Long-term single-molecule tracking in living cells using weak-affinity protein labeling

Single-particle tracking (SPT) has become a powerful tool to monitor the dynamics of membrane proteins in living cells. However, permanent labeling strategies for SPT suffer from photobleaching as a major limitation, restricting observation times, and obstructing the study of long-term cellular processes within single living cells. Here, we use exchangeable HaloTag Ligands (xHTLs) as an easy-to-apply labeling approach for live-cell SPT and demonstrate extended observation times of individual live cells of up to 30 minutes. Using the xHTL/HT7 labeling system, we measure the ligand-induced activation kinetics of the epidermal growth factor receptor (EGFR) in single living cells. Furthermore, we generate spatial maps of receptor diffusion in cells, report non-uniform distributions of receptor activation, and the formation of spatially confined ‘hot spots’ of EGFR activation. This approach represents a general strategy to monitor protein dynamics in a functional context and for extended observation times in single living cells. ### Competing Interest Statement JK and KJ are listed as inventors on a patent application related to the exchangeable HaloTag ligands and filed by the Max-Planck Society. Abberior GmbH Goettingen commercializes the exchangeable HaloTag ligands. All other authors declare no competing interests.