fld. We further propose that the dimer represents the "standby" state to stabilize itself, while the monomer constitutes the "ready" state to bind FMN. Furthermore, we generated a new docking model of cyanobacterial Fld-FNR complex based on the recently reported cryo-EM structures, and mapped the special interactions between Fld and FNR in detail. Copyright © 2022 Elsevier Inc. All rights reserved.

Citation

Shuwen Liu, Yuanyuan Chen, Tianming Du, Wencong Zhao, Xuejing Liu, Heng Zhang, Qing Yuan, Liang Gao, Yuhui Dong, Xueyun Gao, Yong Gong, Peng Cao. A dimer-monomer transition captured by the crystal structures of cyanobacterial apo flavodoxin. Biochemical and biophysical research communications. 2023 Jan 08;639:134-141