Improvement of a FRET-based redox probe Redoxfluor through circular permutation and effects of substitution of cysteine residues on its redox properties

The properties of a FRET-based redox probe Redoxfluor have been improved for its sensitivity and dynamic range. Substitution of the Citrine portion of Redoxfluor with circular permutated (cp) Citrine improved the dynamic range without affecting the redox potential. The cp158 mutant, referred to as Redoxfluor 2, possessed the most extended dynamic range and detected intracellular redox changes in yeast and bacteria, while the original did not. Investigation of the glutathione-redox dependency of the FRET ratio of various cysteine-substituted mutants revealed that Cys230 in the linker between Cerulean and the C-terminal cysteine-rich domain (CRD) and Cys385 in Citrine are essential for glutathione redox sensing. Although neither cysteine residues in CRD is essential for glutathione redox sensing, substitution of the CRD cysteine residues prominently affected the dynamic range of redox sensing and the redox potential titrated with glutathione. One of the CRD cysteine-substituted mutants (C259A) showed a greatly extended dynamic range and a substantially reducing redox potential compared to the original Redoxfluor. Redoxfluor 2 and the C259A mutant are suitable for versatile uses including sensitive detection of aberrant redox states, redox visualization in the more reducing intracellular compartments and high-throughput screening of redox modulators active against pathologically abnormal redox states.