SNAP-Tag-Based Subcellular Protein Labeling and Fluorescent Imaging with Naphthalimides.

Genetically encoded technologies provide methods for the specific labeling and imaging of proteins, which is essential to understand the subcellular localization of these proteins and their function. Herein, we employed naphthalimide, an efficient two-photon fluorophore, to develop O6 -benzylguanine (BG) derivatives for specific labeling of subcellular proteins and fluorescent imaging through the SNAP-tag. Three naphthalimide-BG derivatives, TNI-BG, QNI-BG, and ONI-BG, were conveniently synthesized through modular "click chemistry" in high yields. All of them showed high labeling efficiency with SNAP-tag in solution (≈1-2×103  s-1  m-1 ) and in bacteria. Among them, ONI-BG showed high specificity to diffused, histone H2B and mitochondria COX8A targeted SNAP-tag in mammalian cells. The protein-labeled naphthalimides exhibited high two-photon absorption cross-sections, which indicated their potential application in protein-specific two-photon fluorescent imaging, such as two-photon fluorescent lifetime imaging and two-photon multicolor imaging. Therefore, ONI-BG is a versatile tool that can be used to track subcellular proteins through multiple fluorescent techniques.