Nongenetically Encoded and Erasable Imaging Strategy for Receptor-Specific Glycans on Live Cells.

Glycans on specific receptors play a crucial role in regulating receptor functions and indicating cell pathological states. For a detailed glycosylation regulatory mechanism, live cell imaging of receptor-specific glycans becomes significantly important. In this work, we present a nongenetically encoded labeling strategy to specifically install a fluorescence resonance energy transfer (FRET) pair onto the receptor of interest by ligand-receptor binding and metabolic glycan engineering. This method breaks the limitation that the receptors have to possess an extracellular terminus which can be used to attach a fluorescent tag, avoiding the undesired effects introduced by inserting amino acids into proteins. Furthermore, the donor-equipped ligand can be flexibly competed with an unlabeled compound, leading to an efficient erasure of donor fluorescence signal. We envision that this strategy will have the potential to sequentially identify and characterize multiple receptor-specific glycans on the live cell surface through multiple cycles including labeled ligand binding, FRET-induced fluorescence imaging, and the unlabeled compound competing for fluorescence erasure. Besides, this in situ glycan profiling strategy will have wide applications in molecular diagnosis and cellular targeted therapies.