Overcoming the Limitations of Sortase with Proximity-Based Sortase-Mediated Ligation (PBSL)

S. aureus sortase A (SrtA), a calcium-dependent bacterial transpeptidase, is commonly used to site-specifically label proteins containing a LPXTG SrtA recognition motif with a wide array of chemical moieties. A major limitation of sortase-mediated labeling, however, is SrtA's poor binding affinity to its recognition motif, resulting in long reaction times and poor ligation efficiencies. Here we describe proximity-based sortase-mediated ligation (PBSL), which utilizes the SpyTag-SpyCatcher peptide-protein pair to tether target proteins with a SrtA recognition motif to SrtA, dramatically increasing their local concentrations and overcoming this limitation.