Certain cyanobacteria can utilize both green and red lights for photosynthesis by using their light-harvesting antenna supercomplex called phycobilisome. They can control the absorptive maxima of phycobilisome, which results in remarkable changes in cell color. This phenomenon is regulated by RcaE that belongs to cyanobacteriochrome family of photosensors. RcaE harbors a bilin chromophore and photoconverts green- and red-absorbing states to sense ambient light colors. Although the green and red photoconversion is involved in bilin photoisomerization and subsequent change in bilin protonation state, the structural basis of this photoconversion remains unknown.

The research group comprised Takayuki Nagae (Nagoya University), Masaki Mishima (Tokyo University of Pharmacy and Life Science), Yuu Hirose (Toyohashi University of Technology), Masashi Unno (Saga University), Kei Wada (Miyazaki University), and Yutaka Itoh (Tokyo City University). They determined the high-resolution structure of RcaE in its red-absorbing state via X-ray crystallography. The bilin chromophore showed a conformation with co-planar A-C rings, wherein the nitrogen atoms were facing inward; the nitrogen of the tilted D-ring was facing outward (classified as C15-E,syn structure). Additionally, they identified a porous cavity containing water molecules in the bilin-binding pocket of RcaE. The clustered water molecules were connected with the nitrogen atoms of bilin A-C rings by a hydrogen bond network through the conserved acidic residue, Glu217.