Single-molecule force spectroscopy has revolutionized our understanding of protein mechanical mechanics by relying on versatile protein tags that enable specific tethering while providing unambiguous mechanical signatures. Here we characterize isopeptide bond-forming protein tags—the updated version of SpyTag-SpyCatcher (Spy003) and DogTag-DogCatcher (Dog)—as ideal multifunctional platforms that satisfy this need. Using magnetic tweezers, we demonstrate that these tags exhibit remarkably robust fingerprint signals when both N termini of the tag and catcher are stretched with characteristic critical forces (Spy003: 31.4 pN; Dog: 33.8 pN) and step sizes of approximately $4\mathrm{nm}$ across diverse experimental conditions including different buffers, salt concentrations (10–1000 mM NaCl), temperatures (23–36 °C), and pH values (pH 6–8). In particular, Dog exhibits local unfolding and refolding transitions of DogTag that are 6 times faster than those of Spy003, making it suitable for applications requiring high loading rates, while its compact hairpin structure minimizes structural perturbation when inserted into target proteins. We further proposed a variance-based force-calibration method that exploits the tags’ mechanical transitions to enable rapid, real-time force calibration during experiments, eliminating separate calibration routines. The combination of covalent tethering, environmental resilience, built-in calibration, and distinct mechanical signatures makes these tags powerful tools for studying protein mechanical mechanics in complex physiologically relevant environments.

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• Interactions & forces
• Mechanobiology
• Metrology
• Protein dynamics, structure & function
• Peptides
• Proteins
• Magnetic techniques
• Single molecule techniques