A fluorogenic array for temporally unlimited single-molecule tracking

We describe three optical tags, ArrayG, ArrayD and ArrayG/N, for intracellular tracking of single molecules over milliseconds to hours. ArrayG is a fluorogenic tag composed of a green fluorescent protein–nanobody array and monomeric wild-type green fluorescent protein binders that are initially dim but brighten ~26-fold on binding with the array. By balancing the rates of binder production, photobleaching and stochastic binder exchange, we achieve temporally unlimited tracking of single molecules. High-speed tracking of ArrayG-tagged kinesins and integrins for thousands of frames reveals novel dynamical features. Tracking of single histones at 0.5 Hz for >1 hour with the import competent ArrayG/N tag shows that chromosomal loci behave as Rouse polymers with visco-elastic memory and exhibit a non-Gaussian displacement distribution. ArrayD, based on a dihydrofolate reductase nanobody array and dihydrofolate reductase–fluorophore binder, enables dual-color imaging. The arrays combine brightness, fluorogenicity, fluorescence replenishment and extended fluorophore choice, opening new avenues for tracking single molecules in living cells. A genetically encoded array to amplify signals for live-cell single-molecule fluorescence imaging allows unlimited temporal resolution through exchange of fluorophores and can be used to monitor the dynamics of histones, kinesins and integrins.