Fluorescent labeling of proteins in vitro and in vivo using encoded peptide tags

Epitope tags are a simple and versatile way to label proteins, as their sequences can easily be inserted into protein coding sequences so that the expressed proteins will bear the tag(s). These tags can be used to identify and purify proteins in vitro using Western blots, flow cytometry, affinity chromatography, and other techniques. When labeled with a fluorescent probe, tagged proteins can be visualized in live or fixed cells or tissues using fluorescence microscopy, allowing for the study of protein dynamics. The most widely used epitope tags are those that have affinity to an antibody, which can be used in fixed-sample immunohistochemistry studies. While this will allow insight into a protein's localization, it will not provide any information on its dynamics. Other tags were developed with the intended use in live imaging. In this mini review, we discuss epitope tags that have affinity to antibodies, nanobodies, and small molecules and their use in fluorescence microscopy for fixed and live imaging.