In vivo protein labeling with trimethoprim conjugates: a flexible chemical tag.

The introduction of green fluorescent protein and its variants (GFPs) has allowed protein analysis at the level of the cell. Now, chemical methods are needed to label proteins in vivo with a wider variety of functionalities so that mechanistic questions about protein function in the complex cellular environment can be addressed. Here we demonstrate that trimethoprim derivatives can be used to selectively tag Escherichia coli dihydrofolate reductase (eDHFR) fusion proteins in wild-type mammalian cells with minimal background and fast kinetics.