Intrabody-based FRET probe to visualize endogenous histone acetylation

Post-translational histone modifications are major regulators of gene expression. However, conventional immunoassays do not provide sufficient information regarding their spatial and temporal dynamic changes. Fluorescence/Forster resonance energy transfer (FRET)-based probes are capable of monitoring the dynamic changes associated with histone modifications in real-time by measuring the balance between histone-modifying enzyme activities. Recently, a genetically encoded histone-modification fluorescent probe using a single-chain variable region (scFv) fragment of a specific antibody was developed. The probe, modification-specific intracellular antibody, is capable of monitoring histone-acetylation levels in both cultured cells and living organisms based on the ratio of fluorescence intensities between the cell nucleus and cytoplasm. In this study, we constructed a FRET probe composed of yellow fluorescent protein attached at the N-terminus of an acetyl H3K9-specific scFv, tethered to a cyan fluorescent protein. When the FRET probe was expressed in human cells, both FRET efficiency and fluorescence intensity in the nucleus increased following histone-deacetylase inhibitor treatment. Using these two parameters, endogenous histone-acetylation levels were quantified over a high dynamic range. This probe provides a simple approach to quantify spatial and temporal dynamic changes in histone acetylation.