Chemical tags and beyond: Live-cell protein labeling technologies for modern optical imaging

Imaging proteins with high resolution is crucial for studying cellular physiology and pathology. Fluorescence imaging is a privileged method to visualize proteins with subcellular precision in live cells. In recent years, there has been a tremendous advance in the field of fluorescent dyes that are optically more sophisticated than genetically-encodable fluorescent proteins. In this review, we aim to discuss modern bioconjugation methods to specifically incorporate these dyes into protein-of-interests. We focus on advances in live-cell labeling strategies and fluorescent probes, especially the HaloTag, SNAP-tag, TMP-tag, and unnatural amino acid systems and their applications. These protein labeling methods, along with cutting-edge dyes and novel microscopy methods, have become the infrastructure for biological research in the era of super-resolution imaging.