Autophagy Caught in the Act: A Supramolecular FRET Pair Based on an Ultrastable Synthetic Host–Guest Complex Visualizes Autophagosome–Lysosome Fusion

A supramolecular FRET pair based on the ultrahigh binding affinity between cyanine 3 conjugated cucurbit[7]uril (CB[7]-Cy3) and cyanine 5 conjugated adamantylamine (AdA-Cy5) was exploited as a new synthetic tool for imaging cellular processes in live cells. Confocal laser scanning microscopy revealed that CB[7]-Cy3 and AdA-Cy5 were intracellularly translocated and accumulated in lysosomes and mitochondria, respectively. CB[7]-Cy3 and AdA-Cy5 then formed a host-guest complex, reported by a FRET signal, as a result of the fusion of lysosomes and mitochondria. This observation not only indicated that CB[7] forms a stable complex with AdA in a live cell, but also suggested that this FRET pair can visualize dynamic organelle fusion processes, such as those involved in the degradation of mitochondria through autophagy (mitophagy), by virtue of its small size, chemical stability, and ease of use.