Unexpected Light-Sensing Abilities Discovered in Ancient Bacterial Proteins

Researchers have discovered novel photoreceptors in cyanobacteria that shed new light on the evolution of light-sensing mechanisms. These photoreceptors, called cyanobacteriochromes (CBCRs), are related to phytochromes found in plants but have unique properties. The study identified CBCRs belonging to the extended red/green (XRG) lineage that have a cysteine residue in place of the usual tryptophan in a key motif. Surprisingly, despite lacking other structural features typical of CBCRs that can isomerize their chromophore, these new CBCRs were able to perform this isomerization.

Through detailed spectroscopic analysis and mutagenesis experiments, the researchers pinpointed three amino acid residues crucial for the isomerization ability: a cysteine, valine, and isoleucine. Replacing these with the amino acids found in typical XRG CBCRs eliminated the isomerization capability.

The cysteine residue appears to play the most important role, likely by altering how the chromophore is positioned. The valine and isoleucine residues seem to provide supportive stabilizing interactions. Together, these changes create the right environment to trigger isomerization of the chromophore from phycocyanobilin to phycoviolobilin.

This discovery reveals unexpected diversity and plasticity in the molecular evolution of CBCRs. It demonstrates how relatively small changes in protein structure can lead to new light-sensing capabilities. The findings provide insight into how photoreceptors adapt and diversify to allow organisms to optimally sense and respond to their light environment.

The study highlights the power of combining bioinformatics, spectroscopy, and protein engineering to unravel the intricacies of photoreceptor function and evolution. It opens up new avenues for understanding and potentially engineering novel light-sensing proteins.

This work was published on the journal Protein science : a publication of the Protein Society on 2024-08-01 with the title "Red/green cyanobacteriochromes acquire isomerization from phycocyanobilin to phycoviolobilin." It is available at: 10.1002/pro.5132.