Third-Generation Covalent TMP-Tag for Fast Labeling and Multiplexed Imaging of Cellular Proteins.

Self-labeling protein tags can introduce advanced molecular motifs to specific cellular proteins. Here we introduce the third-generation covalent TMP-tag (TMP-tag3) and showcase its comparison with HaloTag and SNAP-tag. TMP-tag3 is based on a proximity-induced covalent Michael addition between an engineered Cys of E. coli dihydrofolate reductase (eDHFR) and optimized trimethoprim (TMP)-acrylamide conjugates with minimal linkers. Compared to previous versions, the TMP-tag3 features an enhanced permeability when conjugated to fluorogenic spirocyclic rhodamines. As a small protein, the 18-kD eDHFR is advantageous in tagging selected mitochondrial proteins which are less compatible with bulkier HaloTag fusions. The proximal N-C termini of eDHFR also enable facile insertion into various protein loops. TMP-tag3, HaloTag, and SNAP-tag are orthogonal to each other, collectively forming a toolbox for multiplexed live-cell imaging of cellular proteins under fluorescence nanoscopy.