Phaser-Trim: A Phase Separation Based Genetically Encoded Reporter for H3K9 Trimethylation in Living Cells

Histone methylation is a key epigenetic modification that regulates the chromatin structure and gene expression for proper cellular and physiological processes. Aberrant histone methylation patterns are implicated in many diseases. Therefore, monitoring histone methylation dynamics in living cells and species is essential for elucidating its regulatory mechanisms and identifying potential therapeutic targets. However, current methods for detecting histone methylation are limited by their low sensitivity and specificity. To overcome this challenge, we have developed a genetically encoded biosensor named Phaser-Trim (<u>Ph</u>ase <u>se</u>paration based genetically encoded <u>r</u>eporter for H3K9 <u>Trim</u>ethylation) to detect the dynamic changes of H3K9me3 in living cells and species through the generation and disappearance of phase-separated droplets. Phaser-Trim demonstrates advantages of clear phenotypic characteristics, convenient operation, quantitative accuracy, biocompatibility, high specificity, and superior imaging performance with high signal-to-background ratio (SBR) for <i>in vivo</i> animal imaging. Using Phaser-Trim, we have successfully detected the dynamics of the H3K9me3 level during the differentiation of neural stem cells in <i>Drosophila</i>. Furthermore, Phaser-Trim also holds promise for application in high-throughput screening systems to facilitate the discovery of novel anticancer drugs.