A far-red cyanobacteriochrome lineage specific for verdins

Area of Science:

• Biochemistry
• Biophysics
• Molecular Biology

Background:

• Cyanobacteriochromes (CBCRs) are light-sensing proteins with broad spectral absorption.
• Far-red absorbing CBCRs (frCBCRs) are of interest for optogenetics due to far-red light's deep tissue penetration and CBCR's small size.

Purpose of the Study:

• To elucidate the structural basis for far-red absorption in the frCBCR JSC1_58120g3.
• To investigate the chromophore-binding preference of JSC1_58120g3.

Main Methods:

• X-ray crystallography of JSC1_58120g3 with 18¹,18²-dihydrobiliverdin (18¹,18²-DHBV) and biliverdin IXα (BV).
• Structure-guided mutagenesis to identify critical residues for chromophore binding and spectral properties.

Main Results:

• JSC1_58120g3's far-red absorption is mediated by 18¹,18²-DHBV, not the typical phycocyanobilin (PCB).
• JSC1_58120g3 efficiently incorporates the widespread biliverdin IXα (BV), bypassing the need for PCB.
• Key residues governing verdin-binding preference and far-red absorption were identified.

Conclusions:

• The JSC1_58120g3 frCBCR lineage utilizes a distinct chromophore for far-red absorption.
• This frCBCR's ability to bind BV makes it a promising scaffold for developing optogenetic and deep-tissue imaging reagents.