Nanomolar affinity protein trans-splicing monitored in real-time by fluorophore-quencher pairs

High background originating from non-reacted, 'always-on' fluorescent probes remains a key unsolved problem in life science since washing procedures are not easily applicable. Covalent labeling approaches with simultaneous activation of fluorescence are advantageous to increase sensitivity and to reduce background signal. Here, we combined high-affinity protein trans-splicing with fluorophore/quencher pairs for online detection of covalent N-terminal 'traceless' protein labeling under physiological conditions in cellular environments. Substantial fluorescence enhancement at nanomolar probe concentrations was achieved.